Strain-Driven Swelling and Accompanying Stress Reduction in Polymer Gels under Biaxial Stretching

Strain-driven swelling and the accompanying stress reduction in polymer gels are investigated experimentally under general biaxial deformation, with strain being varied independently in two directions. The equilibrium degrees of strain-induced swelling and stress reduction depend markedly on both the magnitude and the type of the imposed deformation. When subjected to a constant elongation in one direction, the strain-induced swelling in biaxial stretching increases with an increase in the elongation in the other direction: When a gel is equibiaxially stretched by a factor of 2, the equilibrium volume is approximately 2.5 times greater than the initial volume. The degree of stress reduction in the direction of the smaller imposed strain is greater than that in the other direction: The stress in the constrained direction of planar extension is reduced by more than 40%. The time course of the stress reduction is governed by a diffusion process, i.e., the dynamics of swelling. The main features of the experimental results are satisfactorily explained by a classical theory for swelling of gels.