Shear-stress fluctuations in free-standing polymer films

TL;DR

This study uses molecular dynamics simulations to analyze shear-stress fluctuations in free-standing polymer films, revealing how shear modulus and relaxation behavior depend on temperature, film thickness, and sampling time, with surface effects linearly superimposed on bulk properties.

Contribution

It provides a detailed analysis of shear-stress fluctuations in polymer films, demonstrating linear superposition of surface and bulk effects and the continuous decrease of shear modulus with temperature.

Findings

Shear modulus and relaxation modulus decrease with temperature.

Observables vary linearly with inverse film thickness.

No jump singularity observed in shear modulus.

Abstract

Using molecular dynamics simulation of a polymer glass model we investigate free-standing polymer films focusing on the in-plane shear modulus $\mu$ and the corresponding shear-stress relaxation modulus $G(t)$ as functions of temperature $T$, film thickness $H$ (tuned by means of the lateral box size $L$) and sampling time $\Delta t$. Various observables are seen to vary linearly with $1/H$ demonstrating thus the (to leading order) linear superposition of bulk and surface properties. In agreement with recent studies on three-dimensional polymer glass-formers, $\mu$ and $G(t)$ are found to decrease continuously with $T$. A jump-singularity is not observed. Confirming the time-translational invariance of our systems, the $\Delta t$-dependence of $\mu$ is traced back to $G(t)$.