Experimental and constitutive analyses of the stress relaxation behavior of glassy polymers

Abstract

The physical mechanism underlying the mechanical behavior of glassy polymers has been studied over decades but remains a long‐standing issue. A consensus view achieved is that the yield, flow, and stress relaxation behaviors are due to structural relaxation in the polymer mainly caused by chain conformation transitions. This is the key physical idea behind the many existing elastic–plastic constitutive models for glassy polymers. In this paper, such a constitutive model was employed for predicting and analyzing the stress relaxation of a glassy polymer. It is found that the model works well in predicting the pre‐yield stress relaxation but significantly underestimates the post‐yield stress relaxation. As considering the chain conformation transition alone leads to a dilemma for the model to concurrently represent the yield/flow and stress relaxation behaviors, the model was extended to incorporate an additional structural relaxation mechanism assumed to originate from the dissociation of weak linkages in the chain network. The extended model succeeds in concurrently representing the yield/flow, and stress relaxation behaviors in the whole deformation region, of which the reasons were analyzed. The knowledge revealed in this paper is instructive and may shed new light on understanding the structural relaxation and mechanical behavior of glassy polymers.

Evaluation of the stress relaxation behavior of poly (L‐lactic acid) self‐expanding braided stents

“…The wavy change was obvious when the temperature was higher (37°C, 42°C), while the wavy could only be observed at low temperature (25°C) through enlarged view. It was speculated that as the temperature increased, the segmental motion between molecules became more free and the range of motion increased, thus leading to a more pronounced wave‐like change 38–40 …”

“…It was speculated that as the temperature increased, the segmental motion between molecules became more free and the range of motion increased, thus leading to a more pronounced wave-like change. [38][39][40] The normalized radial strength curve was presented in Figure 3B, as the hold time increased, the radial strength of braided stent at the same temperature decreased continuously. For stents at human body temperature, its normalized radial strength at time 300 s was decreased by 22.2% compared to that at time 0 s. Compared to the normalized radial strength at time 0 s, the declining ratio of braided stents at 25 and 42 C were 17.5% and 25.7%, respectively.…”