COMPRESSIONAL AND SHEAR BEHAVIOR OF WEAKLY IONIC POLYACRYLAMIDE GELS

The shear and compressional elastic properties of weakly ionic acrylam ide-based hydrogels are discussed. The equilibrium shear modulus of ge ls measured at low deformations is considered as a function of swellin g degree, ionic monomer mole fraction, crosslinker mole fraction and i nitial total monomer concentration. The modulus of a gel does not decr ease as a power of its swelling degree but passes a distinctive minimu m. To describe this non-Gaussian behaviour, we take account of the fin ite extensibility of network chains and the transition of chains to th e globular conformation when deswelling a gel from its initial state. Good agreement is obtained between so predicted and measured modulus v alues. To characterize compressibility of gels, the swelling pressure is measured. We analyse different swelling-pressure contributions as f unctions of swelling degree, ionic monomer mole fraction and concentra tion of salt in the outer solution. The mixing and elastic contributio ns are found to be approximated by the Flory equations. The ionic cont ribution is mainly determined by the difference in concentrations of m obile ions inside and outside the gel. It can be described taking into consideration the ideal Donnan equilibrium and dissociation of ionoge nic groups. Copyright (C) 1996 Elsevier Science Ltd.