Cross-linked polymers in strain: Structure and anisotropic stress

TL;DR

This study uses molecular dynamics simulations to explore how cross-linked polymers develop anisotropic stress under strain, highlighting the roles of bond length, chain structure, density, and temperature effects.

Contribution

It provides detailed insights into the micro-structural factors influencing stress responses in cross-linked polymers under strain, emphasizing bond length variations and chain interactions.

Findings

Bond length distribution varies along the chain due to cross-linkers.

Bond deformations dominate over size and shape changes at high density.

Temperature increases bond length but decreases stress due to excluded volume effects.

Abstract

Molecular dynamic simulation enables one to correlate the evolution of the micro-structure with anisotropic stress when a material is subject to strain. The anisotropic stress due to a constant strain-rate load in a cross-linked polymer is primarily dependent on the mean-square bond length and mean-square bond angle. Excluded volume interactions due to chain stacking and spatial distribution also has a bearing on the stress response. The bond length distribution along the chain is not uniform. Rather, the bond lengths at the end of the chains are larger and uniformly decrease towards the middle of the chain from both ends. The effect is due to the presence of cross-linkers. As with linear polymers, at high density values, changes in mean-square bond length dominates over changes in radius of gyration and end-to-end length. That is, bond deformations dominate over changes in size and shape. A large change in the mean-square bond length reflects in a jump in the stress response. Short-chain polymers more or less behave like rigid molecules. Temperature has a peculiar effect on the response in the sense that even though bond lengths increase with temperature, stress response decreases with increasing temperature. This is due to the dominance of excluded volume effects which result in lower stresses at higher temperatures. At low strain rates, some relaxation in the bond stretch is observed from $\epsilon=0.2$ to $\epsilon=0.5$. At high strain rates, internal deformation of the chains dominate over their uncoiling leading to a rise in the stress levels.