Microscopic study of structure/property interrelation of amorphous polymers during uniaxial deformation: A molecular dynamics approach

Abstract

A molecular dynamics model was established to simulate the uniaxial deformation of polymer materials. Efforts were made on revealing the relation of microstructure evolution on mechanical properties of polymers at different temperature and strain rate. The numerical model semi-quantitatively represented the time-temperature equivalence of polymer materials. Conformational parameters such as the bond length, bond angle, and radius of gyration were measured quantitatively, and it showed that the change of mechanical properties with increasing temperature is due to the increase in atomic kinetic energy. And as shown in the simulation, above Tg, the atomic thermal movement surpassed the particle pair-wise potential and chain entanglement. The in-situ simulation was conducted during uniaxial deformation. The results showed that chain segments are constrained around their equilibrium positions in elastic deformation stage. While after yielding, the entanglement density decrease rapidly, the orientation parameter and content of dihedral angles in trans state increase aggressively after yielding, indicating that the pair-wise forces were surpassed and the bonds and bond angles were stretched. The energy analysis of interatomic potentials also showed that the sliding between the macromolecules is dominating after the chains are stretched and extended. The proposed coefficient, potential ratios, parameterized the influences of different structural elements during deformation. All the potential ratios exhibited extremums at yield points, illustrating that the van der Waals potential is dominating before yielding but overwhelmed by bond and angle potentials afterwards. Consequently, Tg can be described as a critical point above which the effects of excess kinetic energy exceed the pair-wise potential, which explains that yielding point turned to vague at temperatures above Tg.