Microgels

Encyclopedia of Nanoscience and Nanotechnology

www.aspbs.com/enn

Polymeric Nano/Microgels Piotr Ulanski, Janusz M. Rosiak Technical University of Lodz, Poland

CONTENTS 1. Introduction 2. Synthesis 3. Methods Used for Studying Microgels 4. Applications Glossary References

1. INTRODUCTION Soft nanomaterials—polymeric nanogels and microgels— have had a fast and brilliant career from an unwanted byproduct of polymerization processes to an important and fashionable topic of interdisciplinary research in the fields of polymer chemistry and physics, materials science, pharmacy, and medicine. Together with their larger analogs— macroscopic gels, most known in the form of water-swellable hydrogels—they have a broad field of actual and potential applications ranging from filler materials in coating industry to modern biomaterials. There are at least two ways of defining polymeric nanogels and microgels. One of them originates from the definition of polymer gels. A polymer gel is a two-component system consisting of a permanent three-dimensional network of linked polymer chains, and molecules of a solvent filing the pores of this network. Nanogels and microgels are particles of polymer gels having the dimensions in the order of nano- and micrometers, respectively. The other definition says that a nanogel or a microgel is an internally cross-linked macromolecule. This approach is based on the fact that, in principle, all the chain segments of a nanogel or microgel are linked together, thus being a part of one macromolecule. It also reflects the fact that such entities can be synthesized either by intramolecular cross-linking of single linear macromolecules or in a single polymerization event (e.g., initiated by one radical) that in the absence of cross-linking would lead to the formation of a single linear polymer chain. The latter definition allows us to consider nano- and microgels as a specific form of macromolecules, along with linear, branched, comblike, circular, star-shaped, dendrimer, ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

and other forms (Fig. 1). Since usually the shape of a nanoor microgel resembles a linear macromolecule in a coiled conformation, these structures are often seen as permanently “frozen” polymer coils. In fact, molecular weights and dimensions of swollen nanogels are often similar to these of typical single macromolecules in solution, but the presence of internal bonds results in different physicochemical properties, including fixed shape, different rheological behavior, higher resistance to degradation, and the ability to trap other molecules within their structure. It seems logical to use the term nanogel to describe gel particles of submicrometer size and to limit the use of microgel to particles over micrometer size. This rule has, however, not established itself in the literature. More often than not, the more popular term microgels is used to describe all small gel structures, including those having the dimensions of several tens or hundreds of nm. Anyway, a strict differentiation between these two groups is not always unambiguous (shall we measure the particles in their dry or swollen forms?), reasonable, and useful, since may properties, synthetic methods, etc. are common. Therefore, in the following we will use the word microgel as a general term denoting all over-micrometer and submicrometer gel particles, limiting the use of the term nanogel to a subclass of small microgels formed by internal cross-linking of single macromolecules (see Section 2.2). Microgels belong to the large family of cross-linked polymeric microparticles. Their distinct property is the ability to swell in a suitable solvent. Certainly, the choice of such a solvent is a function of the chemical structure of the polymer network. Therefore a cross-linked microparticle that remains impermeable and does not swell in contact with a group of solvents may become a microgel in another group of solvents. Very important and perhaps most studied so far are microgels composed of hydrophilic polymers, thus capable of swelling in water. Such gels, irrespective of their dimensions, form a large group of compounds named hydrogels. Typical hydrogel-forming polymers are those containing hydrophilic groups as OH, COOH, NH2 , CONH2 , CONH , SO3 H, or ether linkages. In principle, nearly all water-soluble polymers, including those of natural origin, Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (845–871)

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(a)

(b)

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A distinct class of microgels is stimuli-sensitive (“smart,” “intelligent”) structures. They are able to react, usually by a pronounced change in dimensions and swelling ability, to external stimuli such as temperature, pH, ionic strength, concentration of a given substance, electric field, light, etc. Such structures may find applications in controlled or selfregulating drug delivery, signal transmission, or micromachinery (see Sections 4.1 and 4.3). First reports regarding microgels date from 1930s, when Staudinger described the formation of a styrenedivinylbenzene microgel [14, 15]. Since then, hundreds of reports have been published on this topic. For a full insight, the reader is directed to the excellent review papers by Funke et al. [16] and Saunders and Vincent [17], covering most of the relevant aspects, as well as to a number of reviews dedicated to more specific issues [18–20]. In this chapter we will first discuss the methods used to synthesize microgels, then list the techniques used in studying their properties, and finally briefly review their actual and potential applications.

(g) Figure 1. Macromolecules of various architectures: (a) linear, (b) branched, (c) comblike, (d) circular, (e) star-shaped, (f) dendrimer, (g) microgel.

can be transformed into hydrogels. Macroscopic hydrogels have been extensively studied since the 1960s [1], and to date a number of large-scale applications emerged from this field (contact lenses, drug-delivery systems, wound dressings, etc.) [2–5]. Hydrogels, due their good biocompatibility and ability to mimic the properties of some tissues like cartilage and muscles, are especially suitable for use as biomaterials (see Section 4.1). All polymer gels, macro- and microscopic, can be divided in two classes with respect to the character of the bonds linking the chains. In a physical gel or pseudogel, the links may be relatively weak van der Waals forces, hydrophobic or electrostatic interactions, or hydrogen bonds. Such gels are usually weak and can be destroyed (reversibly or not) by, for example, increasing the temperature or changing the solvent. Gelatin-based gels may serve as an example of such structures. It should be noted that many polymers in specific solution conditions tend to aggregate or self-assemble, thus forming physical microgels [6–13]. These phenomena will not be discussed here. Chemical gels or permanent gels are polymer networks where the links between the chains are covalent bonds. Therefore, such gels cannot be easily destroyed or dissolved, since their destruction would require the covalent bonds to be broken. The scope of this chapter is limited to the permanent gels only. A further differentiation comes from the fact that, besides homogeneous microgels composed either of a single polymer or a copolymer, there is a growing interest in nonhomogeneous, complex structures. These are either single microgel particles of a special architecture (e.g., core and shell of different composition and properties), or materials where microgels serve as building blocks for larger organized structures.

2. SYNTHESIS A multitude of techniques have been described for the synthesis of polymeric microgels. Most of them can be classified in two groups. The first one are techniques based on concomitant polymerization and cross-linking (where the substrates are monomers or their mixtures), called by some authors “cross-linking polymerization.” The second group is methods based on intramolecular cross-linking of macromolecules (where the starting material is not a monomer, but a polymer).

2.1. Formation of Microgels by Cross-Linking Polymerization Cross-linking polymerization is the most common process used to synthesize polymeric networks. The substrate is usually a mixture of monomers, where at least one of the components contains two or more polymerizable functions. For example, in the case of free-radical cross-linking polymerization of vinyl compounds this would require that at least one of the monomers contains two or more vinyl groups. When radicals are generated in such a system, in the process of propagation monomer molecules add one after another to the free radical at the end of the growing chain. Since some monomer molecules bear two or more active groups, upon incorporation in the (initially) linear chain, some of their functionalities are not yet used and form pendant active groups along the macromolecule. Therefore, the propagating chain may react not only with monomer molecules but also with these pendant groups. If the pendant group belongs to another chain, the two chains become linked together (intermolecular cross-linking). If the propagating radical reacts with a pendant group localized on its own chain, a closed loop is formed (intramolecular cross-linking). Intermolecular cross-linking leads to the formation of branched structures and finally to macroscopic gel filling the whole volume of the reaction vessel (“wall-to-wall” gel). In an ideal case,

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after all the functionalities reacted, the final product would be a single molecule occupying the whole reaction vessel, with all chain segments linked together (Fig. 2a). On the other hand, pure intramolecular cross-linking leads to separate, highly internally cross-linked single macromolecules, that is, nanometer-size microgels (Fig. 2b). In practice we can speak about prevailing inter- or intramolecular cross-linking. Most microgels synthesized by cross-linking polymerization are formed by combination of these two processes.

2.1.1. Cross-Linking Polymerization in Solution Most of the work on the microgel synthesis by cross-linking polymerization in solution has been done on the systems reacting according to the free-radical mechanism. An alternative is the synthesis by anionic cross-linking polymerization in solution (Section 2.1.1). Free-Radical Polymerization Free-radical cross-linking polymerization in solution, although less commonly used than emulsion-based techniques, is a subject of considerable interest, in the areas of both gelation theories and experimental studies. This seems to result from the fact that it is probably the simplest available method for the preparation of microgels, has a broad range of applications, is highly versatile, and does not require the use of surfactants. The latter, employed in most versions of emulsion polymerization techniques (see Section 2.1.2), may be present in the final product and may need to be removed in a separate purification step, thus making the synthetic procedure more complex. On the other hand, microgel formation by cross-linking polymerization in solution requires a very careful choice of reaction parameters. In general, it provides less precise control of the size of the products and yields microgels of broader size distribution than emulsion polymerization.

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Figure 2. Two modes of polymer cross-linking: (a) intermolecular, leading to the formation of macroscopic gel in the whole reaction volume (“wall-to-wall” gel), (b) intramolecular, leading to the formation of microgels.

In order to obtain microgels by cross-linking copolymerization in solution, it is necessary to control the competition between propagation of linear chains, intermolecular cross-linking, and intramolecular cross-linking in such a way that microgels of desired average molecular weight and/or dimensions are formed, but no macrogelation occurs. One of the decisive factors is the concentration of the monomer mixture. In a dilute solution where the growing chains are separated, the local concentration of the pendant reactive groups within the macromolecule is much higher than their average concentration in the whole reaction volume. Therefore, reaction of the growing chain end with a pendant group belonging to its own chain is much more probable than a reaction with a pendant group of another chain. Thus, intramolecular cross-linking is promoted and, at a certain stage, the system contains mainly microgels. On the contrary, in a concentrated solution where the coils of both growing and dead chains overlap, the probability of the chain end reacting with a “foreign” pendant group may be higher than with its own one. This promotes intermolecular crosslinking and, in consequence, macrogelation, often not being preceded by a distinct microgelation stage. For a detailed description of the phenomena related to microgel formation in free-radical cross-linking polymerization, consult the review paper by Funke et al. [16] and a recent theoretical study by Okay [21]. In the course of a cross-linking polymerization reaction proceeding through a microgelation stage, two factors tend to shift the competition between intra- and intermolecular cross-linking toward the latter process. The first factor is that the number (and concentration) of microgels increases, making the interparticle contact and reaction more probable. The second is that within a microgel particle the number of pendant groups available for further intramolecular crosslinking events decreases (probably not only due to their actual decay in the cross-linking reaction, but also due to the increasing sterical constraints within the internally crosslinked structure). Therefore, even in the relatively dilute systems where intramolecular cross-linking initially prevails, one can finally expect macrogel formation. A practical message from this reasoning is that in order to avoid macrogelation and obtain microgels one has to work in a dilute solution (typically below 5%) and stop the synthesis before the formed gel microparticles start to react with each other. The latter means that usually one has to terminate the process well before all of the monomer functionalities have reacted. The works of Graham and co-workers [22–27] indicates that the previously described general picture is not always fully applicable and that some of the formulated precautions may be too conservative. They postulate that for every system where a carefully selected solvent is used with respect to the solubility of the polymer being formed, a “critical gelation concentration” (CGC) can be found, below which no macrogelation takes place, even at a complete monomer conversion. What is surprising is that this CGC may be in some cases as high as 90% (w/w), although values in the order of 20% seem to be more typical. It has been also shown that by varying the concentration at which microgels are prepared, their molecular weight may be controlled. A reason that is evoked for the existence

848 of CGCs and for their relatively high values is that in a good solvent (or solvent combination) the dead chains, polymerizing chains, and polymer loops extending out of the microgel particle being formed act as very effective steric stabilizers that prevent contact between the microgel particles. As a result of that, particle–particle interactions and interparticle cross-linking are hampered, thus preventing macrogelation and shifting the competition to the side of intramolecular cross-linking. The previously described hypothesis has been tested for many polymerizing systems [e.g., for radical polymerization of 2-hydroxyethyl methacrylate with diethyleneglycol methacrylate and for step-growth polymerization of poly(ethylene glycol), 1,2,6-hexanetriol, and biscyclohexylmethane-4,4
-diisocyanate] in various solvent compositions. These observations emphasize the importance of solvent selection in the formation of microgels by this technique. In a recent work, the question of solvent composition and its influence on the microgel formation has been addressed by Frank et al. [28]. They have shown that in the cross-linking copolymerization of divinylbenzene and maleic anhydride at low monomer concentration in methyl ethyl ketone/heptane mixtures, products of three distinct morphologies are formed, depending primarily upon solvent composition. With increasing MEK contents the properties of the obtained cross-linked structures change from microspheres and microgels to macroscopic networks. Basic, extensive research on the mechanism of crosslinking polymerization in solution has been done by Funke and co-workers [16, 29, 30] and other groups [21, 31–35], mostly on 1,4-divinylbenzene and its copolymers. Recent studies on the applications of this method to synthesize microgels include copolymerization of various divinyl compounds with derivatives of maleic and fumaric acids [36], synthesis of functionalized microgels based on sulfoethylmethacrylate, able to bind metal ions (the bound ions can be subsequently reduced to form stabilized metal colloids) [37], and synthesis of reactive microgels as a semiproduct in the formation of oxazaborolidine-based dispersed catalysts for enantioselective reduction reactions [38]. Microgelation of the acrylamide/N ,N
-methylene bis(acrylamide) system has been extensively investigated by Kara and Pekcan, using special techniques enabling real-time observation of the reaction progress [39–41] and precise monitoring of the accompanying thermal effects [42]. An interesting example illustrating how versatile this method may be is the first report on the synthesis of fullerene-containing microgel structures [43]. Fullerenes, acting as multifunctional monomers, were co-polymerized, in a free-radical process in o-dichlorobenzene solutions, with 4-vinylbenzoic acid, as well as with 2- and 4-vinylpyridine to form submicrometer and micrometer size microgels of interesting rheological and electroactive properties. A promising modification of cross-linking polymerization in solution may be the use of chain transfer agents [32, 33, 44, 45]. The aim of this approach is to limit macrogelation processes and in this way to allow the synthesis of highly branched chains and possibly also microgels to be carried out to high degrees of monomer conversion and/or at relatively high monomer concentrations. Catalytic chain transfer agents such as cobalt porphyrin have been shown to perform

Polymeric Nano/Microgels

much better in preventing macrogelation than conventional chain transfer agents exemplified by mercaptans [44, 45]. Another new and elegant way to control the kinetics and yields of free-radical cross-linking polymerization reactions is the idea of radical living polymerization [46–52]. In this technique, a spin-trapping agent (iniferter) is present in the polymerizing system that can reversibly combine with the propagating radical at the end of a growing polymer chain. The free chains, spin trap, and their product—blocked, inactive (“dormant”) radical—are in equilibrium. As a result, at given polymerization conditions the momentary concentration of active propagating (or cross-linking) radicals can be kept at a much lower level than in the absence of the iniferter. This concentration may be controlled by varying the concentration of the spin trap. The use of living polymerization is rapidly expanding in polymer synthesis due to its many advantages (e.g., precisely controlled average molecular weight and molecular weight distribution) and has also found its way to the field of synthesis of microgels. Abrol and co-workers have demonstrated the application of nitroxides as trapping agents in the free-radical formation of t-butylstyrene-co-1,4-divinylbenzene microgels [53, 54]. It has been found that living free-radical polymerization helps one to avoid the macrogelation problem and provides much better control over the formation of microgels compared to classical free-radical polymerization. Both random (homogeneous) and core–shell type microgels (Fig. 3) can be formed using the living polymerization approach. Ward et al. described the use of this method for cross-linking polymerization of multifunctional glycol methacrylates, with p-xylene bis(N ,N
-diethyldithiocarbamate) as the iniferter [55], while Baek et al. carried out detailed studies on the application of metal-ion-mediated living radical polymerization to obtain star polymers with microgel cores [56–60]. A specific case of cross-linking polymerization in solution is precipitation polymerization, when the microgels being formed in solution have a tendency toward deswelling and precipitation. Since the further growth and cyclization reactions are limited mostly to the surface and the inner sphere of such a phase-separated particle (that can be considered a deswollen, collapsed microgel), the resulting products are often nearly monodisperse. It should be noted that use of the term precipitation polymerization is not limited to solution polymerization. There are also special cases of emulsion polymerization

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Figure 3. Microgels of various architectures: (a) random (homogeneous), (b) core–shell.

Polymeric Nano/Microgels

(especially the surfactant-free emulsion polymerization; see Section 2.1.2) which are frequently classified as precipitation polymerization [18]. It seems that precipitation polymerization is especially suitable for preparing thermosensitive microgels, since one can control the precipitation/deswelling tendency by changing temperature. For example, for a polymer that undergoes a phase transition leading to precipitation above a given temperature (lower critical solution temperature, LCST), it is possible to carry out precipitation polymerization above LCST and subsequently resolubilize the product by lowering the temperature. In a broad study on stimuli-sensitive microgels, a range of microgels composed of N -vinyl caprolactam and its copolymers with sodium acrylate has been synthesized in aqueous solution using this method [61–66]. Precipitation polymerization carried out at temperatures above the LCST is also frequently used to obtain thermoresponsive microgels of poly(N -isopropylacrylamide) and poly(N isopropylmethacrylamide) [18, 67–70]. Other examples of utilization for this technique include the synthesis of ionic microgels based on methacrylic and acrylic acids [71–75]. Fukutomi et al. employed a modification of this technique to obtain microgels of poly(vinyl alcohol) [76]. In this case one cannot start from the (nonexistent) monomer. Instead, vinyl acetate was co-polymerized in solution with N ,N
-dimethyl-N N
-divinyl-sebacoyldiamide to yield a precipitate of cross-linked poly(vinyl acetate) microparticles. Upon alkaline hydrolysis of this material, poly(vinyl alcohol) microgels were formed. Recent studies indicate that precipitation polymerization can be also used to fabricate a sort of imprinted microgel structure [77], per analogy to the molecularly imprinted nonswellable microspheres of broad biomedical use, synthesized by a similar technique [78]. Anionic Polymerization In ionic polymerization (cf. [79–82]), the active centers participating in chain growth are not radicals, but ions. In principle, three groups of monomers can be used: hydrocarbon (including vinyl), polar (acrylates, methacrylates), and cyclic (oxiranes, lactones, etc.). Although the ring-opening polymerization of the latter group of compounds is a promising technique used to obtain microspheres [83], so far only the first two groups have gained some attention in the field of microgel synthesis. The carbanions being the active centers in anionic polymerization usually do not undergo any spontaneous deactivation; thus such a process may be considered as a living polymerization. However, since carbanions react rapidly with any substances bearing reactive H atoms (water, carboxylic acids, alcohols), a prerequisite of performing a living polymerization is high purity of the reactant mixture. Since even traces of water from air can prevent the polymerization and in general the concentration of impurities should be kept at a submicromolar level, this requirement is one of the main difficulties encountered when using this technique. In the case of polar monomers, polymerization is often carried out at a low temperature (e.g., below −75  C) in order to eliminate side reactions. One of the main advantages of anionic polymerization is the possibility to obtain precisely defined

849 products of narrow molecular weight distribution. Moreover, block copolymers or other structures of special architecture can be synthesized in this way. In order to obtain a microgel by anionic polymerization, one has to use a bifunctional monomer compatible with this technique. Up to now, most studies in this area concentrated on ethylene glycol dimethacrylate and divinylbenzenes. Studies on the anionic polymerization of ethylene glycol dimethacrylate (EGDMA) revealed interesting features indicating two distinct phases of the process (cf. [16] and references cited therein). In the first phase, just after the formation of growing chains, no strong increase in molecular weight is observed, while the number of pendant unsaturated groups decreases. In this stage, intramolecular reactions (cyclization) dominate, with only minor participation of intermolecular cross-linking. It has been estimated that ca. 50% of the structural units participate in cyclization. The products are “living” microgels that possess the active centers and reactive vinyl groups. At a later stage, as the free volume decreases, these entities come into contact and undergo intermolecular cross-linking, evidenced by a steep increase in average molecular weight. Other works on this topic have indicated a strong influence of the solvent-initiator system on the reaction kinetics as well as on the general possibility of directing the reaction into the formation of microgels rather than insoluble macroscopic networks and avoiding excessive side reactions [84]. A toluene/n-BuLi system works apparently better than most of the other tested combinations. In contrast to the EGDMA system, in the polymerization of 1,4-divinylbenzene in THF cyclization reactions are not very effective, at least at low concentrations of an initiator (n-BuLi, lithium diisopropylamide) [16, 85]. Instead, in the first stage mostly linear polymer is formed, with the pendant vinyl groups left largely unreacted. However, upon an increase in n-BuLi concentration over 2%, a gradual increase in cyclization yield is observed resulting in the formation of microgels. At initiator concentration higher than 15%, insoluble macrogels are formed. The tendency to form microgels is a function of both initiator and monomer concentration. The shapes of these complex dependencies (cf. also [86–88]). and the locations of “microgel formation regions” can be satisfactorily predicted based on kinetic considerations [89]. The resulting microgels are in the size range of 3–30 nm. In comparison with 1,4-divinylbenzene, anionic polymerization of 1,3-divinylbenzene is much faster. Due to the much higher reactivity of pendant vinyl groups in the latter isomer, a strong tendency for cyclization is observed; thus the formation of microgels is favored over reactions leading to the linear chains and macroscopic gels [16]. Another polymer that has been used to obtain microgels by anionic polymerization is 1,4-diisopropenylbenzene [85, 90]. The technique of anionic polymerization, due to the formation of (theoretically) infinitely long-lasting active centers, is particularly useful in the synthesis of structured microgels, mostly based on block copolymers. A typical approach is a two-step procedure. In the first step, only one monomer (A) is polymerized until the monomer is used up but the active centers at the chain ends are still present.

850 Upon addition of a second monomer (B), the polymerization continues. In a simplest case, a block copolymer of the structure (A)n –(B)m is formed. If A is a bifunctional monomer and B is a monofunctional one, this approach leads to the formation of core–shell star-shaped structures, with a microgel as a core and linear chains bound as branches. This technique (called by some authors “corefirst”) has been applied to the systems where the microgel core is built of 1,4-divinylbenzene [91–95]. A reverse technique (“arm-first”), where the bifunctional monomer is added to the living linear chains, was used, for example, to synthesize products having EGDMA microgel core and poly(t-butyl acrylate) arms [96]. Much work has been done on the microgel formation in another block copolymer system, t-butylstyrene-divinylbenzene (TBS-DVB) [97–101]. Various versions of the synthetic procedure have been tested. In one of them the reaction was performed in n-heptane, which is a good solvent for polyTBS but not for polystyrene or the resulting copolymer. The reaction was started by the formation of reactive polyTBS chains. Upon addition of DVB the block copolymers formed precipitated from the solution and further reaction proceeded in this separated phase. This method yields star-shaped microgels with polyTBS arms having gel nuclei molar fractions up to 35%. One could regulate the properties of these entities by changing the length of the living polyTBS chains used in the synthesis. The choice of solvent is an important factor in these syntheses. By selecting THF as a solvent where no precipitation occurs, the molecular weight distribution can be better controlled. It may be expected that in the future anionic polymerization will be one of the methods used to construct even more complex microgels of well-defined architecture. For example, in a recent work a complex triblock poly(2-hydroxyethylmethacrylate-b-styrene-b-2-hydroxyethylmethacrylate) was synthesized forming a “flower-type” microgel (cf. [102– 106]) with closed-loop arms fixed to a polystyrene shell and an additional function enabling precise ultraviolet (UV)-induced cutting of these loops into two equal linear arms [107].

2.1.2. Emulsion Polymerization Macroemulsion In the synthesis of microgels by polymerization is solution, the most important difficulty is how to avoid macrogelation (i.e., how to confine the cross-linking reactions into small, separated spaces). This problem can be overcome by using emulsion polymerization, where each micelle may serve as a separate microreactor, protected from the contact with other micelles by the stabilizing action of a surfactant. Thus, in such a confined space the polymerization and cross-linking reactions can be carried out to a high degree of monomer conversion, resulting in a single microgel particle, with no or very limited macrogel formation. Because of this advantage, emulsion polymerization is a very popular method of microgel fabrication. In a classical free-radical emulsion polymerization [108, 109]. (Fig. 4), the system initially consists of monomer molecules that are dispersed in the liquid phase (usually water) in the form of micelles (of a size in the order of a few nm) and monomer droplets, typically of the size

Polymeric Nano/Microgels

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a) Figure 4. Classical emulsion polymerization, at moderate surfactant concentration: (a) monomer molecules in solution, (b) inactive monomer micelles, (c) active monomer micelles with growing polymer chain(s), (d) monomer droplet. Arrows denote the monomer transport toward the active micelles.

of 0.1–1 mm. Such a microheterogeneous system is stabilized by the presence of surfactants. The radicals are generated in the liquid phase. In some systems, initiation and first propagation steps also take place in the solution. As the growing chain has the tendency to become phaseseparated, surfactant-protected polymer–monomer particles are being formed, where the further polymerization and cross-linking steps take place. In other systems (especially when monomers are poorly soluble in water), initiation may occur within a monomer-filled micelle by an initiator radical that diffuses into the micelle from solution. During the chain growth, monomer molecules diffuse from the droplets and any inactive micelles to the active particles containing growing chain(s). After these outer monomer sources are used up and only the rest of the monomers inside the active particles react, these particles do not grow any longer (or may even contract due to internal cross-linking when multifunctional monomers are present) and, upon nearly complete consumption of the monomer, the process is finally terminated. Emulsion polymerization of monofunctional monomers leads to the formation of coagulated polymer particles (latexes), while in the presence of multifunctional monomers internally cross-linked particles—microgels—are formed, having the ability to swell in a good solvent. Extensive comparative studies on both processes, mostly employing styrene and divinylbenzene, revealed marked differences in the kinetics, mechanism, and product properties [16, 110–114]. Cross-linking polymerization leads to the formation of smaller particles, due to intraparticle cross-linking and to the hampered diffusion of monomer molecules into the structure. The number of polymer particles is higher than in the polymerization of monofunctional monomers, due to the fact that the particle growth rate decreases upon cross-linking, the monomer consumption from the inactive micelles is slower, and their lifetime is longer. Thus, in consequence, their chance of capturing a radical and being transformed into a new polymer particle is higher. A stronger tendency to form interparticle aggregates has

851

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been observed, which is attributed to the possibility of the reaction of pendant groups with the propagating radical of the neighboring particle. Moreover, a tendency of polymer formation in the monomer droplets is evidenced, in contrast to the emulsion polymerization of corresponding monofunctional monomers. It has been suggested that this is due to the fact that if some radicals enter a monofunctional monomer droplet, polymerization is rapidly terminated, but in the case of multifunctional monomer, the onset of gelation slows down termination processes, thus favoring further propagation and cross-linking events. Apart from these effects, size distribution of microgels is narrower than for non-cross-linked particles formed of chemically similar monomers. Recent research on emulsion polymerization in microgel synthesis includes broad studies by Matsumoto and co-workers on allyl methacrylate (an interesting monomer bearing two different vinyl groups), polymerized alone and in the presence of various comonomers (diallyl terephtalate, allyl benzoate, methyl methacrylate) and comparative studies on vinyl methacrylate [115–120]. The same group also investigated microgel formation by emulsion polymerization of lauryl methacrylate/trimethylolpropane trimethacrylate [121]. Ishii synthesized new emulsifiers which, when applied for emulsion co-polymerization of mono- and bifunctional monomers, yielded microgel emulsions of better shelf stability than in the case of a conventional emulsifier, sodium dodecylbenzene sulfonate [122]. Other examples are the synthesis of microgels based on methyl methacrylate/butanediol dimethacrylate [123], N -ethylacrylamide/N ,N
-methylenebisacrylamide [124], N vinylcaprolactam [125, 126], as well as ionic microgels made of methyl methacrylate, 2-ethoxyethyl methactylate, ethylene glycol methacrylate, and 2-(tetradecyldimethylammonio)ethyl methacrylate bromide [127]. The emulsion polymerization technique is also employed as one of the synthetic steps in preparation of complex microgel-based particles of core–shell morphology [128], and in fabrication of reactive microgels having filmforming abilities than can be applied in coating technology [129]. Last but not least, it is applied for the formation of polyorganosiloxane microgels by polycondensation of trimethoxymethylsilane in aqueous emulsions [130–132]. An insight into the kinetics, mechanism, molecular weight distributions, etc. in emulsion polymerization of multifunctional monomers may be gained by performing simulations based on the Monte Carlo method, as demonstrated by Tobita et al. [133, 134] and Jabbari [135]. It is expected that an approach based on a combination of theoretical considerations, simulations, and experimental data (cf. [134]) will lead to a more detailed understanding of this complex process. Microemulsion Some of the disadvantages of the classical emulsion polymerization can be avoided by using miniemulsion of microemulsion polymerization (cf. [16, 20, 136, 137], Fig. 5). In a monomer-containing emulsion, with increasing surfactant concentration the amount of monomer stored in the droplets decreases while more monomer molecules form micelles. When a critical value of emulsifier concentration is reached, no monomer droplets are left, with all the

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Figure 5. Polymerization in microemulsion, at high surfactant concentration: (a) monomer molecules in solution, (b) inactive monomer micelles, (c) active monomer micelles with growing polymer chain(s). No monomer droplets present (compare Fig. 4). Arrows denote the monomer transport toward the active micelles.

monomer being present in micelles (and to some extent in the solution). Such a transparent micellar solution is a starting point for the polymerization in microemulsion. In such a system the polymerization in monomer droplets is avoided. In the absence of this side effect known from the macroemulsion technique, nearly monodisperse microgels can be easily synthesized. Microemulsion polymerization has been used to prepare styrene/1,4-divinylbenzene and styrene/1,3-diisopropenylbenzene microgels of different sizes and swelling abilities [16, 138, 139]. The styrene-based structures formed by this method may be subsequently chemically modified to yield polyelectrolyte microgels of various, defined sizes [140, 141]. Inverse Emulsion Most of the emulsion polymerization syntheses are performed in systems where the continuous liquid phase is water or aqueous solution, and the monomers and polymers are of relatively hydrophobic character. Certainly, it is possible to reverse this situation and polymerize hydrophilic monomers in organic, hydrophobic liquid phase. The mechanism and kinetics of microgel formation in inverse emulsion polymerization have been extensively studied in the case of acrylamide [142]. The influences of the kind of solvent, kind and concentration of emulsifier, monomer content, agitation speed, etc. on the rate of process and product properties were described. Besides synthesizing neutral hydrophilic microgels (cf. [142, 143]), this technique is especially well suited for producing polyelectrolyte microgels of uniform size. For example, spherical gel microparticles of acrylic acid copolymerized with diethylene glycol diacrylate have been fabricated by inverse miniemulsion polymerization [20]. The products had various diameters, depending on the surfactant concentration, and a polydispersity lower than 10%. Neyret and Vincent co-polymerized in inverse microemulsion N ,N
-methylenebisacrylamide with equimolar or nonequimolar amounts of cationic [2-(methacryloyloxy)· ethyl]trimethylammonium chloride (MADQUAT) and anionic sodium 2-acrylamido-2-methylpropanesulfonate

852 and obtained polyampholyte microgels of very interesting swelling and flocculation properties [144]. Surfactant-Free Emulsion Polymerization Emulsion polymerization, besides its numerous advantages, has also some shortcomings, the most important being the presence of surfactants that usually have to be removed from the products in a separate step after the synthesis. A complete removal of a surfactant is not always possible, since its molecules may be in some cases incorporated (bound or trapped) into the products. This problem is especially pronounced in the case of microemulsion polymerization, where relatively large quantities of emulsifiers must be used in order to force all monomer to be present in micelles. A way out of this problem, although limited to some particular systems, is the use of a technique called by some authors “surfactant-free emulsion polymerization,” where the stabilization of emulsions is provided by the monomer and/or polymer itself. This can be realized in at least two ways: either the initiator of free-radical polymerization is an ion that, when incorporated into a growing oligomeric chain, causes this molecule to be surface active, or the substrates for polymerization are unsaturated (i.e., polymerizable) oligomers bearing ionized groups at one or two ends. The application of the former technique in microgel synthesis evolved from a method used for preparation of nonswellable polystyrene latex particles [145, 146]. It has been first used by Pelton and Chibante to synthesize temperature-sensitive poly(N -isopropylacrylamide) microgels [147], and since then it has become the method of choice in the fabrication of these materials [148–152]. Other examples of microgels that can be obtained in this way are systems based on poly(methyl methacrylate) and polystyrene, alone or as a copolymer with methacrylic acid [153–155]. The synthesis is usually carried out with a persulfate salt as the initiator that decomposes into SO•4 radicals capable of adding to monomer molecules and initiating chain growth [17]. The growing oligomer radicals bear the anionic sulfate group at the dead end and behave like a surfactant. The presence of surface-active compounds combined with the tendency of polymer chains to undergo a phase separation (cross-linking polymerization of N -isopropylacrylamide—NIPAM—is carried out above the LCST) results in formation of stabilized particles containing the growing and cross-linking chains. Internally cross-linked particles obtained in this way form swollen microgels when transferred into a good solvent. In the case of poly(NIPAM), the switching from a poor to a good solvent is accomplished simply by lowering the temperature below the LCST. It is possible to synthesize NIPAM-based copolymer microgels by incorporating water-soluble vinyl monomers like acrylamide [147], acrylic acid [152, 156], N -acryloylglycine [157], or 2-aminoethylmethacrylate hydrochloride [158]. The version of surfactant-free emulsion polymerization where self-emulsifying reactive oligomers are used has been extensively studied by Funke and co-workers on unsaturated polyesters [16, 159–161]. In this approach, unsaturated polyester molecules having carboxylic groups at both chain ends were used. For effective self-emulsification, as well as for emulsification of their mixture with comonomers,

Polymeric Nano/Microgels

the average molecular weight of these compounds must lie within a certain range. Molecules of too low molecular weight do not provide the desired solubilization efficiency, while for too long chains their own solubility is insufficient. For example, in one of the systems studied where the polyesters were made of maleic anhydride and 1,6-hexanediol, the optimum molecular weight of these oligomers for solubilization of styrene as a hydrophobic comonomer was in the range 1.7–2.2 kDa [162]. Also the way the initial emulsion is prepared and the relation between the comonomer ratios and overall monomer concentration must be carefully chosen to avoid formation of insoluble fraction (macrogels or agglomerates) [163]. A common property of the products synthesized by surfactant-free emulsion polymerization is the presence of ionic groups at the microgel surface which may be used for further modification, binding various compounds, or interparticle cross-linking.

2.1.3. Cross-Linking Polymerization in the Bulk Bulk polymerization is a possible but, in general, not particularly suitable way of synthesizing microgels, primarily since the polymerizing and cross-linking system tends to form a macroscopic “wall-to-wall” gel in the whole reaction volume. According to classical gelation theory [164] involving homogeneous growth of linear chain and their subsequent linking together, formation of distinct microgels as intermediate stages should not take place. However, in reality most polymerizing and cross-linking systems are not strictly homogeneous (see a brief general discussion in [16] and references cited therein). If, before macrogelation is reached, the polymerization and cross-linking processes proceed through stages characterized by inhomogeneous density of cross-links [i.e., when microgels are formed that are not (yet) linked together], there is some chance that they could be potentially separated. A recent example showing that this may in fact be possible is a study on free-radical bulk polymerization of tetraethoxylated bisphenol A dimethacrylate with styrene or divinylbenzene, where various populations of microgels at different stages of cross-linking (single microgels in the range of 10–40 nm and large microgel clusters) have been observed by atomic force microscopy and by dynamic light scattering upon dissolution of the samples [165, 166]. Also recent works on simulation of polymerization in systems containing multifunctional monomers clearly indicate the onset of structural heterogeneity and the formation of microgels ([167], cf. also [168, 169]).

2.1.4. Polymerization with Nonclassical Initiation Most of the work on the synthesis of microgels by combined polymerization and cross-linking has been done using classical initiation methods, that is, with chemical initiators, which, when activated (mostly thermally decomposed) give rise to reactive intermediates capable of initiating the chain reactions of polymerization and cross-linking. This

Polymeric Nano/Microgels

approach, although most commonly used, has some disadvantages. First of all, the initiator or its fragments usually remain in the products, either chemically bound or entrapped within the polymer structure. This may pose serious problems in these applications where purity of the material is of high importance (biomaterials, optics, electronics). Second, in some cases the heating of the reaction mixture in order to activate the initiator may be undesirable. Therefore there is a need for alternative techniques where initiation is provided by other means. One may envisage that the techniques already used in polymerization processes, like photopolymerization, radiation polymerization, initiation by the action of ultrasound, or microwaves, will find an application in the microgel synthesis as well. Photopolymerization and photocuring are very intensely studied synthetic techniques, widely used in industry (cf. reviews [170, 171]). Photoinduced polymerization combined with cross-linking can be used to produce macroscopic polymer gels based, for example, on 2-hydroxyethyl methacrylate [172], N -isopropylacrylamide [173], acrylic acid, and N -vinylpyrrolidone [174]. Microgels of poly(ethylene glycol) (PEG) were synthesized by photopolymerization, using a method where a mixture of poly(ethylene glycol) dimethacrylates and a photoinitiator was sprayed over a double-layer liquid bath [175]. The droplets formed in the upper layer were subsequently UV-illuminated in the lower layer to form internally cross-linked PEG spheres, able to swell in water. Kazakov et al. proposed an interesting procedure where monomers (acrylamide, N -isopropylacrylamide, vinylimidazole) were encapsulated into liposomes and subsequently polymerized and cross-linked with UV light, resulting in microgels of 30–300 nm size [176]. Radiation-induced polymerization is a well-established, versatile synthetic technique used in polymer science and, to a limited extent, also in technology (for reviews see [177–184]). Polymerization initiated by ionizing radiation (typically gamma rays from isotope sources or fast electrons generated by accelerators) is quite similar to the classical one and can be performed in the bulk, in solution, in emulsion, etc., the main difference being only the initiation step. Ionizing radiation can interact with monomers and polymers by direct or indirect effects. In the former, the energy is absorbed by a monomer molecule, which may result in a radical formation, in the latter the energy is absorbed by the solvent, and reactive products (mostly radicals) resulting from this event may in turn attack monomer to initiate polymerization. In the case of aqueous systems, the species initiating the polymerization is most often the hydroxyl radical. Ionizing radiation can be used in the synthesis of polymer gels (both macro- and microscopic) in two general ways: either by inducing cross-linking polymerization of monomers or by inducing cross-linking of polymer chains in the absence of monomers. The latter technique has some important advantages and will be discussed separately in Section 2.2.2. It is important to know that at late stages of radiationinduced polymerization, when only low quantities of free monomer molecules are left in the system and the radicals are still generated randomly along the chains, intermolecular recombination (cross-linking) reactions may occur with a

853 considerable yield even in the total absence of any bifunctional monomer. Therefore, by using ionizing radiation it is possible to obtain a polymer gel starting from monofunctional monomers. More often than not, multifunctional monomers are used anyway, usually in order to increase the yield of cross-linking and thus reduce the radiation dose necessary to produce a gel of a given cross-link density. Applications of this technique range from the formation of gels based on relatively simple compounds (e.g., acrylamide [185] or N -vinylpyrrolidone [4]) to complex stimuli-sensitive “smart” gels targeted for advanced biomedical purposes (cf. [186–190]). Examples of radiation-induced polymerization employed to synthesize internally cross-linked polymer microparticles are the works of Yoshida et al. [191–195] and Naka et al. [196–198]. Various monomer mixtures, mainly containing diethylene glycol dimethacrylate, were irradiated without any auxiliary substances in organic solution to yield products that were suitable for derivatization or immobilization of biomolecules and intended for biomedical applications. Microgels can be also synthesized by radiation-induced cross-linking polymerization in emulsion, as shown on the example of styrene-based gels co-polymerized with cationic polymerizable surfactants [199]. Ultrasound is often used in microgel or microparticle synthesis, albeit not for initiating chemical reactions, but rather as a tool for solubilization, agitation, homogenization, formation of miniemulsions, etc. It seems that the well-known fact that the same ultrasound can induce polymerization (for reviews see [200–202]) has so far largely escaped the attention of researchers working on microgel synthesis. Ultrasound waves propagating in a liquid may cause the formation of free radicals. This effect is due to the phenomenon of cavitation (i.e., the formation of small gas bubbles). Their collapse and their compression phase of oscillations in the ultrasonic wave are adiabatic processes leading to local, transient increase in temperature inside the bubble, lasting for fractions of microseconds. Since these temperatures may reach over 3000 K, [203, 204] a fraction of solvent molecules (or other molecules) present in the gas phase is decomposed into free radicals. These can diffuse to the liquid phase and initiate chemical reactions, including polymerization. In the case of aqueous systems, hydroxyl radicals are generated [205–212] that are very efficient initiators of polymerization. The efficiency of cavitation and of radical formation in particular depends on frequency and intensity of ultrasound, as well as on temperature, external pressure, presence of gases in the liquid, etc. [200, 212]. Ultrasound-induced polymerization in solution has been mostly performed on vinyl polymers, namely styrene [213–221], methyl methacrylate [213, 215–219, 222, 223], n-butyl methacrylate [221], vinyl acetate [213], vinyl chloride [224], acrylamide [222, 223], N -vinylcarbazole [225], and N -vinylpyrrolidone [200]. Moreover, the suitability of this technique for synthesis of some special polymers has been demonstrated (e.g., polysilylenes [226] and phtalocyanine polymers [227]). Although no extensive studies on the possibility of microgel synthesis by ultrasound have been made so far, there are first reports on the possibility of ultrasound-induced

854 polymerization of bifunctional monomers [228] and fabrication of microspheres [229]. It has been shown that concomitant action of ionizing radiation and ultrasound may lead to interesting results in the synthesis of internally cross-linked microspheres (that would become microgels in a suitable solvent). In a radiation-induced cross-linking polymerization of diethyleneglycol dimethacrylate in ethyl acetate, the action of ultrasound affects the size and shape of formed particles, probably by promoting interparticle interactions [230].

2.2. Intramolecular Cross-Linking of Polymer Chains: Monomer-Free Techniques In most research work and applications, microgels are synthesized using procedures based on polymerization processes starting from monomers as the basic substrates. This is, however, not the only possible way. An alternative approach to the synthesis of microgels, in particular the nanogels of small size (typically <0 1 m), is intramolecular cross-linking of individual macromolecules. An obvious and important advantage of this method is the absence of monomer. This is of great value when the product is intended for biomedical use, where even small quantities of residual monomer may be potentially harmful and thus unacceptable. Furthermore, intramolecular crosslinking may provide means to obtain cross-linked structures of various molecular weight and size, including very small structures, depending on the molecular weight of the parent polymer. Such nanogels obtained from single macromolecules are interesting physical forms of polymers as they are a sort of “frozen” polymer coil of limited segmental mobility. One can also expect that a combination of intraand intermolecular cross-linking (cf. [231–233]) will provide a tool for synthesizing nanogels and microgels of independently chosen molecular weight and dimensions (various internal densities). Last but not least, intramolecular cross-linking of individual macromolecules is an interesting reaction. A number of questions regarding this process, particularly its kinetics, have not been answered yet (cf. [234–236]). Given the commercial availability of a multitude of polymers, including food- and medical-grade products, starting from a polymer rather than from a monomer can be a reasonable synthetic option. Moreover, in some cases, where monomers do not exist [like poly(vinyl alcohol)] or polymerization is either impossible or very difficult (carbohydrates), intramolecular cross-linking of polymers may be the best way to produce microgels.

2.2.1. Chemical Intramolecular Cross-Linking Intramolecular cross-linking, similar to polymerization, can be performed either as a thermally initiated chemical reaction or as a photo- or radiation-induced process. Chemical intramolecular cross-linking of individual polymer chains can be achieved in at least two ways. One is to prepare linear or branched polymer with pendant reactive (e.g., vinyl) groups and initialize the cross-linking by a suitable initiator. Batzilla and Funke synthesized linear poly(4-vinyl styrene) and subsequently carried out a cross-linking of this polymer

Polymeric Nano/Microgels

in dilute solution using 2,2
-azobis(isobutyronitrile) (AIBN) as an initiator [237]. Reaction conditions and time could be chosen where intramolecular cross-linking prevailed. In a similar way, microgels can be made of preformed polymers by photo-cross-linking [238]. Another way does not require any special substrate preparation (no polymerizable pendant groups needed). It has been shown that intramolecular cross-linking of single chains of water-soluble polymers can be carried out by reacting them with a suitable cross-linking agent in dilute solutions. The cross-linker must be capable of reacting with the functional groups ( OH, COOH, etc.) of the polymer and should be at least bifunctional. Synthesis is carried out in solution. Polymer concentration must be chosen sufficiently low to avoid intermolecular cross-linking (i.e., it must be significantly lower than the coil overlap concentration). By varying the concentration of the cross-linker one can influence the internal cross-link density. Burchard et al. used this approach to synthesize internally crosslinked single macromolecules (nanogels) of poly(vinyl alcohol) with glutaraldehyde as the crosslinker [239] and of poly(allylamine) cross-linked with 1,4-dimethoxybutane-1,4diimine dihydrochloride [240] (cf. also [241, 242]). A similar approach is used to obtain microgels of polysaccharides. For example, hydroxypropylcellulose microgels can be produced by (presumably mostly intramolecular) cross-linking of linear chains with divinylsulfone [243]. Analogous processes are utilized for the synthesis of commercially produced preparations of internally cross-linked hyaluronic acid [244–247].

2.2.2. Radiation-Induced Cross-Linking Synthesis of nano/microgels by intramolecular cross-linking of individual polymer chains can be also initiated by ionizing radiation. The main advantage of this method is that it can be carried out in a pure polymer/solvent system, free of any monomers, initiators, cross-linkers, or any other additives. Therefore it seems to be especially well suited for the synthesis of high-purity products for biomedical use. In this approach, to be discussed in more detail, pure aqueous solution of a polymer is subjected to a short (a few microseconds), intense pulse of ionizing radiation. In this way, many radicals are generated simultaneously along each polymer chain, and their intramolecular recombination leads to the formation of nanogels. This approach has been first tested on neutral water-soluble polymers—poly(vinyl alcohol) [235], polyvinylpyrrolidone [248], and poly(vinyl methyl ether) [249, 250]—and later expanded to poly(acrylic acid) as an exemplary polyelectrolyte [236, 251]. The main parameter influencing the competition between inter- and intramolecular recombination of polymer radicals in dilute solutions is the average number of radicals present at each macromolecule at the same time [234]. If this number, under the given synthesis conditions, is much lower than 1, there is only a meager chance that a radical will find a reaction partner within the same chain. In such cases, recombination is only possible between radicals localized on two separate macromolecules. On the other hand, when there are tens of radicals present along each chain, the probability of intramolecular encounters and reactions

855

Polymeric Nano/Microgels

is higher than that of intermolecular ones. The latter processes are relatively slow, since they require that two large entities—polymer coils—diffuse toward each other. In the case of the radiation-induced radical formation, these two opposite conditions (i.e., a very low or very high number of radicals per chain) can be fulfilled by means of a proper choice of irradiation conditions. Continuous irradiation at a relatively low dose rate, such as typical irradiation with gamma rays from isotope sources, leads to a steady-state concentration of polymer radicals in the order of 10−7 M. When the concentration of polymer coils is significantly higher than this value (this condition is usually easily fulfilled), the average number of radicals per chain is much lower than unity and intermolecular cross-linking is observed (Fig. 6a). In order to promote intramolecular cross-linking, short, intense pulses of radiation can be employed, such as pulses of fast electrons from an accelerator, generating radical concentrations in the order of 10−4 – 10−3 M. If the concentration of polymer coils is low (that is to say, 10−6 –10−4 M), many radicals are generated on each macromolecule (typically many tens or even over a hundred), and the conditions for intramolecular recombination are fulfilled (Fig. 6b). Certainly, this does not mean that intermolecular reactions are totally eliminated in such a case. Some coils may come into contact before all the radicals decay, and if there is an uneven number of radicals on a chain, at least one of them must finally find a reaction partner at a neighboring macromolecule. The data on changes in molecular weight, viscosity, and radius of gyration following the pulse irradiation of dilute polymer solutions clearly indicate that strongly internally cross-linked nanogels are formed which, in comparison with the starting macromolecules, have somewhat higher molecular weight but at the same time significantly lower

(a)

(b)

Figure 6. Gel formation by recombinaton of polymer-derived radicals under two different experimental conditions: (a) intermolecular crosslinking leading to macrogelation (high polymer concentration, low steady-state concentration of radicals ⇒ momentary average number of radicals per chain is lower than one), (b) intramolecular cross-linking leading to microgelation (low polymer concentration, pulse-generated high concentration of radicals ⇒ momentary average number of radicals per chain is much higher than one).

dimensions [235, 236, 248, 251]. While the main reason for the increase in molecular weight is the intermolecular crosslinking occurring in the system with very low yields in parallel to intramolecular recombination, the latter process is the dominant reason for the reduction in coil dimensions. A balance between inter- and intramolecular recombination of polymer radicals may be also maintained when continuous irradiation is used. Therefore it is possible to synthesize microgels by cross-linking in a solution using isotope sources, as has been experimentally demonstrated and supported by simulations for poly(vinyl alcohol) by Wang et al. [231–233, 252].

2.3. Disruption of Macroscopic Networks The idea of obtaining microscopic gel particles by disrupting continuous “wall-to-wall” gels seems to be conceptually the simplest of all synthetic approaches, since the procedures used to obtain macroscopic networks are usually simple, with less parameters to be controlled than in the previously described typical microgel synthesis methods—there is no need to control the micelle size or to observe the precautions necessary to avoid macrogelation. A disadvantage of this “nonelegant” method is that the size distribution is very broad (however, for microgels in the scale of many micrometers it can be reduced, e.g., by using mechanical sieves); one cannot usually expect to produce extremely small gel particles in this way nor obtain products of a regular, spherical shape. On the other hand, the disruption method may be of some advantage for synthesizing microgel fractions of various diameters but precisely the same crosslink density (since they are derived from one specimen of a macroscopic gel) [253]. In the authors’ laboratory this method is used routinely to fabricate large amounts of coarse polyvinylpyrrolidone microgel of dimensions below 50 m, following radiation-induced synthesis of macroscopic gel in the bulk [254]. A gel disruption process has been reported to yield cross-linked polysaccharide microgels (of dimensions in the 100 nm range), as one of the steps to construct polysaccharide/phospholipide biovectors for drug delivery [255]. It should be noted that important final steps of almost all synthetic procedures used to obtain microgels are purification and drying. It has been clearly demonstrated that drying methods and conditions may have significant influence on the final structure and properties of microgels [143, 256].

3. METHODS USED FOR STUDYING MICROGELS Since most microgels of micrometer and submicrometer size are soluble in a suitable solvent, their properties can be conveniently studied by the methods developed for macromolecules in solution. These include various versions of viscometry, static and dynamic light scattering, gel permeation chromatography (GPC), and, to a lesser extent, ultracentrifugation and osmometry. Relative changes in hydrodynamic dimensions of microgels, related to their average molecular weight, structure, and size, can be followed by viscosity measurements in dilute

856 solutions, using simple equipment such as an Ubbelohde viscometer. Determination of an intrinsic viscosity of a polymer solution [ ] is one of the simplest and most common ways to determine average molecular weight of a polymer (precisely the viscosity-average molecular weight, M ). Correlation between these values is known as the Mark–Houwink equation,   = K M , where K and  are parameters which are constant for a given linear polymer/solvent pair at a defined temperature and can be found in handbooks (e.g., [257]). Since a microgel particle is more compact than a coil of the same linear polymer, viscosity of microgel solution is lower than that of linear macromolecules of equal concentration. As a result of that, viscosity measurements do not yield proper values of molecular weight for microgels, at least when K and  values for linear chains are used. Although the Mark–Houwink parameters for some microgels have been determined, their practical use is very limited, since the real dependence of viscosity on molecular weight is influenced by cross-link density, way of synthesis etc., and therefore it may vary from one to another preparation. Anyway, it is worth noting that the values of the exponent  for microgels are low (e.g., 0.16 for microgels of poly(vinyl alcohol) [241], 0.25 for microgels based on polystyrene copolymers [258], 0.09 to 0.24 for divinylbenzene-based gels [16, 29]) when compared with values typical for random coils of flexible macromolecules in good solvents (0.5–0.8), indicating only a weak dependence of viscosity on molecular weight. Despite the previously described disadvantage, viscosity is a very convenient tool to measure volume changes in gel structures of constant average molecular weight. Examples of such experiments are synthesis of microgels from linear chains by intramolecular cross-linking [231, 232, 235] or changes in volume of stimuli-sensitive microgels [251]. Senff and Richtering provided large data collections and detailed discussions on the temperature influence on viscosity and rheology of thermosensitive poly(N -isopropylacrylamide) microgels [259, 260]. An extensive study of viscosity-related phenomena in polyelectrolyte microgels has been presented by Antonietti et al. [140, 141, 261]. Viscosity measurements are also frequently used to follow the changes in size of microgels during the synthesis based on polymerization and cross-linking, often in parallel with other methods, indicating the changes in molecular weight. Such studies have been done, for example, on polyacrylamide [142], poly(allyl methacrylate) [115–118], poly(t-butyl acrylate) [96], polyurethanes [22], and poly(vinyl methacrylate) [117]. Relatively low viscosity of microgel solutions is one of the factors that makes them suitable for high-solid-content organic coatings (see Section 4.2), since to maintain the desired viscosity level of a solution or dispersion, higher concentration of microgels than linear chains can be used. Another viscosity-related field of studies is rheology of microgel solutions or dispersions. The information gained is related to the compactness and stiffness of gel particles and their susceptibility to flow-induced deformation, solvent permeability, as well as the interactions between the particles. For some compact microgels (e.g., based on polyester copolymers), almost Newtonian flow has been observed

Polymeric Nano/Microgels

even at concentrations of 40 wt% indicating low interparticle interaction and low deformation [16]. Solutions of other microgel structures show pronounced deviations from the Newtonian behavior, mostly behaving as pseudoplastic, shear-thinning fluids. Discussions on structure–rheology relationships of microgel solutions can be found in [19, 259, 260]. Rheological tests provide valuable information on the behavior of the outer layer of complex microgels—for example, in a core–hair structure one can follow the tendency of the hairs to become extended in a given solvent [262]. Data on the solutions of thermoresponsive microgels of poly(N -isopropylacrylamide) show pronounced temperature dependence of rheological properties, reflecting the change in volume fraction occupied by gel microparticles [18, 263]. Changes in rheological properties of the reaction mixture can yield important information on the mechanism and kinetics of cross-linking polymerization [264]. Precise knowledge on how the composition and synthetic procedures influence the rheological properties of final products is essential for fabrication of microgels used in coating technology [265–268]. Static light scattering, based on the laws discovered and methods developed by Rayleigh, Debye, and Zimm, is an absolute method of determination of weight-average molecular weight of any dissolved macromolecular structure [269–271]. In this method, a laser light beam is passed through a solution of macromolecules or microgels, and the (time-averaged) intensity of scattered light is measured, in relation to the intensity of the incident beam, at various angles, for a few different concentrations of the polymer in the sample. Double extrapolation of these data to zero angle and zero concentration yields the weight-average molecular weight. One has to stress that this method is of particular value for studying microgels, since many other methods (viscometry, simple gel permeation chromatography without light-scattering detection) that require calibration can yield incorrect molecular weights of microgels when calibrated on linear polymer samples. Certainly, static light scattering also has some limitations. The most important in the present context is the upper limit of molecular weight that can be determined. Molecules or gel particles of Mw higher than a few million Da cannot be usually analyzed by this method. Another limitation is uncertainty whether one of the calculation parameters, the refractive index increment (dn/dc), undergoes significant changes when linear chains of a given polymer are transformed into microgels. This problem, often neglected, still requires detailed studies. Examples of static light-scattering measurements on microgels are listed, arranged according to the main chemical component of the microgel: hyaluronic acid [247], organosilicon compounds [131], poly(acrylic acid) [236, 251], poly(allyl aniline) [240], poly(allyl methacrylate) [115–118, 120, 272], polyethylene [273], poly(lauryl methacrylate) [121], poly(methacrylic acid) [274], poly(methyl methacrylate) [275], poly(N -isopropylacrylamide) [151, 259, 260, 274], poly(N -vinylcaprolactam) [61, 63, 64, 66, 125], poly(vinyl alcohol) [231, 235, 239], poly(vinyl methacrylate) [117], and polyvinylpyrrolidone [248]. Static light scattering is not only a tool for determining the weight-average molecular weight, but it yields as well

Polymeric Nano/Microgels

the radius of gyration (a very important parameter in analysis of microgels) and a second virial coefficient, which is useful for investigating polymer-solvent vs. polymer–polymer interactions. For example, a negative value of this parameter indicates a tendency of polymer chains or microgels to undergo reversible aggregation in a given solvent. Static light-scattering techniques can be also used for real-time study of the kinetics of microgel formation. This can be done by coupling a pulse-radiolysis setup with a laser light scattering photometer [276–279]. Cross-linking and/or polymerization reactions are initiated by short (a few microseconds) pulse of ionizing radiation in the form of fast electrons generated by an electron accelerator, and subsequent changes in the intensity of scattered light caused by changes in molecular weight and/or radius of gyration can be recorded, analyzed, and, when possible, recalculated into rate constants of participating reactions. It is worth mentioning that in-situ, real-time monitoring of microgel formation is possible not only by following the increase in the intensity of scattered light but also by measuring the decrease in intensity of transmitted light. A description of the latter technique and its applications can be found in the works of Kara and Pekcan [39–41]. Dynamic light scattering, based on temporal correlations of the intensity of light scattered by the solution of macromolecular structures, does not yield (at least directly) molecular weights, but average values and distributions of diffusion coefficients [271, 280–282]. These data can be recalculated into average values and distributions of hydrodynamic diameters—again very important characteristics of a microgel sample. The range of sizes that can be measured is more shifted toward large structures when compared with static light scattering (up to a few micrometer). It should be stressed that the average radius of gyration determined by static light scattering and the average hydrodynamic radius calculated from dynamic light scattering are different physical parameters. The relation between these two values is a valuable indicator of microgel structure [239, 240]. Examples of application of dynamic light scattering for analyzing microgels can be found in the following papers, arranged according to the main component of the studied material: dextran [283], hydroxypropyl cellulose [243], organosilicon gels [132, 284], polyacrylamide and its derivatives [71, 285], poly(acrylic acid) [285, 286], poly(allyl aniline) [240], poly(n-butyl acrylate) [129], polyesters [287], poly(ethylene oxide) [8], poly(2-hydroxyethyl methacrylate-b-styrene-b-2hydroxyethyl methacrylate) [107], poly(methacrylic acid) [274], MADQUAT [144], poly(methyl methacrylate) [123], poly(methyl vinyl ether) [250], poly(N -isopropylacrylamide) [70, 148–150, 152, 259, 260, 274, 288–295], poly(N -vinylcaprolactam) [126], poly(sodium 2-acrylamido-2-methylpropanesulfonate) [144], polystyrene [129, 139, 296], poly· (tetramethoxylated bisphenol A dimethacrylate) [165], poly(vinyl alcohol [231, 232, 239]), poly(2-vinylpyridine) [297, 298], and poly(2-vinyl pyridine-b-styrene-b-2-vinyl pyridine) [299]. Surface charge effects like electrophoretic mobility can be investigated by measuring the zeta potential or by phase analysis light scattering [67, 144, 300]. A method that in some aspects resembles light scattering is the small angle neutron scattering technique (SANS).

857 Neutrons of suitable energy range are scattered by atomic nuclei of a polymeric sample and give rise to scattering patterns similar to those obtained by light scattering. An advantage of neutron scattering is that it can be used not only in solutions but also in gels and even in the condensed phase, when appropriate samples are used (e.g., containing a small fraction of deuterated macromolecules). This method allowed detection of structural differences in gels of the same chemical composition but synthesized by different methods (chemical and -ray initiation) [301]. Temperaturedependent SANS measurements were applied to follow the structural changes in thermoresponsive microgels of poly(N isopropylacrylamide) [17, 148, 294, 302]. Microgels can be also studied by GPC (size exclusion chromatography; for general reviews see [303, 304]). This method is routinely used in research and industry to determine molecular weight distribution and number-, weight-, and z-average molecular weights of polymers. In its basic form, GPC is not an absolute method and requires calibration on monodisperse polymer standards of precisely known molecular weight. Since macromolecules or microgels are segregated on GPC columns according to their size (hydrodynamic volume) and not molecular weight, the same precautions as mentioned previously for viscometry must be applied if the method is to be used for determination of molecular weight distribution of microgels, since linear polymer standards and microgels of the same molecular weight may have very different dimensions and retention volumes. This problem can be partially overcome in modern GPC setups equipped with a light-scattering detector that in principle allows for absolute determination of molecular weight of each polymer or microgel fraction. Still, the use of rightangle laser light scattering detectors with microgel samples may lead to some systematic errors, since the algorithms for recalculation of the data to zero angle are based on the theory of polymer coils, not microgels. On the other hand, detection at a low angle (multiangle or low-angle detectors) usually gives noisy signals, especially in aqueous solutions. Nevertheless, the method is fast, efficient and for sure a valuable tool in microgel analysis. Exemplary applications described in the literature refer to microgels based on poly(allyl methacrylate) [272], poly(divinylbenzene) [305], poly(methyl methacrylate) [23], polystyrene and poly(t-butyl styrene) [101, 306], polystyrene and poly(butyl acrylate) copolymers [307], poly(t-butyl styrene-co-divinylbenzene) [100], and poly(vinyl alcohol) [232]. Gel permeation chromatography may be also applied for preparative purposes. Microgels based on polyethyleneimine and poly(ethylene glycol) have been successfully fractionated by this technique [308]. Chemical composition of microgels can be determined by any regular analytical technique used for polymers. UVvisible or infrared (IR) spectroscopy is often used to control the conversion of monomers during the microgel synthesis. The use of IR spectroscopy may be facilitated by the fact that many microgels form films that may be analyzed directly. Concentration and structures of polymer-derived free radicals during polymerization or cross-linking can be followed, in some cases, by electron paramagnetic resonance [165, 309–312]. Classical methods of instrumental analysis

858 as conductometric and potentiometric titration may be very useful for analyzing ion-bearing microgels [155, 297]. Nuclear magnetic resonance (NMR) spectroscopy can be used for determination of the chemical structure of microgels [68, 120, 125, 313–316]. Another application of a related technique in the field of polymer systems is the pulsedNMR measurements of spin–spin nuclear magnetic relaxation times. They yield valuable data regarding polymer structure, crosslink, and/or entanglement density in solid polymers [317–319], polymer melts [320], and gels [321–323]. Visualization of microgels can be realized by various microscopic techniques. While the use of optical microscopy is limited to the relatively large structures, most studies are based on transmission electron microscopy (TEM, selected reviews: [324, 325]), scanning electron microscopy (SEM, [325, 326]) and atomic force microscopy (AFM, [327–330]). Transmission electron microscopy is widely used to study microgels. Usually it does not require any complicated sample preparation. In a typical procedure, a drop of dilute microgel dispersion or solution is cast on a suitable support (often a carbon-film-coated copper mesh) and dried at room temperature (RT). This method is well suited for observation of single gel particles or their monolayers. Exemplary applications of TEM in studying microgels are visualization of simple gel particles based on polyacrylamide [142], poly(2-acrylamiddo-2-methylpropanesulfonate-co-(2(methacryloyloxy)ethyl)trimethylammonium chloride) [144], poly(divinylbenzene-co-maleic anhydride) [28], poly(N ethylacrylamide) [124], poly(N -isopropylacrylamide) [149, 150, 156], polyurethanes [22], as well as more complex structures and effects, like core–shell microgels [69, 128, 262, 293], loading of nanogels with drugs [308], binding metal or metal oxide particles [37, 331, 332], and template influence on the synthesis of polysilane microgels [333]. More complex microgel structures, multilayers, foils, and surface features of microgel-based materials can be visualized by scanning electron microscopy. SEM enables the analysis of thick, nontransparent (in the sense of TEM) samples and due to its large focus depth gives sharp pictures of structurally complex materials. Sample preparation is not as simple as in TEM, since here coating the sample with a thin layer of a conducting material is necessary. Examples of applications of SEM in the field of microgels include a study on the influence of monomer ratio on the properties of poly(N -isopropylacrylamide-co-acrylic acid) gel particles [152], thermally induced structural changes in poly(N isopropylacrylamide) microgels [294], various morphologies of poly(divinylbenzene-co-maleic anhydride) [28] and poly(divinylbenzene-co-4-methylstyrene) gels [305], binding microgels onto TiO2 pigments [122] and aluminum surfaces [332], structures of microgel-based ion-exchange resins [334], structure of polypyrrole particles obtained by template polymerization in the presence of poly(vinyl methyl ether) microgels [335], and structural effects accompanying inverse emulsion cross-linking polymerization of acrylamide [142]. AFM is a development of scanning tunneling microscopy. These techniques differ from electron microscopy both in operating principles and in many aspects of their application. Sample preparation is simple, no metal sputtering or vacuum environment, etc. is needed and the native

Polymeric Nano/Microgels

sample structure in its natural environment (e.g., air, liquid) can be visualized. AFM has been successfully used for imaging of polymer systems, from surfaces through colloidal particles and dendrimeric structures down to single macromolecules (extensive review: [336]) In the field of microgels, the AFM technique has been used for example, for studying surface topology of vinyl/dimethacrylate networks [165, 166], poly(acrylic acid) films based on linear chains and on nanogels (see Fig. 9 in Section 4.3) [251], films based on polystyrene and poly(n-butyl acrylate-coacetoacetoxy ethyl methacrylate) gel particles [129], cluster formation in organosilicon micronetworks [284], structures of polystyrene graft microparticles [337], and the fibrous nature of polysaccharide gels [338]. An interesting modification of the AFM technique allowing for measurement of interparticle forces (that might be possibly used in future for measuring the forces between microgel particles) has been described by Sigmund et al. [339]. Another application of AFM that may be of interest in the present context is the determination of elastic properties of microparticles [340].

4. APPLICATIONS 4.1. Biomaterials The possibilities of employing macroscopic polymer gels as biomaterials, mostly in the form of hydrogels based on synthetic polymers, have been explored since 1960s, when these materials were first synthesized [1]. Since then, a number of products reached the stage of commercial application, soft contact lenses, drug delivery systems, and wound dressings being the most widely known examples. Given the number of research groups involved and progress being made in this field, one may anticipate that in the future the number of hydrogel-based biomedical products on the market will be constantly increasing. Broad although not very recent publications on the medical use of hydrogels are the collective works edited by Peppas [2] and DeRossi et al. [341]. Park et al. reviewed the narrower field of biodegradable hydrogels [342]. Out of more recent books and book chapters on this subject [343] provides a more general outlook, while the scope of [344] is limited to silicone-based hydrogels. For exemplary review papers on the medical applications of hydrogels, see [3, 345–352]. Although certainly the characteristics of hydrogels differ from one to another formulation, a few common properties can be listed that make these materials suitable for biomedical applications. In their high water content and hydrophilicity hydrogels are similar to tissues. They also mimic some properties of soft tissues as reversible swelling and elasticity. Due to their network structure they may be loaded with a drug which can be subsequently released at a controlled rate. This rate can be adjusted, one of the main factors being the mesh size. The latter parameter allows also construction of semipermeable membranes or containers, for example an outer shell of a hybrid artificial organ (an implant containing living cells) allowing the transport of water, oxygen, nutrients, and enzymes, but being impermeable to larger entities such as immunoglobulins and other components of the

Polymeric Nano/Microgels

immune system (cf. [353–355]). Due to their fair to excellent biocompatibility, hydrogels are usually well tolerated as implants. An example of a mature biomaterial technology based on classical, homogeneous hydrogels is the large-scale production of wound dressings, by a technique combining radiation-induced cross-linking of polyvinylpyrrolidone and concomitant sterilization of the final product [4, 356]. A number of other products based on similar technology (e.g., systems for local delivery of anticancer drugs and for induction of childbirth) have successfully passed clinical tests [347, 355, 357, 358]. While “regular” hydrogels are already common components of biomaterials, current efforts of the researchers are now concentrated on the stimuli-sensitive (“intelligent,” “smart”) gels (reviews: [5, 18, 341, 359–363]). These materials are able to respond to external stimuli, such as temperature, pH, ionic strength, light, electric field, or even (selective) changes in the concentration of a given chemical species. The latter property can be used, for example, in glucose-responsive insulin-releasing devices [314] or antigen-responsive systems [364]. The response to the stimulus, being induced by conformational changes of the polymer chain segments, usually manifests itself as a pronounced change in the gel volume (strong contraction or expansion) and in the amount of bound liquid (decrease or increase in the degree of swelling). Due to these properties, stimulisensitive gels are tested for applications such as sensors, actuators, chemical valves, controllable or self-regulating drug-delivery systems, or even artificial muscles. Certainly, there is still some gap between the artificial hydrogel fish that moves by swinging its tail in a laboratory bath [365] or electrically driven gel finger working in the air [366] and a future implementation of a hydrogel-based muscle, but fast developments in the field of stimuli-sensitive hydrogels allow one to expect that biomaterials based on these materials will be implemented very soon. Microgels are also intensely studied with respect to their biomedical applications (reviews: [16, 18, 361, 362, 367–370], exemplary patents: [371–375]). Of course, the product range is different than that of macroscopic gels, although there is a significant overlap in the field of controlled drug delivery. The most important microgel applications in the biomedical field are carriers for enzymes, antibodies, etc. used in diagnostics (e.g., immunoassays), drug carriers for therapeutic purposes (local, controlled drug delivery), and, potentially, microdevices (see Section 4.3), artificial biological fluids, and synthetic vectors for drug delivery. Coupling microgels with selective biochemicals leads to materials applicable in biological testing [16, 18, 376]. This technique has been used for some time with solid nanospheres [368, 376, 377]. In some cases, in comparative tests microgels performed better than polystyrene-based microsphere supports, providing testing material of higher sensitivity [378]. A typical action of the microgel-based immunoassay is based on an aggregation of antibody-bearing microgels with the specific antigens. The large particles formed in this way can be detected by microscopic techniques. In a modified immunoassay, magnetic microgels can be applied, containing an encapsulated polymer core with adsorbed magnetic nanoparticles [371, 379]. Magnetically

859 labeled cells can be separated from a cell mixture by applying magnetic field, for example in a section of tubing in which the cell mixture is flowing. Although there is a broad field of potential biomedical applications of conventional microgels, a strong tendency is observed to focus the research on complex microgels and on stimuli-sensitive systems. The preference for using stimulisensitive gels is even more pronounced for microgels than for the “wall-to-wall” gels. One of the important reasons is much shorter response time. While the reaction of macroscopic responsive gels to a stimulus is sometimes unacceptably slow (for example, when the molecules of a chemical stimulus have to diffuse into the whole volume of a gel slab), microgels, due to their small dimensions and high surfaceto-volume ratio, respond much faster. Temperature-sensitive microgels are tested for controlled binding of biomolecules. It has been shown that poly(N -isopropylacrylamide)—pNIPAM—microgels can bind various proteins by physical sorption above the phase transition temperature (i.e., at ca. 40  C) and release them upon lowering the temperature to 25  C [380, 381]. Proteins can be also covalently bound to such gels, and their activity can be controlled by temperature changes [382]. In complex systems such as described by Yasui et al. [383], one can achieve high enzyme activity within a defined, relatively narrow (a few degrees) temperature range. Pichot and co-workers demonstrated the possibility of using thermo- and pH-responsive microgels for binding nucleic acids [362, 384]. It has been shown that the interaction of temperature-sensitive microgels with elements of the immune system like granulocytes (foreign-body attacking cells) can be moderated by changes in temperature [385]. It is worth noting that the interaction of pNIPAM gel particles was much weaker than that of polystyrene microspheres, which may indicate that, in the context of attack by immune system cells, these microgels have higher biocompatibility than solid microspheres. Thermosensitive microgels have been also tested as drug carriers. The structure and hydrophilic/hydrophobic properties of the drug have been identified as important factors influencing the phase transitions and uptake/release characteristics of poly(N -vinyl caprolactam) gel particles [126]. In order to achieve the desired release profile, composite microgels may be used, for example combined nonporous silica/pNIPAM gels [386]. The rate of the thermally triggered drug release may then be controlled by changing the composition of this hybrid product. Another interesting example of a composite structure based on drug-loaded thermosensitive microgels is a wound dressing, where drug-bearing pNIPAM gel particles are incorporated into a self-adhesive film [387]. The product combines adhesive and temperature-controlled absorptive functions and is easy to peel off after use. Compositions containing poly(N -vinylcaprolactam-cosodium acrylate) microgels and gelatin undergo a reversible macrogelation upon temperature increase above ca. 32  C [65]. Such materials, liquid (injectable) at RT but forming a gel at the temperature of the human body, are considered for applications in surgery and drug delivery. Another group of responsive materials tested for use as biomaterials are pH-sensitive and/or ionic-strength-sensitive microgels. For these products, there are at least two mechanisms allowing for controlled drug delivery. One can load

860 the gel particles with a drug at a pH where the particles are fully swollen (expanded), trap it inside by a pH change leading to the collapse of the microgel, and subsequently allow the drug to diffuse out at a pH-controlled rate. A similar mechanism applies as well to the systems where ionic strength is the stimulus for expansion and collapse, or where both pH and ionic strength effects are operating. Another mechanism is pH-dependent reversible ionic binding of drugs. Drug and protein binding and release from anionic microgels have been studied, for example, by Eichenbaum et al. [73, 74] and Soppimath et al. [388], while Vinogradov et al. [308, 375, 389] described the synthesis and properties of some cationic systems. A sophisticated drug-delivery system, mimicking the action of secretory granules, has been constructed by Kiser et al. [390–392]. The core is an anionic microgel particle based on methacrylic acid, loaded with a drug. Subsequently, by lowering pH, a collapse of the microgel is induced. In this form, the particle is coated with a lipid bilayer, to simulate the natural secretory granule and to protect the particle from premature swelling. Poration of the lipid bilayer (e.g., by applying electric field) causes the gel to swell, allowing release of the drug. A different synthetic procedure leading to similar systems is based on encapsulation of hydrogelforming components into liposomes and subsequent polymerization [176]. A further example of this kind is a group of products intended for drug delivery of vaccine formulations, consisting of cross-linked polysaccharide microgel core surrounded by a lipid bilayer [255]. A pH-sensitive microgel preparation based on poly(methacrylic acid-co-ethyl acrylate) has been devised for oral delivery of a novel drug being a HIV-1 protease inhibitor [393]. It has been demonstrated that the absorption of this poorly water soluble drug from the tested system was much better than from the suspension of a free drug. Solid sustained release devices for oral delivery of drugs can be obtained by compressing microgels. Such a system based on polyurethane microgels has been shown to retain its integral structure but become microporous on swelling with water [374]. Microgels can be functionalized not only by coupling them with biomolecules, but also by molecular imprinting. This process leads to microgels bearing structural binding sites specific to target molecules. A general procedure of molecular imprinting of a polymer is as follows. Monomers being in contact with a template molecule are polymerized and cross-linked, the template is removed, and the polymer network contains a complementary binding site able to rebind the same template or analogous molecules. Ye et al. and Biffis et al. demonstrated the applicability of this procedure for synthesizing molecularly imprinted microgels and proved their binding performance [77, 78, 394]. Stability of shape and dimensions of microgels when compared with linear polymer chains may be helpful in their potential use for blocking dental microchannels in cases when there is a need to use a synthetic substitute for the natural gel-like substance performing this function [395]. An interesting and potentially valuable property of microgels is their enhanced resistance against degradation when compared to linear macromolecules [251, 396]. This effect

Polymeric Nano/Microgels

is illustrated in Figure 7. As a result of any intense or longlasting stimulus inducing chain breakage (a mechanochemical action, ultrasound, the formation of peroxyl radicals along the chain, etc.), a linear macromolecule is easily degraded to short fragments. The same number of chain breaks formed in a microgel may cause no or very little fragmentation, since the chain segments are linked together in many points and will not fall apart as a result of a single chain break. This effect has been demonstrated on microgels of poly(acrylic acid) (PAA) [251, 397]. Aqueous solutions of linear chains and microgels of PAA were subjected to the action of ionizing radiation in the presence of oxygen. Under such conditions, no cross-linking takes place in the system. Initially formed carbon-centered radicals are rapidly converted into the corresponding peroxyl radicals, which in turn initiate processes leading to chain scission. As the concentrations of linear and cross-linked chains of similar average molecular weight were identical, the yield of scission events should be equal for both samples. However, the changes in molecular weight and in the radius of gyration in the case of linear and microgel PAA revealed striking differences. While linear PAA is easily degraded even at relatively low doses, which is evidenced by parallel decrease in weight-average molecular weight and radius of gyration, microgels, within the same dose range, seem to remain intact, their molecular weight and radius of gyration being constant. This degradation resistance combined with suitable rheological properties may be used in fabricating materials for medical applications. A commercialized example of such a product is a polymer drug based on microgels of a natural polysaccharide used to enhance the viscoelasctic performance of synovial fluid [246]. These macromolecules are subjected to mechanochemical stress and to the attack of reactive oxygen species, mainly free radicals. It has been shown that under such conditions internally cross-linked

(a)

(b)

Figure 7. Enhanced degradation resistance of microgels (b) when compared to linear macromolecules (a). Upon several chain scission events at locations marked with ⊗ a linear chain is cut into pieces, while the segments of a microgel do not fall apart.

861

Polymeric Nano/Microgels

macromolecules perform better than the corresponding linear ones [247]. Preliminary tests on a substitute synovial fluid containing microgels made of a synthetic polymer proved the high degradation resistance and proper viscoelastic properties of this product [398]. First tests have been performed on the application of microgels as synthetic, nonvirial vectors in gene delivery. The latter is regarded as a powerful tool for curing some hereditary diseases and treating genetically based disorders. Certainly, the issue is a very complex one, since such vectors must be capable of performing many processes such as binding DNA fragments, attachment to cells, internalization, and intracellular plasmid release. First attempts using microgel-like structures for gene delivery were based mainly on chitosan, but synthetic structures based on 2-(dimethylamino)ethyl methacrylate, N -vinylpyrrolidone, and N -isoporpylacrylamide have been tested as well, with promising results [369]. There are also projects to design microgel-based intravenous drug carriers that could remain in blood for a suitable period of time, facilitate the cellular uptake, and possibly also selectively deliver the drug to a target site. Animal tests have shown that by varying properties of such structures (chemical composition, hydrophilicity) one can change the biodistribution patterns of the microgels and that drug-loaded microgels were more efficient than equivalent concentrations of free drug in curing melanoma in mice [369]. Some further perspectives in the application of nano- and microsized particles and devices in drug delivery are discussed in [370].

4.2. Coatings Where actual, large-scale applications of microgels are concerned, the surface coatings industry seems to be the most prominent field. The hundreds of research papers published and patents issued (for a few representative examples see [399–415]) dealing with microgel-containing coatings emphasize the significance of this application. The interest in the use of microgels as components of coatings originated mainly from environment protection needs and regulations. In order to reduce the amount of volatile organic compounds in the coating formulations, the manufacturers tend to increase the total solid content. This is, however, problematic if polymer components of a given molecular weight range are used in binders, since the viscosity of formulations becomes too high. When microgels are used, viscosity can be maintained at a desired level. Another approach besides the high-solid products is to use water-borne coating systems. Microgels can be used both in solvent-borne and water-borne coating products. Besides their advantageous rheological properties, they often exert a reinforcing effect on the cured coating. The use of microgels in coating technologies has been the subject of concise but informative reviews [16, 267] (cf. also book chapters in [416, 417]). Microgel-containing solutions and dispersions are usually characterized by pseudoplastic, strongly non-Newtonian rheological properties. They are highly viscous at low shear rate (they do not flow at a zero shear rate), but their viscosity decreases remarkably with increasing shear rate. This is in a

perfect accordance with the needs for a typical paint application process. In the storage tank (no or low shear rate) the viscosity should be high to prevent pigment settling, during the gun-spraying (high shear rate) low viscosity is desired, and at the object surface (no or low shear rate again) viscosity should increase rapidly so that no sagging effect occurs, even for films of high thickness. Another useful property of microgels is their positive influence on the orientation of flake pigments in metal effect coatings. Microgels also have a positive influence on the mechanical properties of cured paint films such as stone chip resistance, impact flexibility, and elasticity. This is attributed to the cross-linked polymer structure, the high molecular weight of gel particles, and the microheterogeneous structure of the microgel-containing polymer film that probably allow the impact energy to be more efficiently dissipated [267]. In the automotive industry, the primer coating layer is often applied by electrodeposition. It has been shown that compositions containing amine-based cationic microgels, mostly in the form of aqueous dispersions, can be used for electrocoating [418, 419]. Problems that have been recently investigated with respect to the use of microgels in coatings include deeper understanding of the relationship between microgel structure and rheological properties (and ways to adjust the latter) [265], the influence of substrate composition and synthetic procedures on the shelf stability of the product and affinity between microgels and pigments [122], influence of microgels on the drying time [142], and ability to form films suitable for drying at ambient temperatures [129]. Microgels can be used not only as constituents of regular coatings but also as self-adsorbing and self-organizing filmforming layers for protecting metal surfaces against corrosion. It has been shown that core–shell microgels based on styrene, butyl acrylate, and phosphate-substituted acrylates form a layer of a structured molecular order on a surface of technical aluminum and provide efficient protection against corrosion in standard tests [332, 420]. Although the use of synthetic microgels in coatings started in the late 1960s [421] or, depending on what we consider a true microgel, perhaps rather in the 1970s [422, 423], one should mention that physical and chemical studies on a highly durable ancient oriental lacquer used in Asia for millennia revealed a structure containing self-formed, natural core–shell-like microgel particles [424].

4.3. Miscellaneous Besides their applications in coatings (biggest market) and in biomaterials (perhaps the most promising direction for the future), microgels are used or tested for use in a number of other fields. Besides large-scale use of microgels in the coating industry, similar properties make them interesting for the manufacturers of cosmetics, namely nail varnishes [425, 426]. In varnishes based on organic solvents, organophilic clays are used to prevent pigment sedimentation. These compounds, however, exert some unwanted side effects and usually require toluene as a component of the formulation. The presence of microgels allows reduction or elimination of the

862 use of clays while maintaining the proper rheological properties of the varnish and preventing the precipitation of the pigments. Historically one of the first applications of microgels was in papermaking [427–429]. In manufacturing quality products starch and other polysaccharides used as paper and paperboard sizes may be replaced with poly(vinyl alcohol) (PVAL) for better performance. It has been demonstrated that microgels of PVAL show clear advantages over the same polymer in a linear form (e.g., they have lower tendency to penetrate into paper, which is an undesirable effect). Several microgel-containing preparations were used, based either on pure PVAL gels or gel components combining PVAL and polysaccharides. Anionic polysilicate microgels in combination with organic polymers are used to flocculate pulp and filler fines in the water-removal step of the papermaking process [430]. Microgels can be a valuable component of fibers. An exemplary application is the admixture of vinylidene chloride gels to an acrylic fiber, significantly improving the flame retardancy [431]. Similar gels have been shown to improve load-bearing properties and flame retardancy of polyurethane foams [432]. Microgels can be also applied as supports in catalysis. Organo-aluminum compounds coupled to organopolysiloxane microgel particles serve as co-catalysts in the polymerization of olefins [433, 434]. Enantioselective reduction of prochiral ketones can be catalyzed by microgel-bound oxazaborolidines [38]. Good solubility and, at the same time, low viscosity of the solution are the important advantages of microgel-supported catalyst, while the catalytic action and selectivity are comparable to those of low-molecularweight analogs. Microgels bearing trialkylammonio groups have been demonstrated to catalyze alkaline hydrolysis of aryl laurate esters [127]. By using microgel-supported catalyst, a homogeneous catalysis in a flow reactor may be carried out (Fig. 8). If the inlet and outlet of the reactor are equipped with membranes that the gel particles cannot pass, the reactants are in a constant flow but the catalyst remains in the reactor, despite it being present in a nearly molecular dispersion. Particular rheological properties as well as higher mechanochemical resistance of microgel solutions when compared to the solutions of linear chains are the basis of their applications in the fields of liquid thickening, oil recovery, and hydraulics. Solutions of relatively inexpensive polyacrylamide-based microgels are used as thickening agents and agents for restricting the flow of liquids through subterranean formations [435]. Microgel solutions can be applied as a non-Newtonian, shear-resistant, nonleaking hydraulic fluid for hydraulic energy transmission systems and devices absorbing mechanical energy (liquid springs, shock absorbers) [436]. Due to their swelling and water-retaining properties, hydrophilic microgels have been postulated and tested, with good results, for use as soil conditioners [437–439]. In the case of preparations consisting only of homogeneous microgels, one may encounter a problem of poor mixing with soil and spontaneous separation (gel particles come up to the surface of soil), due to differences in specific gravity of soil components and gels. This can be avoided if structured

Polymeric Nano/Microgels

Figure 8. A schematic illustration of a homogeneous catalysis in a flow reactor, based on catalyst coupled to microgel particles, trapped between two membranes.

microgels are used, consisting of solid filler grains of high specific gravity embedded in gel particles [439]. While gelbased soil conditioners are well suited for use in flower pots or on the lawn of a golf course, their price does not allow them to be used in large-scale projects like desert reclamation in countries with very limited financial resources, even if economical synthesis routes are taken into consideration (e.g., radiation-cross-linking of acrylamide in bulk followed by mechanical grinding). An extensively tested and potentially broad field of microgel application is in printing and photographic technology. Since the rheological properties of microgel can be tailored to meet specific needs (a sophisticated method of viscosity adjustment may be varying the hair length of core-hair type microgels), they may be useful in liquid photopolymer formulations used to cover the screen in the screen printing technique [262]. Incorporation of microgels in the photosensitive layer of a kind of modern lithographic plate leads to a product that can be developed directly on a printing press, without a postexposure wet development step [440]. This technology allows one to save time and labor as well as reduce the use of volatile organic compounds. Another technology has been elaborated to obtain printing plates for flexographic printing [441, 442]. Microgel-based photosensitive resin allows the plates to be water-developable, thus eliminating the use of harmful halogen-containing solvents used to develop the plates in a conventional technique. A variety of photosensitive compositions for use in printing and manufacture of printed circuit boards employ microgels to enhance physical properties, eliminate cold flow, improve storage stability, enhance photospeed, and render these materials suitabable for water processing [443–446]. A further interesting example is the use of reactive microgels

Polymeric Nano/Microgels

that constitute a chemically active compound of a hardenable resin composition [447, 448]. Some inks and electrographic liquid developers contain finely dispersed microparticles of magnetic substances. Several problem have to be overcome in producing these “magnetic fluids,” one the most important being the poor dispersion stability. This can be significantly improved when incorporating these solid microparticles, during synthesis, into polymeric microgels [449]. Macroscopic polymer gels can be made photosensitive, not only by introducing chromophores that undergo permanent changes (e.g., cross-linking) upon irradiation, but also by incorporating structures that enable reversible photochemical switching of properties (reversible cross-linking) [450]. This indicates that also microgels themselves can be turned into photosensitive materials, either by using suitable monomers or by postsynthesis modification. In fact, coumarin-containing organosilicon microgels have been demonstrated to undergo photoinduced cluster formation [284]. So far, most often microgels are used in photography and imaging as auxiliary substances, either as binding agents improving the physical properties of the film layers or as additives that help to incorporate the photographically active or other (e.g., antistatic) substances into the photographic layers [451, 452]. Debord et al. have demonstrated that relatively simple microgels made of poly(N -isopropylacrylamide) can be manipulated to form colloidal crystals of specific colors depending on the fabrication parameters [70]. In another approach, polymeric nanospheres are incorporated into a stimuli-sensitive hydrogel [453]. Under the action of a chemical stimulus, the distances between the nanospheres can be varied, thus causing shifts of the wavelength of the Bragg peak of the diffracted light. This leads to the changes in color of the specimen. It is expected that such colloidal crystals can find a wide range of applications in photonics and chemical sensing. One may expect that in the near future interesting applications will be found for the recently described electrically conducting microgels based on pyrrole and aniline polymers bound to a gel core [454]. The products have promising properties in the aspects of electrical conduction, electromagnetic frequency interference shielding, and electrostatic prevention. Sorption and binding of a variety of compounds is an intensely studied application field of polymer gels. Most research in this area concerns the use of hydrogels. Exemplary applications of macroscopic hydrogels being investigated, besides the drug-delivery systems described in Section 4.1, include, for example, binding of metal ions [455], selective removal of pollutants (arsenate and selenite [456], textile dyes [457], organic substances [458]), or collection of uranyl ions from seawater [459]. Reversible sorption of water by hydrogels is tested for an application in sludge dewatering [460]. A number of studies revealed the potential application of microgels in metal ion binding and ion exchange. An advantage of using micro- instead of macrogels is primarily the binding kinetics—in microgels the binding groups are easily accessible, in contrast to most macroscopic gel

863 structures, where considerable time is needed for the substrates to diffuse into the gel volume. Gel particles bearing carboxylate groups bind divalent alkali earth cations (Mg2+ , Ca2+ , Sr2+ , Ba2+ ) more effectively than monovalent cations (as Na+  [75]. Binding of calcium ions has been studied in more detail [63, 65]. Other studies have shown that there may be a pronounced selectivity in binding divalent metal ions. For example, binding of Hg2+ cations by poly(N -vinylcaprolactam-co-sodium acrylate) microgels is much stronger than binging of Cu2+ and divalent alkali metal ions [64]. Strong, albeit reversible, binding of Pb2+ by microgels of poly(N -isopropylacrylamide-co-sodium acrylate) has been reported as well [149, 288]. These observations indicate that microgel-based systems may be used for selective ion binding. Various microgel-containing selective ion-exchange materials have been fabricated. Examples are commercially available macroporous polymeric anion and cation exchangers where every exchanger particle (having the diameter of a fraction of mm) consists of a large number of submicrometer size microgels (cf. [461–463]). Such systems may be applied for example for eliminating trace concentrations of organic compounds that exist in water in an ionic form. It is postulated that thermosensitive microgels may serve as valuable binding/separation agents, due to the ability of these materials to absorb various chemical species below the conformational transition temperature and to release them at elevated temperatures due to the contraction of microgels. The compounds being reversibly absorbed include metal ions [288, 464], polymers [465], and surfactants [18, 150, 302, 466–468]. Most studies in this area have been done on poly(N -isopropylacrylamide)-based gels. Besides, the interactions between surfactants and poly(N vinylcaprolactam) microgels have been investigated [61]. One of the most fascinating prospective uses of stimuli microgels is in microdevices and micromachinery. Due to their fast response time, they may be used, for example, as self-regulated, pH- or temperature-controlled microvalves. Contrary to conventional microactuators (electromagnetic, electrostatic, thermopneumatic, etc.), microgel-based valves are simple and do not require external power for operation. Fully operational exemplary devices of this kind, having short response times, have been recently presented by Beebe et al. [469]. Microgels can be used as building bricks for constructing complex polymer structures and materials. A simple example may be fabrication of a polymer material (foil, coating, etc.) of a well-developed, rough (in the microscale) surface. This may be carried out just by casting a foil from a microgel solution. Such a surface, even after drying and collapse of microgels due to a loss of solvent, remains structured when compared to analogous sample obtained from solution of linear chains of the same polymer (Fig. 9) [251]. Chen et al. have demonstrated that styrene-based microgels bearing carboxyl group could be easily self-organized into ordered multilayered films or latex crystals [155]. Calcium ions may be used to form large structures of spherical poly(N -vinylcaprolactam-co-sodium acrylate) microgels [63]. Poly(vinyl methyl ether) (PVME) microgels can be used as specific nanoporous templates in the polymerization of pyrrole [335]. The resulting products were large,

864

Polymeric Nano/Microgels

[nm] 5.23 0.00 840 [nm]

840 [nm]

0

0

[nm] 11.0 0.0 1442 [nm]

1146 [nm]

0

0

Figure 9. Surfaces of films obtained by casting and drying solutions of poly(acrylic acid): (a) film of linear PAA, (b) film of PAA microgels, as observed by atomic force microscopy.

needle-like particles, in contrast to small spherical particles obtained with linear PVME. Microgel-like, internally crosslinked rubber particles have a high reinforcing effect when incorporated into regular rubber materials [470, 471]. Materials constructed of microgels that are embedded in continuous polymer matrix are considered for use as membranes, filters, and absorbents [472].

GLOSSARY Chemical gel (permanent gel) A gel where the polymer network is composed of polymer chains connected by permanent, covalent bonds. Critical gelation concentration (CGC) 1. The highest monomer concentration at which no macroscopic gel is formed even at a complete monomer conversion (in the crosslinking processes starting from polymers). 2. The highest polymer concentration at which no macroscopic gel can be obtained (in the crosslinking processes starting from polymers). Cross-linking A process of formation of permanent links (covalent bonds) between two separate macromolecules or between two segments of same macromolecule. Cross-linking polymerization A process where polymerization and cross-linking take place side-by-side; a typical phenomenon for polymerization of systems containing multifunctional monomers. Gel A two-component material consisting of a (permanent or labile) network of linked polymer chains and a solvent filling the pores of this network. In this chapter this term is used to denote only the chemical (permanent) gels. Hydrogel A two-component material consisting of a (permanent or labile) network of linked hydrophilic polymer

chains and water (or aqueous solution) filling the pores of this network. In this chapter this term is used to denote only the chemical (permanent) hydrogels. Intermolecular cross-linking A process of formation of permanent links (covalent bonds) between two separate macromolecules. Intramolecular cross-linking A process of formation of permanent links (covalent bonds) between two segments of same macromolecule. Ionizing radiation Any form of radiation (electromagnetic radiation or fast particles) having sufficiently high energy to cause ionization of matter. Gamma rays from isotope sources and beams of accelerated electrons are commonly used in radiation-based polymer technology. Lower critical solution temperature (LCST) Lowest temperature at which the polymer-solvent system becomes phase-separated (for polymer-solvent systems that become immiscible upon temperature increase). Macroscopic gel A continuous gel phase of macroscopic dimensions. It can be easily seen with a naked eye. Macroscopic gel can be filling the whole volume of the sample or the reaction vessel (a “wall-to-wall” gel). Microgel A gel particle of microscopic dimensions (or in other words, an internally cross-linked polymer microparticle), able to swell, without disintegration, in a suitable solvent. In this chapter this term is used to denote only the chemical (permanent) gel particles. For a more detailed discussion of the microgel definition—see Introduction. Microparticle (polymeric) Any polymer particle of microscopic dimensions, regardless of the presence or absence of any links and the nature of the forces acting between the individual polymer chains. Nanogel 1. A gel particle of submicromter size. 2. A subclass of small microgels formed by intramolecular crosslinking of single macromolecules. In this chapter this term is used to denote only the chemical (permanent) gel particles. For a more detailed discussion of the nanogel definition— see Introduction. Physical gel (pseudogel) A gel where the polymer network is composed of polymer chains connected by weak, usually reversible, binding phenomena (ionic bonds, hydrogen bonds, van der Waals forces, entanglements). Surfactant-free emulsion polymerization A particular kind of emulsion polymerization where the emulsifying action is provided not by an added surfactant but by the monomer and/or polymer itself.

ACKNOWLEDGMENTS This work has been supported in part by the State Committee for Scientific Research (Poland), grants 3 T09B 053 19, 3 T09A 128 22, and PBZ-KBN-004/CD/T11/2000, as well as by the International Atomic Energy Agency, research project 302-F2-POL-11513 (F2.20.34). The valuable assistance of Renata Czechowska-Biskup, Katarzyna Filipczak, Artur Henke, Jadwiga Ignaczak, Dr. Ireneusz Janik, Slawomir Kadlubowski, Marek Kozicki, Dr. Piotr Kujawa, and Dr. Przemyslaw Stasica in collecting literature,

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managing the reference database, and preparing the drawings is gratefully acknowledged.

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