Short-Time Elasticity of Polymer Melts: Tobolsky Conjecture and Heterogeneous Local Stiffness

TL;DR

This study uses molecular dynamics to analyze the short-time elastic properties of polymer melts, confirming Tobolsky's conjecture and revealing heterogeneity in local stiffness related to chain ends.

Contribution

It provides detailed insights into the local stiffness distribution and validates Tobolsky's conjecture for polymer melts through extensive simulations.

Findings

Local stiffness varies with monomer position, softer at chain ends.

Infinite-frequency shear modulus increases with chain length.

Plateau modulus is unaffected by chain length and depends on non-bonding interactions.

Abstract

An extended Molecular-Dynamics study of the short-time "glassy" elasticity exhibited by a polymer melt of linear fully-flexible chains above the glass transition is presented. The focus is on the infinite-frequency shear modulus $G_\infty$ manifested in the picosecond time scale and the relaxed plateau $G_p$ reached at later times and terminated by the structural relaxation. The local stiffness of the interactions with the first neighbours of each monomer exhibits marked distribution with average value given by $G_\infty$. In particular, the neighbourhood of the end monomers of each chain are softer than the inner monomers, so that $G_\infty$ increases with the chain length. $G_p$ is not affected by the chain length and is largely set by the non-bonding interactions, thus confirming for polymer melts the conjecture formulated by Tobolsky for glassy polymers.