Mechanism of complicated volume deformation in polymers and its fractional time-based description

In tension and compression of polymers, some abnormal phenomena of volume changes were often observed and its mechanism is still unclear. In this paper, based on the thermodynamics principle, it is shown that the combined effects of internal energy and entropy are the basic reason of causing the anomaly in polymers. Moreover, to describe the complicated deformation through a simple model and have a correct modelization of mechanical behaviors, a volume strain model is presented, in which the variation of volume strain resulted from the combined effects is characterized through a fractional integral expression. Comparison between fitting results and experimental results reveals the proposed model can describe the volume deformation of polymers.