Shear modulus of polyelectrolyte gels under electric field

Abstract

The shear modulus of two polyelectrolyte gels poly(acrylic acid), a weak acid with small side groups, and poly(2-acrylamide-2-methylpropane sulfonic acid), a strong acid with large side groups, have been measured with and without application of an electric field across a gel sample swollen with water to equilibrium. In the absence of electric field the shear modulus of PAMPS gels made to different crosslink degrees is shown to be inversely proportional to the swelling degree, in accord with theoretical prediction. Under a steady electric field the measured modulus of these gels is seen to reduce with time. This is explained by migration of the free counter ions and associated water towards the cathode, resulting in reduced contact between gel and rheometer due to exuded water. Theoretical prediction of G' and G'' in the presence of a thin slip layer are in good quantitative agreement with experimental observation. Preliminary measurements of the reduction of shear modulus under pulsed electric field have been obtained and an unexpected recovery of the initial modulus is seen on all subsequent applications of the field, before the continued reduction to successively lower values. When roughened platens are used a step-wise variation in measured modulus is seen, with a slightly lower modulus being recorded in the presence of an applied electric field.