Physically-based interpretation of abnormal stress relaxation response in glassy polymers

Abstract

Understanding the mechanical responses of glassy polymers remains a grand scientific challenge for many decades, which plays a central role in the application of these materials. In this work, we have performed stress relaxation tests on a series of glassy polymers. An abnormal rate-dependent stress relaxation response is revealed, featuring as a larger stress drop and smaller quasi-steady-state stress with increasing the initial loading rate. To explore the underlying deformation mechanism, we further carry out a coarse-grained molecular dynamics simulation on glassy polymers, which qualitatively demonstrates the experimental observations. The underlying physical mechanism is revealed to be that a larger initial loading rate can lead to a more-activated structure state of polymers. Based on these findings, we develop a physically-based model by introducing the effective temperature model into the shear transformation zone (STZ) theory. The effective temperature, as an internal variable, can describe the deformation-induced structural evolution of amorphous polymers. The Adam–Gibbs model is then adopted to relate the effective temperature with the transformation rates of STZ sites. Comparison with experimental results shows that the micro-mechanical model can capture the rate-dependent stress–strain relationship in the loading process as well as the abnormal stress relaxation responses. In comparison, it is challenging to rationalize this abnormal behavior based on classic viscoplastic models due to a lack of physical mechanisms. Thus, this work promotes our understanding of the origin of complex mechanical responses of glassy polymers that are strongly related to the evolution of microstructure upon deformation.