Polybenzoxazines: Chemistry and Properties

Polybenzoxazines: Chemistry and Properties Dr. K.S. Santhosh Kumar Dr. C.P. Reghunadhan Nair iSmithers – A Smithers G...

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Polybenzoxazines: Chemistry and Properties

Dr. K.S. Santhosh Kumar Dr. C.P. Reghunadhan Nair

iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.ismithers.net

First Published in 2010 by

iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2010, Smithers Rapra

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library.

Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.

ISBN: 978-1-84735-500-3 (hardback) 978-1-84735-501-0 (softback) 978-1-84735-502-7 (ebook)

Typeset by Argil Services Printed and bound by Lightning Source Inc.

P

reface

The chemistry of polybenzoxazines blossomed in the mid-1990s. These thermally stable polymeric materials have become very popular during the last decade. They received a lot of attention in view of their potential use as high-performance polymers. This necessitated meticulous investigation of their chemical, mechanical, thermal and surface characteristics. Even though >200 articles have been published and many patents applied for, a book detailing the potential research and development of polybenzoxazines is lacking. We hope that a book describing the up-to-date development of these high-tech polymers will be an important resource for researchers and those working in the polymer industry. Polybenzoxazines obviate many shortcomings associated with their ‘parental’ phenolic resins. They can form void-free materials (due to addition polymerisation) whereas phenolic resins form voids during curing (because of condensation polymerisation). For polybenzoxazines, no catalyst is needed for curing, the moisture uptake is very low, the glass transition temperature is high, and the char yield is high. This book is divided into four chapters. Chapter 1 describes the competency of the properties of polybenzoxazine against other state-of-the-art polymers. Recent developments in parent phenolic resins, epoxy resins, and cyanate esters are addressed. The hydrogen bonding aspects in the crosslinked network of polybenzoxazines, the regioselectivity and kinetics of polymerisation (curing) are also detailed in this chapter. Chapter 2 is devoted entirely to the diverse types of syntheses of benzoxzines and their properties. The syntheses are grouped into (i) benzoxazine with additional polymerisable groups (e.g., allyl, propargyl); and (ii) benzoxazines derived from various precursors (based on several different amines and diphenols) without additional polymerisable moieties. Tailored polymeric systems such as blends, alloys, copolymers, nano-, micro- and fibre composites are described in a systematic manner in Chapter 3. Finally, the stability and degradation of polybenzoxazines under chemical,

iii

Polybenzoxazines: Chemistry and Properties thermal and photochemical environments are discussed in Chapter 4. A concerted effort has been made to include all significant advancements in polybenzoxazine science. However, in this emerging area of high-temperature polymers, novel polymers are constantly being discovered. Suggestions from readers are welcome and will be acknowledged gratefully. We thank R. Sivaramakrishnan and Dr. P. Radhakrishnan Nair, VSSC, for reviewing the manuscript and for giving fruitful suggestions. Thanks are due to the Editorial Board and Director, VSSC, for giving permission to publish the book. We would like to thank iSmithers Publishers for inviting us to write this book. K.S. Santhosh Kumar C.P. Reghunadhan Nair India, 2010 to Vikram Sarabhai Space Centre, Thiruvananthapuram, India, 2010

iv

C

ontents

1

Polybenzoxazines and State-of-the-Art High-Temperature Polymers.................................................................................................1 1.1

Introduction.................................................................................1

1.2

Epoxy Resins...............................................................................1

1.3

1.4

1.2.1

Chemistry of Epoxy Resins..............................................2

1.2.2

Reactions of Epoxy Resins and Curing Mechanisms.......3

1.2.3

Recent Developments......................................................3

Bismaleimides..............................................................................5 1.3.1

Synthesis and Features of BMI.........................................5

1.3.2

Modified BMI..................................................................6

1.3.3

Recent Developments......................................................8

Cyanate Ester Resins..................................................................12 1.4.1

Thermal Curing.............................................................12

1.4.2

Thermal Stability...........................................................13

1.4.3

Modified Systems..........................................................14 1.4.3.1

1.5

Phenolic-triazine Resins.................................15

Phenolic Resins..........................................................................17 1.5.1

Allyl-functional Phenolics..............................................19

1.5.2

Bisoxazolineñphenolics..................................................19

1.5.3

Phenolic ResinñEpoxy Systems......................................20

1.5.4

Propargyl Ether and Ethynyl Novolacs..........................21

1.5.5

Recent Developments in Phenolics.................................23

v

Polybenzoxazines: Chemistry and Properties

1.6

1.7 2

3

Polybenzoxazines.......................................................................24 1.6.1

Features of PBZ.............................................................28

1.6.2

Regioselectivity..............................................................28

1.6.2

Hydrogen-bonding Aspects...........................................31

1.6.4

Cure Monitoring and Kinetics.......................................34

Conclusion.................................................................................36

Structure-Property Relationships..........................................................51 2.1

Introduction...............................................................................51

2.2

Functionalised BZ (BZ Containing Addition-curable Moieties)....................................................................................52 2.2.1

BZ-Containing Allyl Groups.........................................52

2.2.2

BZ Containing Maleimide Groups................................55

2.2.3

BZ-Containing Propargyl Groups..................................59

2.2.4

BZ-Containing Furan Groups........................................61

2.2.5

BZ-Containing Acetylene Groups..................................63

2.2.6

BZ-Containing Nitrile Groups.......................................65

2.2.7

Main-chain or High-molecular-weight PBZ...................66

2.3

BZ Derived from Various Precursors (Non-functionalised BZ)............................................................77

2.4

Conclusion.................................................................................91

Blends and Composites of Polybenzoxazines........................................95 3.1

Introduction...............................................................................95

3.2

PBZ-Epoxy Blends.....................................................................95

3.3

PBZ-Poly(e-Caprolactone) Blends..............................................97

3.4

PBZ-Polyimide Blends................................................................98

3.5

Other Blend Systems................................................................100

3.6

Nanocomposites of PBZ..........................................................112 3.6.1

vi

PBZ-Clay Systems.......................................................112

Contents

4

3.6.2

PBZ-Polyhedral Oligomeric Silsesquioxane (POSS) Systems........................................................................114

3.6.3

Miscellaneous Nanocomposites...................................115

3.7

Fibre Composites and Microcomposites..................................115

3.8

Conclusion...............................................................................126

Stability, Degradation Chemistry and Applications.............................131 4.1

Introduction.............................................................................131

4.2

Chemical Stability....................................................................132

4.3

Thermal Stability and Degradation..........................................133

4.4

UV Stability and Photochemical Degradation..........................140

4.5

Degradation Mechanism of PBZ: A Comparison with other Thermosets.............................................................141

4.6

Applications.............................................................................145

4.7

Conclusion...............................................................................149

Abbreviations................................................................................................153 Index ..........................................................................................................161

vii

Polybenzoxazines: Chemistry and Properties

viii

1

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

1.1 Introduction High-temperature polymers are leading the polymer market because of their variety of applications. They find widespread use in areas such as adhesives, structural applications in aerospace, printed circuit boards, conductive polymer elements, and encapsulation materials for electronic applications [1]. The aerospace industry and space programmes have created new demands for high-temperature polymers which can withstand further higher temperatures owing to their excellent thermal and thermo-oxidative stability, high char yield, good chemical inertness, abrasion resistance and flame retardance. The best known members of the thermoset family are phenolic resins, epoxy resins, unsaturated polyesters, isocyanate-derived polymers, bismaleimides (BMI), acrylates, and cyanate esters [2]. Several challenges must be overcome regarding processing of these materials. This includes higher melting and processing temperatures as well as void formation during curing. In recent years, development of the benzoxazine (BZ)-based family of phenolic resins has attracted significant attention. Polybenzoxazines (PBZ) are a special type of phenolic resins that do not form voids during curing due to addition polymerisation. ‘Traditional’ phenolic resins result in void-full polymers because the polymerisation is of the condensation type. The attractive characteristics of BZ polymers include: low melt viscosity; no release of volatiles during cure and no need of harsh catalysts for curing; high thermal stability; good mechanical properties; excellent electrical properties; and wide flexibility with respect to molecular design. Before the discussion of PBZ chemistry, the state-of-the-art and other competing thermally stable polymeric candidates are addressed in this chapter. This is followed by discussion of the chemistry and fundamentals of PBZ.

1.2 Epoxy Resins Epoxies are the most versatile class of polymers and have diverse applications; they are probably the best-known thermoset polymers. The largest use of epoxies is in

1

Polybenzoxazines: Chemistry and Properties protective coatings; other applications include printed circuit boards, laminates, electronic materials and structural composites. Cured epoxies provide excellent mechanical strength; toughness; outstanding resistance to chemicals, moisture and corrosion; good thermal, adhesive and electrical properties; absence of volatiles and low shrinkage on cure; and good dimensional stability. This unique combination of properties is, in general, not found in any other plastic material. The patent literature indicates that the synthesis of epoxy resins was discovered as early as the late 1890s [3–5]. These materials are used as antenna reflectors, solar panel arrays, and optical support structures in satellites. Epoxy-polyaromatic composite motor cases have replaced the heavy metallic counterparts in modern launch vehicles. Satellites and other components are also made of epoxy/carbon and epoxy/polyaromatic composites.

1.2.1 Chemistry of Epoxy Resins The monomers of epoxy resins are characterised by more than one epoxy ring which undergoes ring opening to produce epoxy resins. Most commercially important epoxy resins are prepared by the coupling reaction of compounds containing at least two active hydrogen atoms (including polyphenolic compounds, monoamines and diamines, aminophenols, heterocyclic imides and amides, aliphatic diols and polyols and dimeric fatty acids) with epichlorohydrin followed by dehydrohalogenation. Epoxy resins derived from epichlorohydrin are termed as ‘glycidyl-based resins’. The most widely used epoxy resins are the diglycidyl ethers of bisphenol A (DGEBA) (Scheme 1.1).

HO

O

OH

CH 2 Cl

Epichlorohydrin

Bisphenol A

OH

O H2 C

O

O

CH 2

O CH 2 O

Typical bisphenol A epoxy resin Scheme 1.1 Typical synthesis of epoxy resins

2

O

CH 2

Polybenzoxazines and State-of-the-Art High-Temperature Polymers Approximately 75% of epoxy resins currently used worldwide are derived from DGEBA. Others include brominated bisphenol A (which imparts fire resistance), epoxy phenol novolac (EPN) resins, bisphenol F epoxy resins, epoxy cresol novolac resins, and tetraglycidyl-4,4′-diaminodiphenylmethane.

1.2.2 Reactions of Epoxy Resins and Curing Mechanisms Crosslinking of the resin can occur through the epoxide or hydroxyl groups. It proceeds by two types of curing mechanism, i.e., direct coupling of resin molecules by a catalytic homopolymerisation, or coupling through a reactive intermediate. The ability of this ring to react by several paths and with various reactants gives epoxy resins their great versatility. The chemistry of most curing agents currently used is based on polyadddition reactions that result in coupling as well as crosslinking. The most widely used curing agents are compounds containing active hydrogen such as polyamines, polyacids, polymercaptans and polyphenols [6–15].

1.2.3 Recent Developments The fully cured fluorinated epoxy resin 1,1-bis (4-glycidylesterphenyl)-1-(3trifluoromethylphenyl)-2,2,2-trifluoroethane was shown to have good thermal stability with a glass transition temperature (Tg) of 170–175 °C and temperature at 5% weight loss of 370–382 °C in N2. It also showed mechanical properties as good as the commercial epoxy resins. It also possessed low water absorption due to the presence of a hydrophobic fluorine atom [16]. A novel imide ring and siloxanecontaining cycloaliphatic epoxy monomeric compound 1,3-bis[3-(4,5-epoxy-1,2,3,6tetrahydrophalimido) propyl] tetramethyl disiloxane (BISE) was thermally cured with alicyclic anhydrides, hexahydro-4-methylphthalic anhydride (HMPA) and hexahydrophthalic anhydride (HHPA), respectively. The fully cured BISE epoxy resins have good thermal stability, high thermal decomposition temperature, and excellent mechanical and dielectric properties. However, they gave a relatively low Tg due to the presence of flexible propyl and siloxane segments in the epoxy backbone [17]. Blends of diethyl tolylene diamine-cured DGEBA epoxy resin have been modified by flexible diamines, which show that the addition of flexible diamines improves the elongation at break and impact strength [18]. New mercaptan-terminated polythiourethanes were used as curing agents for epoxy resin, which found use as effective surface-coating materials [19]. The effect of multiwalled carbon nanotubes (MWNT) on the cure reaction of the epoxy resin showed that, at the early stage of the cure reaction, a low mass fraction of MWNT reduces the activation energy of the reaction. However, excess MWNT hinder the contact

3

Polybenzoxazines: Chemistry and Properties between functional groups, which elevates the activation energy [20]. The epoxycardanol resin exhibits better properties as compared with epoxy resin (DGEBA) in terms of increase in tensile strength, elongation, bonding with steel, and lowering of water vapour transmission of the film [21]. Diallyl bisphenol A (DABA) was used as a curative for novolac epoxy resin (EPN). The reaction was catalysed by triphenyl phosphine (TPP). The activation energy of the system diminished from 91 kJ/mol to 67 kJ/mol on addition of 0.5 wt% TPP [22]. The matrix resin provided a low Tg (80 °C). On initiating further crosslinking by the 100% curing of the allyl groups, the mechanical properties were diminished even though Tg was increased (Table 1.1) [23]. However, the glass composites of the matrix system showed a marked increase in flexural strength from 369 MPa (40% allyl curing) to 458 MPa by complete curing of allyl groups.

Table 1.1 Effect of allyl curing on the properties of a DABA-EPN system Property

40% Allyl cured neat

100% Allyl cured neat

Tensile strength (MPa)

118

77

Flexural strength (MPa)

113

110

Tg (°C)

79

91

A polyhedral oilgomeric silsesquioxane containing eight functional hexafluorine groups called octakis (dimethylsiloxyhexafluoropropyl ether) silsesquioxane (OF) is a nanoporous additive. OF has been synthesised and blended with the ultraviolet (UV)-cured epoxy resin, which resulted in remarkable thermal characteristics. OF containing (10%) epoxy has a significantly lower dielectric constant (2.65) than the neat epoxy (3.71) [24]. Epoxy-phenolic-based foams have also been studied [25]. Epoxy-clay nanocomposites were prepared with organically modified layered clay with varying clay contents (1–8 wt%). The tensile modulus of the nanocomposites increased by 47%, but improvement in tensile strength and Tg was not observed due to the intercalated morphology of clay layers in the epoxy resin systems [26]. Epoxy-coiled carbon nanotube composites have shown high hardness and elastic modulus [27].

4

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

1.3 Bismaleimides Polyimides are manufactured in various forms which find use in the electronic, medical, structural, and adhesive fields. Polyimide films are very popular, particularly in the aerospace field, and are known as ‘Kapton films’ (DuPont). In film form, they do not display a Tg, have good radiation resistance, and are used as bagging material for composites, belts, wire wrapping, and thermal insulation for satellites. Condensation and addition polyimides are used as high-performance matrices. Condensation polyimides are typically based on high-molecular-weight poly(amic acid) precursors that release voids during curing and decompose below their melt temperature. Addition polyimides are low-molecular-weight resins containing unsaturated moieties such as maleimido, ethynyl, and nadimido. They do not form voids when they undergo curing and a void-free matrix is theoretically possible. Among addition polyimides, BMI is the most important system used for advanced material applications due to its high performance-to-cost ratio. However, unmodified BMI show brittleness due to high crosslink density and lower toughness.

1.3.1 Synthesis and Features of BMI BMI resins are synthesised by reacting diamines with maleic anhydride in two steps: formation of bismaleimic acid and imidisation [28]. A typical synthesis is given in Scheme 1.2. O

O

O NH

O O

H2 N

Ar

R

NH

O

N H2

N C O

OH

OH

O

C

C

O

Ar

N O

O

Scheme 1.2 General synthesis of a bismaleimide monomer BMI resins undergo curing initiated at ~150–250 °C and peak temperatures ~230– 330 °C dependent upon the structure. In general, the onset and peak exotherm temperatures increase with the size of the bridging group between two maleimido groups. These results arise from the reduced concentration of the maleimido groups. Cured BMI have excellent heat and thermal resistance, and high Tg and decomposition temperatures. Bis(4-maleimidophenyl)methane and its modified resins are the most important advanced thermosetting materials. These resins possess a differential

5

Polybenzoxazines: Chemistry and Properties scanning calorimetry (DSC) exotherm peak temperature of 260 °C and melting point of 148–158 °C. The mechanical properties are good and have a Tg of 230–290 °C. They have high property retention at elevated temperatures. Electrical properties such as dielectric strength, volume resistivity, dielectric constant, and dissipation factor are generally good for the fabrication of electronic components [29]. However, low elongation to failure and poor toughness are concerns for structural applications. Even though basic BMI resins possess good performance after thermal curing, the toughness of the cured resin is a major concern with respect to material durability. BMI resins modified with soft segments are useful for flexibility improvement but drastically reduce their glass transition temperatures and property retention at elevated temperatures. Incorporation of ether linkages in a BMI resin reduces the brittleness of the cured thermoset. BMI resins are incorporated with aliphatic polyether to modify the fracture strength. Introduction of engineering thermoplastic segments between two bismaleimido groups is an attractive way to achieve high heat resistance and improved toughness. BMI-poly(arylene ethers) show significant increase in their Tg values after curing. Because of the improved solubility (processability) and toughness compared with simple BMI resins, these materials are useful for the preparation of structural composites. These composites have low tensile strength, elongation, good heat resistance and electrical properties similar to phenolic and epoxy composites [30]. BMI resins with low-molecular-weight compounds and oligomers require a significant increase in molecular weight and structure modification to become useful. A few curing reactions, such as thermal polymerisation, addition reactions, and the Diels–Alder reaction have been developed to increase the application performance of BMI resins.

1.3.2 Modified BMI In general, aromatic BMI resins produce high char yield at elevated temperatures and are self-extinguishable. The fire retardance of aromatic BMI can be enhanced by bridging with phosphorus-containing segments [31]. They have a Tg >380 °C and anaerobic char yield of 60–71% at 800 °C. Maleimide-terminated oligomers and maleimide end-capping agents have been investigated for high-temperature applications [32–34]. BMI undergo Michael additions with hydrogen-active moieties such as phenol, thiols, and amines. These reactions are used to prepare various high-performance thermosets. One important class of high-temperature thermoset is derived from BMI resins and allylphenyl oligomers. Upon thermal curing, the mixtures copolymerise to produce polymers with a high Tg. For example, the mixture of 3,3′-DABA and bis(4-maleimidophenyl)methane displays low- and high-temperature exotherms at ~130 °C and ~255 °C, respectively. The low-temperature exotherm was recognised as an ene-reaction between allyl and maleimido groups. The high-temperature exotherm

6

Polybenzoxazines and State-of-the-Art High-Temperature Polymers was related to a Diels–Alder reaction of the functional groups produced in the reaction [35]. The overall reaction, known as the ‘Alder-Ene reaction’, is illustrated in Scheme 1.3. A stiff, heat-resistant ring structure is formed by Diels–Alder polymerisation. It is an excellent approach to increase polymer performance. Soluble BMI with good electrical properties have been synthesised from siloxane-containing bis furans and BMI monomers [36]. The brittleness of the cured thermoset is a major concern for structural applications. Rubber toughening is an effective way to improve the impact resistance, but heat and thermo-oxidative resistance goes down due to the reduction in the Tg and the poor oxidative stability [37].

O HO

N

OH

O R

O

o,o′ diallyl bisphenol-A

O

Bismaleimide Ene addition O

O

HO

O

N

OH

R

O

N

N

HO

O

O

Ene adduct O

Wagner-Jauregg

N

O R

O R

O

O O

N

O

N OH

O

O R

N

HO

N

OH O

O

O

Ene and Diadduct Further crosslinking Crosslinked product

Scheme 1.3 Co-reactions in the blend of DABA and BMI 7

Polybenzoxazines: Chemistry and Properties Diamine-terminated amide resins were prepared to modify bis(4-maleimidophenyl) methane to lower the crosslinking density and to increase the toughness of the cured materials [38, 39]. The same resin has also been chain-extended with aliphatic amines [40]. Polyoxyalkyleneamines are effective chain extenders that significantly improve the flexibility and elasticity of modified resins [41]. Copolymerised BMI materials have been prepared to improve the processability and the performance of the final thermoset [42]. A technology using alkenyl phenols and alkenyl phenol ethers to react with BMI to form processable prepolymers was developed by Zahir and Renner to improve the toughness and humidity resistance of BMI resins [43]. During the crosslinking process, allylphenyl ethers undergo thermal Claisen rearrangements to the corresponding allylphenols, and then proceed with the crosslinking reactions through the Ene–Alder reaction and Diels–Alder reaction. A class of thermosetting resins, N-allyloxyphenyl maleimides, having both allyl and maleimide groups in a molecule have been developed [44]. Polybenzimidazole is a high-temperature thermoplastic polymer with a Tg of 435 °C that has been incorporated into BMI to increase the fractural energy and toughness [45]. Combination of the rigid rod benzimidazole structure, Michael addition of BMI resins, and BMI-allylphenol reactions have resulted in the preparation of molecular composites with the simultaneous improvement of tensile strength, modulus, and elongation [46]. Commercial addition-cure formulations based on co-reaction of diallyl phenols and BMI such as Matrimide-5292 from Ciba-Geigy which typically contain DABA and 4,4′-bismaleimido diphenyl methane (BMM) (Scheme 1.4) are leading matrix resins for carbon fibre composites for advanced aerospace applications.

O

O

N

CH 2

O

N

O

4,4′ Bismaleimido diphenyl methane (BMM)

Scheme 1.4 Structure of BMM

1.3.3 Recent Developments Interpenetrating polymer networks (IPN) of BMI-modified polyurethane-epoxy

8

Polybenzoxazines and State-of-the-Art High-Temperature Polymers systems were prepared by curing polyurethane-modified epoxy with aliphatic or aromatic BMI in the presence of 4,4′-diaminodiphenylmethane. Incorporation of aromatic BMI into the polyurethane-modified epoxy system increased the Tg, thermal stability, and electrical properties. Decreased values of the Tg and heat distortion temperature were obtained in the case of aliphatic BMI-modified polyurethaneepoxy systems [47]. Epoxy systems modified with cyanate ester (CE) were made with diaminodiphenyl methane as the curing agent. The cyanate ester-toughened epoxy systems were further modified with the BMI N,N′-bismaleimido-4,4′-diphenylmethane and N,N′-bismaleimido-4,4′-diphenylsulfone. Incorporation of BMI into unmodified epoxy and CE enhanced the thermal and mechanical properties according to its percentage content [48]. Epoxy-novolac resin modified with 4,4′-bismaleimido diphenyl methane showed that when BMI concentration was high, the adhesive force, degradation temperature and thermal stability were enhanced, whereas the shear strength showed a decreasing trend [49]. Epoxy-based laminate properties were improved with the incorporation of BMI [50]. The BMI modified-novolac resin was synthesised by allylation of the novolac resin and its ‘ene’ reaction with BMI. The BMI-modified allyl novolac resin with 48% degree of allylation has the best thermal properties and the highest dynamic modulus [51]. The curing process of BMI and dicyanate ester monomers containing a naphthalene ring involves copolymerisation between these monomers, and self-additional polymerisation may occur in the last course of the curing process. Investigation into the co-curing of bisphenol A-based CE and the BMI 2,2-bis[4-(4-maleimido phenoxy) phenyl]propane (BMP) indicates no co-reaction between the two, and the system finally formed a sequential IPN [52, 53]. The cured polymer blends were found to undergo two-stage decomposition, with each stage corresponding to the components of the blend. Compositions richer in BMP were found to be relatively brittle and possessed a high propensity to develop microvoids on curing. The system becomes brittle with BMP content of >40%. The 2,2-bis(4-cyanatophenyl)propane (BACY)-rich systems were good only for neat resin casting. The increase in BMP content decreased the tensile properties, whereas these blends possessed improved flexural strength and fracture toughness. The BACY-rich blends showed a single Tg which was observed at a slightly lower Tg than that of polycyanaurate (~250 °C). However, the blends with BMP content of >50% showed two glass transitions due to phase separation. Due to the formation of IPN structure, these blends showed low moisture absorption (1.491.25 wt%). The mechanical properties of BACY/BMP sequential IPN are shown in Table 1.2.

9

Polybenzoxazines: Chemistry and Properties

Table 1.2 Mechanical properties of BACY/BMP IPN matrix systems Content of Fracture BMP in the IPN toughness (MN/ (wt%) m3/2)

Tensile strength (MPa)

Tensile modulus (MPa)

Flexural strength (MPa)

0

3.8

70

3140

95

10

3.5

68

3100

100

20

3.4

64

3531

105

30

4.1

59

3825

114

40

5.3

46

3603

117

The BMI–triazine resins (effectively a blend of CE and BMI) with different proportions showed good thermal stability, their Tg values were >440 °C, and the residual char ratio at 700 °C was ~60% [54]. The onset cure temperatures of the blend of bis propargyl ether bisphenol A (PBPA) with 4,4′-bismaleimide diphenyl methane (BDM) resins were about 20–30 °C lower than that of pure PBPA, and the cure exothermic enthalpy of the resins also significantly reduced from 1320 J/g (PBPA) to 493 J/g (PBPA-BDM (1.0:2.0)) [55]. Thermosetting resin systems with very high Tg values were formulated on the basis of BMI and allylated novolac. When the allylation degree of the novolac resin was sufficiently high, the BMI proportion was not critical to the heat resistance of the cured resin [56, 57]. BMI and biscitraconimides with bisallyl groups and brominated BMI also showed enhanced thermal and mechanical features [58, 59]. A series of BMI monomers with amide groups were prepared and characterised [60]. A new type of BMI resin containing an epoxy unit and phosphorus in the main chain was synthesised [61]. The polymers, obtained through the reactions between BMI and diamine agents, also demonstrated excellent thermal properties and high char yield. Novolac resin based on 2,2′-diallyl bisphenol A when co-reacted with bisphenol A bismaleimide provided a resultant high-temperature resin with good adhesive strength at higher temperature. The moderately crosslinked blend was conducive for achieving optimum adhesive properties on aluminium substrates. Retention of adhesive properties was >100% at 150 °C [62]. Phenol-(4-hydroxy)phenylmaleimide-

10

Polybenzoxazines and State-of-the-Art High-Temperature Polymers formaldehyde (PMF) resins were prepared from phenol, hydroxyphenylmaleimide (HPM) and formaldehyde. These matrix resins were investigated for their adhesive properties by blending with EPN. The lap shear strength (LSS) of PMF resin was poor (~5 MPa) at ambient temperature. The PMF-29 resin (which contains 29% of HPM) with EPN in a 1:1 ratio provided optimum adhesive properties with 84% and 47% retention of the LSS at 150 °C and 175 °C, respectively, and could serve as a potential structural adhesive for moderately load-bearing applications. The cure reactions in the PMF-EPN system are shown in Scheme 1.5. The adhesive strength was improved by toughening the materials through blending with high-temperature thermoplastics [63, 64].

CH2 OH O

CH2

CH2

CH O

CH2

O

N

O

O

N

O

CH2

CH2

m

CH2

n

170 °C

CH2

OH

n

O

OH

CH2

EPN

PMF

HO

O

CH CH2

CH2

O O

CH2

m

CH2 HO

CH O

O

N

H2 C

CH2

CH2

m

CH2

0 °C -25 180 n

O CH2 HO

CH2 O

H2C

O

CH

CH2 HO

CH O

CH2

Scheme 1.5 Cure reactions in PMF-EPN blend

11

Polybenzoxazines: Chemistry and Properties

1.4 Cyanate Ester Resins CE resins have received considerable attention in the past few years due to their importance as thermosetting resins for use as encapsulants in electronic devices, hightemperature adhesives, and structural materials in aerospace applications because of their outstanding mechanical, thermal, and adhesive properties [65–70]. This new generation of thermoset resins encompasses the processability of epoxy resins, thermal characteristics of BMI, and the heat resistance and fire resistance of phenolic resins. CE resins have their own unique properties such as good strength, low dielectric constants, radar transparency, low water absorption, and superior metal adhesion, which make them the resin of choice in high-performance structural applications in the electronics and aerospace industries [71, 72]. CE are formed in excellent yields by the reaction of the corresponding phenols with cyanogen halides (Scheme 1.6).

R

OH

CNBr

HBr

R

OCN

Scheme 1.6 Synthesis of cyanate esters

1.4.1 Thermal Curing CE can undergo thermal or catalytic polycyclotrimerisation to give polycyanurates. The catalysts are usually Lewis acids, transition metal complexes or amines. In general, they are cured with a transition metal catalyst or chelate catalyst in the presence of a hydrogen donor such as nonyl phenol. It is accepted that the cyanate cure takes place through cyclotrimerisation to give polycyanurates. However, evidence for this mechanism is inconclusive. The thermal cure reaction of the bisphenol A cyanate BACY using a monofunctional model compound suggested that trimerisation was the major reaction (>80%) in the curing process [73]. There was very little outgassing during the polymerisation reaction, which allowed for easy fabrication of void-free composites. The thermal stability of the thermoset was much higher than that of most epoxy-based systems.

12

Polybenzoxazines and State-of-the-Art High-Temperature Polymers A widely studied CE known as BACY can be processed and cured above its melting point (80 °C). The resulting thermoset exhibits good thermal stability (95% weight retention at 430 °C) and a Tg as high as 290 °C if fully cured [67, 74]. However, the resulting polymer is brittle, which limits its use in many applications. Several factors such as impurities and environment [75, 76], solvent [77], and catalysts [78] influence the cure reaction. It has been reported that no reaction occurs if absolutely pure CE is heated [79]. In the absence of an externally added catalyst, the reaction is believed to be catalysed by water and residual hydrogen-donating impurities such as phenol [80]. It was found that the Tg of the network was lowered if it cured in the presence of solvent. Like other thermoset resins, CE are amenable to processing by a large variety of conventional techniques. Their processing versatility has gained them widespread acceptability in composites for various applications. The flexibility is further enhanced by blending with other resins such as epoxies, BMI, additives, and toughening agents. The cure cycle is dependent upon the catalytic level. Partially polymerised thermosetting resins retain the ability to fuse and to form further crosslinks achieving good tackiness.

1.4.2 Thermal Stability The cyanurate is a thermally stable crosslinking polymer responsible for the high mass loss temperature (450 °C) of these thermosets. Polycyanurates derived from phenol novolac cyanate esters known as phenolic-triazine (PT) resins have a high Tg (>350 °C), which approaches their thermal decomposition temperature [81]. In addition to high thermal stability, polycyanurates form a carbonaceous char during burning that protects the underlying material and further enhances the fire resistance. Thermogravimetric studies of polycyanurates in air have indicated that thermo-oxidative degradation proceeds via rapid hydrolysis of the ether oxygen bond between the phenyl and triazine rings in the presence of moisture at 350–420 °C [82, 83]. Purely thermal degradation under anaerobic conditions is claimed at higher temperature (545 °C) via homolytic cleavage of the hydrocarbon backbone over a narrow temperature range (450–500 °C) independent of the chemical structure of the linking groups between the cyanurate rings. Polycycanuarte structure is given in Scheme 1.7.

13

Polybenzoxazines: Chemistry and Properties

R

R OCN

R O

NCO

OCN

R

Heat

O

N N

N O

R R

Cyanate ester monomer

Polycyanurate

Scheme 1.7 Structure of cyanate ester monomer and polycyanurate

1.4.3 Modified Systems Even though most commercial CE possess good flammability and high-temperature properties, they are too brittle to be widely used in structural applications. Many additives have been used to strengthen the resulting CE thermoset, including epoxies [74, 76, 84], polyesters [85] and BMI [66, 86] with varying success. Co-curing the CE with these polymers could result in non-miscibility, which has desired and undesired effects on the physical and thermal properties of the polymeric matrix [87, 88]. CE could be substantially toughened by the addition of rubbery or rigid thermoplastic components [89–97]. The most effective toughening approach has been incorporation of thermoplastics with a high modulus and high Tg [89, 91, 94–98]. In recent years, the addition of nanoscale fillers such as layered silicates has been used as an alternative approach to enhance performance [99–105]. The major reason for introducing these fillers at the nanoscale level was the pronounced improvements in properties at low clay contents. CE resins have recently attracted the attention of composite fabricators due to their high Tg (typically ~290 °C) [99, 106] and relatively easy, epoxy-like processing. However, to obviate the brittleness of CE, thermoplastic resins with high Tg values were used for blending. Resins such as polyether ketones [107], polyether imides [108-110], polyethersulfone [111, 112] were also incorporated for improving properties.

14

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

1.4.3.1 Phenolic-triazine Resins PT precursor resin is a reaction product between novolac resin and cyanogen halides. A PT network is formed by the thermal cyclotrimerisation of the cyanate ester of novolac [113] (Scheme 1.8). The networks have low melt viscosity, resinous consistency, long gel time, good thermal expansion and a Tg up to 399 °C dependent upon post-cure conditions. However, the ultimate cure temperature must be high (>300 °C) to achieve optimum cure and a higher Tg (>300 °C). PT resins possess better thermo-oxidative stability and char yield than conventional phenolics because they are mostly crosslinked by triazine groups. The decomposition starts at ~420 °C and the char yield is ~65–70%. PT resin is commercially available under the trade name Primaset PT-15, PT-30, PT-60 and PT-90. They essentially differ in their molar masses [114].

OH

O

OCN CH 2

CNBr

CH 2

Triethyl amine CH 2

Novolac

N

Heat O

N N

O

CH 2

Novolac cyanate ester

Phenolic-triazine network

Scheme 1.8 Synthesis of PT resins

Many structural alterations have been attempted to confer specific properties to polycyanurate matrices. Thus, a flame-retardant and atomic oxygen-resistant cyclomatrix phosphazene-triazine network was derived by employing Alder-ene chemistry. Co-reaction of a blend of allyl phenoxy triazine and allyl phenoxy phosphazene with BMI gave rise to a phosphazene-triazine network [115] (Scheme 1.9).

15

Polybenzoxazines: Chemistry and Properties NC O CH3 C CH3

O

P PhO N

CH3 C

N

P

N P O OPh

N

CH3 C CH3

O CN CH3

PNC-3

NCO

O P PhO N

OPh O

Heat

Ph O P O N

N

O

P O OPh

O

O

O N

N

N

N

O N

O

N

P

O Ph N

Ph P O O P O N Ph O

O

CH3

Where,

=

C CH3

Scheme 1.9 Formation of phosphazene-triazine network polymers

16

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

1.5 Phenolic Resins Despite the emergence of several new classes of high-performance thermosets that are superior in some aspects, phenolic resins retain industrial and commercial interest one century after their introduction. These resins have several desirable characteristics such as superior mechanical strength, heat resistance, and dimensional stability, as well as high resistance against various solvents, acids and water. Although phenolics cannot be substituted for epoxies and polyimides in many engineering areas, their composites find a major market in thermo-structural applications in the aerospace industry due to good heat resistance and flame resistance, excellent ablative properties, and low cost. These key properties mean that phenolic resins can cope with the ever-changing requirements and challenges of advanced aerospace technologies [116–120]. The simple and inexpensive synthesis of phenolic prepolymers is well established. The commonest types of phenolic-based prepolymers are resoles and novolacs. Syntheses of resole and novolac prepolymers are carried out using phenol or its derivatives and formaldehyde under acidic or basic conditions. Novolacs are formed under highly acidic conditions with an excess of phenol. Resoles are synthesised under basic conditions with an excess of formaldehyde. The conversion of novolacs into insoluble and infusible networks requires catalysts, whereas resoles require only heating over a certain time period [121–123]. The phenolic networks are highly aromatic and resistant to thermal oxidation. However, some of the inherent qualities derived from their special chemical structures hamper their acceptance as universal polymeric materials in many engineering areas. These resins cure at moderately high temperature by a condensation mechanism with the evolution of volatiles. The need for a catalyst for curing and the limited shelf-life of resins at ambient conditions are major shortcomings of these systems. When compared with many known thermally stable polymers, their thermo-oxidative stability is low. The rigid aromatic units tightly held by short ethylene linkages make the matrix brittle. The degradation of phenolic resins is depicted in Scheme 1.10. In view of this, a new chemistry is needed to modify the cure of phenolic resins. Addition cure phenolic resins can obviate the shortcomings associated with condensation-type phenolics. The major strategies in designing addition-cure phenolics are: (i) incorporation of thermally stable addition-curable groups onto a novolac backbone; (ii) structural modification (transformation) involving phenolic hydroxyl groups; (iii) curing of novolac by suitable curatives through addition reactions of OH groups; and (iv) reactive blending of structurally modified phenolic resin with a functional reactant [124–127]. A brief account of addition-cure phenolic resin chemistry is discussed in the following section.

17

Polybenzoxazines: Chemistry and Properties

HO

CH 2 OH O2

OOH

HO

O

HO

CH

CH

OH

HO

OH

OH CH

O

HO

C

OH

OH

OH

OH +

CO OH

OH O C.

. +

O C O

OH .

+

C O

Scheme 1.10 Thermal degradation of phenolic resins

18

OH

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

1.5.1 Allyl-functional Phenolics Allyl phenol–formaldehyde novolac, synthesised by the allylation of novolac, can cure thermally at 180 °C without the evolution of volatiles. The allyl derivatives of phenols have been used for the manufacture of glass fibre-reinforced plastics and mouldings, as well as casting or impregnating compositions with high heat resistance, mechanical strength and chemical stability [126]. However, the cured matrix is not thermally stable due to the thermal fragility of the crosslinks arising from the polymerisation of allyl groups [127]. The thermal stability of the allyl phenolic novolac resins could be further improved by reactive blending with BMI compounds. Studies by Enoki and co-workers [128] showed that the reaction between the two components proceeds via the Ene reaction. The unsaturated Ene adduct intermediate undergoes a further Diels–Alder type reaction with BMI to give the -bis and -tris adducts. The intermediate step (Diels–Alder) is sometimes referred to as the Wagner–Jauregg reaction [129]. Ideally, a ratio 1:3 (allyl:maleimide) provides maximum crosslinking and enhanced thermal stability, but this could lead to brittle matrices. In most cases, a compromise of various properties was achieved at an allyl:BMI ratio of 1:2. Commercial addition-cure formulations based on the co-reaction of diallyl phenols and BMI are available such as Matrimide-5292. Matrimide-5292 is used as a matrix resin in carbon fibre composites for advanced aerospace applications. Suitably formulated, the resin system made up of 4,4′-bismaleimido diphenyl methane, DABA and desirable catalysts can give a high Tg (315 °C) matrix that is stable up to 450 °C [130]. The Alder–ene polymers can be conferred good ablative properties by introducing boron into the molecular backbone of allyl compounds [131]. Allyl naphthols can replace allyl phenols in Alder–ene adducts [132].

1.5.2 Bisoxazoline–phenolics The unusual addition co-reaction of novolac phenolic resins with phenylene bisoxazoline has been explored to derive a new class of non-conventional phenolic thermosetting resins by Culbertson and co-workers [133]. The polymerisation involves a tertiary phosphine-catalysed reaction of bisoxazoline with a phenol-free novolac resin, leading to an ether–amide copolymer. The reactions and cure chemistry are shown in Scheme 1.11. The systems are suitable for high-performance composite applications [134]. These materials have low cure shrinkage, high neat resin modulus, no volatiles during cure, low coefficient of thermal expansion, and excellent toughness. The fibre-reinforced copolymers possess the low smoke and heat-release requirements of materials for aircraft interior applications [135]. Based on this chemistry, several compositions with many interesting properties have been patented. Electrical, physical and mechanical properties of the neat resin suggest that these new thermosets could be

19

Polybenzoxazines: Chemistry and Properties useful in various electrical applications. Incorporation of siloxane and montmorillonite clay is very effective in enhancing the flame resistance of the bisoxazoline–phenolic system [136]. A Tg of 220 °C was observed for the 8% polysiloxane-modified material, compared with the Tg of 248 °C for the unmodified system.

O

N

N

C

C

OH

OH

O

1,3-Phenylene bisoxazoline (PBOX)

PHEN 175-225°C

O C

O NH

CH 2 CH 2 O

O

CH 2 CH 2 NH

C

PBOX-PHEN

Scheme 1.11 Addition polymerisation of the bisoxazoline-phenolic system

1.5.3 Phenolic Resin–Epoxy Systems Curing of epoxy with novolac by making use of the OH–epoxy reaction appears to be the simplest way to design an addition-cure phenol system. Although less preferred, polyphenols are used as curative for epoxies because the addition-curing results in void-free products that are comparatively tougher due to the formation of flexible ether networks [137–140]. Interest in these systems has been revived further by the need for void-free, low moisture-absorbing matrices with low dielectric properties for various electronic applications. Their cure kinetics have been studied extensively [141–155].

20

Polybenzoxazines and State-of-the-Art High-Temperature Polymers Novel phenolic novolac resins bearing maleimide groups (PMF resin) can undergo cure mainly through addition polymerisation of these groups. They were synthesised by polymerising a mixture of phenol and HPM with formaldehyde in the presence of an acid catalyst [156] (Scheme 1.12). The thermal curing of the PMF system through polymerisation of the maleimide group resulted in comparatively brittle matrices [157].

O O

N

O

HCHO

+

OH

H+

H2 C

N

CH 2

OH

OH

O CH2OH CH2

CH2 OH

OH

HPM

Soluble PMF resin

Scheme 1.12 Synthesis of PMF resin from N-(4-hydroxy phenyl) maleimide and phenol

1.5.4 Propargyl Ether and Ethynyl Novolacs Although less commercially exploited, propargyl ether-functional phenolic resins (PN resins) were developed as a potential hydrophobic substitute for epoxies in advanced composites, electronics, adhesives and coatings. Most thermosets such as epoxy and BMI absorb moisture up to 5%, resulting in low hot/wet physico-chemical properties. This problem could be avoided by using propargyl phenolics. The structural similarity of propargyl ether to epoxy resins is useful for the preparation, processing and development of thermally stable polymers [158]. The curing of propargyl ether resins proceeds by a Claisen rearrangement followed by addition polymerisation of the resultant chromene [159]. A Tg of ~300 °C and moisture absorption of ~0.3–0.4% have been observed. Addition-curable phenolic resins bearing terminal ethynyl groups anchored to a benzene ring through a phenyl azo linkage such as ethynyl phenyl azo novolac were realised by a simple synthetic strategy involving the coupling reaction between novolac and 3-ethynyl phenyl diazonium sulfate [160] (Scheme 1.13). These resins showed a broad cure exotherm in DSC in the 140–240 °C range due to the curing of acetylene groups. The cured resin retained a char yield of 70%.

21

Polybenzoxazines: Chemistry and Properties

PN polymer

Scheme 1.13 Synthesis and polymerisation of propargyl ether-functional novolac resins (PN resins)

Phenyl ethynyl functional phenol–formaldehyde (novolac-type) addition-curable resins were synthesised by reacting a mixture of phenol and 3-(phenylethynyl)phenol with formaldehyde in the presence of an acid catalyst [161]. The polymerisation reaction was done at 75 °C. The resin underwent thermal curing at ~250–275 °C. The cure mechanism was proposed (Scheme 1.14) as a combination of acetylene addition [162] and by addition of phenol to the triple bond as implied in a model study [163]. These addition-cure phenolics provided an overall char yield of ~70%.

22

Polybenzoxazines and State-of-the-Art High-Temperature Polymers OH

OH

OH H 2C

HCHO

H+

C

CH2

HO

C

OH CH2

OH CH 2 C

C

C

C

CH2

Phenylethynylphenol (PEP) PEPFN

CH 275ºC

-

OH H2 C Further addition and crosslinking

OH CH2

CH2

C

OH CH 2

Heat

HO

C C

CH

CH2

C OH

CH 2

O

CH 2 CH2 CH 2

Scheme 1.14 Crosslinking of phenyl ethynyl functional phenolics phenyl ethynyl functional novolac resins (PEPFN)

1.5.5 Recent Developments in Phenolics By incorporation of propargyl and methylol groups onto a novolac backbone, a series of addition-curable phenolic resins and condensation-addition dual-cure-type phenolic resins (novolac modified by propargyl groups is referred to as PN; novolac modified by propargyl and methylol groups simultaneously is referred to as MPN) were synthesised and PN and MPN resins exhibited excellent processing properties [164, 165]. It was recognised that PN resin is an ideal candidate for advanced composite matrices in thermo-structural and ablative applications [164]. Modified novolac resins with BZ rings were prepared and copolymerised with a glycidyl phosphinate. The materials exhibited a high Tg and retardation on thermal degradation rates [166]. Novel molybdenum-phenolic resins were prepared. When the mixing ratio of the molybdenum-phenolic resin (with 12% molybdenum) to the curing agent was

23

Polybenzoxazines: Chemistry and Properties 100/10 (w/w), the curing temperature and activation energy were at a minimum, the thermal degradation stability of the cured product was optimal, and the temperature corresponding to the maximum extent of curing was 200 °C. The curing mechanism was similar to that of conventional phenolic systems [167]. A series of MWNT were obtained as products from catalytic pyrolysis of the crosslinked phenol-formaldehyde resins with different ferrocene content under an inert atmosphere. The amount of nanotubes increased with iron content released from the ferrocene catalyst during pyrolysis [168]. A composite prepared from 1% alkali-treated glass fibre and 55% resin showed the highest tensile strength, whereas a 5% alkali-treated glass fibre and 55% resin composites showed maximum flexural properties [169]. The MWNT and carbon fibres (CF) were added to the phenolic resin to fabricate MWNT/phenolic, MWNT/CF/phenolic nanocomposites and CF/phenolic composites by the hot press method. The MWNT/phenolic nanocomposites had the lowest Tg among the three types of composites, which indicated the better thermal conductivity of MWNT [170]. The foregoing discussion has presented a consolidated view of the recent developments in non-conventional, addition-curable phenolic resins. PBZ is another interesting addition-cure phenolic which is the focus of this book.

1.6 Polybenzoxazines The preceding sections highlighted the significance of high-performance thermosets such as BMI, phenolic resins, advanced epoxy resins, CE and others that have been engineered to meet the demands of industry. Phenolic resins are widely used as a high-technology material in the aerospace arena. Although these resins have improved thermal stability, thermo-oxidative resistance, chemical resistance, and high mechanical strength, they face numerous processability issues due to their high viscosity, short shelf-life, brittleness, high processing temperature, and low solubility in organic solvents. Most importantly, they release byproducts during processing which adversely affects the material performance due to formation of microvoids. From the family of phenolic resins, the novel polymer PBZ has been the focus of attention because it overcomes many of the problems associated with existing stateof-the-art polymers. PBZ is a newly developed addition-polymerised phenolic system having a wide range of interesting features. It has the capability to overcome several shortcomings of conventional novolac- and resole-type phenolic resins while retaining their beneficial properties. It is an addition-cure phenolic system based on oxazine-modified phenolic resin that undergoes a ring-opening polymerisation. PBZ exhibit: (i) near-zero volumetric change upon curing; (ii) low water absorption; (iii) high char yield; (iv) no

24

Polybenzoxazines and State-of-the-Art High-Temperature Polymers strong acid catalysts required for curing; and (v) release of no toxic byproducts during curing. PBZ was developed to combine the thermal properties and flame retardant properties of phenolics and the mechanical performance and the flexibility of molecular design of advanced epoxy systems. From the synthesis viewpoint, PBZ resins offer enormous design flexibility, allowing tailoring the properties of the cured materials for a wide range of applications. PBZ resins are expected to replace traditional phenolics, polyesters, vinyl esters, epoxies, BMI, CE and polyimides in many respects. They have tremendous advantages over other state-of-the-art thermosetting resins. However, some drawbacks of these materials must be addressed. Hence, blends, compsoites, and alloys have been derived from PBZ. The precursors (BZ monomers) of PBZ are formed from phenols and formaldehyde in the presence of aliphatic or aromatic amines [171]. The choice of phenol and amine permits design flexibility and tailoring of polymer properties. The most investigated PBZ is derived from bisphenol A and aniline (BA-a). This BZ is treated as a ‘benchmark’ among PBZ, and the properties of new PBZ are compared with it. The as-synthesised mixture consists of monomer, and oligomers that contain phenolic groups. For practical applications, the mixture is sufficient but, for controlled structure and properties, the monomer is freed of the oligomers. The synthesis and polymerisation of BZ monomers is depicted in Scheme 1.15.

OH

O

N

R OH

OH N R

4 HCHO

2R-NH2

N OH

N

O

OH

R

R

Scheme 1.15 General protocol for the synthesis and polymerisation of benzoxazine based on bisphenol A (when R= phenyl group, the monomer is denoted as BA-a)

25

Polybenzoxazines: Chemistry and Properties A property correlation of state-of-the-art matrices and PBZ is given in Table 1.3. This helps us to understand the properties of PBZ and its significance as a hightemperature material [172]. The relative advantages of PBZ are obvious. They are usually cured in the temperature window of 160–220 °C. The ring-opening polymerisation of these new materials occurs with near-zero shrinkage or even with a slight expansion upon cure. It is proposed that the volumetric expansion of the BZ resin is mostly due to the consequence of molecular packing influenced by inter- and intramolecular hydrogen bonding. The polymers exhibit a Tg in the 160–340 °C range dependent upon the structure. This new family of phenolic resins features a wide range of mechanical and physical properties that can be tailored to various needs. Dynamic mechanical analysis (DMA) reveals that these are candidate resins for composite applications and that they possess high moduli and Tg at low crosslink densities. Long-term immersion studies indicate that these materials have a lower rate of water absorption and low saturation content. Impact, tensile, and flexural properties are also good. Dielectric analyses on these polymers demonstrate their suitability for electrical applications. Char yield of ≤82% has been claimed. Their composites are comparable with polyimides and other highperformance polymers, and they are readily processable [173, 174]. In comparison with the other known expanding monomers and spiro ortho compounds, PBZ resins have been shown to have good potential for structural/engineering applications. PBZ have the lowest heat release during combustion and are, therefore, more flameresistant, surpassing that of phenolics and polyether imides (the current aerospace matrices of choice).

26

Table 1.3 Comparative properties of various high-performance polymers (neat resins) [172] Property

Phenolics

Toughened BMI

Bisoxazolinephenolics (40:60)

Cyanate ester

PT resin

PBZ

1.2–1.25

1.24–1.32

1.2–1.3

1.3

1.1–1.35

1.25

1.19

180

200

~200

250

150–200

300

130–280

Tensile strength (MPa)

90–120

24–45

50–90

91

70–130

42

100–125

Tensile modulus (GPa)

3.1–3.8

3–5

3.5–4.5

4.6–5.1

3.1–3.4

4.1

3.8–4.5

3–4.3

0.3

3

1.8

2–4

2

2.3–2.9

Dielectric constant (1 MHz)

3.8–4.5

4–10

3.4–3.7

–

2.7–3.0

3.1

3–3.5

Cure temperature (°C)

RT–180

150–190

220–300

175–225

180–250

177–316

160–220

>3

0.002

0.007

<1

~3

~3

~0

Thermogravimetric analysis onset (°C)

260–340

300–360

360–400

370–390

400–420

410–450

380–400

Tg (°C)

150–220

170

230–380

160–295

250–270

300–400

170–340

GIC (J/m2)

54–100

–

160–250

157–223

–

–

168

0.60

–

0.85

Density (g/cm3) Max use temperature (°C)

Elongation (%)

Cure shrinkage

KIC (MPa-m1/2) PT = Phenolic-triazine

–

–

–

0/94

27

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

Epoxy

Polybenzoxazines: Chemistry and Properties

1.6.1 Features of PBZ As stated above, PBZ that can overcome many shortcomings associated with traditional phenolic resins have been synthesised and characterised extensively by Ishida and co-workers [175–178]. They possess excellent resistance to chemicals [179], UV light [180] and amazingly high Tg values [181]. A wide variety of BZ monomers can be readily prepared by varying the precursor components by the Mannich condensation reaction. They are therefore of great interest in synthetic fields. Many studies on the fundamental features accountable for the unique properties of PBZ have been conducted [182–192]. Hydrogen bonds are known to play an important part in determining the properties of a range of materials [193–195]. PBZ properties are mainly derived from inter- and intramolecular hydrogen bonding prsent in the polymeric structures. The network structure of PBZ is supported by the interaction of polymer chains due to strong hydrogen bonding, as well as chemical crosslinking. Therefore, taking into account the low crosslinking density of PBZ, the role of hydrogen bonding in the polymer system is of great importance in interpreting structure–property relationships. By understanding the polymerisation mechanism, we can define the polymer structure via manipulation of the monomer chemistry for achieving excellent properties. Hydrogen bonding and regioselectivity play major parts in the polymerisation mechanism.

1.6.2 Regioselectivity In general, the thermal curing of BZ monomers generates a PBZ structure via the attack of iminium ions on the ortho position (with respect to phenol) of the monomers (see mechanism in Scheme 1.15). PBZ derived from monofunctional monomers (containing one oxazine ring) forms linear polymers, whereas di- or polyfunctional BZ (having two or more oxazine rings) display crosslinking networks which are very constructive for commercial applications. The widely studied PBZ (‘traditional PBZ’, PBA-a) network is synthesised from bisphenol A and aniline. BA-a has been shown to polymerise via a thermally induced ring-opening reaction via ortho attack to form a phenolic structure characterised by a Mannich base bridge (CH2-NR-CH2) instead of the methylene bridge structure associated with traditional phenolic resins [191, 196–200]. Ishida and co-workers examined the regioselectivity of polymerisation of monofunctional and difunctional oligomers exclusively [182, 184] using 1H- and 13 C-NMR spectroscopic analyses. They were derived from 2,4-dimethyl phenol (24DMP-a) and 4-t-butyl phenol (4TBUPH-a) based on various aromatic amines, aniline (a), o-toluidine (ot), m-toluidine (mt), p-toluidine (pt), and 3,5-xylidine (35x)

28

Polybenzoxazines and State-of-the-Art High-Temperature Polymers (Scheme 1.16). The addition of methyl substituents at the free ortho and para sites of the phenolic ring is sufficient to prevent the polymerisation of 24DMP-ot and 24DMPpt. The stabilities of the oxazine rings in these compounds are such that they do not open even if exposed to higher temperatures. The presence of methyl substituents at one or both meta positions facilitates ring opening at temperatures near or below 200 °C. As the temperature is increased, many oxazine rings open in 24DMP-35x (2,4dimethyl phenol and 3,5 xylidene-based BZ). These open rings can thermally cleave and may react at para positions on the aryl amine ring to yield arylamine Mannich bridges, which may degrade due to the absence of stabilisation by intramolecular hydrogen bonding with the phenolic group. In the case of 24DMP-mt, ring cleavage has not occurred until slightly higher temperatures such that the reaction to activated sites on the arylamine ring occurs in a significant amount.

O

O

N

O

N

N CH 3

CH 3

CH 3 O

N

O

N

CH 3

O

N

CH 3

Scheme 1.16 Structures of benzoxazine monomers. Reproduced with permission from H. Ishida and D.P. Sanders, Polymer, 2001, 42, 7, 3115. ©2001, Elsevier Science [182]

The polymerisation site is regioselective and properties were varied in diamine-based BZ with different phenolic substitutions. Methyl substitution on phenols decreased the

29

Polybenzoxazines: Chemistry and Properties reactivity, and a higher temperature was required for curing. The rate of polymerisation for these BZ decreased as a function of chain length. Phenol substitution slowed the polymerisation process, with the reaction rate higher for the non-substituted BZ, followed by para substitution, and then slowest for ortho-substitution [201].

Phenolic mannich bridge network

Bisphenolic methylene bridge network

Arylamine mannich bridge network

Arylamine methylene bridge network

Scheme 1.17 Regioselectivity of the polymerisation reaction leads to various chemical bridges. Reproduced with permission from H. Ishida and D.P. Sanders, Macromolecules, 2000, 33, 22, 8149. ©2000, American Chemical Society [184]

30

Polybenzoxazines and State-of-the-Art High-Temperature Polymers To determine the ring-opening polymerisation at activated arylamine sites even if there is a free ortho site available on the phenolic ring, a series of monomers based on 4-tbutyl phenol was synthesised [182]. Methyl substituents on the ortho position of the arylamine ring serve to significantly reduce the basicity of the monomer and sterically hinder the polymerisation process. Methyl substituents in the meta position on the arylamine ring facilitate ring-opening/cleavage at lower temperatures. This leads to the secondary reaction, which generates bisphenolic methylene linkages. The presence of methyl substituents that activate the para position on the arylamine ring increases the extent of ring-opening and produces species of higher molecular weight. Ishida and co-workers also studied the regioselectivity of difunctional PBZ derived from bisphenol A with the same series of amines used for the monofunctional study [184]. They envisaged several possible network structures for PBZ. If the BZ polymerise only at the ortho phenolic sites, a phenolic Mannich bridge network structure would develop (Scheme 1.17). In this structure, two possible main chain pathways can be visualised, both of which include a -C-N-C Mannich bridge. However, Fourier-transform–infrared spectroscopy (FT-IR) data and thermal analyses suggest that some pendant rings also react. Thus, if one imagines a case in which only these activated sites react and no ortho phenolic positions react, using BA-mt (bisphenol A and a m-methyl toluene-based monomer) as an example, an arylamine Mannich bridge network structure could form (Scheme 1.17). Again, two main-chain pathways can be visualised. However, a new main-chain pathway, path D, is present that proceeds through a -CN-Mannich bridge through the arylamine ring and then through the next -C-N- Mannich bridge. With the aromatic carbon-nitrogen bond being significantly stiffened by resonance and the absence of one of the methylene ‘hinges’ in the Mannich bridge, this arylamine Mannich bridge should be significantly more rigid, resulting in a higher Tg at the same extent of conversion. Alternatively, if the arylamine ring serves as the site for formation of the methylene linkages, an aryl amine methylene bridge network would result. The actual network structure is expected to be in the continuum between these four extremes.

1.6.2 Hydrogen-bonding Aspects The distribution of hydrogen bonding in PBZ has been evaluated exclusively by Ishida and co-workers by the assignment of the vibrational spectra of these molecules [191]. In the study by Dunkers of hydrogen bonding for a BZ model dimer using methylamine and aniline [201], it was shown that several types of hydrogen-bonded species can be formed. This included –OH….O intermolecular hydrogen bonding, -OH….O intramolecular hydrogen bonding, and -OH….N intramolecular hydrogen

31

Polybenzoxazines: Chemistry and Properties bonding. This hydrogen bonding aspect helped to establish the generalised explanation between the physical properties of PBZ and the network structure. The hydrogen bonding in the Mannich bridge is strongly affected by the basicity of the amine functional group [183]. From the similar spectra for both model dimer crystals (i.e., methyl-dimer and aniline-dimer), it seems that the basicity of the amine functional group has little influence on the hydrogen-bonding structure. It has been shown that the simpler structure of the asymmetric dimers closely simulates the hydrogen-bonded network structure between polymer chains, whereas the structure of symmetric dimers reflects the hydrogen bonding related to the end-groups of polymer chains. It is described that the BA-m polymer (derived from bisphenol A and methylamine) consists mainly of –OH…N and -O-…H+N intramolecular hydrogen bonding, whereas the BA-a polymer has considerable intermolecular hydrogen bonding and relatively weak intramolecular hydrogen bonding in the polymer network structure. Asymmetric model dimers having only one -OH group in the structure have only two types of hydrogen bonding. One is –OH….O intermolecular hydrogen bonding, and the other is –OH….N intramolecular hydrogen bonding. Possible hydrogen bonding for symmetric and asymmetric aniline dimmers is depicted in Scheme 1.18. If the amine constituents have similar basicities, the nature of hydrogen bonding is very similar [185]. The steric aspect of bulkier amines has a more profound effect on the stability of the Mannich base, resulting in heterogeneous products during the ringopening reaction. It has also been demonstrated that bifunctional BZ monomers based on extremely bulky amines cannot form completely crosslinked polymer networks due to the extensive degradation process.

32

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

a)

b)

CH 3 O

H 3C

H

N

N

H 3C

H 3C

c)

OH

d)

H 3C

CH 3 H

O

H 3C

CH 3

O H

N N H O

H

CH 3

O

H 3C

H 3C CH 3

a)

Asymmetric aniline dimers

CH 3 O

H 3C

N

N

H

b)

CH 3

HO

N

CH 3

CH 3 O

CH 3

H N

H 3C

CH 3 O H

c)

H 3C CH 3

O

H

O

H O N

H

N

H O H 3C CH 3

CH 3

H 3C

Symmetric aniline dimers

Scheme 1.18 Various inter- and intramolecular hydrogen bonding in symmetric and asymmetric aniline dimers. Reproduced with permission from H.D. Kim and H. Ishisa, Journal of Physical Chemistry A, 2002, 106, 14, 3271. ©2002, American Chemical Society [183] 33

Polybenzoxazines: Chemistry and Properties The temperature dependency of various types of hydrogen bonding has been reported to enhance further understanding of the hydrogen-bonded structure in PBZ molecules [192]. There can be two types of hydrogen bonds. One is the hydrogen bond that is favoured by a low-energy conformation with a narrow structural distribution, and is known as the ‘conformationally preferred hydrogen bond’ or the ‘chelated hydrogen bond’. The other type is more opportunistic and the structure of the hydrogen bond more statistically distributed. It is found that temperature influences statistically distributed hydrogen bonding but has little effect on conformationally preferred hydrogen bonding below the Tg. A combination of molecular modelling, density functional theory, chemical-shift calculations, and advanced solid-state nuclear magnetic resonance (NMR) experiments such as 1H-NMR–magic angle spinning (MAS) and double quantum MAS (DQ MAS) spectroscopy were used to elucidate the supramolecular and helical structure of BZ oligomers [186–188]. Ishida and Allen reported that PBZ have low crosslink densities but high Tg values and high moduli when compared with epoxy resin [175]. This phenomenon is unexpected because, in general, the higher the degree of crosslinking, the higher and the Tg. This behaviour is attributed to hydrogen bonding that leads to stiffness, and a high Tg and modulus. Despite the abundance of hydrophilic phenolic and tertiary amine groups in each chemical repeat unit, PBZ do not absorb water as much as their phenolic or epoxy resin counterparts. After 600 days in water at room temperature, the saturation water content of BA-a was 1.9% by weight; it was 1.3% by weight for BA-m. The low uptake of water can be explained by the strong hydrogen bonding of the phenol and the nitrogen atom of the Mannich base. Intramolecular hydrogen bonds are characterised and identified to be the ‘driving force’ for ring-shape and helical conformations of trimeric and tetrameric units. This geometry, in which the N….H….O and O….H….O hydrogen bonds are protected on the inside of the helix, can account for many of the exemplary chemical properties of PBZ materials [8]. The conformational statistics of PBZ have also been reported, indicating the significant influence of hydrogen bonding on physical and mechanical properties [190].

1.6.4 Cure Monitoring and Kinetics BZ monomers undergo polymerisation via thermally/photochemically induced ringopening. To make the optimum use of BZ resins, it is important to understand the nature of the curing process, the structure of the cured material, and how the kinetic parameters can be influenced by temperature and time. The final properties of the crosslinked BZ resins are significantly dependent upon the kinetics of the curing

34

Polybenzoxazines and State-of-the-Art High-Temperature Polymers reaction. Several techniques have been used to examine the kinetics of PBZ curing, e.g., DSC, FTIR, and rheokinetic measurements [202–215]. An understanding of the gel time of BZ curing is imperative for resin processing. Monitoring of the viscoelastic behaviour of the PBZ during isothermal cure by DMA [203] shows that the BZ is transformed from a low-molecular-weight monomer and oligomer into a high-molecular-weight, crosslinked PBZ structure. Isochronic measurements show that, although the aniline-based BZ has a lower activation energy for the gelation process than the methylamine-based resin, it has a slower rate of reaction. The torsional braid analysis (TBA) of PBA-a has also been reported. It describes the three regions of BA-a curing as: melt transition (100–148.7 °C), gelation transition (148.7–237.2 °C) and glass transition (237.2–327 °C). According to the TBA of the curing of BZ, the curing reaction occurs between 148 °C and 237 °C [211]. The curing reactions of BZ precursors based on bisphenol A and aniline were studied by Ishida and co-workers using DSC to determine the feasibility of processing them into final phenolic products. Curing was found to have an autocatalytic character with an overall reaction order of 2 [202]. The curing of BZ precursors is an autocatalysed reaction until vitrification is reached, and diffusion begins to control the curing process afterwards. The activation energy is 102–116 kJ/mol. Some evidence from isothermal and scanning experiments suggests that further structural rearrangements may be occurring at high temperatures. The isothermal heat of reaction follows a quadratic relationship with the cure temperature, whereas the onset of curing and the exotherm maximum during the isothermal tests were found to decay exponentially with temperature. The curing kinetics of polyfunctional benzoxazine resins based on 3,5-xylidine and bisphenol A (designated as BA-35x) was also investigated [207]. The BA-a resin shows only one dominant autocatalytic curing process with an average activation energy of 81–85 kJ/mol, whereas BA-35x exhibits two dominant curing steps. We reported the kinetics of the thermal crosslinking polymerisation of bisphenol-Abased BZ monomer by rheological analyses at different isothermal curing temperatures [216]. In that study, the effect of autocatalysis (Figure 1.1 ‘S’-shaped curve) was observed predominantly, and this effect was attributed to phenolic -OH groups generated by the ring-opening polymerisation, which aids further polymerisation. An autocatalytic kinetic model was also proposed for the curing of BZ monomers. The empirical model derived from of the rheological studies confirmed the activation energies to be 198 kJ/mol (for a first-order reaction) and 62 kJ/mol (for autocatalysis). The low activation energy for autocatalysis implies the prominence of this phenomenon in the overall kinetics. The overall reaction order was 1.6–1.8. The photo-initiated cationic polymerisation of BZ monomers by onium salts has also been reported [217, 218].

35

Polybenzoxazines: Chemistry and Properties

190 °C 195 °C 205 °C 210 °C

Figure 1.1 isothermal fractional conversion as a function of time at different temperatures

1.7 Conclusion We have discussed the recent developments in major high-temperature polymers and the fundamental chemistry of PBZ. BZ monomers undergo polymerisation via thermally/photochemically induced ring-opening. The curing of BZ monomers generates a PBZ structure via the attack of iminium ions on the ortho position (with respect to phenol) of the monomers. The curing was found to have an autocatalytic character, with an overall reaction order of 2 for a typical bisphenol A- and anilinederived PBZ. The effect of autocatalysis was observed to be predominant, and this effect was attributed to the presence of phenolic -OH groups generated by the ringopening, which aids further polymerisation. PBZ properties are mainly dictated by inter- and intramolecular hydrogen bonding present in the polymeric structures. The network structure of PBZ is supported by the interaction of polymer chains due to strong hydrogen bonding, as well as chemical crosslinking.

36

Polybenzoxazines and State-of-the-Art High-Temperature Polymers

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Polybenzoxazines and State-of-the-Art High-Temperature Polymers 131. G.Z. Liang and J. Fan, Journal of Applied Polymer Science, 1999, 73, 9, 1623. 132. B. Dao, J.H. Hodgkin and T.C. Morton, High Performance Polymers, 1997, 9, 4, 413. 133. B.M. Culbertson, O. Tiba, M.L. Deviney and T.A. Tufts in the Proceedings of the 34th International SAMPE Symposium, Reno, NV, USA, 1989, p.2483. 134. M.L. Deviney and J.J. Kampa in the Proceedings of the 42nd International SAMPE Symposium, Anaheim, CA, USA, 1997, p.24. 135. B.M. Culbertson and T.A. Tufts, inventors; Ashland Oil Inc., assignee; US 4,430,491, 1984. 136. A. Dekar, H. Stretz and J. Koo, Polymeric Material Science and Engineering, 2000, 83, 105. 137. M. Kaji and T. Endo, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1999, 37, 16, 3063. 138. M. Ogato, N. Kinjo and T. Kawata, Journal of Applied Polymer Science, 1993, 48, 4, 583. 139. C.S. Tyberg, K. Bears, M. Sankarapandian, P. Shih, A.C. Loos, D. Dillard, J.E. Mcgrath, J.S. Riffle and U. Sorathia, Polymer, 2000, 41, 13, 5053. 140. C.S. Tyberg, K. Bears, M. Sankarapandian, P. Shih, A.C. Loos, D. Dillard, J.E. Mcgrath, J.S. Riffle and U. Sorathia, Construction Building Materials, 1999, 13, 6, 343. 141. S. Han, W.G. Kim, H.G. Yoon and T.J. Moon, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1998, 36, 5, 773. 142. A.T. Di Benedetto, Journal of Polymer Science, Part B: Polymer Physics Edition, 1987, 25, 9, 1949. 143. S. Han, W.G. Kim, H.G. Yoon and T.J. Moon, Bulletin of Korean Chemical Society, 1997, 18, 11, 1199. 144. S. Han, H.G. Yoon, K.S. Suh, W.G. Kim and T.J. Moon, Journal Polymer Science, Part A: Polymer Chemistry Edition, 1998, 37, 6, 713. 145. S. Han, W.G. Kim, H.G. Yoon and T.J. Moon, Journal of Applied Polymer Science, 1998, 68, 7, 1125. 45

Polybenzoxazines: Chemistry and Properties 146. A. Fainleib, J. Galy, J.P. Pascault and H.J. Sue, Journal of Applied Polymer Science, 2001, 80, 4, 580. 147. R.W. Biernath and D.S. Soane in Advances in New Materials, Eds., J.C. Salamone and J.S. Riffle, Contemporary Topics in Polymer Science, Volume 7, Plenum Press, New York, NY, USA, 1992. 148. C.P. Reghunadhan Nair, K. Ambika Devi, K. Krishnan, and K.N. Ninan in the Proceedings of Thermans 2001, Mumbai, India, 2002, p.134. 149. X.W. Luo, Z.H. Ping, J.P. Ding, Y.D. Ding and S.J. Li, Journal of Macromolecular Science, Part A: Pure and Applied Chemistry, 1997, A34, 11, 2279. 150. A. Nagai, H. Kokaku and T. Ishii, Journal of Applied Polymer Science, 2002, 85, 11, 2335. 151. H.J. Tai, Journal of Polymer Research, 2000, 7, 4, 221. 152. A. Matsumoto, K Hasegawa, A. Fakuda and J-S. Pae, Polymer International, 1993, 31, 3, 275. 153. W.G. Kim, H.G. Yoon and J.Y. Lee, Journal of Applied Polymer Science, 2001, 81, 11, 2711. 154. T.H. Hsieh, T.L. Wang, K.S. Ho and Y.Z. Wang, Polymer Engineering Science, 2000, 40, 2, 418. 155. W.G. Kim and J.Y. Lee, Journal of Applied Polymer Science, 2002, 86, 8, 1942. 156. C.P. Reghunadhan Nair, R.L. Bindu and K.N. Ninan, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2000, 38, 3, 641. 157. R.L. Bindu, C.P. Reghunadhan Nair and K.N. Ninan, Journal of Applied Polymer Science, 2001, 80, 5, 737. 158. S.K. Dirlikov, High Performance Polymers, 1990, 2, 1, 67. 159. M.F.L. Grenier and C. Sangler, European Polymer Journal, 1997, 33, 7, 1125. 160. C.P. Reghunadhan Nair, R.L. Bindu and K.N. Ninan, Polymer, 2002, 43, 9, 2609.

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Polybenzoxazines and State-of-the-Art High-Temperature Polymers 161. C.P. Reghunadhan Nair, R.L. Bindu and K.N. Ninan, Journal of Material Science, 2001, 36, 17, 4151. 162. P.M. Hergenrother and J.G. Smith, Jr., Polymer, 1994, 35, 22, 4857. 163. K.H. Wood, R.A. Orwoll, B.J. Jensen, P.R. Young and H.M. McNair in the Proceedings of the 42nd International SAMPE Symposium, Anaheim, CA, USA, 1997, p.1271. 164. W. Mingcun, W. Liuhe and Z. Tong, Journal of Applied Polymer Science, 2006, 99, 3, 1010. 165. W. Mingcun, W. Liuhe and Z. Tong, European Polymer Journal, 2005, 41, 5, 903. 166. M.A. Espinosa, V. Cadiz and M. Galia, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2004, 42, 2, 279. 167. H. Youqing, Z. Zhengxi and Q. Qian, Journal of Applied Polymer Science, 2003, 88, 6, 1410. 168. A. Morozan, I. Stamatin, A. Dumitru, V. Ciupina, G. Prodan, J. Niewolski and H. Figiel, Physica E, 2007, 37, 1/2, 44. 169. D. Debasish, B. Adhikari and D. Debapriya, Polymers for Advanced Technologies, 2007, 18, 1, 72. 170. M-K. Yeh, N-H. Tai and Y-J. Lin, Key Engineering Materials, 2007, 334/335, 6, 713. 171. H. Ishida and D.J. Allen, Journal of Polymer Science, Part B: Polymer Physics Edition, 1996, 34, 6, 1019. 172. C.P. Reghunadhan Nair, Progress in Polymer Science, 2004, 29, 5, 401. 173. S.B. Shen and H. Ishida, Polymer Composites, 1996, 17, 5, 710. 174. H. Ishida and H.Y. Low, Macromolecules, 1997, 30, 4, 1099. 175. M. Nakamura and H. Ishida, Polymer, 2009, 50, 12, 2688. 176. X. Ning and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 1994, 32, 5, 921.

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Polybenzoxazines and State-of-the-Art High-Temperature Polymers 194. P. Brunet, M. Simard and J.D. Wuest, Journal of American Chemical Society, 1997, 119, 11, 2737. 195. R.K. Castellano and J. Rebek Jr., Journal of American Chemical Society, 1998, 120, 15, 3657. 196. R. Andreau, J.A. Reina and J.C. Ronda, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2008, 46, 10, 3353. 197. W.J. Burke, E.L. Glennie and C. Weatherbee, Journal of Organic Chemistry, 1964, 29, 4, 909. 198. W.J. Burke, J.L. Bishop, E.L.M. Glennie and W.N. Bauer, Jr., Journal of Organic Chemistry, 1965, 30, 10, 3423. 199. G. Reiss, J.M. Schwob, G. Guth, M. Roche and B. Lande in Advances in Polymer Synthesis, Eds., B.M. Cuthbertson and J.E. McGrath, Plenum Press, New York, NY, USA, 1985. 200. D.J. Allen and H. Ishida, Polymer, 2009, 50, 2, 613. 201. J.P. Dunkers, A. Zarate and H. Ishida, Journal of Physical Chemistry, 1996, 100, 32, 13514. 202. H.Y. Low and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 1998, 36, 11, 1935. 203. H. Ishida and D.J. Allen, Journal of Applied Polymer Science, 2001, 79, 3, 406. 204. D. Rosu, C.N. Cascaval, F. Mustata and C. Ciobanu, Thermochimica Acta, 2002, 383, 1/2, 119. 205. H. Ishida and Y. Rodriguez, Journal of Applied Polymer Science, 1995, 58, 10, 1751. 206. S. Vyazovkin and N. Sbirrazzuoli, Macromolecules, 1996, 29, 6, 1867. 207. C. Jubsilp, S. Damrongsakkul, T. Takeichi and S. Rimdusit, Thermochimica Acta, 2006, 447, 2, 131. 208. Y-C. Su, D-R. Yei and F-C. Chang, Journal of Applied Polymer Science, 2005, 95, 3, 730. 209. Y-X. Wang and H. Ishida, Polymer, 1999, 40, 16, 4563. 49

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50

2

Structure-Property Relationships

2.1 Introduction The development of novel polybenzoxazine (PBZ) systems that exhibit excellent thermo-mechanical performance to expand their end-use applications in harsh environments (e.g., against heat, moisture, and chemicals) is a big challenge. Towards that goal, several benzoxazine (BZ) monomers were prepared, and their polymer properties reported. Synthesis of novel monomers was divided mainly into two categories: BZ monomers containing addition-curable moieties, and synthesis via the manipulation of various precursors (i.e., varying amine or phenol components in the Mannich condensation reaction by exploiting their wide flexibility in molecular design). BZ monomers are typically synthesised using a phenol, formaldehyde and aliphatic or aromatic amine by employing a solvent or solvent-less method. The slow reaction rate, and large amount of solvent required for the synthesis (and, in some cases, the poor solubility of the precursors) were the major problems concerned with the solvent method. To overcome these problems, Ishida and coworkers developed a solvent-less method (see Chapter 1). Due to the wide variety of starting compounds, several BZ can be synthesised by experimenting mainly with phenol and amine structures. In this way, monofunctional, difunctional and polymerisable group-substituted BZ were prepared. High-molecular-weight PBZ and main-chain BZ are the novice materials in this thermoset class. This chapter is an account of various PBZ reported that have been categorised into two broad classes: (i) functionalised BZ (containing addition-curable sites) and (ii) non-functionalised BZ (of varying structural features but no additional polymerisable groups). The dependency of their properties on structural features is examined.

51

Polybenzoxazines: Chemistry and Properties

2.2 Functionalised BZ (BZ Containing Addition-curable Moieties) 2.2.1 BZ-Containing Allyl Groups 3-Allyl-3,4-dihydro-2H-1,3-benzoxazine (P-ala), bis (3-allyl-3,4-dihydro-2H1,3-benzoxazinyl) isopropane (B-ala) and 3-phenyl-3,4-dihydro-8-allyl-2H-1,3benzoxazine (also called bisphenol A- and allyl amine-based benzoxazine; P-alp) were the first allylbenzoxazines reported [1] (Scheme 2.1). Of these, P-alp showed difficulty in polymerising because the active ortho sites for polymerisation were blocked, so the exotherm was observed at a higher temperature. P-ala exhibited two exotherms: the first was attributed to allyl polymerisation and the second to oxazine polymerisation. B-ala showed a broader exotherm with higher exothermic features than normal bisphenol A- and aniline-based BZ (BA-a). This was attributed to the polymerisation of allyl and BZ groups. The polymers of P-ala, and B-ala showed a glass transition temperature (Tg) of 297 °C and 322 °C, respectively, whereas P-alp had poor thermal properties with a Tg of 107 °C.

O

N

O

N

P alp

O

O

N

N

P ala

B ala

Scheme 2.1 Chemical structure of allyl functional benzoxazine monomers

52

Structure–Property Relationships To enhance thermal characteristics, a bis BZ monomer with additional polymerisable allyl groups substituted on the active ortho sites of bisphenol-A (BA-allyl) was synthesised [2] (Scheme 2.2). Differential scanning calorimetry (DSC) analyses of BA-allyl compound manifested a dual cure pattern due to the polymerisation of allyl groups and curing of BZ (Figure 2.1). A weak exotherm observed at 210 °C was attributed to allyl polymerisation and an exotherm at 273 °C to oxazine ringopening.

N

OH

O

NH 2 + (CH 2O) n +

O

N

OH

Scheme 2.2 Synthesis of BA-allyl

0.8 0.7

oxazine

Bz a

0.6 Heat Flow (W/g)

0.5 0.4 0.3 0.2

oxazine

0.1 0.0 0.1

Bz allyl

0.2 0.3 0.4

DABA 50

100

150

200

250

300

350

400

Figure 2.1 Result of analyses of differential scanning calorimetry for BA-allyl, BA-a and diallyl bisphenol-A (DABA)

53

Polybenzoxazines: Chemistry and Properties The possible reaction sites and network formed in the polybenzoxazine were investigated by evaluating the thermal degradation products. Pyrolysis – gas chromatography was used to identify the thermal degradation products. The analysis of evolved gases revealed that the aniline content in this PBZ was only 32%. It was low compared with other PBZ, implying the possibility of additional crosslinking through the aniline moiety. The ortho sites for polymerisation in the BZ were blocked, so an altered ring-opening polymerisation mechanism was proposed. It was concluded that the pendant aniline ring was the preferred site for the addition of the iminium cation during ring opening. The crosslinked structure of BA-allyl polymer is given in Scheme 2.3.

Scheme 2.3 Crosslinked structure of BA-allyl 54

Structure–Property Relationships The PBZ derived from the allyl functional BZ exhibited a high Tg of 298 °C. It manifested the 5% and 10% weight loss temperatures as 395 °C and 425 °C, respectively, and had unambiguous superiority over the other allyl BZ. The anaerobic char yield of the diallyl bisphenol A- and aniline-based PBZ (PBA-allyl) was 42% compared with 32% for BA-a (Figure 2.2). This enhancement in char yield demonstrated the effect of allyl groups.

110 100

Weight loss (%)

90 80 70 60 PBz allyl

50 40

PBz a

30 0

200

400 600 T (˚C)

800

1000

Figure 2.2 Thermogravimetric analyses of BA-allyl and BA-a polymers

2.2.2 BZ Containing Maleimide Groups A series of novel hybrid monomers of BZ and maleimide moieties were made because the maleimide group can serve as an additional polymerisable group to produce high-performance PBZ resins. A BZ monomer with a maleimide pendant [N-(4hydroxyphenyl) maleimide]-benzoxazine (HPM-Ba) has shown attractive processing and thermal properties [3]. HPM-Ba could be polymerised to produce a fusible polymer with BZ pendants (PHPM-Ba-I). The further polymerisation of PHPM-Ba-I to PHPM-Ba-II via crosslinking of the pendent BZ groups enhanced the Tg to 204 °C (Scheme 2.4).

55

Polybenzoxazines: Chemistry and Properties

OH N

O

n O

N

O O

O N

OH

O N N

OH

O

PHPM-Ba-I

PHPM-Ba-II

Scheme 2.4 Structures of polymerised HPM-Ba. Reproduced with permisson from Y.L. Liu, J.M. Yu and C.I. Chou, Journal of Polymer Science, Part A: Polymer Chemistry Edtion, 2004, 42, 23, 5954. ©2004, John Wileys & Sons [3]

The anaerobic temperature at 5% weight loss (T5%) and temperature at 10% weight loss (T10%) of PHPM-Ba-I were found to be 250 °C and 282 °C, respectively. PHPMBa-II showed T5% and T10% values at 330 °C and 366 °C, respectively. The relatively high thermal stability of PHPM-Ba-II was due to the restriction of initial-stage thermal degradation by the formation of BZ crosslinked networks. A high char yield of 50% and a limiting oxygen index (LOI) value >33 were also observed with the cured PBZ, reflecting its good flame-retardant characteristics.

56

Structure–Property Relationships Another series of maleimide-containing BZ were prepared from hydroxyphenylmaleimide and various amines (e.g., aniline, allylamine, and amino phenyl propargyl ether) [4]. The structures and synthesis are given in Scheme 2.5. The cure of BZ and maleimide were shifted to lower temperatures (probably because of the phenolic OH that remained in the monomer). The cure temperatures for a BZ derived from maleimidophenol and allyl amine (Mal-BZ-Al) and hydroxyphenylmaleimide and phenyl propargyl ether-based BZ (Mal-BZ-Pg) (see Scheme 2.5) were also shifted to lower temperatures. The ring-opening polymerisation of BZ occurred at a faster rate than the polymerisation of maleimide. However, in the presence of allyl functionality, the cure of maleimide and allyl groups occurred faster than that of BZ.

R O

O OH

R NH 2

N

HCHO

O

O

O

Mal BZ R

Mal BZ Al O

CH 2

C

CH

Mal BZ Pg

Scheme 2.5 Preparation and chemical structure of maleimido benzoxazine monomers. Reproduced with permission from T. Agag and T. Takeichi, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2006, 44, 4, 1424. ©2006, John Wiley & Sons [4] The cured Mal-BZ-Al showed a Tg of 299 °C and the cured Mal-BZ-Pg exhibited a Tg of 355 °C. High values of Tg were due to participation of allyl and propargyl groups in crosslinking. Overall, the thermal cure of the monomers produced novel thermosets with high Tg values ranging from 241 °C to 335 °C with excellent thermal stability. Monofunctional BZ containing norbornene functionalities have also been reported [5] (Scheme 2.6). Maleimido benzoxazine (MIB) and norbornene benzoxazine (NOB) showed BZ polymerisation exotherms at 213 °C and 261 °C, respectively. MIB has a maleimide group, so it could be further polymerised by a free-radical mechanism by thermal activation or with an initiator. Char yields >55% and Tg >250 °C have been observed for maleimide and norbornene functional BZ structures.

57

Polybenzoxazines: Chemistry and Properties O

O N

N

O

O

O

O

N

MIB

N

NOB

Scheme 2.6 Maleimido and norbornene benzoxazines. Reproduced with permission from Y.L. Liu and J.M. Yu, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2006, 44, 6, 1890. ©2006, John Wiley & Sons [6] The polymerisation features of the mixtures of BZ and N-phenyl maleimides have been investigated [6]. Three N-phenyl maleimides, which have carboxylic acid, hydroxyl, and hydrogen moieties, respectively, attaching on the phenyl group, were employed in the studies (Scheme 2.7).

O N

N

N

O

O

P-a O

O

HPM-Ba O

O

N O

OH

N

N O

O

MI-OH

COOH

MI-COOH

MI-H

Scheme 2.7 Chemical structures of benzoxazine and maleimide compounds. Reproduced with permission from Y.L. Liu and J.M. Yu, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2006, 44, 6, 1890. ©2006, John Wiley & Sons [6] 58

Structure–Property Relationships The peak curing temperature of phenol- and aniline-based BZ (P-a) was observed at ~220 °C and that of MI-OH at ~316 °C. However, a relatively low polymerisation temperature was observed with the mixture of P-a/MI-OH (1:1 molar) compared with the polymerisation temperature of individual components. The ring-opening polymerisation of BZ was an acid-catalytic reaction; therefore the acidic MI-OH promoted the polymerisation of P-a. Conversely, the presence of BZ compounds may also catalyse the reaction of the maleimide groups of MI-OH. P-a/MI-H showed only one exothermic peak at 186 °C, corresponding to the polymerisation of P-a. This indicated that MI-H did not promote the polymerisation of BZ. Conversely, polymerisation of the maleimide group in MI-H was catalysed in the presence of P-a because the polymerisation temperature of MI-H in the blend was shifted from 280 °C (corresponding to pure MI-H) to the low-temperature region, and overlapped with the polymerisation of P-a. Therefore, the amine group of P-a was presumed to help catalyse the maleimide polymerisation. Further, P-a and MI-H polymerise simultaneously in the P-a/MI-H co-curing system. However, a shoulder of the exothermic peak indicated that the polymerisations of P-a and MI-OH occurred independently. To investigate this issue, another maleimide possessing a carboxylicacid moiety (MI-COOH) was used in the co-curing composition. The carboxylic acid group in MI-COOH strongly catalysed the ring-opening reaction of BZ groups. Interestingly, the reaction of maleimide groups in the co-curing compositions occurred at very different temperatures. The reaction temperatures were in the order MICOOH < MI-OH < MI-H. However, the mechanism of catalysis of maleimide polymerisation by BZ has not been proved.

2.2.3 BZ-Containing Propargyl Groups Propargyl functional groups are interesting because they can polymerise independently and impart thermal stability to the PBZ structure. Hence, monofunctional (phenol- and propargylamine-based, P-appe) and difunctional (bisphenol A and propargylamine, B-appe) propargyl BZ monomers were synthesised and their polymerisation investigated [7] (Scheme 2.8). P-appe showed an exotherm at 191 °C with a maximum at 235 °C, whereas the exotherm of B-appe was observed at 223 °C with a maximum at 249 °C. The polymerisation of propargyl and BZ occurred simultaneously and, as a result, only one exotherm was observed. However, from infrared (IR) studies, it was substantiated that the propargyl group underwent polymerisation earlier than oxazine ring-opening. The polymers P-appe and B-appe showed Tg values of 251 °C and 318 °C, respectively. They also showed better thermal stability, and P-appe showed a char yield of 66% whereas the value for B-appe was 61%.

59

O

CH

O

O

N

N

N CH

HC C

P-appe

C O

B-appe

O

Scheme 2.8 Chemical structures of propargyl functionalised benzoxazine monomers. Reproduced with permission from T. Agag and T. Takeichi, Macromolecules, 2001, 34, 21, 7257. ©2006, ACS [7]

Polybenzoxazines: Chemistry and Properties

60 OCH2C

Structure–Property Relationships Propargyl BZ have been used to produce polyacetylenes (in which PBZ served as side groups) which undergo irreversible cis–trans isomerisation. These polymers have a higher char yield than propargyl PBZ [8].

2.2.4 BZ-Containing Furan Groups The furan-containing BZ monomers 3-furfuryl-3, 4-dihydro-2H-1, 3-benzoxazine (P-af) and bis(3-furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl)isopropane (BA-af) were prepared using furfurylamine as a raw material (Scheme 2.9) [9]. P-af exhibited an exothermic peak centred at 241 °C, and a relatively broad peak centred at 247 °C was observed for BA-af. The polymerisation temperature of P-af and BA-af was 233 °C. After initial polymerisation, difunctional BA-af experienced steric hindrance during further polymerisation, resulting in a shift of the exothermic peak to a higher temperature and broadening of the exothermic peak. The activation energies of polymerisation were 96 kJ/mol and 98 kJ/mol for P-af and BA-af, respectively, by the Kissinger method. This implied that functionality does not have a bearing on reactivity. The polymerisation is given in Scheme 2.10.

O

O

N

O

N

N O

O

O

P-af

BA-af

Scheme 2.9 Furan-containing benzoxazine monomers. Reproduced with permission from Y.L. Liu and C.I. Chou, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 21, 5267. ©2006, John Wiley & Sons [9]

61

Polybenzoxazines: Chemistry and Properties

O O

O N O

N

N OH

O

N

O O

O O N

N

O OH

O

N N

HO

O OH N

N O

Scheme 2.10 Polymerisation of furan-containing benzoxazines. Reproduced with permission from Y.L. Liu and C.I. Chou, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 21, 5267. ©2006, John Wiley & Sons [9]

62

Structure–Property Relationships The Tg of phenol- and aminofuran-based BZ (PP-af) and bis(3-furfuryl-3,4-dihydro2H-1,3-benzoxazinyl)isopropane (PBA-af) was 315 °C and 308 °C, respectively. Introduction of furan groups into the crosslinked polymers can also increase their Tg, with formation of hydrogen bonding between furan and hydroxyl groups. The coefficients of thermal expansion (CTE) were 45 ppm and 37 ppm for PP-af and PBA-af, respectively. These values were lower than the typical phenol-aniline-based PBZs (PP-a) because the latter exhibited CTE values of 56 ppm at <120 °C and 152 ppm at >161 °C (Tg of PP-a). The thermal stability of the PBZ is in the order PP-af >PBA-af. Their char yields are 53% and 47%; T5% values are 336 °C and 347 °C; T10% values are 382 °C and 391 °C; and the LOI values are 30 and 31, respectively.

2.2.5 BZ-Containing Acetylene Groups High char-yielding PBZ were obtained from acetylene-functional BZ monomers. Polymerisation of acetylene functional groups (in addition to oxazine polymerisation) contributes to thermal stability. The reaction exotherm of acetylene polymerisation broadly overlaps with the reaction exotherm of BZ ring-opening polymerisation [10]. A significant change in char yield was found for these PBZ in air and an inert atmosphere [11]. These BZ polymerised in air, resulting in a higher char yield and thermal stability than those polymerised under an inert atmosphere due to the different concentration and structure of newly formed polyene chains by acetylene-group polymerisation, and due to the different extent of ring-opening polymerisation. Much effort has been made to investigate thermally curable acetylene-containing materials (acetylene-terminated pre-polymers) because they can be polymerised under moderate conditions without the evolution of volatiles [12]. The high char yield achieved for this class of materials is 71–81% by weight at 800 °C in a nitrogen atmosphere and 30% by weight at 700 °C in air; T10% is 520–600 °C. These polymers provide many desirable properties, such as resistance to solvents and moisture, and good physical properties as well as high thermal stability. These characteristics make them good candidates for matrix resins for advanced composite materials. The acetylenic group can also react under cationic, coordination, free-radical, photolytic, and thermal inducement. Dynamic mechanical analysis (DMA) was carried out on these PBZ, bisphenol A- and 3-aminophenylacetylene-based BZ (BA-apa), BAF-apa, as well as phenol- and 3-aminophenylacetylene-based BZ (Ph-apa) monomers (Scheme 2.11). The storage modulus (G′) of these polymers was in 1.9–2.2 GPa at room temperature. The resulting Tg values were 330 °C to 368 °C, which were significantly higher than those of the analogous PBZ and much higher than their polymerisation temperature (190 °C). Their higher Tg values were attributed to acetylene functional 63

64

BA-apa

Ph-apa

N C CH

CH

CH

N C

BPFP-apa

CF3

O

C

N

O

Scheme 2.11 Acetylene functional benzoxazine monomers. Reproduced with permission from H.J. Kim, Z. Brinovska and H. Ishida, Polymer, 1999, 40, 23, 6565. ©1999, Elsevier [11]

CH

CH

O

C

N

N

C

O

O

CF3

Polybenzoxazines: Chemistry and Properties

groups that provided an additional crosslinking site. These monomers polymerise at moderate temperatures without added catalysts.

Structure–Property Relationships

2.2.6 BZ-Containing Nitrile Groups Phthalonitrile PBZ (Scheme 2.12) showed a lower cure temperature (250 °C) and high char yield (68%) at 800 °C as a result of polymerisation of nitrile groups at a higher temperature [13]. The reactive phthalonitrile terminal group contributed to the formation of highly thermally stable crosslinked structures. A polymerisation temperature of 250 °C was sufficient to achieve material of high thermal stability and a Tg of 275–300 °C. Similarly, the phenylnitrile crosslinking sites containing polymers showed excellent thermal properties and improved mechanical properties [14] (Scheme 2.13).

N

C

N

N

N

O

C

N

CH3 N

C

C

I

C

N C

O

N

N CH3

O C

N C

N

N O

CH3

II C

O

O

C

O

CH3 N

N

C C

C

N C

N

III

N

N

IV

Scheme 2.12 Phthalonitrile benzoxazines. Reproduced with permission from Z. Brunovska, R. Lyon and H. Ishida, Thermochimica Acta, 2000, 357/358, 195. ©2000, Elsevier [13]

65

Polybenzoxazines: Chemistry and Properties

O

N O

O N CN

PN-1

NC

O

PN-2

N

CN

Scheme 2.13 Phenylnitrile benzoxazines. Reproduced with permission from H. Qi, H. Ren, G. Pan, Y. Zhuang, R. Huang and L. Du, Polymers for Advanced Technologies, 2009, 20, 3, 268. ©2009, John Wiley & Sons [14]

2.2.7 Main-chain or High-molecular-weight PBZ Several new synthetic techniques have been introduced for preparing diverse BZ, including polymers with BZ units in the main chain, which enables BZ technology to be much more versatile to tailor desired properties in the final product. These polymers can be processed into self-supporting flexible films. During this phase of processing, they behave as thermoplastics. A linear PBZ molecule with oxazine rings in the main chain was synthesised with a molecular weight of ~10 kDa. The resultant polymer had a moderately broad polydispersity index [15]. However, insolubility of the products due to extreme rigidity resulted in a low molecular weight and broad polydispersity. An attempt to overcome this difficulty using a flexible and thus more soluble segment (i.e., an aliphatic amine) was made (Scheme 2.14).

Scheme 2.14 A main-chain benzoxazine polymer. Reproduced with permission from A. Chernykh, J. Liu and H. Ishida, Polymer, 2006, 47, 22, 7664. ©2006, Elsevier [15] 66

Structure–Property Relationships Two distinctive exothermic peaks (161 °C and 242 °C) appeared in the DSC analysis. The first peak could be assigned to the crosslinking reaction due to methylol end groups. The higher temperature peak was due to conventional BZ polymerisation. The derivative weight-loss curve showed that the polymer degraded in a three-stage weight-loss process. The crosslinked polymer seemed to be thermally more stable (T1% was 270 °C and T5% was 302 °C). High-molecular-weight polyether esters containing BZ units showed thermal stability more or less close to the stability of BA-a, but they exhibited better film properties. The resultant polymer showed better toughness induced by the soft ether-ester group, but the aliphatic ether ester group modules had some drawbacks such as high water absorption and low thermal stability [16]. Linear main-chain PBZ with molecular weight in the range 20–40 kDa range were also prepared by the ‘click chemistry approach’ (Scheme 2.15). Azide-containing BZ monomers produce PBZ with a triazole unit in the chain, enhancing the Tg up to 278 °C with low moisture absorption [17].

67

CH3

O

CH3

N

O

O

N3

CH3 CH3

O

N O

N N N

CuI/pyridine/DMF

CH3

O

CH3

n

N O H3C N O

N3

O

N N N

O

N

N

O

CH3

O

N

CuI/pyridine/DMF N

CH3 O

N

O

CH3

O

N

O

O

N N N

O

N

O

O

N

O

n

Scheme 2.15 Polybenzoxazines via the click chemistry approach (dimethylformamide (DMF)). Reproduced with permission from A. Chernykh, T.Agag and H. Ishida, Polymer, 2009, 50, 2, 382. ©2009, Elsevier [17]

Polybenzoxazines: Chemistry and Properties

68 O

N N N

Structure–Property Relationships A polycaprolactone-naphthoxazine (PCL-NZ)-containing polycaprolactone (PCL) has been reported (Scheme 2.16). This naphthoxazine macromonomer does not exhibit the exotherms usually observed with low-molecular-weight BZ due to the polymeric nature of the macromonomers [18]. However, 1H-nuclear magnetic resonance (NMR) spectroscopy and Fourier-transform infrared (FTIR) investigations confirmed the ring opening of the naphthoxazine groups. The cured products contain chemically incorporated PCL segments, which may significantly influence physical and mechanical properties.

Scheme 2.16 Chemical structure of PCL-NZ. Reproduced with permission from B. Kiskan and Y. Yagci, Polymer, 2005, 46, 25, 11690. ©2005, Elsevier [18]

Thermally curable naphthoxazine-functionalised polymers were synthesised by the reaction of linear (diamines) and branched (triamines) polypropyleneoxides (PPOA) of various molecular weights with paraformaldehyde and 2-naphthol [19] to prepare thin films (Scheme 2.17). The DSC results showed that the maximum cure temperature increased with increase in the molecular weight of PPOA, and that it reduced the exothermicity. The films of polybenzoxazine had a water contact angle of 67°. The water contact angles of the BA-a incorporated system increased with the increase in the amount of monomer in the film and by curing.

69

N

N

O n

O

or

R1 O

N

R2

O

O

O

x

O y

N

O

R1= H, CH 2 CH3

R2=

CH2

z

OCH2 CH CH3

O

g

CH 2 CH

NH2

CH 3

Scheme 2.17 Naphthoxazine-functionalised polymers. Reproduced with permission from A. Yildirim, B. Kiskan, A.L. Demirel and Y. Yagci, European Polymer Journal, 2006, 42, 11, 3006. ©2006, Elsevier [19]

Polybenzoxazines: Chemistry and Properties

70 O

Structure–Property Relationships The controlled synthesis of a series of PBZ model oligomers was also reported [20]. A synthetic strategy was developed in which bromine was used as an ortho positionblocking group, allowing a stepwise synthesis of structurally uniform compounds. The report opened a path to prepare many different structurally uniform compounds to aid the characterisation and deeper understanding of this new class of resin. Table 2.1 provides an overall idea of the performance of various functional PBZ.

Table 2.1 Thermal properties of various polybenzoxazines Structure of benzoxazine monomer Tg T5% T10% Char (°C) (°C) (°C) yield (%)

O

N

107 288 356

45

297 348 374

44

322 343 367

28

P-alp O

N

P-ala O

O

N

N

B-ala

71

Polybenzoxazines: Chemistry and Properties

O

O

N

N

298 395 425

42

278 349 376

62

299 394 412

59

355 390 421

70

BA-allyl

N

O

N

O

O

O

Mal-BZ

N

N

O

O

Mal-BZ-Al O

O

CH 2

C

CH

N N

O

O

72

Mal-BZ-Pg

Structure–Property Relationships

O O N

N O

158 330 366

50

252 375 392

56

-

365 383

58

251 362 400

61

318 352 388

66

HPM-Ba

O O

N O

N

MIB O O

N O

N

NOB OCH2 C

O

CH

N

P-appe O

O

N

N CH

HC C

C O

O

B-appe

73

Polybenzoxazines: Chemistry and Properties

O N O

315 336 382

53

308 347 391

47

-

415 513

75

-

470 575

78

(BMO-apa) -

478 547

80

362 494 539

71

P-af O

O

N

N

O

O

BA-af X

O

O

N

N

C

when

CH

C

CH

X = -O- (BO-apa)

=

CH2

(BM-apa)

O

=

C

CF3

=

74

(BPFP-apa) CF3

Structure–Property Relationships

=

S

(BS-apa)

-

489 592

79

(BSO-apa) -

440 540

78

(BA-apa) 347 458 524

74

O

=

S O

CH 3

= CH 3

= Nil (BP-apa) -

O

N

C

462 492

73

380 428

76

319 450 560

76

CH

N

O

C N

N

C N

CH

O

C

I

75

Polybenzoxazines: Chemistry and Properties

N

C N

N

O CH 3

C CH 3

C

73

-

544 596

80

330 423 468

68

N C

O

414 505

II C

O

-

N

N O

N

C C

N

N

III C

N C

CH3 C

O

N O

CH3

N

C C

N

N

N

IV

BO-apa = 4,4′-oxydiphenol- and 3-aminophenylacetylene-based benzoxazine MAL-BZ = 4-Maleimidophenol- and aniline-based benzoxazine BM-apa = 4,4′-Methylenediphenol- and 3-aminophenylacetylene-based benzoxazine BMO-apa = Bis(4-hydroxy phenyl)methanone- and 3-aminophenylacetylenebased benzoxazine BPFP-apa = 2,2 Bis(4-hydroxyphenyl)perfluoropropane- and 3-aminophenylacetylene-based benzoxazine BS-apa = 4,4′-Thiodiphenol- and 3-aminophenylacetylene-based benzoxazine BSO-apa = Bis(4-hydroxyphenyl)sulfone- and 3-aminophenylacetylene-based benzoxazine BP-apa = Biphenyl-4,4′-diol- and 3-aminophenylacetylene-based benzoxazine 76

Structure–Property Relationships

2.3 BZ Derived from Various Precursors (Non-functionalised BZ) Dihydrobenzoxazines were also synthesised from 4,4′-biphenol (BIP), and dicyclopentadienephenol adduct (DCPD) (Scheme 2.18) [21]. The DCPD BZ resins exhibited properties such as low dielectric constants and low dissipation factors for high-frequency application, whereas the PBZ with a rigid biphenyl structure provided high values of Tg and mechanical properties. The BZ polymer resulting from DCPD (DCPDBZ) had a dielectric constant of 2.94, which was better than that of polymers derived from BA-a (3.31), BIP (3.45), and traditional phenolic resin (3.9–4.0). The nonplanar structure of DCPD led to more spacing between polymer molecules, resulting in less efficient chain packing and an increase in the free volume of the polymer.

O

O

N

N

BIPBZ

N

O

O

N

DCPDBZ

Scheme 2.18 Structure of some dihydrobenzoxazines (BIPBZ = 4,4′-Biphenol- and aniline-based benzoxazine). Reproduced with permission from J.Y. Shieh, C.Y. Lin, C.L. Huang and C.S. Wang, Journal of Applied Polymer Science, 2006, 101, 1, 342. ©2006, John Wiley & Sons [21]

The key factors affecting the viscosity of a difunctional BZ resin are the bulky diphenol part between two oxazine rings and the pendant amine groups. Hence, aromatic amine-based PBZ have better properties than their aliphatic amine counterparts. Monofunctional BZ resins based on non-substituted phenol and primary aromatic amines that were liquid at room temperature were synthesised. With non-substituted phenol, formaldehyde, and primary amines as starting materials, a series of monofunctional BZ resins with low viscosities at room temperature were developed [22]. They showed highly improved thermal stability and a high Tg. A series of linear aliphatic diamine-based BZ monomers has been successfully polymerised into transparent, crosslinked materials that are free of voids and with good mechanical integrity (Scheme 2.19) [23]. The density of these PBZ was shown to decrease as a function of the chain length of the aliphatic diamine. In DMA, the linear aliphatic diamine-based PBZ exhibited two fairly strong, aliphatic chain length77

Polybenzoxazines: Chemistry and Properties dependent, low-temperature relaxation processes. The room temperature modulus was also shown to be a strong function of diamine length, decreasing from a rather stiff 2.1 GPa for P-ad2 to 0.87 GPa for the polymer with the longest aliphatic chain (P-ad12). The Tg of P-ad2 and P-ad6 surpassed the 170 °C of bisphenol A-based PBZ (Table 2.2). Thus, linear aliphatic diamine-based polymers possess inherently flexible PBZ network structures.

Pad-8 Pad-4 O N

(CH2)

N O

n

n

2, 4, 6, 8, 12 Pad-2 Pad-6 Pad-12

Scheme 2.19 Aliphatic diamine-based benzoxazine monomers. Reproduced with permission from D.J. Allen and H. Ishida, Journal of Applied Polymer Science, 2006, 101, 5, 2798. ©2006, John Wiley & Sons [23]

Table 2.2 Tg of the diamine-based series of polybenzoxazine as a function of the length of the aliphatic chain

78

Polymer

Tg (°C)

Pad-2

184

Pad-4

160

Pad-6

169

Pad-8

151

Pad-12

118

Structure–Property Relationships The polyfunctional BZ 8,8′-bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine) (22P-a) and 6,6′-bis(2,3-dihydro-3-phenyl-4H-1,3-benzoxazinyl)ketone (44O-a) were successfully cured to produce void-free resins in an autoclave with a pressure of 1.33 MPa (Scheme 2.20) [24]. The maximum Tg achieved for the 22P-a and 44O-a PBZ was 200 °C and 365 °C, respectively. The highly crosslinked 44O-a PBZ exhibited a Tg that was higher than the Tcure. The polymer was processed at 290 °C and showed a Tg of 365 °C. The maximum service temperatures were found to be 177 °C and 290 °C, respectively.

O O

O

N

N

Scheme 2.20 Structure of 44O-a

A series of BZ resins having different amine moieties have been synthesised [25] that, upon polymerisation, produce varying amounts of phenolic Mannich bridges, arylamine Mannich bridges, and methylene linkages (Scheme 2.21).

79

O

N

N

BA-a O

O

N

N

CH 3

H3C

BA-ot

H 3C

O

O

N

N

O

N

N

BA-mt

H 3C

H 3C CH 3 H 3C

BA-pt

O

CH 3

O

O

N

N

BA-35X

CH 3

CH 3

Scheme 2.21 Benzoxazine monomers possessing methyl substituents (BA-ot = bisphenol A- and o-methyl aniline-based benzoxazine). Reproduced with permission from H. Ishida and D.P. Sanders, Journal of Polymer Science, Part B: Polymers Physics Edition, 2000, 38, 24, 3289. ©2000, John Wiley & Sons [25]

Polybenzoxazines: Chemistry and Properties

80 O

Structure–Property Relationships BA-ot is the least thermally stable compound, with an onset degradation temperature of <220 °C. BA-pt and BA-a exhibited similar thermal stabilities. However, the materials with meta-substituted aniline rings exhibited significantly higher thermal stabilities, with no weight loss up to ~300 °C. The meta-substituted compounds BAmt and BA-35x possessed the highest T5% (350 °C), but the ultimate char yield at 800 °C remained similar (~30–31%). Some novel approaches have been reported for the synthesis of PBZ [26]. In one approach, a novel BZ was synthesised from a phosphorus-containing triphenol (dopotriol), formaldehyde, and methylamine (dopot-m, Scheme 2.22); dopot-m was then copolymerised with a commercial BZ [6′,6-bis(3-phenyl-3, 4-dihydro-2H-1, 3-benzoxazineyl)methane (F-a), Scheme 2.22] or the diglycidyl ether of bisphenol A (DGEBA). The thermal properties and flame-retardant characteristics of the F-a/ dopot-m copolymers increased with the content of dopot-m. The Tg of the dopot-m/ DGEBA copolymer was 252 °C, which was higher than that of poly (dopot-m). The T5% of the dopot-m/DGEBA copolymer increased from 323 °C to 351 °C because of the higher crosslinking density caused by the reaction of phenolic OH and epoxy groups.

O

O

P

O

O

O

N

N

CH3

CH3

H3 C

N

O

N

CH 2

O

F-a

N

Dopot-m

Scheme 2.22 Structure of Dopot-m and F-a. Reproduced with permission from C.H. Lin, S.X. Cai, T.S. Leu, T.Y. Hwang and H.H. Lee, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2006, 44, 11, 3454. ©2006, John Wiley & Sons [26] 81

Polybenzoxazines: Chemistry and Properties In another approach, phosphorus was incorporated into BZ via the curing reaction of dopotriol and F-a. After curing, the thermal properties of the F-a/dopotriol copolymers were almost the same as those of neat poly (F-a). This implies that the flame-retardant element phosphorus can be incorporated into the PBZ without sacrificing thermal properties. In an another method, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10oxide (DOPO, Scheme 2.23) was reacted with electron-deficient BZ (F-a, excess) to incorporate the element phosphorus. After curing, the Tg of PBZ decreased slightly with the content of DOPO, mainly because of the smaller crosslinking density of the resultant PBZ.

O

P H

O

DOPO Scheme 2.23 Structure of DOPO

It is well-known that low dielectric systems are used as insulating materials, and the incorporation of fluorinated substituents into polymers can decrease dielectric constants because of the small dipole and low polarisability of the C–F bond. A fluorinated BZ (F-1 benzoxazine) has been synthesised by incorporating the trifluoromethyl groups into the monomer. This fluorinated copolybenzoxazine (Scheme 2.24) possesses a high Tg and high thermal stability, which is suitable for high-temperature operation for certain special processes of interlayer dielectrics [27]. Incorporation of fluorine atoms into the BZ structure also leads to materials with higher thermal stability because fluorination can improve thermal properties. This is partly because the C–F bond is stronger than the C–H bond. The copolymer of BA-a and F-1 (BA-a-co-F-1) exhibited only one Tg, strongly implying that all these copolymers are homogenous and that increasing the F-1 benzoxazine content results in a substantial increase in the Tg. This study showed that a strong intermolecular 82

Structure–Property Relationships interaction exists in BA-a-co-F-1. The Tg increases by ~113 °C, from 170.6 °C of the BA-a benzoxazine to 283.4 °C (F-1 benzoxazine). The copolymers possess low water absorption due to their high crosslink density and hydrophobic properties, and their percentages of water absorption are <1 wt%, even in the underwater environment. The results indicate that the fluorinated copolybenzoxazines possess outstanding properties in resisting moisture uptake.

BA-a

F-1

BA-a-co-F-1

Scheme 2.24 Polymerisation in BA-a-co-F-1. Reproduced with permission from Y.C. Su and F.C. Chang, Polymer, 2003, 44, 26, 7989. ©2003, Elsevier [27]

Incorporation of various transition metal salts (2 mol%) has been shown to improve the char yield of PBZ by 10–20% [28]. It has been shown that the transition metal salts initiate ring-opening but do not catalyse the chain propagation in the polymerisation of BZ. Further, metal salts were found to promote the formation of carbonyl groups during the polymerisation of BZ. It was proposed that the flammability of PBZ is also reduced by the presence of metal salts because a higher concentration of CO2 is observed upon thermal degradation. It was also revealed that the mechanism of degradation was not significantly affected by the presence of metals, except that the rate of weight loss was lower. A char yield ≤70% (at 800 °C under a nitrogen environment) has been achieved without sacrificing the ease of processing for a PBZ based on 2,2′-biphenol and aniline (22BP-a, Scheme 2.25)

83

Polybenzoxazines: Chemistry and Properties

N

O

O

N

Scheme 2.25 Structure of 22BP-a The polymer–cyclodextrin (CD) inclusion complexes are also attractive because they render stiffness to the polymer chain and enhance thermal stability and the Tg. The adamantane unit was used to modify BZ because it can fit precisely into the slightly polar-CD cavity [29]. b-Cyclodextrin (b-CD) forms inclusion complexes with adamantane-modified BZ, whose structure is given in Scheme 2.26.

R

OH

N

n

R = Phenyl or methyl

CD

Scheme 2.26 Cyclodextrin-inclusion complexes of benzoxazine. Reproduced with permission from Y.C. Su, W.C. Chen and F.C. Chang, Polymer, 2005, 46, 5, 1617. ©2005, Elsevier [29]

84

Structure–Property Relationships The Tg values of poly-2-benzoxazine and poly-3-benzoxazine are 109.4 °C and 188.8 °C, respectively. The Tg of poly (b-CD/2 benzoxazine inclusion complex) and poly (b-CD/3 benzoxazine inclusion complex) are 216.6 °C and 243.4 °C, respectively. These results are reasonable because of the greater steric congestion existing in these complexes, and because PBZ main chains become stiffer, resulting in higher Tg values. X-ray diffraction and solid-state 13C-cross polarisation/magic angle spinning NMR spectroscopic studies indicate that the resulting polypseudorotaxane is a channel-type structure with stiffer main chains. Two novel structures of adamantane-modified BZ were synthesised from 4-(1-adamantyl)-phenol through incorporation of adamantane as a pendant group into the PBZ backbone [30]. The rigid structure of adamantane tended to hinder the chain mobility (‘boat anchor effect’) and substantially enhanced the thermal properties, including the Tg and decomposition temperature, especially for poly(6-adamantyl-3-methyl-3,4-dihydro-2H-1,3-benzoxazine). In the poly(6adamantyl-3-phenyl-3,4-dihydro-2H-1,3-benzoxazine) system, however, the opposite result for the Tg was observed, and it was attributed to a lower crosslinking density. The phenyl group was bulkier than the methyl group, and the movement of the molecular chain was hindered between bridging points during the curing process; this resulted in a lower crosslinking density and a lower Tg than those of poly(6adamantyl-3-methyl-3,4-dihydro-2H-1,3-benzoxazine). With phosphorus pentachloride as a cationic initiator, substituted 3-methyl-3,4dihydro-2H-1,3-benzoxazine-based semi-crystalline and amorphous thermoplastics with significant molecular weights have been synthesised [31]. Dependent upon the different substitution positions of the monomers, two polymerisation mechanisms competed during the polymerisation process, giving rise to phenoxy-type and phenolictype polymers. Phenolphthalein- and allylamine-based BZ was also reported [32]. A new type of PBZ (derived from p-hydroxy toluene and p-amino toluene), poly(3-(pmethyl) benzyl-3, 4-dihydro-6-methyl-2H-1, 3-benzoxazine), has been synthesised by an electrochemical method in acetonitrile-alkali aqueous solution [33]. The obtained film showed good heat-resistance properties, and the polymer remained stable up to 360 °C in air. All the materials mentioned above are monoamine-based PBZ. High-molecularweight PBZ precursors have been synthesised from aromatic or aliphatic diamine and bisphenol-A with paraformaldehyde (Scheme 2.27) [34]. These PBZ films exhibited significantly improved toughness than the typical PBZ, PBA-a, due to the long linear backbone and high crosslink density. The bisphenol A- and dimethyl diamine-based PBZ (PB-mda) and bisphenol A- and hexamethyl diamine-based PBZ (PB-hda) films were very tough and showed good bending flexibility. The Tg values of PBZ from the high-molecular-weight precursors were 238–260 °C, much higher than the Tg of the typical PBA-a (171 °C).

85

Polybenzoxazines: Chemistry and Properties

O

N

OH NH 2

H 3C

CH 2 O

CH 3

H 3C

CHCl3 n

CH 3

Reflux 5h

NH2 N

O

n

OH

(CH2)2

PB mda

,

(CH2)6

,

CH2

PB hda

Scheme 2.27 Synthesis of high-molecular-weight polybenzoxazines. Reproduced with permission from T. Takeichi, T. Kano and T. Agag, Polymer, 2005, 46, 26, 12172. ©2005, Elsevier [34]

Various approaches for the improvement in the performance of PBZ have been attempted. This includes alloying with other high-performance polymers, hybridisation with inorganics, and designing of novel monomers as well as high-molecular-weight polymeric precursors. By these approaches, lowering of the cure temperature, improvement in toughness, and enhancement of thermal and mechanical properties were achieved. BZ precursors containing phenol hydroxyl groups (using bisphenol A and 4,4′diaminodiphenyl methane (DDM) in a 2:1 ratio) have been reported [35]. As expected, the cure temperature was lowered (215 °C) due to the presence of hydroxyl groups. The thermal stability at T5% and T10% was 346 °C and 375 °C, respectively. However, the Tg was found to be only ~180 °C. DDM-based PBZ derived from phenol, p-cresol, 2-naphthol have also been reported. The thermal polymerisation temperature of phenol (broad and centred at 226 °C)-based PBZ was lower than the p-cresol (265 °C) and naphthol-based systems (256 °C). Phenol-based PBZ was found to have better thermal stability (T10% at 432 °C and char yield of 65%). The rigid structure of naphthol did not help increase the high-temperature properties, which showed poor characteristics compared with the other two systems [36].

86

Structure–Property Relationships The issue of improving the properties of PBZ stimulated many researchers to design polymers with different structural features. We recently synthesised a few new PBZcontaining cyclohexyl moieties [37] (Scheme 2.28).

HO

OH

NH 2 + 4 HCHO +

N

N

O

O

R R R=

-H, -C15H31 ,

Scheme 2.28 Synthesis of cyclohexyl-benzoxazine monomers

These new BZ monomers exhibited better processability with lower peak cure temperature (Tp) and a lower processing window (Tp–Ti) (Table 2.3).

Table 2.3 DSC characteristics of cyclohexyl-benzoxazine monomers Tm

Ti (°C)

Tp (°C)

Tf (°C)

ΔH (kJ/mole)

BZ-CH

69

120

221

276

139

BZ-PD-CH

54

124

212

311

133

BZ-PHC-CH

93

130

223

280

134

BA-a

82

180

249

294

123

Monomers

Ti = Initial cure temperature Tp = Peak cure temperature Tf = Final cure temperature

87

Polybenzoxazines: Chemistry and Properties The Tg of PBZ (derived from the maximum of the tan δ peak, Figure 2.3) varied from 170 °C to 205 °C. The Tg of BZ-CH, BZ-PD-CH and BZ-PHC-CH are 191, 170 and 206 °C, respectively. The conventional PBZ (BA-a) exhibited a Tg of 215 °C. Due to the flexibility of the C15 aliphatic chain, the Tg of BZ-PD-CH drops in comparison with BZ-CH. Conversely, the rigidity of the cardo group in BZ-PHC-CH results in a higher Tg (206 °C) relative to BZ-CH.

PBZ-CH

1.0

PBZ-PD-CH

PBA-a

tan δ

0.5

0.0 PBZ-PHC-CH

0

100

200 T(°C)

300

400

Figure 2.3 Change in loss factor of cyclohexyl-based polybenzoxazines with temperature

A semi empirical equation has been used for calculating the crosslink density of highly crosslinked systems. log 10 G′= 7 + 293 Xdensity

88

Structure–Property Relationships where G′ is the storage modulus of the cured polymer in the rubbery plateau region in dynes/cm2 above the Tg (i.e., Tg+40 °C), Xdensity is the crosslink density of the polymer. The crosslink density of the cured BA-a was found to be 2900 mol/m3. The crosslink densities of the polymers BZ-CH, BZ-PD-CH and BZ-PHC-CH were 4760, 4390, and 7590 mol/m3, respectively. The new PBZ BZ-CH and BZ-PD-CH showed almost identical crosslink density compared with that of BA-a. However, the higher crosslink density of BZ-PHC-CH is attributed to the perhydrocumene structure attached to the backbone. The lower Xdensity of BZ-PD-CH is due to the incorporation of a flexible aliphatic chain. The DMTA results are compiled in Table 2.4.

Table 2.4 DMTA characteristics of cured monomers Tg (°C)

Xdensity (mol/m3)

BZ-CH

191

4760

BZ-PD-CH

170

4390

BZ-PHC-CH

206

7590

BA-a

215

2900

Cured monomers

Xdensity = Crosslink density

The thermal stability of the PBZ was studied by thermogravimetric analysis (TGA). The T5%, T10%, and char yield at 800 °C were used to compare the thermal stability of the polymers. The T5% of BZ-CH, BZ-PD-CH and BZ-PHC-CH were 290 °C, 338 °C and 313 °C, respectively, in comparison with the T5% of BA-a (275 °C). The T10% of BA-a was 335 °C, whereas those for the three polymers BZ-CH, BZ-PD-CH and Bz-PHC-CH were 324 °C, 366 °C and 343 °C, respectively. The higher values of T5% and T10% showed the better thermal stability of the novel PBZ. This is due to the higher crosslink density of these systems. However, the residue at high temperature is very low for the three PBZ in comparison with cured BA-a. The char yield is usually proportional to the aromatic content of a polymer. The aromatic content is a maximum for the BA-a polymer and minimum for BZ-PD-CH among the four polymers investigated. The char yield of BZ-PD-CH was very low at 800 °C (5%) due

89

Polybenzoxazines: Chemistry and Properties to the high content of the aliphatic pentadecinyl cyclohexyl moiety, which stripped off very easily from the network at high temperature. Though the new PBZ have higher initial decomposition temperature, their high temperature stability is inferior to that of BA-a polymer. The TGA results are compiled in Table 2.5.

Table 2.5 Thermal stability of cured benzoxazines Cured monomers

T5% (°C)

T10% (°C)

Char yield at 800 °C (%)

BZ-CH

290

324

17

BZ-PD-CH

338

366

05

BZ-PHC-CH

313

343

16

BA-a

275

335

35

T5% = Temperature at 5% mass loss T10% = Temperature at 10% mass loss These PBZ showed improved surface de-wetting properties. BZ-PHC-CH (112 ± 1°) showed the highest contact angle, whereas BZ-CH (98 ± 1°) exhibited the lowest value. For BZ-PD-CH, the contact angle was 105 ± 1°; a value of 102 ± 1° was observed and for BA-a polybenzoxazine. Except for BZ-PHC-CH, the difference in contact angle was marginal among the PBZ. The T5% of BZ-CH, BZ-PD-CH and BZ-PHC-CH were 290 °C, 338 °C and 313 °C, respectively, in comparison to the T5% of BA-a (275 °C). The T10% of BA-a was 335 °C, whereas those for the three polymers BZ-CH, BZ-PD-CH and BZ-PHC-CH were 324 °C, 366 °C and 343 °C, respectively. The higher values of T5% and T10% showed the better thermal stability of the novel PBZ. The water contact angle values are given in Table 2.6.

Table 2.6 Water contact angle of cyclohexyl polybenzoxazines Polybenzoxazine

Water contact angle (±1°)

PBA-a

102°

PBZ-CH

98°

PBZ-PD-CH

105°

PBZ-PHC-CH

112°

90

Structure–Property Relationships

2.4 Conclusion The structure–property relationships among various PBZ have been detailed. Several polymerisable groups have been incorporated into the PBZ structure. In the case of propargyl PBZ, the propargyl group underwent polymerisation earlier than oxazine ring-opening. Allyl BZ showed a broad exotherm with high exothermicity than typical BA-a. This was attributed to the polymerisation of allyl and BZ groups. The furan groups, nitrile, phthalonitrile, and maleimide in the PBZ confer enhanced thermal and mechanical properties. Varying phenolic and amine components in the monomer leads to PBZ with a high Tg (≤350 °C) and increased char yield (≤80%). PBZ surfaces are hydrophobic in nature. PBZ without addition-curable moieties also exhibited good high-performance properties dependent upon the strength of their backbone.

References 1.

T. Agag and T. Takeichi, Macromolecules, 2003, 36, 16, 6010.

2.

K.S. Santhosh Kumar, C.P. Reghunadhan Nair and K.N. Ninan, European Polymer Journal, 2007, 43, 6, 2504.

3.

Y.L. Liu, J.M. Yu and C.I. Chou, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2004, 42, 23, 5954.

4.

T. Agag and T. Takeichi, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2006, 44, 4, 1424.

5.

H. Ishida and S. Ohba, Polymer, 2005, 46, 15, 5588.

6.

Y.L. Liu and J.M. Yu, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2006, 44, 6, 1890.

7.

T. Agag and T. Takeichi, Macromolecules, 2001, 34, 21, 7257.

8.

B. kiskan and Y. Yagci, Polymer, 2008, 49, 10, 2455.

9.

Y.L. Liu and C.I. Chou, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2005, 43, 21, 5267.

10. H.J. Kim, Z. Brunovska and H. Ishida, Polymer, 1999, 40, 7, 1815. 11. H.J. Kim, Z. Brunovska and H. Ishida, Polymer, 1999, 40, 23, 6565.

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Polybenzoxazines: Chemistry and Properties 12. H.J. Kim, Z. Brunovska and H. Ishida, Journal of Applied Polymer Science, 1999, 73, 6, 857. 13. Z. Brunovska, R. Lyon and H. Ishida, Thermochimica Acta, 2000, 357/358, 195. 14. H. Qi, H. Ren, G. Pan, Y. Zhuang, R. Huang and L. Du, Polymers for Advanced Technologies, 2009, 20, 3, 268. 15. A. Chernykh, J. Liu and H. Ishida, Polymer, 2006, 47, 22, 7664. 16. B. Kiskan, Y. Yagci and H. Ishida, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2008, 46, 2, 414. 17. A. Chernykh, T. Agag and H. Ishida, Polymer, 2009, 50, 2, 382. 18. B. Kiskan and Y. Yagci, Polymer, 2005, 46, 25, 11690. 19. A. Yildirim, B. Kiskan, A.L. Demirel and Y. Yagci, European Polymer Journal, 2006, 42, 11, 3006. 20. H. Ishida and C.M. Krus, Macromolecules, 1998, 31, 8, 2409. 21. J.Y. Shieh, C.Y. Lin, C.L. Huang and C.S. Wang, Journal of Applied Polymer Science, 2006, 101, 1, 342. 22. Y.X. Wang and H. Ishida, Journal of Applied Polymer Science, 2002, 86, 12, 2953. 23. D.J. Allen and H. Ishida, Journal of Applied Polymer Science, 2006, 101, 5, 2798. 24. S.B. Shen and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 1999, 37, 23, 3257. 25. H. Ishida and Sanders, Journal of Polymer Science, Part B: Polymer Physics Edition, 2000, 38, 24, 3289. 26. C.H. Lin, S.X. Cai, T.S. Leu, T.Y. Hwang and H.H. Lee, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2006, 44, 11, 3454. 27. Y.C. Su and F.C. Chang, Polymer, 2003, 44, 26, 7989. 28. H.Y. Low and H. Ishida, Polymer Degradation and Stability, 2006, 91, 4, 805.

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Structure–Property Relationships 29. Y.C. Su, W.C. Chen and F.C. Chang, Polymer, 2005, 46, 5, 1617. 30. Y.C. Su, W.C. Chen and F.C. Chang, Journal of Applied Polymer Science, 2004, 94, 3, 932. 31. Y.X. Wang and H. Ishida, Macromolecules, 2000, 33, 8, 2839. 32. H.W. Cao, R.W. Xu, H. Liu, and D.S. Yu, Designed Monomers and Polymers, 2006, 9, 4, 369. 33. L. Li, J. He, X. Wan, D. Zhou; G. Xue, Y. Wang, S.W. Choi and H. Ishida, The Journal of Adhesion, 2003, 79, 4, 351. 34. T. Takeichi, T. Kano and T. Agag, Polymer, 2005, 46, 26, 12172. 35. W. Men, Z. Lu and Z. Zhan, Journal of Applied Polymer Science, 2008, 109, 4, 2219. 36. W. Men and Z. Lu, Journal of Applied Polymer Science, 2007, 106, 4, 2769. 37. K.S. Santhosh Kumar, C.P. Reghunadhan Nair, K.N. Ninan, A. D. Kulkarni and P.P. Wadgoankar, Polymers for Advanced Technologies, 2009, 20, 12, 1107.

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Polybenzoxazines: Chemistry and Properties

94

3

Blends and Composites of Polybenzoxazines

3.1 Introduction Modification of one polymer with other outstanding polymers is an old and successful technique to obtain polymers with diverse properties. It is ‘property-oriented’ moulding of micro- and macrostructures of polymers. Non-reactive and reactive blends, polymer alloys and copolymers are produced to meet the demands of industry. This includes features such as low cost, high temperature resistance, light weight and also to the daily needs of the general population such as polymeric bottles and carrier bags. A ‘multi-component polymeric material’ refers to the combination of two or more polymers, including polymer alloys, blends and composites in all forms. Such materials have been widely investigated through a huge number of polymers in the scientific and engineering fields. Polybenzoxazines (PBZ) have been investigated for their blends, alloys and copolymers with linear polymers and other crosslinking polymers. This chapter unravels the preparation and properties of tailored systems derived from PBZ and their micro- and nano-composites.

3.2 PBZ–Epoxy Blends Incorporation of epoxy resin into the PBZ network structure resulted in a copolymer that had a higher crosslink density and glass transition temperature (Tg) than its homopolymers. The copolymerisation reaction between benzoxazine (BZ) monomer and epoxy occurs via the opening of the epoxide ring by the phenolic hydroxyl functionalities present in the PBZ (Scheme 3.1). In the BZ–epoxy copolymers investigated, with epoxy content >45%, the copolymers experience a sharp decrease in Tg, and the copolymers containing equal amounts of BZ and epoxy exhibited a Tg lower than that of the pure PBZ. The epoxy-rich samples (those with >45% epoxy) showed poor mechanical properties because phenolic groups generated by the oxazine ring-opening reaction not only serve to catalyse the copolymerisation, but also participate as reactants and are therefore consumed by the reaction. Thus, as the stoichiometric ratio of components is approached, non-reacted or small-molecularweight epoxy molecules may remain and interfere with network formation or act as plasticisers.

95

Polybenzoxazines: Chemistry and Properties Increasing molecular weight between epoxy groups by chain extension has afforded copolymers with reduced crosslink density, improved storage modulus, and reduced Tg [1]. Copolymerisation with epoxy reduced char yields compared with pure PBZ, but chain-extended epoxy slightly increased the char yield. The addition of epoxy as a reactive diluent in the BZ matrix reduced the viscosity of the resin, but the curing was shifted to a higher temperature [2].

OH R O

N

O

H 2C

HC

CH2 O

OH O HC R

N

O

O

CH2

CH2

O

OH R

O

H 2C

HC CH2 OH

Scheme 3.1 Copolymerisation of benzoxazine with epoxy. Reproduced with permission from B.S. Rao, K.R. Reddy, S.K. Pathak and A.R. Pasala, Polymer International, 2005, 54, 10, 1371. © 2005, John Wiley & Sons [1] On the contrary, addition of phenolic novolac into the BZ resin resulted in a mixture that could be cured at a lower temperature. Phenolic novolac resin acts mainly as an initiator for these ternary systems, whereas the low melt viscosity, flexibility and improved crosslink density of the materials are attributed to the epoxy fraction. PBZ imparts thermal stability and mechanical properties, as well as low water-uptake, to the ternary systems [3]. The properties of a high-performance thermosetting resin can be improved by tightening its network structure. Hence, the modified novolac resins with BZ rings were prepared and cured with isobutyl bis(glycidylpropylether) phosphine oxide as the crosslinking agent [4]. The maleimide-functionalised BZ was copolymerised with diglycidyl ether bisphenol A (DGEBA) in various compositions in which three polymerisation reactions involving 96

Blends and Composites of Polybenzoxazines an epoxy ring, a BZ ring, and a vinyl group were observed [5]. For copolymers with a high content of epoxy, the polymerisation of the maleimide group was incomplete, whereas polymerisations of epoxy and BZ were less affected. The resulting polymer achieved a high Tg of 278 °C at 10 mol% DGEBA, and was higher than that of the homopolymer (Tg = 253 °C). Meanwhile, flexural strain at breakage increased with the increase in epoxy content. The flexural modulus was ~5.0 GPa for 10 mol% DGEBA and decreased to 4.2 GPa with 50 mol% DGEBA. The toughness of PBZ can effectively be improved by alloying with isophorone diisocyanate-based urethane prepolymers (PU) or with flexible epoxy [6].

3.3 PBZ–Poly(ε-Caprolactone) Blends Hydrogen bonding is adequate to induce rigidity and constrain the mobility in the glassy state for thermosetting resins. Polymer blends of bisphenol A and an anilinebased benzoxazine (BA-a), and poly(e-caprolactone) (PCL) indicated that hydrogenbonding interactions occur between the carbonyl groups of PCL and the hydroxyl groups of PBA-a upon curing [7]. The most pronounced effect of the addition of PCL was to broaden the glass-transition region, and a decrease in the value of the loss tangent (tan δ) in the transition region upon increasing the PCL content. Before curing, the benzoxazine BA-a/PCL blends were miscible, but the phase separation induced by polymerisation was observed after curing at elevated temperature [8]. The Tg of the PCL/BZ monomer blend exhibited a continuous increase as the PCL composition increased from 0 wt% to 33 wt% [9]. The intermolecular hydrogen bonding between PBZ and polycarbonate (PC) is illustrated in Scheme 3.2 [10].

CH3 O

C CH3

CH3 O C O

C

O

O C O

CH3

O H

O

OH N

O H N

N

Scheme 3.2 Intermolecular hydrogen bonding between poly(ε-caprolactone) and polybenzoxazine. Reproduced with permission from H. Ishida and Y.H. Lee, Journal of Polymer Science, Part B: Polymers Physics Edition, 2001, 39, 7, 736. © 2001, John Wiley & Sons [9]

97

Polybenzoxazines: Chemistry and Properties

3.4 PBZ–Polyimide Blends Benzoxazine monomer (BA-a) was blended with soluble polyimide-siloxanes (PDMS) with and without pendant phenolic groups [11]. The onset and maximum of the exotherm due to the ring-opening polymerisation for the pristine BA-a appeared on differential scanning calorimetry (DSC) curves at ~200 °C and 240 °C, respectively. In the presence of poly(imide-siloxane)s, the exothermic temperatures were lowered: the onset to 130–140 °C and the maximum to 210–220 °C. The cured blends containing poly(imide-siloxane) with -OH functionality showed two Tg values, at a low temperature ~255 °C and at a high temperature ~250–300 °C, displaying phase separation between PDMS and the combined phase consisting of polyimide and PBZ (PBA-a) components due to the formation of AB-crosslinked polymers. For the blends containing poly(imide-siloxane) without OH functionalities, in addition to the Tg due to PDMS, two Tg values were observed in high-temperature ranges at 230–260 °C and 300–350 °C. This indicated further phase separation between the polyimide and PBZ components due to the formation of semi-interpenetrating networks (IPN). In both cases, the Tg increased with increasing poly(imide-siloxane) content. Tensile measurements showed that the toughness of PBA-a was enhanced by the addition of poly(imide-siloxane). Thermogravimetric analysis (TGA) revealed that the thermal stability of PBA-a was also enhanced by the addition of poly(imidesiloxane). Polymer alloys of PBZ (BA-a) and soluble polyimide (PI) or its precursor, poly(amide acid) (PAA) were prepared and characterised [12]. The BA-a/PI formed a IPN structure. The ring-opening polymerisation of BA-a occurred in situ with the imidisation of PAA, accordingly causing a complicated reaction. It was proposed that, in addition to the imidisation of PAA, the reaction between the phenolic OH of polyBA-a and the carboxylic acid of PAA could occur, affording an AB-crosslinked structure (Scheme 3.3). The polymer alloy films from PI/BA-a and PAA/BA-a showed only one Tg. The Tg values and thermal stabilities were increased remarkably as the content of PI increased. As the content of BA-a increased, the modulus of the polymer alloy films was also enhanced.

98

Blends and Composites of Polybenzoxazines

Scheme 3.3 A possible copolymerisation between polyBA-a and PAA. Reproduced with permission from T. Takeichi, Y. Guo and S. Rimdusit, Polymer, 2005, 46, 13, 4909. ©2005, Elsevier [12] Polysiloxane-block-polyimide (SPI) has several attractive characteristics such as low moisture absorption and excellent thermal stability. Hence, this flexible material was incorporated into a PBZ matrix (PBA-a). The Tg of blends with different SPI content showed a slight increase from the neat matrix, i.e., 160 °C to 169 °C, and the system exhibited partial miscibility as evidenced from an opaque appearance of cured blends. A noted advantage of blending is that the flexibility of PBZ is improved by SPI. In addition, the temperature at 5% weight loss (T10%) increased from 360 °C to 450 °C (75% weight of SPI) and the char yield increased from 30% (PBA-a) to 45% at 5% weight of SPI [13].

99

Polybenzoxazines: Chemistry and Properties

3.5 Other Blend Systems Any chemical modification or additive to enhance toughness (with a minimum sacrifice of the original mechanical and physical properties of the PBZ) is attractive. Sequential IPN, based on PU and PBZ, was synthesised. The kinetics indicated the presence of physical bonding only in the resulting IPN. Morphological investigations revealed slight phase separation behaviour in all of the IPN studied [14]. In the blend of PBZ/ PC, the ring-opening reaction and subsequent polymerisation reaction of the BZ were significantly inhibited by the presence of PC [15]. The hydrogen-bonding interaction in the blends occurs between the hydroxyl groups of the PBZ and the carbonyl groups of the PC. This is the ‘driving force’ that results in the miscibility of the PC/ BZ blend in the entire composition range along with possible copolymer formation. The Tg of the resulting blends decreased as the concentration of PC increased and deviated markedly from the Fox equation. In addition, an earlier degradation event appeared in the blend with 11 wt% and 33 wt% of PC. The possibility of the exchange reaction (which can occur in the blends containing PBZ and PC) was also confirmed [16]. The rubber-modified system of this PBZ-PCL matrix showed poorer thermal properties [17]. Study of thermosetting blends composed of BA-a and polyethylene oxide indicated that the phenolic hydroxyl groups could not form favourable intermolecular hydrogenbonding interactions at elevated temperatures (e.g., the curing temperatures), i.e., the phenolic hydroxyl groups existed mainly in the non-associated form in the system [18]. Therefore, the phase separation is ascribed to the decrease of the entropic contribution to mixing energy due to the increase in molecular weight. The occurrence of the trans esterification replaced the original hydroxyl groups from the BZ main chain to the phenolic chain ends of the PC, and ‘scissored’ the long chain of PC into short segments. The result of the former can facilitate the ring-opening polymerisation, whereas the latter sacrificed the thermal properties of the blends. Polybenzoxazine/ poly(N-vinyl-2-pyrrolidone) exhibited strong hydrogen-bonding interactions between PBA-a and polyvinyl pyrrolidone segments [19]. Ortho-, meta-, and para-phenylnitrile functional BZ were polymerised at different compositions with phthalonitrile-functional monomers (structures given in Scheme 2.12 in Chapter 2) and which produced copolybenzoxazines of high thermal stability and easy processability [20]. The Tg also dramatically increased from 180 °C for neat ortho-phenylnitrile polymer to 294 °C for the copolymer with 30 mol% of phthalonitrile-functional monomer. Additionally, the high melt viscosity of phthalonitrile-functional BZ decreases upon blending with phenylnitrile-functional monomer. It has been demonstrated that only 30 mol% of phthalonitrile-functional BZ added to the ortho-phenylnitrile-substituted monomer significantly improves the

100

Blends and Composites of Polybenzoxazines char yield from 59 wt% to 77 wt%, which is the value of the neat phthalonitrilebased PBZ. Amine-terminated butadiene–acrylonitrile copolymer (ATBN) and carboxyl-terminated butadiene acrylonitrile rubber (CTBN) were introduced to PBZ. On a comparative scale, ATBN was more effective than CTBN in improving the fracture toughness of PBA-a. This was attributed to the better distribution of rubber particles in an ATBNmodified matrix than for the CTBN-modified matrix. Dynamic mechanical analysis (DMA) showed the existence of two networks in the ATBN-modified matrix [21]. Poly(urethane-benzoxazine) films as novel PU/phenolic resin composites were prepared by blending a BZ monomer (BA-a) and PU prepolymer [22]. All the films had only one Tg from viscoelastic measurements, indicating no phase separation in poly(urethanebenzoxazine) due to in-situ polymerisation. The films containing <15% of BA-a had elasticity characteristics with good elongation and excellent recovering behaviour, whereas those containing >20% of BA-a had plastic characteristics. Polymer alloys composed of epoxy-terminated polyurethane and high-molecularweight PBZ (derived from bisphenol A and methylenedianiline) showed good electrical and mechanical properties. These blends have excellent solvent resistance and a moisture uptake of 1.21–1.61%. However, the blends exhibited two alpha transitions in dynamic mechanical thermal analysis (DMTA) related to each of the components, which showed the phase-separated nature of the blends. The tensile strength of blends was of 24.0–30.5 MPa, which was lower than that of PBZ (30–40 MPa) [23]. A blend of bisphenol A-based BZ (BA-a) and a bismaleimide (2,2-bis[4(4maleimidophenoxy) phenyl] propane (BMI) was thermally polymerised in varying proportions, and the cure and thermal characteristics investigated. The DSC analysis, supplemented by rheology, confirmed lowering of the cure temperature of BMI in the blend, implying catalysis of the maleimide polymerisation by BZ. The peak cure temperature (Tp) of the blend decreased to 211 °C in comparison with those of BMI (270 °C) and BA-a (218 °C). Moreover, the final cure temperature (Tf) of the blend was 284 °C, compared with 339 °C for BMI. Hence, from the viewpoint of BMI, the processing characteristics of the blend are improved. A wide cure regimen between 142 °C and 284 °C was observed for the blend (Table 3.1). Hence, by realising a blend of these resins, the initial and peak curing temperatures are lowered in comparison with those of the component resins, i.e., the processing is facilitated [24].

101

Polybenzoxazines: Chemistry and Properties

Table 3.1 DSC results of BA-a/BMI blends Samples

Ti (°C)

Tp (°C)

Tf (°C)

CCW (°C)

Cure window (°C)

Heat of reaction (J/g) Experimental

Theoretical

BA-a

156

218

270

62

114

257

–

BMI

195

270

339

75

144

130

–

BA-a/BMI (1/0.5)

128

212

293

81

165

225

209

BA-a/BMI (1/1)

142

211

284

69

142

180

191

BA-a/BMI (1/3)

130

224

342

118

212

174

157

Ti: Initial curing temperature Tp: Peak curing temperature Tf: Final curing temperature CCW: Cure-controllable window (Tp–Ti) Cure window: Tf–Ti

Fourier-transform infrared (FTIR) studies provided evidence for the hydrogen bonding between the carbonyl group of BMI and the –OH group of PBZ in the cured matrix. In the BA-a/BMI blend, there are two areas of interest in the FTIR spectra of the blends. One is the carbonyl region (1800–1650 cm–1) and the other is the hydroxyl region (3600–3000 cm–1). In the cured blend, the characteristic carbonyl band was observed at 1714 cm–1. In addition, two relatively weak bands were observed at 1702 cm–1 and 1687 cm–1. These two bands are assigned to the hydrogen-bonded carbonyl groups generated in different environments in the blend. The two possibilities are depicted in Scheme 3.4. The hydroxyl group may be hydrogen-bonded with the carbonyl moiety of the bismaleimides, and hydroxyl groups which are already hydrogen-bonded (intermolecular hydrogen bonding) may be bonded to the carbonyl moiety of another bismaleimide group. Hydrogen bonding is expected to facilitate a homogeneous phase in the IPN or co-reacted matrices.

102

Blends and Composites of Polybenzoxazines Hydrogen bonding of carbonyl moiety with free hydroxyl group

H O

O

N

O

O O

H O

N

O H

H

H-bonding of carbonyl moiety with intermolecular H-bonded hydroxyl grou

p

O

Scheme 3.4 Hydrogen bonding in BA-a/BMI (1/1) blend

The cured matrix manifested dual-phase behaviour in scanning electron microscopy (SEM) and DMTA with the minor phase constituted by PBZ dispersed in an IPN of PBZ and cured BMI. The DMTA of the monomers and their blends implied a mutual catalysis of the two monomers. The maleimide groups are electron deficient whereas BZ groups possess electron-rich centres. The likely nπ–pπ interaction between the amine group of the BZ and the unsaturated group of maleimide may render the O-CH2-N bond weaker and facilitate its cleavage. The π bond in maleimide becomes electron-rich and susceptible to addition polymerisation (Scheme 3.5) [25]. Similar situations have been encountered for maleimide mixed with electron-rich monomers (e.g., copolymerisation with styrene).

103

Polybenzoxazines: Chemistry and Properties

O

Weakening of -C-O bond N

N

O

O

Scheme 3.5 Weakening of the –C-O bond of the oxazine group by nπ–pπ interaction in the BA-a/BMI blend The DMTA of the cured resins is shown in Figure 3.1. Two distinct peaks in tan δ at 145 °C and 267 °C are seen for the BA-a/BMI system. This is due to some microlevel phase separation as also seen in SEM analysis. The BA-a/BMI IPN resulted in a higher Tg. The lower Tg is attributed to the phase-separated PBZ segments.

Cured BA-a/BMI

1.0 0.8 0.6 tan δ

Cured BA-a

0.4 Cured BMI

0.2 0.0 0

100

200 T(°C)

300

400

Figure 3.1 Tan-δ behaviour of cured BA-a/BMI (1/1) in comparison with BA-a and BMI

104

Blends and Composites of Polybenzoxazines The curing of allyl containing BZ–bismaleimide blends (BA-allyl/BMI) was also studied. In the 1/0.5 (BA-allyl/BMI) blend, two exotherms were observed, at 228 °C and 269 °C, respectively. The first exotherm is the co-curing of allyl-bismaleimide and the second exotherm corresponds to the ring-opening of BZ. Further addition of bismaleimide to BZ resulted in a decrease in the overall curing temperature. As the BMI concentration was enhanced from a molar ratio of 0.5 to 1, a broad exotherm was observed, predominantly due to the reaction between the allyl groups in BZ and the maleimide groups. The Tp was lowered to 229 °C from 273 °C. Beyond this composition, bismaleimide did not significantly influence the cure because the Tp and heat of reaction remained constant. The heat of reaction for independent curing of BA-allyl is 40 J/g and is 130 J/g for BMI. The heat of reaction in the 1/1 blend was found to be 190 J/g, indicating a change in the mechanism of cure. A proposed mechanism is shown in Scheme 3.6, involving the Alder-Ene reaction and ring opening of BA-allyl.

105

Polybenzoxazines: Chemistry and Properties O

N Ph

N R N

O

Ph N

O

O

O

Ph N

O

N

O

O

N R N

O O

O

O Ph

O

O

Ph

Wagner-Jauregg

R

N

N

O

N R N O

O

N

Ene adduct

O

O

O O

O

O

R N

O

N R N N

OH N

HO

O Ph

HO

O Ph

OH

N

Further crosslinking

R=

O

O

Scheme 3.6 Cure reaction in BA-allyl and BMI blends

106

N

O O Ph

R

Blends and Composites of Polybenzoxazines BA-allyl/BMI blend formed predominantly a co-reacted network, so a homogeneous morphology was observed in SEM analysis. The Tg of cured BMI and cured BAallyl are 307 °C and 298 °C, respectively, (Figure 3.2). The BA-allyl/BMI system demonstrated a single Tg at 274 °C owing to the formation of co-reacted networks. The thermal stability of BA-a/BMI and BA-allyl/BMI blends was improved compared with PBZ (Table 3.2).

Cured BA-allyl/BMI

0.3

tan δ

0.2 Cured BA-allyl

0.1 Cured BMI 0.0 0

100

200 T(°C)

300

400

Figure 3.2 DMTA of BA-allyl/BMI, BA-allyl and BMI

However, polymer alloys were reported by blending 1,1′-(methylenedi-4,1-phenylene)bismaleimide (BMI) and BA-a. The obtained alloy films had improved toughness which increased with increase in BMI content. The authors proposed a thermal reaction between the double bond of BMI and the hydroxyl groups of BA-a. They suggested

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Polybenzoxazines: Chemistry and Properties that the reactions of the double bond and ring opening of BA-a occurs simultaneously, leading to a AB-co-crosslinked structure. The Tg of alloys also increased from 154 °C to 268 °C at 76% of BMI content [26].

Table 3.2 Thermal stability of BA-allyl/BMI blends Cured samples

T5% (°C)

T10% (°C)

Char yield (%) at 800 °C

BA-allyl

395

422

44.0

BA-allyl/BMI (1/0.5)

350

397

36.0

BA-allyl/BMI (1/1)

387

418

39.0

BA-allyl/BMI (1/1.75)

373

412

38.5

BA-allyl/BMI (1/2.5)

385

418

39.0

BMI

410

430

43.4

BA-a

275

335

35.6

BA-a/BMI (1/1)

335

385

43.2

As described in Chapter 1, cyanate ester (CE) resins have good thermal and hygrothermal properties for use in the electronics, aerospace and adhesive arenas. To incorporate the properties of CE into PBZ, thermosetting polymer blends composed of benzoxazine (BA-a) and bisphenol A cyanate ester (BACY; structure is shown in the inset of Figure 3.3) was investigated [27]. The blend was prepared by powder mixing of two monomers in a mortar and by using a solvent method (AR acetone). The DSC of the powder-blended system was conducted immediately after mixing. Four distinct exotherms were observed in the DSC analysis. The exothermic heat for curing of BA-a and BACY are 262 J/g and 760 J/g, respectively, but the exothermicity of the (1/1) blend was reduced by as much as 210 J/g (powder blend).

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Blends and Composites of Polybenzoxazines

CH3

BACY

NCO

Exo Heat flow

h

f b

CH3

BACY

g e

OCN

solution blended (1/1)

c

a

d

powder blended (1/1) BA-a 0

100

200

T (°C)

300

Figure 3.3 DSC traces of BA-a/BACY blend (1:1) (inset shows the structure of BACY)

The first exotherm below 100 °C (marked as a) was due to a reaction between the two components before the polymerisation. The second exotherm (marked as b) and the third (marked as c) correspond (in all probability) to a certain co-reaction between the two components, leading to polymers. The fourth (marked as d) can be ascribed to the hompolymerisation of BACY. The exotherm corresponding to the exotherm a in the thermogram of the powder-blended system is also observed in the solution-blended system but with a relatively weak intensity (e). This indicates that the reaction corresponding to the first exotherm (corresponding to a) may have occurred during the solution blending of the two resins because the solvent was evaporated by keeping it at 60 °C for 24 h under vacuum. Apparently, the four exotherms observed in the powder-blended system are also seen in the solution-blended system but with weak intensities and a shift towards lower temperature regimens. The exothermicity of solution-blended system was lower than that of the powder-mixed system. This

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Polybenzoxazines: Chemistry and Properties implies that the reaction corresponding to exotherm b in the powder-mixed blend may have partly occurred during evaporation of the solvent for the solution-blended system. The reaction corresponding to exotherm a in the powder-blended system was completed in a solution-blended system at 60 °C under vacuum. In this case, the investigations show that the cyanate trimerisation and ring opening of BZ take place almost simultaneously (in a mutually catalysed manner). During polymerisation, it is likely that some of the phenolic hydroxyl groups resulting from the ring opening of the oxazine moiety may react with the cyanate moiety via an iminocarbonate intermediate that ultimately forms the triazine network. It is probable that part of BACY may be trapped in the PBZ and triazine matrix that could cure independently (accounting for the third exotherm in DSC analysis; marked as d in Figure 3.3). A triazine ring as a part of a PBZ network (Scheme 3.7) could be the major structure. Based on these evidences, a reaction between oxazine and cyanate groups (i.e., a cycloaddition reaction) followed by a crosslinked structure comprising triazine as a part of a PBZ structure (formed via an iminocarbonate intermediate) can be postulated (Scheme 3.7).

N

RO N

OR RO N

N

O N

OR

N N O

N

N

Scheme 3.7 Postulated co-reacted network in a BA-a and BACY blend

PBZ decomposes at 275 °C and polycyanurate at ~400 °C. The blends start decomposing at a temperature closer to that of PBZ (Figure 3.4). The degradation pattern of the blend at a higher temperature matches that of the pure polycyanurate. The initial decomposition could be related to the degradation of PBZ units, and the degradation at higher temperature to polycyanurate. This observation again confirms the presence of these two matrix segments in the network, whereby the initial degradation is decided by the PBZ groups. Interestingly, the char residue for the blend is closer to that of polycyanurate. If the matrix was a blend of the two

110

Blends and Composites of Polybenzoxazines individual components, the decomposition could have occurred in two stages, and the residue would have been the average of the two homopolymers. The TGA behaviour also confirms that the matrix in the co-cured blend is different from the mixture of two independent hompolymers.

Residual Weight (%)

100

0/1

80

60 1/1

1/0

40

0

200

400

600 T(°C)

800

1000

Figure 3.4 Thermal stability of homopolymers and BA-a/BACY blends under N2 (heating rate, 10 °C/min)

The Tg and crosslink density of the BA-a/CE blends were calculated from DMTA data. The crosslink density of the cured BA-a is 2900 mol/m3 and that of polycyanurate is 4380 mol/m3. The BA-a/BACY blend exhibited a higher crosslink density over neat PBZ and polycyanurate. The Tg of the blend is in-between those of the component polymers, implying a near homogeneous matrix. The Tg values and crosslink density of the blend are shown in Table 3.3.

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Polybenzoxazines: Chemistry and Properties

Table 3.3 DMTA analysis of BA-a/BACY blends Samples (molar ratio) BA-a/BACY

Tg (°C)

Crosslink density (mol/m3)

1/0

215

2900

1/1

230

6270

0/1

252

4380

The incorporation of phenol aniline-type into bisphenol A aniline-type (BA-a) BZ improved the processability, but the viscosity of the latter was significantly decreased [28]. PBZ is also copolymerised with the siloxane-containing difunctional BZ monomer 1,3-Bis(3-aminopropyl)tetramethyldisiloxane) (BATMS-BZ), which resulted in toughening of the polymer [29]. In the phenol and aniline based benzoxazine (P-a) and BATMS-BZ copolymers, the chains formed strong intermolecular hydrogen bonding which resulted in a positive deviation of Tg. However, in phenol and aminofuran based benzoxazine, this behaviour as well as toughening were not observed.

3.6 Nanocomposites of PBZ Recently, PBZ-based nanomaterials have been the centre of attention for researchers because they offer tremendous improvement on many properties, such as tensile strength and modulus, thermal stability, solvent resistance, gas permeability, and flammability, with a small content of nanoparticles in the polymer matrix. The enhancement in overall properties is due to the nanoscale dispersion of nanoparticles through the pristine polymer matrix. A substantial number of nanomaterials have been reported, with a strong bias towards nanocomposites [30–33]. Dependent upon the nanoparticles employed for the modification of the PBZ matrix, studies are categorised and described in the next sections.

3.6.1 PBZ-Clay Systems Clay-based PBZ nanocomposites are the most widely investigated nanomaterials of PBZ. Polymer-layered silicate nanocomposites are a class of materials that have attracted considerable interest because of their potential technological applications. They exhibited superior properties over pristine BA-a matrices using organically

112

Blends and Composites of Polybenzoxazines modified montmorillonite (OMMT) as a type of layered silicate [30]. The ringopening polymerisation of BA-a in the presence of OMMT was observed at 170–190 °C compared with the onset of pristine BA-a at 223 °C. This decrease in the onset of the ring-opening polymerisation was due to the catalytic effect of acidic onium protons present on the montmorillonite (MMT) surface on BZ ring opening. Studies on BA-a/modified MMT (using stearyl and dodecyl ammonium chloride) showed that there is no sharp difference in the onset of the exotherms with change in the type of surfactant or amount of OMMT. The thermal stability and Tg of the nanocomposites were also enhanced compared with the pristine PBZ matrix. MMT was also modified using the amines tyramine, phenyl ethylamine, aminolauric acid and dodecylamine. DSC analyses showed that inclusion of all OMMT increased the thermal stability and marginally increased Tg from pristine resin PBA-a [31]. A report on immiscible polymer-clay nanocomposites using organically modified clay stated that the aggregation of silicate layers could be due to entrapment of clay in the pristine matrix [32]. The results indicated that the BZ monomer became intercalated into the galleries of the clay; the nanocomposite possessed an exfoliated structure at 3% clay content. The curing reaction in the synthesis of PBZ–MMT nanocomposites showed autocatalytic characteristics [33, 34]. X-ray diffraction (XRD) analysis showed that the chains of PBZ were successfully integrated into the interlayers of the OMMT to form the intercalated nanocomposite because of the good compatibility between the PBZ and OMMT [35]. Also, aminolauric acid-modified mica has been incorporated into BA-a resin. The polymerisation occurred rapidly in the presence of modified mica, and the storage modulus increased due to the nano-reinforcement. As expected, it showed a wide glass-transition window because of the restriction of segmental motion by mica. The authors reported that the increase in the glassy modulus at ambient temperature is an indication of better adhesion, but it is quite likely that it implies polymer brittleness [36]. A series of polyurethane-PBZ/clay hybrid nanocomposites (PU/P-a–OMMT) films were successfully prepared using a solvent method by in-situ copolymerisation of PU prepolymer and P-a (phenol- and aniline-based monomer) in the presence of OMMT [37]. An exfoliated structure was achieved for <5% of OMMT loading, and a mixed or intercalated configuration was observed for >7% of OMMT loading. The peak curing temperature of PU/P-a was lowered by hybridisation with OMMT due to the catalytic effect of the clay surface on the ring-opening polymerisation of P-a, which enabled complete cure of P-a without thermally decomposing the PU component. The tensile strength and modulus of the PU/P-a films increased remarkably, whereas the elongation decreased with the increase of OMMT loading. The effects of 2,2′(1,3-phenylene)-bis (4,5-dihydro-oxazoles) (PBO) content on the cure of pristine

113

Polybenzoxazines: Chemistry and Properties BA-a monomer (molar ratio of PBO to BA-a, 0.8:1) and their nanocomposites were studied [38]. On the introduction of OMMT, the onset curing temperature of the copolymerisation of BA-a and PBO decreased, and dispersion of OMMT in the PBA-a/ PBO matrix resulted in the exfoliated structure of OMMT. The nanocomposites exhibited much higher values of Tg than the PBA-a/PBO resin and pristine PBZ.

3.6.2 PBZ-Polyhedral Oligomeric Silsesquioxane (POSS) Systems Another interesting nanomaterial of PBZ is based on POSS. The octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane (OpePOSS) was incorporated into PBA-a to prepare PBA-a nanocomposites with POSS concentrations up to 40 wt% [39]. An inter-component reaction between the phenolic hydroxyls of PBA-a and the epoxide groups of OpePOSS, i.e., the phenolic hydroxyls are converted into the secondary alcohol hydroxyls in hydroxy ether structural units (-O-CH2-CH (OH)- CH2-O-), was noted. Polymer properties were enhanced due to nano-reinforcement. Octa-aminophenyl polyhedral oligomeric silsesquioxane (OAPS) was also used to modify BZ in the presence of PBO [40]. The nanocomposites exhibited higher Tg values than the pristine PBZ and PBZ-PBO resin, and the storage modulus of the nanocomposites was maintained at higher temperatures (although only a small amount of OAPS was incorporated into the systems). The thermal stability of the hybrid was also improved by the inclusion of OAPS. To improve the thermal stability of PBZ, a hydrosilane-functionalised polyhedral oligomeric silsesquioxane was incorporated into the vinyl-terminated benzoxazine monomer (VB-a) [41]. Hybrids from a non-reactive POSS and VB-a were also studied. These POSS-containing composite materials displayed significant improvements in their thermal stability relative to the typical PBZ formed in the absence of POSS. The Tg increased from 307 °C for the polyvinyl-terminated BZ monomer to 333 °C for the copolymer hybrid incorporating 5 wt% of POSS. The degradation temperature and char yield under nitrogen increased with the increase in POSS content. A similar class of PBZ/POSS nanocomposites with a network structure was prepared by reacting multifunctional benzoxazine POSS (MBZ-POSS) with BZ monomers (P-a and BA-a) at various compositional ratios [42]. Octafunctional cubic silsesquioxane (MBZPOSS) was used as a curing agent. The Tg and decomposition temperature of these nanocomposites was improved by incorporating the MBZ-POSS on the BA-a- or P-atype PBZ. However, the Tg values of POSS/PBZ hybrids with 10 wt% POSS content were lower than that of 5 wt% POSS, probably due to hindrance of BZ crosslinking caused by hard POSS-rich particles resulting from phase separation.

114

Blends and Composites of Polybenzoxazines

3.6.3 Miscellaneous Nanocomposites PBZ (PBA-a) nanocomposites containing multi-walled carbon nanotubes (MWNT) with hydroxyl, carboxyl, and isocyanate groups on the surface of carbon nanotubes have been reported [43]. The carboxyl groups on the surface catalysed the ring-opening reaction of BZ and decreased the curing temperature of the system. The isocyanate groups reacted with the phenolic hydroxyl groups generated by the ring opening of BZ, resulting in a significant improvement in the adhesion between PBA-a and MWNT. The Tg and storage modulus were increased by the addition of MWNT into PBA-a. MMT organoclay has been modified with allyl-dimethylstearylammonium and propyldimethylstearylammonium groups, and they were used for the preparation of PBZ nanocomposites [44]. The results showed no significant improvement in properties. The Tg and thermal stability of the nanomaterials were lower than those of the pristine PBZ matrix (derived from allylamine and bisphenol A: B-ala). In addition, in the DSC analysis of B-ala, the broad peak attributed to the allyl and oxazine polymerisation became separated in the nanocomposites, and allyl polymerisation became more distinct compared with oxazine polymerisation. Clay could loosen the PBZ matrix, which would be reflected in the Tg value and thermal properties. The porous materials of BZ (BA-a) containing various molecular weights of PCL as the labile constituent showed low dielectric constants (1.95 × 105 Hz at 25 °C) relative to that of the virgin polymer (3.56 × 105 Hz at 25 °C) [45]. When the P-a containing PCL (P-a-PCL) was eliminated by hydrolysis of the P-a-PCL/PBA-a-copolymer, pores were generated. The slight degree of hydrogen bonding that exists between the two polymers results in micro-phase separation without an excess degree of aggregation. Organic–inorganic hybrids were prepared from PBZ and titania using a sol–gel process by blending titanium isopropoxide as a precursor for titania with a typical BZ monomer (BA-a) [46]. The acidity of the formed Ti–OH group in the titania network could initiate ring opening of the oxazine ring and lower the curing temperature. The Tg of the neat PBZ was shifted slightly to 179 °C from 151 °C with the inclusion of titania. The increase in thermal stability was attributed to the thermal insulation effect of a metal-oxide network that protects the underlying polymer matrix. Thermally curable BZ ring-containing polystyrene macromonomers were synthesised and characterised [47]. The macromonomers could undergo thermal curing and formed thermoset networks comprising polystyrene segments.

3.7 Fibre Composites and Microcomposites Various PBZ micro composites and fibre composites have been reported. The PBZ matrix has been modified by using fibres or other fillers. In PBZ-glass composites, the untreated PBZ composite suffers a 53% loss in interlaminar shear strength

115

Polybenzoxazines: Chemistry and Properties (ILSS). However, if a benzoxazine silane-coupling agent is used to treat the glass fibre, the composite retains 100% of its strength even after treatment in boiling water. Surprisingly high wet strength retention of 100% (after boiling for 12 h in distilled water) was obtained with this silane treatment on the PBZ composite [48]. A thermal conductivity of 32.5 W/mK was achieved for a boron nitride-filled PBZ at its maximum filler loading of 78.5% by volume [49]. The specific heat capacity of boron nitride-filled PBZ showed a value of 1098 J/K/kg for 50% filler by weight and 860 J/K/kg for 90% filler content [50]. The stiff boron nitride filler could highly restrict the mobility of the polymer matrix, which adhered on the filler surface and could lead to the large increase in the Tg values of their composites. Interestingly, water absorption of the filled systems at 24 h was <0.1% and decreased with increasing filler content. The boron nitride surface was found to inhibit the curing of BZ coatings in the interfacial region at lower coating thickness. The boron nitride-filled composites showed the greatest reduction in flexural strength and strain to break [51]. Investigations showed that the mechanical properties of PBZ can be tuned by reinforcement with carbon fibre (CF) of varying length [52]. The tensile and flexural strength increased and optimised at a fibre length of ~17 mm, whereas a diminution was observed for the compressive strength. The variation in tensile strength (σt) with fibre length is shown in Figure 3.5. It is clearly visible that, irrespective of the fibre length, the σt of the composites increases and exhibits a value at least double that for neat PBZ. The increase in strength of composites with fibre length is attributed to the enhancement in contact area of a given fibre with the matrix. The experimental results showed that there was an increase in flexural strength of about 250% and an increase in tensile strength of 370% of the composites vis-à-vis the neat PBZ. The longer fibres were conducive for plastic deformation at higher strains. The breaking of fibres and debonding were the major failure patterns in short-fibre composites as evidenced in SEM analyses of the fractured surfaces (Figures 3.6 and 3.7). The defibrillation was associated with all composites irrespective of fibre length.

116

Blends and Composites of Polybenzoxazines

Tensile strength (MPa)

160

120

80

40 0

7

12

17

22

27

Fibre length (mm)

Figure 3.5 Variation of tensile strength with fibre length

Figure 3.6 Fibre clustering (BC-22, composite with a fibre length of 22 mm)

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Polybenzoxazines: Chemistry and Properties

Figure 3.7 Fibre debonding under flexural strength (BC-12, composite with a carbon-fibre length of 12 mm)

The tensile modulus showed higher values compared with neat resin, and it increased with fibre content. An optimum of 250% increase was observed for the tensile modulus. Meanwhile, the flexural modulus displayed an increase of ~800% vis-à-vis the neat resin. This study revealed that composites of PBZ with good strength, ductility, and modulus could be developed using CF of appropriate length and composition (Table 3.4). The general observation from this study was that the percentage of debonded fibres diminishes on increasing fibre length because the contact area of adhesion between fibres-to-matrix increases. Hence, in short fibres, the less-adhered fibres are quite readily debonded from the matrix. In moderately long fibres, there will be more fibre portions that are in the resin pool (embedded fibres). Consequently, the tendency for debonding or breaking up of fibres decreases. However, in composites of longer fibre length, bundling, curling, or entanglement increases, thereby restricting the effective stress transfer. The lowering of stress values at higher fibre lengths can be attributed to the fibre entanglements and curling formed at higher lengths. However, this phenomenon is conducive to imparting more ductility to the composite as a whole. Maximum properties were obtained for composites having a fibre length of 17–22 mm. From these investigations, it is seen that properties are optimised between a fibre length of 17 mm and 22 mm. Hence, a fibre length of 20 mm was selected for investigating the effect of fibre content on mechanical properties. The mechanical properties of BA-a PBZ with a CF of length 20 mm are shown in Table 3.5.

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Blends and Composites of Polybenzoxazines

Table 3.4 Variation of the modulus of composites with fibre length References

Tensile modulus Flexural modulus (GPa) (GPa)

Compressive modulus (GPa)

PBZ

3.0* (ca.)

4.0*

–

BC-7

4.3±0.6

32.0±2.4

2.6±0.2

BC-12

4.8±0.8

30.8±2.8

3.4±0.2

BC-17

6.2±0.8

30.1±0.6

3.3±0.1

BC-22

3.7±0.1

27.5±0.9

2.8±0.1

BC-27

2.3±0.3

24.4±2.0

2.7±0.1

*Approximate values Note: The number associated with the ‘references’ column indicates the length of carbon fibre (millimetres) used for composite preparation.

Table 3.5 Strength and moduli of composites with carbon-fibre loading (fibre length of 20 mm) Fibre content (wt%)

Tensile strength (MPa)

Tensile modulus (GPa)

Flexural strength (MPa)

Flexural modulus (GPa)

Compressive Compressive strength modulus (MPa) (GPa)

PBZ

35*

3.0*

95*

4.0*

150.0*

–

40

80 ± 07

3.4 ± 0.4

214 ± 14

20.9 ± 0.5

122 ± 12

3.2 ± 0.4

50

88 ± 11

5.1 ± 0.5

220 ± 08

22.4 ± 2.7

134 ± 16

3.0 ± 0.1

60

110 ± 10

5.1 ± 0.1

151 ± 05

27.4 ± 1.4

74 ± 14

2.5 ± 0.3

70

64 ± 10

6.7 ± 0.7

150 ± 13

17.8 ± 1.0

82 ± 10

2.5 ± 0.2

*Approximate values

119

Polybenzoxazines: Chemistry and Properties The tensile strength increases with fibre loading and optimises at ~60 wt% fibre loading. The tensile strength of all composites is well above the tensile strength of neat resin (~30–40 MPa). The composite with 60% fibre content showed a tensile value of nearly 120 MPa (3–4 times higher than that of the neat resin). The tensile modulus also increases with increase in fibre content, and is also higher compared with the neat resin. The composite with 70% fibre content exhibited a more than twofold increase in modulus vis-à-vis the neat resin (neat resin has a tensile modulus of ~3 GPa). Flexural strength exhibited high reinforcement with fibre loading. The flexural strength increases and optimises at 50% fibre content. The carbon fibre reinforced polybenzoxazine composites displayed a ~2.5-times increase vis-à-vis the neat resin (the flexural strength of neat resin is ~90–100 MPa). The flexural modulus manifested a pronounced effect on fibre loading, which showed a sevenfold increase compared with neat resin (in which the modulus is ~4 GPa). The flexural modulus increased with fibre content and optimised at 60% fibre loading. The compressive strength decreased continuously with fibre content. The compressive modulus also showed a similar trend. This again substantiates the observation mentioned above that CF determines the compressive property of the composites. CF/PBZ composites were fabricated to examine the toughening effect of ATBN and CTBN on PBZ [53]. Delamination toughness of CF/PBZ composites was increased by the rubber modification of PBZ. The better interfacial bonding in the ATBN-modified system induced the growth of some cracks, allowing the intrinsic resin toughness to be fully extracted during composite delamination. In the case of CTBN modification, the delamination toughness of the composite was barely increased because of the weak interfacial bonding. The ILSS of the CF/PBZ composites decreased notably with CTBN content compared with ATBN modification. Oxygen plasma treatment and nitric-acid treatment were effective in improving the mechanical properties of CF/PBZ composites [54]. However, nitric-acid treatment enhanced the mechanical properties of CF/PBZ composites compared with oxygen plasma treatment due to a large increase in fibre surface roughness. The rubber interlayer approach enhanced the mechanical properties of BA-35X (bisphenol A- and 3,5 dimethyl aniline-based BZ)/CF composites and the adhesion between the resin and CF was improved by ATBN rubber, which led to the improvement in delamination toughness [55]. The char yields of CF composites of other PBZ matrices also improved, which rendered them as good candidates for precursors of carbon–carbon composites [56]. The PBA-a composite systems could exceed bismaleimide composites and compete with polyimide composites in terms of mechanical and thermal properties. Syntactic foams are lightweight materials made of hollow microspheres/microballoons dispersed in a resin, and can be used for structural applications. Composites of PBZ filled with syntactic foams exhibit a light weight and moderate strength [57]. The tensile and compressive strength was optimised at 40% by weight (68% by volume),

120

Blends and Composites of Polybenzoxazines whereas the flexural strength exhibited a general decreasing trend with increase in microballoon content.

Sp.Mechanical Strength (MPa/kg/m 3 )

Specific mechanical strengths (the ratio of mechanical strength of the material to its density) of these syntactic foams are plotted against microballoon volume percentage in Figure 3.8. The specific tensile strength of syntactic foams showed an initial increase (up to 68% by volume of microballoons) followed by a diminution. The values did not increase beyond 74% by volume of microballoons. The diminution in specific tensile strength on increasing filler content indicates that the relative reduction in strength is higher than that for density. It can be concluded that ~40% by weight (68% by volume) of microballoons imparts the maximum specific tensile strength to syntactic foams.

Sp.Tens.str. Sp.Flex.str. Sp.Comp.str.

0.018

0.015

0.012

0.009

0.006

0.003

60

66

72

78

Microballoon volume (%)

Figure 3.8 Variation of specific mechanical strength of polybenzoxazine syntactic foams with microballoon content

The matrix porosity and interfacial bonding between the matrix and microballoon were the two major factors determining the strength of syntactic foams. The failure

121

Polybenzoxazines: Chemistry and Properties in lower-density foams was controlled primarily by the fracture of microballoons. The air voids present in syntactic foams may be isolated or accumulated (see the SEM micrograph shown in Figure 3.9). Though both types of voids lead to deterioration in mechanical properties, the existence of interconnected or accumulated voids is highly undesirable. Several non-accumulated voids present a comparatively better situation than a single accumulated void having the same effective volume. With increase in microballoon content, the possibility for interconnected voids increases. In the present case, the accumulated voids are present in all composites irrespective of the percentage of resin because the present case is, in general, resin-depleted.

Figure 3.9 Accumulation of voids in polybenzoxazine-syntactic foams

PBZ filled with chopped silica fibres (BS systems) and their syntactic foams of varying composition were processed and investigated [58]. Silica fibre-filled composites did not exhibit enhancement in mechanical strength vis-à-vis the neat matrix. This indicates that the ‘filler effect’ of silica powders reduces the strength of the matrix.

122

Blends and Composites of Polybenzoxazines Though the addition of microballoons reduces the strength, there is also a consequent decrease in density. Specific properties (strength/density, i.e., properties normalised to the mass) allow for the comparison of the performance of fibre-reinforced syntactic foams to silica filled materials. The variations in the specific mechanical strength of BS (benzoxazine-silica composites) and benzoxazine-silica-microballon (BSM) composites are shown in Figures 3.10 and 3.11, respectively. The specific strength shows a general decreasing trend for the silica-filled materials.

Sp.Mechanical strength (MPa/kg/m 3 )

In microballoon-embedded PBZ, the specific tensile strength increased upon inclusion of microspheres up to ~47% by volume of microballoon, beyond which a decreasing trend was observed. This occurs because the relative reduction in strength is larger than the reduction in density. Specific compressive strength also followed a similar trend. For the low-density materials, the specific flexural strength decreased marginally and practically stagnated beyond 57% volume of microspheres. A composition with ~55 volume% of the microballoon is therefore probably sufficient for moderate load bearing thermo-structural applications (low-density, moderate-strength materials) without a great penalty on mass. Such systems are advantageous for use as thermal protection of satellites while entering the Earth’s atmosphere.

Sp.T Sp.F Sp.C

0.10 0.08 0.06 0.04 0.02 30

35

40 45 50 55 Fibre volume (%)

60

65

Figure 3.10 Specific mechanical strength of polybenzoxazine filled with silica fibres (BS)

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Polybenzoxazines: Chemistry and Properties

Sp.Mechanical Strength (MPa/kg/m 3 )

0.08 Sp.T Sp.F Sp.C

0.06

0.04

0.02

0.00

35

40 45 50 55 60 65 Volume of microballoon (%)

70

Figure 3.11 Specific mechanical strength of BSM systems

Both BS and BSM composites showed high storage modulii and good resistance against temperature (Figure 3.12). The composites revealed an improved damping property in contrast with the unfilled polymer. The Tg (deduced from the tan-δ maximum from DMTA data) decreased in silica fibre-containing materials and on incorporating the microballoon; the values reverted back to that of the neat polymer. Silica and microballoons conferred better thermal and thermo-oxidative stabilities to the PBZ. The composition of BS2 is 56.5/38.5 by vol% (benzoxazines/silica fibre) and BSM3 is 19.8/11.9/56.9 by vol% (benzoxazines/silica fibre/microballoon). The coefficient of thermal expansion (CTE) of cured PBZ was 50–60 ppm/°C. The CTE of fibre-filled materials were 15–50 ppm/°C (Figure 3.13). The CTE of all silicafilled syntactic foams was still lower (~4.7 ppm/°C) and showed no change with microballoon incorporation. These results show that fibre-embedded PBZ syntactic foams have better dimensional stability under thermal stress conditions than the corresponding fibre-incorporated PBZ.

124

Blends and Composites of Polybenzoxazines 10

Storage modulus(GPa)

BS2

1 BSM3

0.1

0.01

1E 3

BA a

0

100

200

300

400

T(°C)

Figure 3.12 Storage modulus curves of BS and BSM materials

a (°C -1 ) x 10 -5

5 4 3 2 1

20

30 40 50 Volume of fibre (%)

60

70

Figure 3.13 Coefficient of thermal expansion of silica fibre-incorporated polybenzoxazines (BS)

125

Polybenzoxazines: Chemistry and Properties CaCO3 filler decreased the tensile strength and flexural strength but increased the modulus of PBZ [59]. Surface treatment of CaCO3 with stearic acid caused a reduction in composite strength by producing poorer adhesion with the matrix than the untreated CaCO3. Kenaf fibre was incorporated in a PBZ (PBA-a) resin matrix to form a unidirectional-reinforced composite containing 20 wt% fibre by a resin transfer moulding technique [60]. PBZ composites with 20 wt% fibre content have lower flexural and impact strength, but higher flexural moduli were noted compared with unsaturated polyester composites with the same fibre content. Comparison of the properties of composites from both resins shows that the flexural strength and impact strength of the PBA-a composite were less than those of the unsaturated polyester composites, but that its flexural modulus was much higher. An increase in the wood-flour content enhanced the modulus of PBZ-wood-flour composites due to the reinforcing capability of the filler [61]. The effects of the woodflour content and particle size on the thermal, mechanical, and physical properties of PBZ-wood composites have been reported [62]. The storage modulus (G´) of the PBZ-wood composites was found to increase with wood-flour content and reached an optimum value at 70.5% by volume (75% by weight) of the wood-flour. The Tg of the composites was found to increase with wood-flour content, i.e., from 160 °C in the unfilled system to 220 °C in the 70.5% by volume of wood-flour. The char yield of the PBZ-wood composite showed synergistic characteristics, with a peak value of 36% compared with 28% of the neat resin and 18% of the wood-flour.

3.8 Conclusion This chapter is a complementary review of the tailored polymeric systems of PBZ focusing on blends and composites. The properties of PBZ can be tailored by preparing their blends, alloys, copolymers, nanocomposites and microcomposites. The commercially important polymers such as epoxy resins, phenolics, cyanate esters, and bismaleimides have been blended with PBZ. In most cases, blends showed remarkably high thermal and mechanical properties. PBZ blends have comparative or better properties to the other state-of-the-art blend systems. Nanomaterials such as nanoclay and POSS are incorporated into the PBZ matrix to produce exceptionally good polymers with high thermal and mechanical properties. In addition, high strength and lightweight microcomposites have been prepared with silica, carbon, glass fibres and wood. PBZ-based syntactic foams and silica fibre-filled syntactic foams offer materials for lightweight structures and thermo-structures.

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Blends and Composites of Polybenzoxazines

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10. C.P. Reghunadhan Nair, Progress in Polymer Science, 2004, 29, 5, 401. 11. T. Takeichi, T. Agag and R. Zeidam, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2001, 39, 15, 2633. 12. T. Takeichi, Y. Guo and S. Rimdusit, Polymer, 2005, 46, 13, 4909. 13. S. Tiptipakorn, S. Damrongsakkul, S. Ando, K. Hemvichian and S. Rimdusit, Polymer Degradation and Stability, 2007, 92, 7, 1265. 14. Y. Cui, Y. Chen, X. Wang, G. Tian and X. Tang, Polymer International, 2003, 52, 8, 1246. 15. H. Ishida and Y.H. Lee, Journal of Applied Polymer Science, 2001, 81, 4, 1021. 16. H. Lu and S. Zheng, Polymer, 2003, 44, 16, 4689. 17. H. Ishida and Y.H. Lee, Journal of Applied Polymer Science, 2002, 83, 9, 1848.

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Polybenzoxazines: Chemistry and Properties 18. H. Ishida and Y.H. Lee, Polymers and Polymer Composites, 2001, 9, 2, 121. 19. Y.C. Su, S.W. Kuo, D.R. Yei, H. Xua and F.C. Chang, Polymer, 2003, 44, 8, 2187. 20. Z. Brunovska and H. Ishida, Journal of Applied Polymer Science, 1999, 73, 14, 2937. 21. J. Jang and D. Seo, Journal of Applied Polymer Science, 1998, 67, 1, 1. 22. T. Takeichi, Y. Guo and T. Agag, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2000, 38, 22, 4165. 23. H. Yeganeh, M. Razavi-Nouri and M. Ghaffari, Polymers for Advanced Technologies, 2008, 19, 8, 1024. 24. K.S. Santhosh Kumar, C.P. Reghunadhan Nair, R. Sadhana, and K. N. Ninan, European Polymer Journal, 2007, 43, 12, 5084. 25. K.S. Santhosh Kumar, C.P.Reghunadhan Nair and K.N. Ninan in the Proceedings of the International Conference on Polymeric Materials in Power Engineering, ICPMPE-2007, Bangalore, India, 2007. 26. T. Takeichi, Y. Saito, T. Agag, H. Muto and T. Kawauchi, Polymer, 2008, 49, 5, 1173. 27. K.S. Santhosh Kumar, C.P. Reghunadhan Nair and K.N. Ninan, European Polymer Journal, 2009, 45, 2, 494. 28. C. Jubsilp, T. Takeichi and S. Rimdusit, Journal of Applied Polymer Science, 2007, 104, 5, 2928. 29. Y.L. Liu, C.W. Hsu and C.I. Chou, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2007, 45, 6, 1007. 30. T. Agag and T. Takeichi, Polymer, 2000, 41, 19, 7083. 31. T. Takeichi, R. Zeidan and T. Agag, Polymer, 2002, 43, 1, 45. 32. P. Phiriyawirut, R. Magaraphan and H. Ishida, Material Research Innovations, 2001, 4, 2/3, 187. 33. D.R. Yei, H.K. Fu, W.Y. Chen and F.C. Chang, Journal of Polymer Science, Part B: Polymer Physics Edition, 2006, 44, 2, 347.

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Blends and Composites of Polybenzoxazines 34. Z. Shi, D.S. Yu, Y. Wang and R. Xu, European Polymer Journal, 2002, 38, 4, 727. 35. Z. Shi, D. Yu, Y. Wang and R. Xu, Journal of Applied Polymer Science, 2003, 88, 1, 194. 36. T. Agag, V. Taepaisitphongse and T. Takeichi, Polymer Composites, 2007, 28, 5, 680. 37. T. Takeichi and Y. Guo, Journal of Applied Polymer Science, 2003, 90, 14, 4075. 38. Q. Chen, R. Xu and D. Yu, Journal of Applied Polymer Science, 2006, 100, 6, 4741. 39. Y. Liu and S. Zheng, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 2006, 44, 3, 1168. 40. Q. Chen, R. Xu, J. Zhang and D. Yu, Macromolecular Rapid Communications, 2005, 26, 23, 1878. 41. J. Lee, J.M. Huang, S.W. Kuo, J.K. Chen and F.C. Chang, Polymer, 2005, 46, 7, 2320. 42. Y.J. Lee, S.W. Kuo, C.F. Huang and F.C. Chang, Polymer, 2006, 47, 12, 4378. 43. Q. Chen, R. Xu and D. Yu, Polymer, 2006, 47, 22, 7711. 44. B. Kiskan, D. Colak, A.E. Muftuoglu, I. Cianga and Y. Yagci, Macromolecular Rapid Communications, 2005, 26, 10, 819. 45. Y.C. Su, W.C. Chen, K.I. Ou and F.C. Chang, Polymer, 2005, 46, 11, 3758. 46. T. Agag, H. Tsuchiya and T. Takeichi, Polymer, 2004, 45, 23, 7903. 47. T. Agag and T. Takeichi, Polymer Composites, 2008, 29, 7, 750. 48. H. Ishida and H.Y. Low, Journal of Applied Polymer Science, 1998, 69, 13, 2559. 49. H. Ishida and S. Rimdusit, Thermochimica Acta, 1998, 320, 1/2, 177. 50. H. Ishida and S. Rimdusit, Journal of Thermal Analysis and Calorimetry, 1999, 58, 3, 497.

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Polybenzoxazines: Chemistry and Properties 51. M.T. Huang and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 1999, 37, 17, 2360. 52. K.S. Santhosh Kumar, C.P. Reghunadhan Nair and K.N. Ninan, Polymers for Advanced Technologies, 2008, 19, 7, 895. 53. J. Jang and H. Yang, Composite Science and Technology, 2000, 60, 3, 457. 54. J. Jang and H.J. Yang, Journal of Material Science, 2000, 35, 9, 2297. 55. H. Ishida and T. Chaisuwan, Polymer Composites, 2003, 24, 5, 597. 56. S.B. Shen and H. Ishida, Polymer Composites, 1996, 17, 5, 710. 57. K.S. Santhosh Kumar, C.P. Reghunadhan Nair and K.N. Ninan, Journal of Applied Polymer Science, 2008, 107, 2, 1091. 58. K.S. Santhosh Kumar, C.P. Reghunadhan Nair and K.N. Ninan, Journal of Applied Polymer Science, 2008, 108, 2, 1021. 59. N. Supwakorn, S. Dhamrongvaraporn and H. Ishida, Polymer Composites, 1998, 19, 2, 126. 60. N. Dansiri, N.Y. Anumet, J.W. Ellis and H. Ishida, Polymer Composites, 2002, 23, 3, 352. 61. S. Rimdusit, W. Tanthapanichakoon and C.J. Jubsilp, Journal of Applied Polymer Science, 2006, 99, 3, 1240. 62. H. Ishida, Modern Plastics International, 1998, 28, 6, 87.

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4

Stability, Degradation Chemistry and Applications

4.1 Introduction For the optimum use of polymers in high- and intense-heat environments, analysing and identifying the ‘real picture’ of thermal stability and degradation of polymers under such harsh conditions are important. The flammability of polymeric materials raises serious concerns because they generate toxic degradation products when they burn. Polymers degrade by thermal, photochemical and radical means. Various factors that affect the thermal degradation process are the environment of degradation, types of additives and most importantly, the structure of the polymer (e.g., molecular weight, degree of crosslinking, end-group structures, and the structure of the substituents). Unlike in the case of mechanical properties, the structural effect on thermal stability is not easily predictable. It has long been recognised that, to achieve heat resistance, the presence of an aromatic ring and heterocyclic ring in the polymer main-chain structure is necessary. For example, it was found that an increase in the number of aromatic rings increases the thermal stability of certain polymers [1]. However, this is not always the case because it was found that polyimides with dianhydrides that have one aromatic ring are thermally more stable than those consisting of two aromatic rings [2]. Moreover, it is difficult to fully utilise such highly stable structural units in polymers because they tend to make polymers brittle, insoluble, and infusible. When polymers are exposed to ultraviolet (UV) radiation, they can undergo destructive molecular changes, leading to discolouration and/or fracture of the materials, mostly due to the formation of various oxidation products. These destructive effects become important if polymers are utilised in outdoor applications where long-wavelength UV radiation becomes a critical factor [3] Therefore, to compensate for UV-induced molecular degradation, it is necessary to fully understand the changes that occur in polymeric materials upon UV exposure. From the foregoing discussion, it is realised that the understanding of thermal and photochemical degradation is a prerequisite for ultimate application, and the decomposition mechanism and products vary with the structures of the polymers. A well-accepted polymer that exhibits superior thermal stability over other polymers is phenolic resin. The char yield of conventional phenolic resin is 30–60% [4]. In

131

Polybenzoxazines: Chemistry and Properties this milieu, the stability and degradation of polybenzoxazine (PBZ), a new class of phenolic resin, gains particular attention. This is because it has been shown to have a high char yield (30–82%) and retains the qualities of phenolic resins and obviates many shortcomings associated with it (e.g., void formation during processing and high process viscosity). Undoubtedly, major efforts have been devoted to the understanding of the degradation of the PBZ class of polymers. This chapter is a glimpse into the chemical stability and degradation of PBZ in different environments and with different structures. The potential applications of PBZ are also been described.

4.2 Chemical Stability The PBZ network contains basic amine groups, so the stability of PBZ in an acid medium is unlikely. The chemical stability of typical PBZ based on bisphenol A and primary amines in a carboxylic acid solution has been studied [5]. The initial weight loss for the acid-treated bisphenol A and methyl amine-based benzoxazine polymer (BA-m) was observed at <250 °C with a weight loss of 10–15%, whereas the untreated BA-m polymer remained stable over this temperature range and BA-a strongly resisted the acid attack. The BA-m polymer rapidly disintegrated to a small fragmented powder in the carboxylic-acid treatment. It was found that the Mannich base is stable, and it was proposed that the nature of the primary amine is responsible for the interactions between the carboxylic acid and PBZ. From the result of the model dimer study, several possibilities are proposed to explain the change in the stability of different PBZ network structures in acid media. First, the BA-m polymer chains are highly curled so, due to the strong intramolecular hydrogen bonding during ring-opening polymerisation, the network structure of the BA-m PBZ may be formed ‘less tight’ than that of the BA-a PBZ. Further, strong salt formation may occur between the carboxylic acid and the nitrogen atom in the Mannich bridge of the BA-m PBZ. This affinity of the Mannich bridge for carboxylic acid in the BA-m polymer makes the carboxylic acid molecules penetrate into the network structure more readily by osmotic pressure, resulting in well-known macroscopic stress cracking by solvents. However, it is difficult to strongly protonate the tertiary nitrogen of the Mannich bridge of BA-a due to the difference in electronegativity from the methyl counterpart. Also, the network structure of the BA-a polymer has more chemically crosslinked sites due to the active reaction sites of aniline, so the entangled and crosslinked network structure of the BA-a polymer is barely affected by carboxylic acid. Therefore, the PBZ from BA-a can more effectively resist the carboxylic acid environment than the BA-m counterpart.

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4.3 Thermal Stability and Degradation PBZ are thermo-oxidatively stable. This is exemplified by the thermogravimetric analysis (TGA) of the cured resin in air and N2 [6]. Nitrogen-containing phenolic resins are also found to increase the non-flammability of polymers [7]. An increase in initial decomposition temperature (Tid) of the silica and silica-micro cell systems is indicative of the thermal stabilisation effect of the filler on the polymer [8]. The thermograms are shown in Figures 4.1 and 4.2. This enhancement in thermal stability is observed in air and N2. The Tid values are complied in Table 4.1. Interestingly, the Tid values are higher in air and than in N2. Therefore, it can be concluded that the PBZ is thermo-oxidatively more stable. This also suggests that the degradation is non-oxidative. These results are exemplified with the compositions BS2 and BSM3, respectively. The composition of BS2 is 56.5/38.5 by vol% (benzoxazines/silica fibre) and BSM3 is 19.8/11.9/56.9 by vol% (benzoxazines/silica fibre/microballoon).

Weight loss(%)

100 80

BSM3

60 BS2

40 BA-a

20 0

200

400

600

800

1000

T(°C)

Figure 4.1 Comparative thermogram of filled systems in N2

133

Polybenzoxazines: Chemistry and Properties

100

Weight loss (%)

80 BSM3

60 40

BA a

BS2

20 0 0

200

400

600

800

1000

T(°C)

Figure 4.2 Comparative thermogram of filled systems in air

Table 4.1 Initial decomposition temperature (Tid) of filled systems Sample references

Tid (°C) N2

Air

Cured BA-a

311

328

Cured BS2

350

372

Cured BSM3

330

368

Study of their thermal decomposition revealed that the initial decomposition is related to the loss of amine fragments from the network structure [9]. Hence, one strategy for thermal stabilisation is to introduce crosslinking sites on the amine moieties. Thus, amines containing reactive sites/polymerisable moieties are utilised to enhance thermal stability. Allyl, propargyl, and cyano amines are examples in such a category. 134

Stability, Degradation Chemistry and Applications It has been shown that the thermal stability of PBZ is substantially promoted by these reactive amines. Various diphenols are found to affect the thermal stability of this series of reactive amine-based PBZ. The introduction of nitrile [10, 11] groups into the backbone has been fruitful for conferring thermal stability. The reactive phthalonitrile terminal group contributed to the formation of highly thermally stable crosslinked structures which were revealed from dynamic and isothermal TGA. It was confirmed by TGA–Fourier-transform infrared (FTIR) spectroscopy that the remaining nitrile groups which did not react during polymerisation undergo further reaction during thermal degradation and provide high char yields. A polymerisation temperature of 250 °C was sufficient to achieve material of high thermal stability and exhibited values of glass transition temperature (Tg) in the range of 275–300 °C. Additionally, these polymers exhibit low flammability. The structural modifications and compounding are conducive to enhance resistance to thermal degradations of polymers. It has been seen that thermally stable phenol precursors such as naphthols also ensure thermal stability for the resultant PBZ [12]. Ishida and coworkers identified the decomposition product of aromatic aminebased PBZ through TGA and gas chromatography–mass spectroscopy (GC-MS) techniques [7, 13]. Several degradation products were identified under nitrogen and oxygen atmospheres (Scheme 4.1) derived from the degradation of the polymer and subsequent recombination of the degradation products. Benzene derivatives, amines, phenolic compounds and Mannich bases emerge directly from the polymer. Benzofuran is derived from further degradation of phenols. Biphenyl compounds are obtained from recombination of phenyl radicals after the loss of substituents from benzene, amine and phenol derivatives. Isoquinoline and phenathridine derivatives result from Mannich bases by loss of OH groups and dehydrogenation. Scheme 4.1 lists the degradation products of BA-a polymer under inert and oxidative environments. The overall degradation pattern of PBZ is shown in Scheme 4.2.

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Polybenzoxazines: Chemistry and Properties

Aniline (27%) Phenol (3%) p -Amino toluene (2%) N -M ethyl aniline (1%) 2,4- Dimethyl phenol (1%) Toluene (0.5 %) p -Xylene (0.5%)

O2 atmosphere

PBA-a N2 atmosphere

Aniline (56%) 2-Methyl phenol (8%) Phenol (7%) N-Methyl aniline (3%) p-Amino toluene (3%) 2,6-Dimethyl phenol (3%) Isocyanato benzene (1%)

Scheme 4.1 Major degradation products of PBA-a under oxidative and nonoxidative environments. Reproduced with permission from H.Y. Low and H. Ishida, Polymer, 1999, 40, 15, 4365. ©1999, Elsevier [13]

The thermal decomposition of a model compound containing a Mannich bridge and a series of PBZ model dimers led to similar observations [14]. In this case, the 2,4-dimethylphenol-based BZ dimers degraded into smaller and highly volatile compounds, leaving no char at the end of degradation. The p-cresol-based BZ dimers also degraded into smaller and highly volatile products. Some of these could undergo crosslinking and aromatisation processes and form char. The major decomposition products for modified-cresol-based dimers are amines and ester compounds.

136

Stability, Degradation Chemistry and Applications OH

OH

OH

N

N

N

C

C

N OH

HO products

Primary decomposition NH2

OH

OH

N

NH2

Benzene derivatives

OH

Amines

Mannich base compounds

Phenolic compounds

Secondary decomposition products O N Biphenyl compounds

2, 3 Benzofuran derivatives

Isoquinoline derivatives

N Phenanthridine derivatives

Char formation

Scheme 4.2 Decomposition of polybenzoxazine. Reproduced with permission from K. Hemvichian and H. Ishida, Polymer, 2002, 43, 16, 4391. ©2002, Elsevier [13]

137

Polybenzoxazines: Chemistry and Properties The thermal decomposition of PBZ based on bisphenol A and various aliphatic amines have been investigated under a nitrogen environment [15]. Various types of phenols, amines, and Schiff bases are released upon thermal decomposition. Various secondary amines are detected at 400 °C, whereas different phenols are identified at temperatures >400 °C. Of particular interest is the presence of various Schiff bases, and it has been shown that Schiff bases are present in the decomposition products which are the result of Mannich base cleavage. The thermal decomposition processes in the aliphatic amine-based PBZ model trimer and tetramer were investigated by TGA and GC-MS [16]. The end groups containing a methyl-substituted ortho position with respect to the hydroxyl group have no contribution to char formation. Conversely, the repeating units in the main chain had a profound effect on increasing the char yield. The presence of a phosphorus compound plays an important part in the high-temperature performance of PBZ due to its ability to inhibit ignition and promote char formation [17]. GC-MS analyses from the degradation of various acetylene-functional PBZ (3-aminophenylacetylene and bisphenol A) under a nitrogen environment showed that the initial degradation products were CO2, water, and ammonia. The degradation of BA-a is anchored by phenols, i.e., amine evaporation is the first degradation event and, in acetylene-functional PBZ, the amine part reacts due to the presence of an acetylene moiety. This is supported by the lower content of aniline in the degradation products of acetylene functional PBZ. In acetylene-PBZ [6], after cleavage of the Mannich base, a terminal amine group is created which may undergo further scission. The aromatic amine does not evaporate easily even after the thermal cleavage of the Mannich base has occurred because it is anchored by the polymerised acetylene-functional group. Rather, NH3 is released as a result of the Mannich base cleavage, followed by further cleavage of the terminal aniline group. Two routes are possible from the scission of the amine moiety. Route 1 results in the evolution of NH3 and substituted benzene. NH3 is detected at a relatively early stage of degradation, and its evolution persists at >700 °C, so route 1 is a favoured degradation pattern. Route 2 produces aniline and an unsaturated hydrocarbon, both of which were detected from FTIR and GC-MS analyses of the degradation products at higher temperatures. Therefore, degradation route 2 is also possible, but most likely at higher temperatures. A substantial amount of CO2 was detected from the degradation under nitrogen and air environments. The evolution of CO2 and NH3 during the thermal degradation is beneficial from a flammability viewpoint because these are non-flammable volatiles. The degradation of acetylene containing PBZ is shown in Scheme 4.3.

138

OH

OH

CH 3

CH 2

1 R

N

C CH

te

R

N

NH 2

H abstarctio n

R C CH

C

CH

C CH

OH H 2C

NH 2

Ro

ute

2

Substituted benzenes HC CH

OH H 3C R R

Scheme 4.3 Degradation of acetylene-functional polybenzoxazines. Reproduced with permission from H.Y. Low and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 1999, 37, 7, 647. ©1999, John Wiley & Sons [6] 139

Stability, Degradation Chemistry and Applications

R

NH3

OH

Ro u

OH

Polybenzoxazines: Chemistry and Properties

4.4 UV Stability and Photochemical Degradation The UV stability of PBZ has also been investigated [18, 19]. It has been determined that a photo-oxidation reaction occurred in BA-m PBZ upon irradiation in air at room temperature under the experimental conditions employed. Model compound studies demonstrated that the isopropylidene linkage was the site at which oxidation and cleavage occurred to form a 2,6-disubstituted benzoquinone. Model compound studies also indicated that the backbone structure of the Mannich bridge did not oxidise in any situation, nor did it cleave by non-oxidatively means, resulting in the formation of a detectable amount of double-bond-containing Schiff base. By the detection of substituted benzoquinone, it is evident that intramolecular hydrogen bonding decreased and intermolecular hydrogen bonding increased as the hydroxyl groups of the phenolic linkages were converted into quinone carbonyls. However, the depth of penetration of UV radiation into the polymer (to produce active radicals), coupled with the distance of diffused oxygen into the polymer to react with the radical species formed, dictated the extent of photo-oxidation in BA-m PBZ. The photo-oxidation of BA-m is proposed as given in Scheme 4.4.

O

N

CH 3

O

N

CH 3

O

ROO hu

O O OR

H H 2C

N

H 3C

N

O

H 3C

N

O

H 3C

N

O

O

O

Scheme 4.4 Proposed mechanism of photo-oxidation of BA-m monomer. Reproduced with permission from J.A. Macko and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 2000, 38, 20, 2687. ©2000, John Wiley & Sons [18]

BA-a is shown to have the highest degree of substituted benzoquinone formation followed by those polymers derived from hydroquinone, i.e., 4,4′-(hexafluoroisopropylidene)

140

Stability, Degradation Chemistry and Applications diphenol, 4,4′-thiodiphenol, 4,4′-dihydroxybenzophenone, p-cresol and phenol. The nature of the para-position in phenolic substituents was found to have an impact on the oxidation process and affect the degrees of substituted benzoquinone formation. For the irradiated BA-m monomer, the benzoquinone was the primary photooxidation product. Some secondary reactions were also found to occur as a result of photo-oxidation. The photo-oxidative behaviour of several PBZ based upon various phenols with different phenolic substitution but with the same amine functionality (methylamine) was examined upon exposure to UV radiation (290 nm). Each of the PBZ samples investigated had a substituted benzoquinone photo-oxidation product developed upon irradiation [20]. The thermal characteristics of major thermosetting polymers along with phenolic resin and PBZ were illustrated in Chapter 1. However, for comparison, the degradation mechanism and decomposition products of PBZ are summarised along with those of phenolic resin, cyanate ester (CE) and phenolic-triazine (PT) resins.

4.5 Degradation Mechanism of PBZ: A Comparison with other Thermosets The predecessors of BZ are phenolic resins, so their degradation is significant. Several studies have been carried out to explore the degradation of phenol formaldehyde resins [21–25]. The degradation of phenolic resin under an inert atmosphere involves three stages and finally forms char. The volatiles in the first step of degradation (<450 °C) contained structures such as diphenylether, and benzene nuclei bonded with methylene bridges [3, 8]. In this stage, mainly H2O was evolved, arising from a condensation reaction between phenolic groups. In the second step (450–700 °C), the benzene nuclei combined directly with one another to form biphenyl by the breaking of –CH2- bridges and –O- bridges. At 500–560 °C, some unpaired electrons were produced due to the decomposition of bridges, and subsequently the direct bonding of benzene nuclei occurred. The larger evolution of gas and the greatest loss of weight occurred and eventually the network collapsed in this step. The reactions that occur in this stage are: cracking, dehydration and dehydrogenation [7]. In the third step (700–1,000 °C), the remaining hydrogen atoms were removed as H2 as the dominant product, which resulted from the splitting of hydrogen atoms directly bonded to benzene nuclei. The destruction of crosslinks finally led to the formation of clusters of the aromatic units (500–800 °C). The process of carbonisation was completed at 900 °C. In air, the degradation is initiated at 300 °C and results in the formation of substituted phenols, CO2, hydroquinones and carboxylic acids (Scheme 4.5).

141

Polybenzoxazines: Chemistry and Properties

Scheme 4.5 Oxidative degradation of phenolic resins

With reference to the degradation of CE resins, they do not show major degradations below 400 °C. The first step in the degradation process is the thermal decyclisation of the cyanurate to isocyanate with thermal isomerisation of the cyanate to isocyanate, which starts at 400 °C and extends up to 475 °C. Above 475 °C, the cyanic acid 142

Stability, Degradation Chemistry and Applications (HOCN) is eliminated. The isocyanates subsequently react with moisture to form carbamates that could further hydrolyse to carbamic acid, an unstable intermediate that spontaneously decomposes into CO2 and ammonia [26–30]. A common model for the degradation of cyanurate involves the evolution of the volatile compounds CO, CO2, HOCN, and NH3. The GC-MS analysis further indicated that the common degradation products of cyanate ester were substituted benzene and phenols. The most investigated CE, bisphenol A cyanate ester (BACY) has the major degradation products shown in Figure 4.3.

Furanes ( 5.1 %)

Degradation products

Bisphenol A( 16.1 %)

Substituted phenols (29.1%)

Substituted benzene (10.1 %)

CO 2 /HOCN ( 21.2 % ) 0

5

10

15

20

25

30

Percentage of degradaed volatiles

Figure 4.3 Degradation products of cyanate ester (BACY)

PT precursor resin is a reaction product of novolac resin and cyanogen halide. The PT network is formed by the thermal cyclotrimerisation of the CE of novolac. The thermal degradation of PT resin has been investigated and the evolved gases analysed by FTIR and GC-MS. A three-step degradation was concluded. The steps are: (i) random scission and crosslinking of the hydrocarbon backbone at 400–450 °C, (ii) breakdown of the triazine ring with the liberation of low-molecular-weight volatile

143

Polybenzoxazines: Chemistry and Properties

compounds and formation of a primary residue, and (iii) decomposition of the primary residue with the elimination of alkenes and hydrogen, leaving a secondary carbonaceous char containing residual oxygen and nitrogen (500–750 °C). Substituted benzene and phenols are also identified in the decomposition products. The strong peak in the gas chromatogram at m/z = 44 was attributed to a mixture of CO2 and HOCN. In PT resin, this amounts to 38.52%. The benzene derivative and phenol derivatives are 8.7% and 39.8%, respectively. The remaining 10.46% could be due to products such as aryl cyanates, aryl cyanates, and amines. It is obvious that the degradation profiles and products of CE and PBZ are different. Amines are the major decomposition product in PBZ (see Scheme 4.1) whereas CO2/HOCN, along with substituted phenols or benzene derivatives, dominated the degradation products of CE. In CE and PBZ, major decomposition starts at >400 °C. However, the degradation pattern and products vary with structures. In phenolic resins, the major decomposition products are water, H2 gas and finally the fusion of aromatic units to form char, which bring excellent thermal resistance. In PT resin, it combines the degradation products of phenolic resin and CE. In CE, the ratio of CO2/HOCN to substituted phenols is, in general, 21/29% whereas, in PT resins, it is almost equal (~39/40%). Comparative thermograms of these resins are shown in Figure 4.4. The thermal stability indicators (values) of PBZ with these thermosets are compiled in Table 4.2. The onset temperature of the thermal decomposition of PBZ is higher than those of the other two notwithstanding the fact their thermal stability is a slightly inferior compared with others.

Table 4.2 Thermal stabilities of PBZ and other major thermosets Property

PBZ

Phenolic resin

CE resin

PT resin

Tg (°C)

170–340

170

250–270

300

T5 (%)

275–350

335

375–425

420

T10 (%)

325–450

410

440–490

435

Char yield at 700 °C (%)

30–80

58

40–60

63

144

Stability, Degradation Chemistry and Applications 110 100

BA-a Phenolic resisn Cyanate ester P-T resin

90 80

Weight loss (%)

70 60 50 40 30 20 10 0

0

100

200

300

400

500

600

700

800

900

1000

Temperature (° C)

Figure 4.4 Comparative thermogravimetric profiles of BA-a, phenolic resin, cyanate ester and PT resin

4.6 Applications Though a new entrant, PBZ appear to have a lot of applications, as evident from several patents having been registered in this regard. This section is a review of potential patents which go directly to the applications; their properties are also discussed. PBZ find major uses in microelectronics, the aerospace industry, and also in fuel cells. A BZ monomer which was cured with a dicarboxylic acid as a catalyst led to the formation of PBZ surfaces which found use as coating materials in electronic devices such as circuit boards and semiconductors [31]. The processed PBZ have high Tg, low flammability, good electrical properties (e.g., dielectric constant), expansion upon demoulding, post-curing, and cooling and near-zero percentage shrinkage. In addition, this curing technique required only 1–5 min for completion at 150–250 °C. In the fabrication of microchips, semiconductor wafers are processed and sliced into individual chips. These separated chips are then protected by means of package forms.

145

Polybenzoxazines: Chemistry and Properties The protective packages prevent damage to the chip and provide an electrical path to the circuitry of the chip. In general, an epoxy adhesive in liquid form is used as an underfill material. The PBZ-based materials were developed as wafer-level underfill composition as an alternative underfill compound to epoxy resins which have better properties than present epoxy systems [32]. Another application of PBZ is in the form of same underfill material but instead one utilises its lower coefficient of thermal expansion (CTE) [33]. Differences in CTE values between the semiconductor die and the package substrate cause relative shrinkage or expansion, which deteriorates the final performance in the microelectronics industry. The underfill material serves to reduce stresses on the conductive members due to such relative expansion or contraction. It has recently found that PBZ preferably comprising a filler material can be used as an electrical insulation system, for example, for bushings, instruments and distribution transformers [34]. PBZ is anhydride-free and easy to process. It has surprisingly good electrical properties and near-zero shrinkage upon cure. PBZ further offers new manufacturing possibilities which allow a decrease in the production cycle time and also offers the possibility of manufacturing bulky parts with limited residual stresses. The polymers from BZ are useful as precursors for char-yielding material (e.g., precursors to aircraft brake pads). They are also useful as temperature- and flameresistant polymers for electrical components, planes, cars, and buses. PBZ material has the following characteristics that make it desirable as a microelectronic or optoelectronic underfill material: (i) low moisture uptake; (ii) low CTE (iii) good film-forming properties (i.e., thickness control and tackiness at room temperature); and (iv) low volume shrinkage during curing. Table 4.3 compares the properties of a typical PBZ with a typical epoxy resin. Most PBZ-based monomers are solids at room temperature, but they usually have low melt viscosity at slightly elevated temperatures (e.g., 70–80° C). Cured pure PBZ-based film materials also have a lower CTE than a typical epoxy material, which allows lower filler loading to reach the same CTE value as a typical epoxy. The low melt viscosity and lower filler loading facilitate material flow and wetness on contact surfaces, and therefore provide a wider window for processing. The problem of heat dissipation in microelectronics is becoming increasingly important as the demands for denser and faster circuits intensify. To date, the maximum thermal conductivity for commercially available polymer materials remains substantially below 5 W/mK independent of the filler material and/or epoxy resin formulation. Most commercially available moulding compounds presently used in plastic microelectronic packaging typically have thermal conductivity values of ~0.7 W/mK. A composition of PBZ with high thermal conductivity has been introduced. The composition comprises BZ resin and boron nitride as a filler material, which confers thermal conductivity

146

Stability, Degradation Chemistry and Applications between ~3 W/mK and 37 W/mK [35]. The minimum concentration of boron nitride filler is 60 wt%. It was also discovered that very high thermal conductivity is attainable independent of the BZ precursors (in several cases). Polymer compounds with high thermal conductivity are also useful for other products such as computer cases, battery cases, electronic controller housings, and for other encasements where heat removal is an important consideration.

Table 4.3 Properties of a typical PBZ and a typical epoxy resin Property

Typical PBZ

Typical epoxy

Tg (°C)

170

150

Tensile strength (MPa)

130

120

Curing shrinkage (%)

0/expansion

3–4

Moisture uptake (%)

1.5

3

Modulus (GPa)

4

2–3

CTE (ppm/°C)

55

65

Impact strength (J/m)

30

30

Another invention is related to the preparation of novel epoxy moulding resins containing benzoxazines for encapsulation purposes. Known epoxy moulding encapsulants are, in general, prepared from blend epoxy resins, phenol hardeners, silica fillers, catalysts, flame-retardant materials, processing aids and colourants. The addition of PBZ as a co-reactant with one or several epoxy resins provided a product with reduced moisture adsorption while maintaining a high Tg [36]. To minimise the stresses that the package encounters at elevated temperatures in microelectronics, it is desirable that the encapsulant has low moisture adsorption and a Tg as high as possible. Table 4.4 lists the properties of resultant encapsualant material from epoxy-containing BZ. The advantages of these PBZ/epoxy compounds, therefore, include a combination of low moisture adsorption, a high Tg, low viscosity, and good processability. This unique set of properties has not been matched by any other epoxy moulding compound.

147

Polybenzoxazines: Chemistry and Properties

Table 4.4 Properties of epoxy-containing benzoxazine for encapsulation use Epoxy moulding compounds containing PBZ Property

Value

Flexural strength (MPa)

109–120

Flexural modulus (MPa)

17,900–20,700

Tg (°C)

130–197

Water absorption at 85% RH/85 °C for 7 days (%)

0.3–0.6

CTE (ppm/°C)

14.4–16.8

Ash content (%)

76.4–79.4

RH = Relative humidity

Polymer electrolyte membrane fuel cells (PEMFC) have been developed as highly efficient cells. A PEMFC comprises a membrane electrode assembly (MEA) including an anode layer, a cathode layer and a polymer electrolyte membrane which is interposed between the two electrode layers. A voltage generated between the anode and the cathode of one fuel cell is ~0.7 V. Therefore, to obtain an appropriate available voltage (10 V to 100 V), several fuel cells must be laminated to form a stack, and adjacent fuel cells separated by bipolar plates are preferable. Hence, the MEA is laminated using a bipolar plate with fluid flow channels formed thereof. The bipolar plate provides an electric connection between the cathode and the anode. It also provides the cathode with a gas flow channel and has strong corrosion resistance and gas impermeability. A composite material for a bipolar plate for application of fuel cells was developed from a PBZ matrix comprising dispersed conductive carbon [37]. These polymers have good workability because there is little volume change during polymerisation, good mechanical and chemical properties, and they can be manufactured at a low cost. Partially polymerised PBZ was introduced into a mould for a bipolar plate, and further polymerisation carried out at 300 °C for 30 min to obtain a bipolar plate. The crosslinked form of PBZ has been developed for fuel-cell applications which have a strong acid-trapping capacity with respect to the BZ monomer and good mechanical properties due to crosslinking. Also, the insolubility of the crosslinked object in

148

Stability, Degradation Chemistry and Applications polyphosphoric acid is desirable, so the crosslinked object is also chemically stable [38]. The electrolyte membrane including the crosslinked object has excellent phosphoric acid-supplementing capacity at a high temperature, as well as good mechanical and chemical stability. Specifically, even if the impregnated amount of a proton carrier (e.g., phosphoric acid) is increased to enhance proton conductivity, the electrolyte membrane has excellent mechanical and chemical stability. Accordingly, the electrolyte membrane can be used in a fuel cell at high temperature and without humidification. A naphthoxazine benzoxazine-based monomer has also been developed with improved performance for use in fuel cells [39]. Conventionally, prepregs are prepared from a matrix resin that is based on one or more epoxy resins. Recently, epoxy resins blended with BZ were found to be potentially useful prepregs [40] because the epoxy resins could reduce the melt viscosity of BZ, facilitating the possibility of higher filler loading while maintaining a processable viscosity. Ceramic–polymer composites were also fabricated from PBZ and barium strontium titanate [41]. It was found that, the composite dielectric constant increases with increasing ceramic content. By adding 80 wt% (48 vol%) of ceramic fillers, the dielectric constant was ≤28. The dielectric constants of the composites were nearly stable, with a frequency range of 1 kHz to 10 MHz and temperature range of 20–130 °C.

4.7 Conclusion This chapter summarised the chemical, thermal and photochemical stability of various PBZ and their degradation features. The ultimate performance of PBZ in harsh environments and outdoor applications is dictated by their thermo-oxidative stability. The degradation of PBZ is non-oxidative. In the thermal degradation of PBZ, the amine fragment is the first decomposition product, which is liberated from the polymer moiety. The thermal stability can be substantially improved by using reactive amines such as acetylene- and nitrile-containing amines. Various decomposition products have been identified by GC-MS analyses. Amines are the major product at temperatures <400 °C, and phenols are detected at >400 °C. The chemical stability of PBZ in acids is structure-dependent. BA-a is stable against acids, but BA-m decomposes. UV irradiation has a profound effect on PBZ. In the case of BA-a, the isopropylidine linkage is the site for oxidation. In all polymers studied under UV radiation, benzoquinone was the major oxidation product. The applications of PBZ have been found use in microelectronics, laminate prepregs, and in fuel cells.

149

Polybenzoxazines: Chemistry and Properties

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D.W. van Krevelen, Polymer, 1975, 16, 8, 615.

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R.A. Dine-Hart and W.W. Wright, Die Makromolekulare Chemie, 1972, 153, 1, 237.

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Plastics Additives Handbook: Stabilisers, Processing Aids, Plasticisers, Fillers, Reinforcements, Colorants for Thermoplastics, 4th Edition, Eds., R. Gächter and H. Müller, Hanser/Gardner Publishing, Cincinnati, OH, USA, 1993.

4.

R. Antony and C.K.S. Pillai, Journal of Applied Polymer Science, 1994, 54, 4, 429.

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H.D. Kim and H. Ishida, Journal of Applied Polymer Science, 2001, 79, 7, 1207.

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H.Y. Low and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 1999, 37, 7, 647.

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H.Y. Low and H. Ishida, Polymer, 1999, 40, 15, 4365.

8.

K.S. Santhosh Kumar, Investigations on Polybenzoxazines, Their Blends and Composites, Kerala University, India, 2008. [PhD Thesis]

9.

Z. Brunovska, R. Lyon and H. Ishida, Thermochimica Acta, 2000, 357/358, 195.

10. Z. Brunovska and H. Ishida, Journal of Applied Polymer Science, 1999, 73, 14, 2937. 11. S.B. Shen and H. Ishida, Journal of Applied Polymer Science, 1996, 61, 9, 1595. 12. W. Men and Z. Lu, Journal of Applied Polymer Science, 2007, 106, 4, 2769. 13. K. Hemvichian and H. Ishida, Polymer, 2002, 43, 16, 4391. 14. K. Hemvichian, A. Laobuthee, S. Chirachanchai and H. Ishida, Polymer Degradation and Stability, 2002, 76, 1, 1. 15. H.Y. Low and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 1998, 36, 11, 1935.

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Stability, Degradation Chemistry and Applications 16. K. Hemvichian, H.D. Kim and H. Ishida, Polymer Degradation and Stability, 2005, 87, 2, 213. 17. S-W. Choi, S. Ohba, Z. Brunovska, K. Hemvichian and H. Ishida, Polymer Degradation and Stability, 2006, 91, 5, 1166. 18. J.A. Macko and H. Ishida, Journal of Polymer Science, Part B: Polymer Physics Edition, 2000, 38, 20, 2687. 19. J.A. Macko and H. Ishida, Polymer, 2001, 42, 1, 227. 20. J.A. Macko and H. Ishida, Macromolecular Chemistry and Physics, 2001, 202, 11, 2351. 21. K.A. Trick and T.E. Saliba, Carbon, 1995, 33, 11, 1509. 22. J. Hetper and M. Sobera, Journal of Chromatography A, 1999, 833, 2, 277. 23. M.C. Roman-Martinez, D. Cazorla-Amoros, A. Linares-Solano, C. SalinasMartinez de Lecea and F. Atamy, Carbon, 1996, 34, 6, 719. 24. L.B. Manfredi, O. Osa, N.G. Fernandez and A. Vazquez, Polymer, 1999, 40, 13, 3867. 25. C. Morterra and M.J.D. Low, Carbon, 1985, 23, 5, 525. 26. M.L. Ramirez, R. Walters, R.E. Lyon and E.P. Savitski, Polymer Degradation and Stability, 2002, 78, 1, 73. 27. I. Hamerton, Chemistry and Technology of Cyanate Ester Resins, Blackie Academic and Professional, London, UK, 1994. 28. D.A. Shimp, S.J. Ising and J.R. Christenson in the Proceedings of the 34th International SAMPE Symposium, Reno, NV, USA, 1989, p.222. 29. D.A. Shimp and S.J. Ising in the Proceedings of the American Chemical Society National Meetings PMSE Division, San Francisco, CA, USA, 1992. 30. M.L. Ramirez, R. Walters, E.P. Savitsky and R.E. Lyon, Thermal Decomposition of Cyanate Ester Resins, Report No. DOT/FAA/AR-01/32, US Department of Transportation (Federal Aviation Administration), Washington, DC, USA, 2001. 31. G.A. Anthony, inventor; Loctite Corporation, assignee; US 6,376,080, 2002.

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Polybenzoxazines: Chemistry and Properties 32. L. Wang, S. Song-Hua and C. Tian-An, inventors; Intel Corporation, assignee; US 6,730,542, 2004. 33. S. Song-Hua, L. Wang and C. Tian-An, inventors; Intel Corporation, assignee; US 6,899,960, 2005. 34. K. Xavier, K.Kurt, R. Jens and W. Reto, inventors; ABB Research Limited, assignee; EP 1,901,312, 2008. 35. H. Ishida, inventor; Edison Polymer Innovation Corporation, assignee; US 5,900,447, 1999. 36. G. D. William, inventor; Cookson Singapore PTE Limited, assignee; US 6,437,026, 2002. 37. K. Hyoung-Juhn, E. Yeong-Chan, C. Sung-Yong, K. Ho-Jin, M. Jin-Kyoung, L. Dong-Hun, K. Ju-Yong and A. Seong-Jin, inventors; Samsung SDI Co Limited, assignee; US 7,510,678, 2009. 38. C. Seong-Woo, S. Hee-Young, L. Myung-Jin and J. Woo-Sung, inventors; Samsung SDI Co Limited, assignee; EP 1,760,110, 2007. 39. C. Seongwoo and P. Jungock, inventors; Samsung Electronics Co Limited, assignee; EP 2,055,706, 2009. 40. L.L. Stanley, L. W. Helen and W. S. Raymond, inventors; Henkel Corporation, assignee; US 7,537,827, 2009. 41. G. Panomsuwan, H. Ishida and H. Manuspiya in the Materials Research Society Symposium Proceedings, Ed., E. Charson, Volume 993E, 2007, 0993E03-08. [Electronic papers only]

152

A

bbreviations

22BP-a

2,2′-Biphenol- and aniline-based benzoxazine

22P-a

8,8′-Bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)

24DMP-a

2,4-Dimethyl phenol- and aniline-based benzoxazine

35x

3,5-Xylidine

44O-a

6,6′-Bis(2,3-dihydro-3-phenyl-4H-1,3-benzoxazinyl) ketone

4TBUPH-a

4-t-Butyl phenol- and aniline-based benzoxazines

AR

Analar reagent

ATBN

Amine-terminated butadiene–acrylonitrile copolymer

BA-35x

Bisphenol A- and 3,5-xylidine-based benzoxazine

BA-a

Bisphenol A- and aniline-based benzoxazine

BA-a-co-F-1

Copolymer of BA-a and F-1

BA-af

Bis(3-furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane

BA-allyl

Diallyl bisphenol A- and aniline-based benzoxazine

BA-apa

Bisphenol A- and 3-aminophenylacetylene-based benzoxazine

BACY

Bisphenol A cyanate [2,2-bis (4-cyanatophenyl) propane]

BAF-apa

Bisphenol F- and aminophenylacetylene-based benzoxazine

B-ala Bisphenol A- and allylamine-based benzoxazine. IUPAC name: [Bis

153

Polybenzoxazines: Chemistry and Properties (3-allyl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane] BA-m

Bisphenol A- and methyl amine-based benzoxazine

BA-mt

Bisphenol A- and m-methyl aniline-based benzoxazine

BA-ot

Bisphenol A- and o-methyl aniline-based benzoxazine

B-appe

Bisphenol A- and propargylamine-based benzoxazine

BA-pt

Bisphenol A- and p-methyl aniline-based benzoxazine

BATMS-BZ 1,3-Bis(3-aminopropyl)tetramethyldisiloxane) containing a difunctional benzoxazine monomer BDM

4,4′-Bismaleimide diphenyl methane

BIP

4,4′-Biphenol

BIPBZ

4,4′-Biphenol- and aniline-based benzoxazine

BISE 1,3-Bis[3-(4,5-epoxy-1,2,3,6-tetrahydrophalimido) propyl] tetramethyl disiloxane BM-apa 4,4′-Methylenediphenol- and 3-aminophenylacetylene-based benzoxazine BMI

Bismaleimide(s)

BMM

4,4′ Bismaleimido diphenyl methane

BMO-apa Bis(4-hydroxy phenyl) methanone- and 3-aminophenylacetylenebased benzoxazine BMP

2,2-Bis[4-(4-maleimido phenoxy)phenyl]propane

BO-apa 4,4′-Oxydiphenol- and 3-aminophenylacetylene-based benzoxazine BPA-FBZ

Bis(3-furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane

BP-apa Biphenyl-4,4′-diol- and 3-aminophenylacetylene-based benzoxazine

154

Abbreviations BPFP-apa 2,2 Bis(4-hydroxyphenyl) perfluoropropane- and 3-aminophenylacetylene-based benzoxazine BS

Benzoxazine-silica fibre composites

BS2

Benzoxazines/silica fibre

BS-apa 4,4′-Thiodiphenol- and 3-aminophenylacetylene-based benzoxazine BSM

Benzoxazine-silica-microballon composites

BSM3

Benzoxazines/silica fibre/microballoon

BSO-apa Bis(4-hydroxyphenyl)sulfone- and 3-aminophenylacetylene-based benzoxazine BZ

Benzoxazine(s)

BZ-CH 1,1′ Bis(4-hydroxyphenyl) cyclohexane- and aniline-based benzoxazine BZ-PD-CH 1,1′ Bis(4-hydroxyphenyl) pentadecyl cyclohexane- and anilinebased benzoxazine BZ-PHC-CH 1,1′ Bis(4-hydroxyphenyl) perhydrocumene cyclohexane- and aniline-based benzoxazines CCW

Cure-controllable window

CD

Cyclodextrin

CE

Cyanate ester(s)

CF

Carbon fibre(s)

CTBN

Carboxyl-terminated butadiene acrylonitrile rubber

CTE

Coefficient(s) of thermal expansion

DABA

Diallyl bisphenol A

DCPD

Dicyclopentadienephenol

155

Polybenzoxazines: Chemistry and Properties DCPDBZ

DCPD-based benzoxazine monomer

DDM

4,4′-Diaminodiphenyl methane

DGEBA

Diglycidyl ether bisphenol A

DMA

Dynamic mechanical analysis

DMF

Dimethylformamide

DMTA

Dynamic mechanical thermal analysis

DOPO

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide

Dopot-m

Dopotriol- and methylamine-based benzoxazine

DSC

Differential scanning calorimetry

EPN

Epoxy phenol novolac resins

F-1

Fluorinated benzoxazine

F-a

6′,6-Bis(3-phenyl-3, 4-dihydro-2H-1, 3-benzoxazineyl) methane

FTIR

Fourier-transform infrared

G′

Storage modulus

G′′

Loss modulus

GC-MS

Gas chromatography–mass spectroscopy

HHPA

Hexahydrophthalic anhydride

HMPA

Hexahydro-4-methylphthalic anhydride

HPM

N-(4-Hydroxy phenyl) maleimide

HPM-Ba

[N-(4-Hydroxyphenyl) maleimide]-benzoxazine

ILSS

Interlaminar shear strength

IPN

Interpenetrating polymer network(s)

IR

Infrared

156

Abbreviations LOI

Limiting oxygen index

LSS

Lap shear strength

Mal-BZ

4-Maleimidophenol- and aniline-based benzoxazine

Mal-BZ-Al

Hydroxyphenylmaleimide- and allyl amine-based benzoxazine

Mal-BZ-Pg Hydroxyphenylmaleimide- and phenyl propargyl ether-based benzoxazine MAS

Magic angle spinning

MBZ-POSS

Octafunctional cubic silsesquioxane

MEA

Membrane electrode assembly(ies)

MIB

Maleimido benzoxazine

MI-COOH

4-Maleimido benzoic acid

MI-H

4-Maleimido benzene

MI-OH

4-Maleimido phenol

MMT

Montmorillonite

MPN Novolac modified by propargyl and methylol groups simultaneously mt

m-Toluidine

MWNT

Multi-walled carbon nanotube(s)

NMR

Nuclear magnetic resonance

NOB

Norbornene benzoxazine

OAPS

Octaaminophenyl polyhedral oligomeric silsesquioxane

OF

Octakis (dimethylsiloxyhexafluoropropyl ether) silsesquioxane

OMMT

Organically modified montmorillonite(s)

157

Polybenzoxazines: Chemistry and Properties OpePOSS

Octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane

ot

o-Toluidine

P-a

Phenol- and aniline-based benzoxazine

PAA

Poly(amide acid)

Pad-12 Phenol- and dodecamethyl diamine-based benzoxazine with 12 methylene groups Pad-2 Phenol- and dimethyl diamine-based benzoxazine with 2 methylene groups Pad-4 Phenol- and tetramethyl diamine-based benzoxazine with 4 methylene groups Pad-6 Phenol- and hexamethyl diamine-based benzoxazine with 6 methylene groups Pad-8 Phenol- and octamethyl diamine-based benzoxazine with 8 methylene groups P-af

3-Furfuryl-3,4-dihydro-2H-1,3-benzoxazine

P-ala Phenol- and allylamine-based benzoxazines (3-allyl-3,4-dihydro2H-1,3-benzoxazine) P-alp 2-Allylphenol- and aniline-based benzoxazines (3-phenyl-3,4dihydro-8-allyl-2H-1,3-benzoxazine) P-a-PCL

P-a containing PCL

P-appe

Phenol- and propargylamine-based benzoxazine

PBA-a

Bisphenol A- and anline-based polybenzoxazine

PBA-af

Bis(3-furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane

PBA-allyl

Diallyl bisphenol A- and aniline-based polybenzoxazine

PB-hda

Bisphenol A- and hexamethyl diamine-based polybenzoxazine

PB-mda

Bisphenol A- and dimethyl diamine-based polybenzoxazine

158

Abbreviations PBO

2,2′-(1,3-Phenylene)-bis (4,5-dihydro-oxazoles)

PBPA

Bis propargyl ether bisphenol A

PBZ

Polybenzoxazine(s)

PC

Polycarbonate

PCL

Polycaprolactone

PCL-NZ

Polycaprolactone-naphthoxazine

PDMS

Poly(dimethylsiloxane)

PEMFC

Polymer electrolyte membrane fuel cells

PEP

Phenylethynylphenol

PEPFN

Phenyl ethynyl functional novolac resins

Ph-apa

Phenol- and 3-aminophenylacetylene-based benzoxazine

PHPM-Ba-I N-(4-Hydroxyphenyl) maleimide]-benzoxazine in which only maleimide groups are polymerised PHPM-Ba-II N-(4-Hydroxyphenyl) maleimide]-benzoxazine in which both maleimide and benzoxazines groups are polymerised PI

Polyimide

PMF resin

Phenolic novolac resins bearing maleimide groups

PN resins Propargyl ether-functional phenolic resins POSS

Polyhedral oligomeric silsesquioxane

PP-a

Polymer of phenol and aniline based benzoxazine

PP-af

Polymer of phenol and aminofuran based benzoxazine

ppm

Parts per million

PPOA

Branched (triamines) poly(propyleneoxide)(s)

159

Polybenzoxazines: Chemistry and Properties pt

p-Toluidine

PT

Phenolic-triazine

PU

Polyurethane

PU/P-a–OMMT Polyurethane-benzoxazine/clay hybrid nanocomposites RH

Relative humidity

SEM

Scanning electron microscopy

SPI

Polysiloxane-block-polyimide

T10%

Temperature at which 10% weight loss occurs

T5%

Temperature at which 5% weight loss occurs

TBA

Torsional braid analysis

Tf

Final cure temperature

Tg

Glass transition temperature

TGA

Thermogravimetric analysis

Ti

Initial cure temperature

Tid

Initial decomposition temperature(s)

Tp

Peak cure temperature

UV

Ultraviolet

VB-a

Vinyl-terminated benzoxazine monomer

Xdensity

Crosslink density

160

I

ndex

A Acetylene, polymerisation of 63 Acrylates 1 3-Allyl-3,4-dihydro-2H-1,3-benzoxazine 52 Allylation, degree of 9 Allyl benzoxazine 91 Allyl, curing of 4 Allyl naphthols 19 Allyl phenoxy phosphazene 15 Allyl phenoxy triazine 15 Allyl phenyl oligomers 6 Aniline dimmers, asymmetric 33 Arylamine Mannich bridge network 29, 30-31, 79

B β-Cyclodextrin 84 Benzoquinone 141 Benzoxazine 25, 29-30, 51, 55-61, 64, 66, 71, 80-84, 95, 101, 105, 115, 141 allyl based 53-55, 72, 106-107 2-allylphenol and aniline based 52 3-aminophenylacetylene based 64 aniline-based/bismaleimide system 104 2,2-bis(4-hydroxyphenol)perfluoropropane and 3-aminophenylacetylene based 64 bisphenol A and allylamine based 71 bisphenol A and aniline based 25, 28, 34-36, 53, 55, 61, 63, 80-81, 87, 89-90, 98, 100, 108, 112, 114-115, 132, 145 bisphenol A and m-methyl aniline based 80 bisphenol A and o-methyl aniline based 80-81 bisphenol A and p-methyl aniline based 80-81 bisphenol A and propargylamine based 59-60, 73 bispenol A and 3,5-xylidine based 80-81 curing 35

161

Polybenzoxazines: Chemistry and Properties epoxy copolymers 95 functionalised 51-52 hydroxyphenylmaleimide- and allyl amine based 72 hydroxyphenylmaleimide- and phenyl propargyl ether based 72 maleimido 57, 73 4-maleimidophenol- and aniline based 72 methyl amine based 132, 141, 149 model dimer 31 norbornene 57-58, 73 oligomers 34 polymerisation 67 ring 96-97 ring-opening polymerisation 63 phenol and allylamine based 52 phenol-and aminofuran based 63 phenol- and 3-aminophenylacetylene based 64 phenol and propargylamine based 59-60, 73 phthalonitrile 65-66 propargyl 61 silica composites 123-125 silica-microballon 123-125 silane-coupling agent 116 4,4'-Biphenol 77 Bis(3-allyl-3, 4-dihydro-2H-1,3-benzoxazinyl) and isopropane 52 6',6-Bis(3-phenyl-3,4-dihydro-2H-1,3-benzoxazineyl) methane 82 Bis propargyl ether bisphenol A 10 Bis(3-furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane 63 Biscitraconimides 10 4,4'-Bismaleimide diphenyl methane resins 10 Bismaleimide resins, aromatic 6 Bismaleimides 1, 5-6, 10, 12-15, 19, 21, 24-25, 101, 103, 107 4,4'-Bismaleimido diphenyl methane 8 Bisoxazoline-phenolic system 19-20 Bisphenol 86 Bisphenol A 25 Bisphenol A and dimethyl diamine-based polybenzoxazines 85 Bisphenol A cyanate ester (BACY) 12-13, 143 Bisphenol F epoxy resins 3 Bisphenolic methylene bridge network 30 Bismaleimide-modified polyurethane-epoxy systems 8 Bismaleimide-triazine resins 10 Bisphenol A, brominated 3

162

Index Butadiene-acrylonitrile copolymer, amine-terminated 101, 120 Butadiene acrylonitrile rubber, carboxyl-terminated 101, 120

C Carbon fibre 24, 116, 118-120 loading 119 Carbon nanotube composites, epoxy-coiled 4 Carboxylic acid treatment 132 Casting 19 Ceramic-polymer composites 149 Click chemistry 67-68 Coefficient of thermal expansion 63, 124, 146 Composites, fibre 115 Composites, silica fibre-filled 122 Composites, void-free 12 Copolybenzoxazines 83 Copolymerisation 96 Cracking 141 Cure window 102 Curing 1, 3, 20, 24, 36, 53. 69, 82, 96 temperature 79 initial 102 thermal 6, 12 Cyanate trimerisation 110 Cyclodextrin 84 Cyclohexyl-benzoxazine monomers 87 Cyclotrimerisation 12 thermal 15, 143 Cynate ester resins 1, 9, 12-15, 25, 108 141, 142-144

D Decomposition temperature, initial 134-135 Decyclisation, thermal 142 Degradation, oxidative 142 Dehydration 141 Dehydrogenation 141 Dehydrohalogenation 2 Diallyl bisphenol A 4, 8 Diamine-terminated amide resins 8 4,4'-Diaminodiphenyl methane 86 Dicyclopentadiene phenol adduct 77 Dicyclopentadiene phenol – epoxy phenol novolac resins 4

163

Polybenzoxazines: Chemistry and Properties Dielectric constant 149 Diels-Alder polymerisation 7 Diels-Alder reaction 6-8, 19 Differential scanning calorimetry 5, 21, 35, 53, 67, 69, 87, 98, 108-110, 113 Diglycidyl ether bisphenol A 2-3, 96-97 Dihydrobenzoxazines 77 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide 82 Dimers, asymmetric model 32 Dopotriol 82 Dopton-m 81 Double quantum magic angle spinning spectroscopy 34 Dynamic mechanical analysis 26, 63, 77, 124 Dynamic mechanical thermal analysis 89, 101, 103, 111

E Ene-Alder reaction 8 Epichlorohydrin 2 Epoxies 13-14, 25 Epoxy moulding compounds 148 Epoxy moulding resins 147 Epoxy, phenolic-based 4 Epoxy ring 97 Ether resins 21

F Fibre clustering 117 Fibre loading 120 Final cure temperature 101-102 Formaldehyde 11 Fourier-transform infrared spectroscopy 31, 35, 69, 102, 138, 143 Free-radical mechanism 57 (3-Furfuryl-3,4-dihydro-2H-1,3-benzoxazinyl) isopropane 61 3-Furfuryl-3, 4-dihydro-2H-1, 3-benzoxazine 61

G Gas chromatography-mass spectroscopy 135, 138, 143, 149 Glass fibre-reinforced plastics 19 Glass transition temperature 5-6, 8-10, 13-14, 19, 23-24, 26, 28, 31, 34-35, 55, 63, 65, 78-79, 82, 85, 88-89, 91, 95, 98, 100, 104, 108, 111-112, 114-115, 124, 135, 147 Glycidyl-based resins 2

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Index

H H-nuclear magnetic resonance spectroscopy 69 Hompolymerisation 109 [N-(4-Hydroxyphenyl)maleimide]-benzoxazine 56, 73 Hydrogen bond, chelated 34 Hydrogen bonding 31-32, 34, 36, 102-103, 141 Hydrolysis 13, 115 Hydroxyphenylmaleimide 11, 21 [N-4(4-Hydroxyphenyl) maleimide]-benzoxazine 55 1

I Interlaminar shear strength 116 Interpenetrating polymer networks 8-9, 100 Isocyanate-derived polymers 1 Isomerisation, thermal 142

K Kapton films 5 Kenaf fibre 126 Kissinger method 61

L Lap shear strength 11 Limiting oxygen index 56

M Macroscopic stress cracking 132 Maleimide end-capping agents 6 Maleimide polymerisation 101 Maleimide-terminated oligomers 6 Mannich base bridge 28, 32, 34, 132, 136 Membrane electrode assembly 148 Microballoons 121-123 Molecular degradation, ultraviolet-induced 131 Molybdenum-phenolic resins 23 Montmorillonite 113, 115 organically modified 113 Moulding, property-oriented 95 Mouldings 19 Multi-component polymeric material 95 Multi-walled carbon nanotubes 3, 24, 115

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Polybenzoxazines: Chemistry and Properties

N Nanocomposites, epoxy-clay 4 Nanocomposites, polymer-clay 113 Nanocomposites, polymer-layered silicate 112 Naphthoxazine benzoxazine based monomer 149 Novolac resin, ethynyl 21 Novolac resins, epoxy cresol 3 Novolac resins, epoxy phenol 1-4, 11, 21, 24, 146, 149 Novolac resins modified by propargyl and methylol groups 23 Novolac resins, phenolic 96 Nuclear magnetic resonance 28, 34, 69, 85 solid-state 13C-cross polarisation/magic angle spinning 85

O Octa-aminophenyl polyhedral oligomeric silsesquioxane 114 Octakis 4

P Peak cure temperature 87, 101-102 Phenol-(4-hydroxy) phenylmaleimide-formaldehyde resins 10, 11, 21 Phenolic Mannich bridges 79 network 30 Phenolic resin 1, 17-18, 24, 77, 131-133, 141-142 epoxy systems 20 Phenolic system, addition-cure 24 Phenolic-triazine resins 13, 15, 141, 143-144 Phenolics, allyl-functional 19 Phenolics, condensation-type 17 Phenols, alkenyl 8 3-Phenyl-3-4,-dihydro-8-allyl-2H-1,3-benzoxazine 52 Phenyl ethynyl functional novolac resins 23 N-phenyl maleimides 58 Phosphazene-triazine network polymers 15-16 Phenol novolac resin with maleimide groups – epoxy novolac resin 11 Polyacetylenes 61 Polyacids 3 Polyamines 3 Polybenzimidazole 8 Polybenzoxazine acetylene-functional 140 bisphenol A and aniline based 99 composites 126

166

Index curing 35 cyclohexyl-based 88 hexamethyl diamine-based 85 microballoon-embedded 123 micro composites 115 model 137 model oligomers 71 nanocomposites, clay-based 112 nanomaterials 112 syntactic foams 121-122, 126 phthalonitrile 65 poly(ε-caprolactone) 97, 115 polyhedral oligomeric silsesquioxane systems 114 polyimide blends 98 wood composites 126 Polycaprolactone-naphthoxazine 69 Polycyanurate 12-14 Polycyclotrimerisation 12 Polyesters 14, 25 unsaturated 1 Polyether imides 14 Polyether ketones 14 Polyethersulfone 14 Polyimides 5, 25 Polymer electrolyte membrane fuel cells 148 Polymer matrix 116 Polymers, Alder-ene 19 Polymers, high-performance 1, 27 Polymercaptans 3 Polymerisation 12, 21-22, 25, 28-31, 59, 61, 79, 83, 96, 100, 103, 109, 113, 135, 148 allyl 52-53 oxazine 52, 115 ring-opening 24, 26, 31, 35, 59, 63, 113 Polyoxyalkyleneamines 8 Polyphenols 3 Polypropyleneoxides 69 Polysiloxane-block-polyimide 99 Polythiourethanes, mercaptan-terminated 3 Powder-blended system 109 Powder-mixed blend 110 Powder-mixed system 109

167

Polybenzoxazines: Chemistry and Properties Pre-polymers, acetylene-terminated 63 Propargyl ether 21 Propargyl ether-functional novolac resins 21-23 Pyrolysis 24 Pyrolysis gas chromatography 54

R Regioselectivity 28 Relaxation processes, low-temperature 78 Resin transfer moulding technique 126 Rheokinetic measurements 35 Rubber toughening 7

S Scanning electron microscopy 103-104, 107, 116 Schiff base 138, 140 Semi-interprenetrating networks 98 Solution-blended system 109-110 Solvent method 108 Solvent-less method 51 Storage modulus 126 Symmetric aniline dimers 33 Syntactic foams 121-124, 126

T Tetraglycidyl-4,4-diaminodiphenylmethane 3 Thermogravimetric analysis 90, 111, 133, 135 dynamic 135 isothermal 135 Thermogravimetric analysis - Fourier-transform infrared spectroscopy 135 Thermal oxidation 17 Thermo-oxidative degradation 13 Thermoplastic resins 14 Thermoset polymers 1 Thermoset resins 13 Thermosetting resins 25 Torsional braid analysis 35 Toughening 14, 112

V Vinyl esters 25

168

Index

W Wagner-Jauregg reaction 19 Water contact angle 90 Weight-loss curve, derivative 67

X X-ray diffraction analysis 85, 113

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Polybenzoxazines: Chemistry and Properties

170