Structure, molecular dynamics, and stress in a linear polymer under dynamic strain

TL;DR

This study investigates how molecular structure and dynamics influence the stress response of linear polymers under dynamic strain, highlighting the roles of bond deformation, chain length, and entanglement effects.

Contribution

It provides new insights into the molecular mechanisms governing stress in polymers, especially the impact of chain length and entanglement on their mechanical behavior.

Findings

Bond deformations dominate stress at high densities.

Stress behavior changes significantly beyond chain length 50.

Temperature and loading rate have mild effects on stress.

Abstract

The structural properties of a linear polymer and its evolution in time have a strong bearing on its anisotropic stress response. The mean-square bond length and mean bond angle are the critical parameters that influence the time-varying stress developed in the polymer. The bond length distribution along the chain is uniform without any abrupt changes at the ends. Among the externally set parameters such as density, temperature, strain rate, and chain length, the density as well as the chain length of the polymer have a significant effect on the stress. At high density values, changes in mean-square bond length dominates over changes in radius of gyration and end-to-end length. In other words, bond deformations dominate as opposed to changes in size and shape. Also, there is a large change in the mean-square bond length that is reflected as a jump in the stress. Beyond a particular value of the chain length, $n = 50$, called the entanglement length, stress-response is found to have distinctly different behavior which we attribute to the entanglement effects. Short chain polymers more or less behave like rigid molecules. There is no significant change in their internal structure when loaded. Further, temperature and rate of loading have a very mild effect on the stress. Besides these new results, we can now explain well known polymeric mechanical behavior under dynamic loading from the point of view of the evolution of the molecular dynamics and the derived structural properties. This could possibly lead to polymer synthesis with desired mechanical behavior.