A systematic description by BKZ model of strain hardening or softening in uniaxial elongation of polymer melts

Abstract

Homogeneous polymer melts show strain hardening, while recent uniaxial elongational studies have revealed that some heterogeneous polymer melts show strain softening. A systematic description and explanation about strain hardening of homogeneous polymers and strain softening of heterogeneous polymers is still needed. Here, the Bernstein, Kearsley, and Zapas (BKZ) model is applied to give a systematic description using model relaxation spectra and model damping functions. The effects of damping function and the longest relaxation time on the strain hardening or softening are investigated in terms of intensity and critical strain. The intensity, that is, the degree of hardening or softening, is defined as the degree of positive or negative deviation from the Trouton rule. The critical strain is defined as the strain at which the deviation starts. The intensity is strongly affected by the damping function. If α, the damping parameter in the said damping function, is more than 2, strain hardening appears. With a less than 2, strain softening occurs. It is suggested that strain hardening or softening in various polymer melts is able to be characterized by showing the location of each polymer in the proposed figures. It is proposed that a systematic description in terms of the BKZ model is capable of explaining not only the strain hardening of homogeneous polymer melts, but also the strain softening of heterogeneous polymer melts.