SWELLING EQUILIBRIA FOR POSITIVELY IONIZED POLYACRYLAMIDE HYDROGELS

Swelling equilibria are reported for polyacrylamide gels in water and for copolymer gels containing acrylamide and [(methacrylamido)propyl]trimethylammonium chloride (MAPTAC) in aqueous NaCl solutions. Gel swelling was investigated as a function of gel structure (cross-link density and monomer concentration), degree of gel ionization (relative amount of charged comonomer) and solution ionic strength. A gel-swelling model is presented which describes polymer/solvent mixing effects using a recently proposed lattice model developed for aqueous/polymer systems; this model accounts for hydrogen bonding in aqueous solutions by distinguishing between different types of contact sites on a solvent molecule or polymer segment. The elastic contribution to swelling is represented using a network theory which accounts for nonaffine displacement of network junctions under strain; polyelectrolyte effects on swelling are described using ideal Donnan theory. Swelling equilibria for uncharged polyacrylamide networks in water are correlated using the gel-swelling model. The model describes reasonably well the effect of cross-link density on swelling but fails to reproduce accurately the dependence of swelling on monomer concentration at preparation. Exchange-energy parameters obtained from polyacrylamide gel-swelling measurements are used to predict swelling in salt solutions for acrylamide/MAPTAC copolymer gels. The model predicts well the effect of gel charge density and solution ionic strength on swelling; however, the effect of monomer concentration is not accurately predicted. Further work is needed to quantify the effect of monomer concentration on the swelling and elastic properties of polyacrylamide hydrogels. © 1990, American Chemical Society. All rights reserved.