Molecular Dynamics Simulations of the Thermal Glass Transition in Polymer Melts: Alpha-Relaxation Behaviour

TL;DR

This study uses molecular dynamics simulations to analyze the structural and alpha-relaxation behavior during the thermal glass transition in dense polymer melts, highlighting ensemble-dependent differences in relaxation dynamics.

Contribution

It introduces a model with competing length scales to prevent crystallization and compares relaxation behaviors under different thermodynamic ensembles.

Findings

Relaxation behavior fits mode coupling theory predictions.

Critical temperature and relaxation exponents depend on the thermodynamic ensemble.

Structural properties vary with temperature and simulation conditions.

Abstract

We present Molecular Dynamics simulations of the thermal glass transition in a dense model polymer liquid. We performed a comparative study of both constant volume and constant pressure cooling of the polymer melt. Great emphasis was laid on a careful equilibration of the dense polymer melt at all studied temperatures. Our model introduces competing length scales in the interaction to prevent any cristallisation tendency. In this first manuscript we we analyse the structural properties as a function of temperature and the long alpha-relaxation behaviour as observed in the dynamic structure factor and the self-diffusion of the polymer chains. The relaxation can be consistently analysed in terms of the mode coupling theory (MCT) of the glass transition. The mode coupling critical temperature and the exponent gamma, defining the power law divergence of the alpha-relaxation timescale both depend on the thermodynamic ensemble employed in the simulation.