Transition from linear to nonlinear viscoelastic behavior. Part II. stress relaxation of polycarbonate

Below the glass transition temperature Tg (ca. 150° C), the vis-coelastic behavior of the polycarbonate of bisphenol A is linear up to a strain level of about 1%, whereas, at higher strains, the iso-chronous relaxation modulus drops steadily with increasing strain. The critical strain level diminishes from about 1.2 to about 0.7% as the temperature of glassy polycarbonate increases from 23° to 130° C. Comparison of this finding with earlier independent observations above Tg indicates rather unexpectedly that, in terms of the strain level alone, linear viscoelasticity theory should be minimally applicable near Tg. These conclusions emerge from a detailed study of stress relaxation at constant uniaxial strain performed on amorphous, elastically isotropic specimens of this polycarbonate.