SUPERPOSITION OF SMALL STRAINS ON LARGE DEFORMATIONS AS A PROBE OF NONLINEAR RESPONSE IN POLYMERS

The incremental response DELTA G(t), obtained by superposing a deformation, DELTA gamma , on a large deformation, gamma //1, has been determined in step shear experiments for a polyisobutylene solution and for a poly(methylmethacrylate) glass in torsion. For both systems DELTA G(t) at gamma //1 was found to be smaller than the linear viscoelastic modulus, G(t), at zero prestrain. The results for the solution are well described by the nonlinear constitutive equation of the BKZ elastic fluid theory. It is found that DELTA G(t) for the glass falls between the behavior predicted by the BKZ theory and the linear viscoelastic behavior.