Compressional and shear behaviour of weakly ionic polyacrylamide gels

Abstract

The shear and compressional elastic properties of weakly ionic acrylamide-based hydrogels are discussed. The equilibrium shear modulus of gels measured at low deformations is considered as a function of swelling degree, ionic monomer mole fraction, crosslinker mole fraction and initial total monomer concentration. The modulus of a gel does not decrease as a power of its swelling degree but passes a distinctive minimum. To describe this non-Gaussian behaviour, we take account of the finite extensibility of network chains and the transition of chains to the globular conformation when deswelling a gel from its initial state. Good agreement is obtained between so predicted and measured modulus values. To characterize compressibility of gels, the swelling pressure is measured. We analyse different swelling-pressure contributions as functions of swelling degree, ionic monomer mole fraction and concentration of salt in the outer solution. The mixing and elastic contributions are found to be approximated by the Flory equations. The ionic contribution is mainly determined by the difference in concentrations of mobile ions inside and outside the gel. It can be described taking into consideration the ideal Donnan equilibrium and dissociation of ionogenic groups.