Nonlinear Elasticity and Diffusio-Mechanical Coupling of Elastomeric Polymer Networks Reveald by Multiaxial Stretching

General biaxial stretching with varying the strains independently in the two orthogonal directions covers a wide range of accessible deformation for incompressible materials such as gels and elastomers. A custom-built biaxial tester enables us to characterize the nonlinear elasticity of novel polymer gels including the Tetra-PEG gels with uniform network structure, the slide-ring gels with movable cross-links along the network strands, and the T-PEG double network gels showing the pronounced upturn of stress at high elongation. General biaxial stretching experiments are also employed to characterize the pronounced stress-softening in filled elastomers (Mullins effect). We also demonstrate that the volume change and the accompanying force reduction driven by imposed strain for fully swollen gels considerably depend on the degree of strain as well as the type of stretching.