Biaxial strain testing of extremely soft polymer gels

Abstract

We present a biaxial tensile tester to characterize the nonlinear stress–strain behavior of extremely soft polymer gels with very low shear moduli, of the order of 10² Pa, under general (equal and unequal) biaxial strain. Stretching of gel sheet specimens in a solvent bath can avoid finite self-weight bending deformations that have precluded biaxial tensile experiments with such extremely soft gels. General biaxial strain covers a wide range of physically accessible deformations in contrast to the conventional uniaxial strain that is only a special one among them. We have used this tensile tester to examine the biaxial stress–strain relations of two types of hydrogel with water contents of more than 95 wt%, namely “chemical gels” with covalent cross-links and “physical gels” with noncovalent cross-links (microcrystallites acting as cross-links). Biaxial data provide a definite basis for the discussion of the differences in the nonlinear elasticities of these hydrogels at very low network concentrations. In particular, the biaxial data for fully swollen chemical gels reveal that the exceptional agreement of the data in a wide range of uniaxial strain with predictions of the simplest rubber elasticity model (ideal gas model), which has long been known, is superficial because the model evidently fails to describe the biaxial data. This new biaxial tester will be a powerful tool for the full characterization of the large deformation behavior of extremely soft materials, including biological soft tissues.