Nonmonotonic dependence of polymer glass mechanical response on chain bending stiffness

TL;DR

This study reveals that the mechanical response of amorphous polymers exhibits a nonmonotonic dependence on chain bending stiffness, with initial softening followed by hardening, due to a balance between backbone resistance and material density.

Contribution

It introduces a combined simulation and theoretical approach to uncover the nonmonotonic relationship between chain stiffness and polymer glass strength.

Findings

Small increases in bending stiffness soften the material initially.

Further increases in stiffness lead to material hardening.

The nonmonotonic behavior results from a competition between resistance and density decrease.

Abstract

We investigate the mechanical properties of amorphous polymers by means of coarse-grained simulations and nonaffine lattice dynamics theory. A small increase of polymer chain bending stiffness leads first to softening of the material, while hardening happens only upon further strengthening of the backbones. This nonmonotonic variation of the storage modulus $G'$ with bending stiffness is caused by a competition between additional resistance to deformation offered by stiffer backbones and decreased density of the material due to a necessary decrease in monomer-monomer coordination. This counter-intuitive finding suggests that the strength of polymer glasses may in some circumstances be enhanced by softening the bending of constituent chains.