Preface, Introduction - American Chemical Society

Downloaded by 80.82.77.83 on April 20, 2018 | https://pubs.acs.org Publication Date: October 15, 2002 | doi: 10.1021/bk-2002-0833.pr001

Preface Polymer gels, which have both solid and liquidlike properties, are a novel state o f matter. Once cross-linked, either chemically or noncovalently, the polymer chains lose their individual identity and become part o f a large three-dimensional interconnected network pervading through the entire solution volume. Such a system has both fluidity and elasticity, and the dynamics o f such a phase is quite revealing. For physical gels, which are interconnected v i a noncovalent linkages, the fluidity and elasticity become a matter o f the time scales o f the ob servations relative to the lability o f those interconnections. Such intrigu ing considerations have given rise to great activity in gel theory and experimentation, which are further motivated by the development o f novel uses for polymer gels. Gels have applications in numerous fields, including food and chemical processing, pharmaceuticals, biotechnology, agriculture, c i v i l engineering, and electronics. Chemomechanical systems, soft actuators, drug delivery systems ( D D S ) , permselective membranes for selective extraction, and chemical valves are examples o f applications o f stimuliresponsive polymer gels. Modulation o f swelling forces in gels by chem ical or physical stimuli enables dynamic control o f the gel hydration and desirable diffusion and permeability o f solutes can be thereby obtained. Because a polymer gel can absorb water up to several thousands times its original weight (depending on the chemical structure o f the gel and number o f charges) disposable diapers, sanitary napkins, and perfumes are everyday-life examples o f the applications o f highly water-absorbing hydrogels. Considerable interest and activity exists in the application o f synthetic and biological polymer gels in medicine, particularly hydrogels for biomedical applications. The bulk property o f swelling is o f interest for "swelling implants," which can be implanted in a small dehydrated state v i a a small incision and then swell to fill a body cavity and/or to exert a controlled pressure. The wide range o f biomedical applications for hydrogels can be attributed both to their satisfactory performance

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upon either blood- or tissue-contact in vivo implantation and to their fabrication in a wide range o f morphologies. In this book, Chapter 16 by R a v i et al. describes hydrogels as potential soft-tissue substitutes. A m p h i p h i l i c gels capable o f containing active pancreatic beta cells are described by Kennedy et al. (Chapter 19); and alkylthioacrylate hydro gels prepared for possible contact lens development are covered by M u k k a m a l a et al. in Chapter 11. The use o f hydrophilic systems for drug delivery provides the motivation for the preparation o f hylauronate gels by Y o m o t a and Okada (Chapter 22), and o f biodegradable insulinreleasing injectables utilized by Y o u n g Jin K i m and Sung W a n K i m (Chapter 20). Gels can be considered to be formed o f a macromolecular net work consisting o f a number o f small cavities that are filled with the fluid. Chemical gels are cross-linked by covalent bonds, whereas physical gels are cross-linked by weak forces such as hydrogen bonds, hydrophobic, or ionic interactions or combinations o f them. The forma tion o f polymer gels can thus proceed by conventional synthesis or by physical gelation. Polymer gel synthesis is addressed here from such various perspectives. A l l c o c k anf A m b r o s i o (Chapter 6) describe novel gels based on radiation cross-linking o f organophosphazene polymers, and gels with inorganic constituents but formed through multifunctional siloxane monomers are also discussed by Kajiwara and co-workers (Chapter 5). Hydrogels can also be formed through photocrosslinking, as shown by K u c k l i n g et al. (Chapter 21) who prepared gels at the submicrometer size range. Physical gelation processes are usually reversible and are called sol-gel transitions. A n overview o f physical gelation is provided by Ross-Murphy (Chapter 4) with special focus on biopolymers. Kressler and co-workers (Chapter 8) describe gelation o f amphiphilic block co polymers in water. Gels with a wide range o f properties result from the relaxation time scale o f these physical cross-linkages, as discussed by M o r i s h i m a et al. (Chapter 2), and by Tribet and co-workers (Chapter 18). Arglielles-Monal et al. (Chapter 7) present the viscoelastic behavior o f physical gels from alkali-soluble chitin. That physical gelation can take place for less polar polymers also is shown by the network formation o f block copolymers in nonaqueous solution presented by Spontak and co workers in Chapter 17. Gels, in general, are considered to be composed o f hetero geneous domains from a few angstroms to several micrometers. Because o f this heterogeneity and the swelling property that gives the network

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motion, detailed gel structures have been difficult to visualize satis factorily. Scattering techniques such as small-angle neutron scattering ( S A N S ) , small-angle X-ray scattering ( S A X S ) , light scattering have been extensively employed to characterize polymer networks as discussed by Candau in 1982. The structure factors, proportional to the scattered intensity, for gels are usually discussed by classifying gels into two categories: non-ionic neutral gels and ionic gels. Here, Oppermann et al. (Chapter 3) use static light scattering to probe the spatial inhomogeneities o f networks formed from both ionic and nonionic gels. Waterswellable gels can also possess hydrophobic domains creating yet anoth er form o f inhomogeneity. Such gels are the subject o f the chapters by Gitsov et al. (Chapter 15), by R a v i et al. (Chapter 16), and by Kressler and co-workers (Chapter 8). The volume o f a polyelectrolyte gel is a manifestation o f the competition among three osmotic pressures acting on the polymer network; positive pressure o f counterions, negative pressure due to the affinity among polymers, and the rubber elasticity that keeps the network in a state o f moderate expansion. The combined effect o f these three pressures determines the equilibrium volume. Temperature, p H , and salt ions affect both positive and negative pressures, whereas solvent composition influences only the negative pressure. For some hydrophilic gels, the volume change is discontinuous, as observed by Tanaka in 1980. Here, Harmon and Frank (Chapter 1) describe the kinetics o f L C S T volume transitions in gels. Several other chapters, in addition to the contribution by Morishima and co-workers (Chapter 2), deal with polyelectrolyte gels. Kudaibergenov et al. (Chapter 10) examine the response o f polyampholyte gels to electrical fields, and also the ability o f these hydrogels to bind proteins. Tribet's gels arise in part from charge interactions between amphiphilic polyacids and proteins. Opperman describes the response o f ionic gels to added salt. Several contributions point to the importance o f dynamic studies for the investigation o f gel structure. Rheological methods are featured in the chapters by Eloundou et al. (Chapter 14) who compared gelation and vitrification o f epoxyamine systems with percolation theory, and by Dutta and co-workers (Chapter 13) who offer rheological examination o f physical gels. Rheology is also central to the studies by R a v i et al. (Chapter 16), who measured creep-recovery in hydrogels; and by Tribet and co-workers (Chapter 18), who used proteins to cross-link amphi philic polyacids.

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The chapters in this volume are derived from a symposium on Polymer Gels held by the American Chemical Society ( A C S ) at their Spring National Meeting in San Francisco, California, M a r c h 26-30, 2000 in which 40 authors from 10 countries contributed oral and poster presentations. Support for the travel o f overseas speakers was supplied by T h e Petroleum Research Fund, the A C S D i v i s i o n o f Polymer Chemistry, Inc., Rohm & Haas Company, and GelTex, Inc.


Himadri B. Bohidar School o f Physical Sciences J. N. University N e w D e l h i 11067, India

Paul Dubin Chemistry Department Indiana-Purdue University Indianapolis, I N 46202

Yoshihito Osada D i v i s i o n o f Biological Science Hokkaido University Sapporo, Japan

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Introduction The history o f gels goes back for many centuries, probably even many millennia. Some scientists would even claim that life itself is based on gels. Their scientific study, however, is more recent, dating essentially from the work o f Thomas Graham in the nineteenth century. The saying that the gel state is easier to recognize than to define still holds true today, because the term is used by different communities to describe a host o f seemingly different objects. These include networks formed by long flexible polymers (e.g., neutral polymers such as poly(dimethylsiloxane), natural rubber, polyacrylamide, or polyelectrolytes such as sodium polyacrylate) cross-linked by permanent covalent bonds and containing a mobile solvent. They include as w e l l quasi-rigid structures (agarose, caragheenan, and so on) in which the connecting struts are bundles o f stiff macromolecules. The latter are frequently thermoreversible and the constituent molecules are generally o f biological origin. Other gel systems are formed from smaller molecules in solution that, under appropriate conditions o f temperature and composition, self-assemble into columns and thence into a three-dimensional network. A further category that is often classified among the genus gels includes arrays o f molecular units undergoing fleeting associations and that therefore flow over time. Y e t another category is aerogels and xerogels, in which no solvent is present in the pores. The common characteristic o f all these systems is that their shear elastic modulus is several orders o f magnitude smaller than their bulk modulus: This condition is met in structures where the sequence o f molecules that connect cross-links is much longer than the size o f the cross-links themselves. The ability o f such structures to absorb large amounts o f solvent makes them ideal vehicles for the storage or transport o f active ingredients. Applications o f gels have become extraordinarily widespread, notably in food processing, cosmetics, and paint manufacturing. Polyelectrolyte gels have been developed as superabsorbent materials in diapers and for moisture control for arid soils. A c t i v e research is being conducted into these systems for use as

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decontaminants for heavy metals and organic pollutants. Recently biomedical applications have taken on immense importance, not only for soft contact lenses, eye implants or for gel chromatography, but in many other fields, such as biosensors, drug release vectors, encapsulating media, and so on. In the 1940s and 1950s, the study o f gels was dominated by the pioneering contributions o f Flory, Stockmeyer, Gee, Treloar, and others who established the basic statistical concepts governing the macroscopic elastic and osmotic properties o f these systems. A t that time, details o f the structure at a local scale could only be guessed at, with even less being known about the dynamics. Subsequent investigations by Dusek, Prins, Silberberg, and others based on a variety o f techniques, such as classical light scattering or permeability measurements, demonstrated that gels are intrinsically inhomogeneous in their structure. The innovative work o f Tanaka, who used quasi-elastic light scattering to study the dynamics o f gels, was an important stimulus for theoreticians, notably Edwards and de Gennes, to seek a more local description o f these systems. The contemporaneous development o f small-angle neutron scattering ( S A N S ) techniques with the possibility o f using partial deuteration opened totally new horizons, enabling measurements to be made o f the properties o f individual marked coils inside networks. In these advances the contributions o f several French groups, notably inspired by Benoit, were outstanding. Although many other techniques, such as N M R or fluorescence spectroscopy, have also helped in reaping a rich harvest o f knowledge on gels, scattering techniques in general have proved outstandingly successful in resolving the structure and the dynamics o f swollen networks. It has been demonstrated for example, that the thermodynamic response o f these systems is the same, whether it is measured by S A N S , by dynamic light scattering, or by macroscopic osmotic observations. It has also been shown that the intrinsic nonuniformities in concentration are governed by the same thermodynamic forces. Lists o f scientific advances such as this, however, are necessarily partial and can reflect only one individual's personal view. In the past few years significant progress has been made in the understanding o f gels. It is fair to say, however, that in spite o f considerable effort, several areas still remain poorly understood. Polyelectrolyte systems fall into this category, owing to the many length scales that govern their properties. A l s o , our knowledge o f gels containing filler particles, whether as reinforcing agents or as molecules for drug

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release, is incomplete, particularly with respect to their dynamics and their structural response to external forces. Sophisticated experimental techniques, such as neutron spin echo, anomalous small-angle X - r a y scattering, or even X-ray photon correlation spectroscopy, are opening promising avenues, whereas computer simulation techniques are also beginning to be able to handle such complex systems.


Recently, the potential of these disordered systems for performing complex functions in biomedical applications has become increasingly visible, whether as drug-release systems, biosensors, or biocompatible encapsulation devices. It seems likely that biomedicine w i l l become the focus for major developments and the driving force for research into gels in the coming years. It is for this reason that this volume is particularly timely.

Erik Geissler Laboratoire de Spectrometrie Physique Universite de Joseph Fourier de Grenoble B.P. 87, 38402 St. Martin D'Heres France

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Preface, Introduction - American Chemical Society

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