Conformation-Induced Stiffening Effect of Crosslinked Polymer Thin Films

TL;DR

This study combines computational modeling and experiments to uncover how conformational changes in crosslinked polymer thin films lead to increased stiffness, providing insights for tailoring their mechanical properties at the nanoscale.

Contribution

It introduces a coarse-grained molecular dynamics framework and a unified theory to explain the microscopic origin of stiffening in polymer films, validated by experiments.

Findings

Polymer conformations significantly influence elastic modulus.

A unified theory links chain end-to-end distances to stiffness.

Experimental results show up to two orders of magnitude difference in modulus.

Abstract

Nanoscale polymeric thin films are widely used in diverse applications such as energy devices, flexible electronics and biosensors, where a satisfactory mechanical performance is of vital importance to realize their full functionality. It has been evidenced that the elastic properties of polymer films are often strongly affected by their thickness; however, the underlying mechanism of this phenomenon, especially a thorough understanding at the microscopic level, has yet to be achieved. Here we established a coarse-grained molecular dynamics (CGMD) based computational framework, combining with experimental verifications, aiming to reveal the conformational origin of the stiffening behavior of crosslinked polymeric thin films. By imposing systematic controls over the polymer network structures, we found that the bi-axial modulus changes are essentially consequent of the alteration of polymer conformations. A unified theory was then proposed, to quantitatively clarify the correlation between the elastic properties of the system and the distributional variations of the chain end-to-end distances, with predicting a significant hardening effect on top of the conventional entropic elasticity with largely uncoiled chains. Adopting processing protocols inspired by the modeling, our experiments showed that PDMS films at approximately the same thickness may exhibit a two order of magnitude difference in their moduli. The good agreement between experiments and simulations illustrated our findings as an effective guideline for tailoring the elastic properties of polymer films at nanoscale.