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

Homogeneous polymer melts show strain hardening, while recent uniaxial elon gational studies have revealed that some heterogeneous polymer melts show s train softening. A systematic description and explanation about strain hard ening of homogeneous polymers and strain softening of heterogeneous polymer s 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 r elaxation time on the strain hardening or softening are investigated in ter ms of intensity and critical strain. The intensity, that is, the degree of hardening or softening, is defined as the degree of positive or negative de viation from the Trouton rule. The critical strain is defined as the strain at which the deviation starts. The intensity is strongly affected by the d amping function. If alpha, the damping parameter in the said damping functi on, is more than 2, strain hardening appears. With alpha less than 2, strai n softening occurs. It is suggested that strain hardening or softening in v arious polymer melts is able to be characterized by showing the location of each polymer in the proposed figures. It is proposed that a systematic des cription in terms of the BKZ model is capable of explaining not only the st rain hardening of homogeneous polymer melts, but also the strain softening of heterogeneous polymer melts.