SUPERPOSITION OF SMALL DEFORMATIONS ON LARGE DEFORMATIONS: MEASUREMENTS OF THE INCREMENTAL RELAXATION MODULUS FOR A POLYISOBUTYLENE SOLUTION.

The incremental relaxation modulus DELTA G(t) for a concentrated solution of polyisobutylene has been determined from step-shear experiments in which a small deformation DELTA gamma was superimposed on a large deformation gamma //1; DELTA G(t) was found to decrease with increasing gamma //1 and to increase with the time t//e after the imposition of the large deformation. It was also observed that the 'apparent relaxation spectrum' associated with DELTA G(t) narrows and shifts to shorter times when compared to the spectrum associated with the linear viscoelastic relaxation modulus G(t). The results are well described by the nonlinear constitutive equation of the BKZ elastic fluid theory.