Microgels

Microgels Microgels are discrete gel particles with an overall size in the colloidal range (5 nm R 500 nm) which are composed of cross-linked polymer chains, that is the inner structure typical for a polymer network. As molecular or colloidal structures, they easily dissolve or disperse so that they are accessible to the different techniques of polymer analysis; molecular properties such as a molecular weight or average size can be defined. Microgel structures are regularly found in biological systems, e.g., starch (amylopectin), and most proteins are linear peptide chains inter- or intramolecularly cross-linked by secondary interactions or sulfur linkages. In synthetic polymer science and technology, microgels have become increasingly popular because it is recognized they are close analogues to dendrimers and hyperbranched polymers and show a number of their advantageous properties, i.e., high molecular mass, a relative low viscosity, and high chemical functionality. Compared to their counterparts, microgels are more simple to synthesize and also accessible in larger amounts. Recent reviews focusing on microgels include Antonietti (1995), Saunders and Vincent (1999), Funke et al. (1998), and Ishii (1998). 1. Synthesis Up to now, two different ways of microgel synthesis are commonly found in the literature. The first technique (type A) makes use of crosslinking copolymerization in a restricted volume, e.g., the small droplets of microemulsions or emulsions can be made of monomer mixtures which are polymerized forming the microgel. By using standard techniques of heterophase polymerization the size can be easily adjusted between 5 nm R 50 nm (microgels from microemulsion polymerization), 30 nm R 300 nm (miniemulsion polymerization), or 50 nm R 500 nm (emulsion polymerization) where R denotes the radius of spherical, practically monodisperse microgel. It has been shown that the microgels fabricated in this way essentially have the structures and behavior of networks (Antonietti et al. 1990). Microgels made of polar monomers (water-soluble microgels) can be fabricated with inverse heterophase polymerization techniques or by precipitation polymerization, isolated, and redissolved in water. Nonpolar microgels are made in reverse systems, swell in organic solvents, and can also be applied in polymer melts. In the second technique (type B) microgels are obtained when a cross-linking reaction is carried out in dilute solution. According to the Ziegler dilution principle, the probability of inner cyclization increases with dilution, and, below a critical threshold, all crosslinking reactions lead to microgels only. Pure divinyl-

benzene, for instance, polymerizes to pure microgels when the concentration is below 80 g l−". This was first pointed out in 1935 by Staudinger, but later examined in more detail. In this case, the microgels do not behave as a polymer network, but parallel the behavior of hyperbranched molecules. The inner structure of such systems can be described by the laws of fractal geometry (Antonietti and Rosenauer 1991), and their mechanical behavior resembles a network at the gelpoint. Type A and type B microgels have very different properties and applications and should be carefully distinguished.

2. Properties and Applications Due to the fact that microgels are composed of classical monomers, but show special properties because of the molecular structure, they are used technically in a number of applications. The most classical application is as polymer binders in solvent-based paints, where microgels are advantageous because of rather low viscosities at high polymer concentrations and their ability for direct pigment stabilization (Ishii 1998). They have also been used as reactive fillers or multivalent cross-linking centers for the reinforcement of rubbers. In the case of a special cross-linking topology close to critical cross-linking and appropriate functional groups, microgels also can be used as superb adhesives (Tobing and Klein 2000). For water, intelligent thickeners are developed based on microgels with tunable swellability. Microgels made of poly-N-isopropylacrylamide are widely examined because of their temperature-dependent swelling characteristics (e.g., Pelton 2000). Charged polyelectrolyte microgels were used to examine ion interactions and ion exchange as model systems (Eichenbaum et al. 2000). Similar ionic microgels with sufficient amphiphilicity in water can also adsorb drugs in their interior or proteins onto their surface, keeping their molecular identity and colloidal stability. This makes them ideal objects for drug delivery with controlled release. Cationic microgels were employed to mimic histons and to complex deoxyribonucleic acid (DNA) strands on their surface for applications in gene delivery. In basic research, microgels turned out to be valuable model systems for the understanding of transport phenomena in complex fluids and polymer melts or the examination of interactions in electrostatically coupled systems. This is due to their strict monodispersity and their adjustable size and interaction potentials. Relaxation and transport phenomena in densely packed systems and model glasses were examined that way, and transport mechanisms slower than reptation were characterized by observation of the mobility of very small, spherical microgels in their 1

Microgels melts (Antonietti et al. 1995). The intermolecular nature of the polyelectrolyte effect, as well the presence of attractive forces in similarly charged polyelectrolytes, were also examined with charged microgels. Microgels give the polymer chemist the ability to design macromolecules with a controlled molecular three-dimensionality. The application side is attracted by the fact that microgels represent new polymer systems with new properties made, however, from old monomers and with old technologies. From the viewpoint of basic research, microgels with their model properties allow examination of complex cooperative phenomena such as phase or binding transitions on a subset or part of the structure which can be characterized by standard techniques of polymer analysis. Therefore, it is expected that the science of microgels will continue to grow.

Bibliography Antonietti M 1995 Microgels—colloidal models for the crosslinked state. Macromol. Symp. 93, 213–25 Antonietti M, Bremser W, Schmidt M 1990 Microgels: model

polymers for the cross-linked state. Macromolecules 23, 3796–805 Antonietti M, Pakula T, Bremser W 1995 Rheology of small spherical polystyrene microgels: a direct proof for a new transport mechanism in bulk polymers besides reptation. Macromolecules 28, 4227–33 Antonietti M, Rosenauer C 1991 Properties of fractal divinylbenzene microgels. Macromolecules 24, 3434–42 Eichenbaum G M, Kiser P F, Shah D, Meuter W P, Needham D, Simon S A 2000 Alkali earth metal binding properties of ionic microgels. Macromolecules 33, 4087–93 Funke W, Okay O, Joos-Mu$ ller B 1998 Microgels—intramolecularly crosslinked macromolecules with a globular structure. AdŠ. Polym. Sci. 136, 139–234 Ishii K 1998 Synthesis of microgels and their application to coatings. Colloid Surface A 153, 591–5 Pelton R 2000 Temperature sensitive aqueous microgels. AdŠ. Colloid Interfac. Sci. 85, 1–33 Saunders B R, Vincent B 1999 Microgel particles as model colloids: theory, properties and applications. AdŠ. Colloid Interfac. Sci. 80, 1–25 Tobing S D, Klein A 2000 Mechanistic studies in tackified acrylic emulsion pressure sensitive adhesives. J. Appl. Polym. Sci. 76, 1965–76

M. Antonietti

Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 5635–5637 2