Molecular-Dynamics Simulation of a Glassy Polymer Melt: Incoherent Scattering Function

TL;DR

This study uses molecular dynamics simulations to analyze monomer dynamics in a supercooled polymer melt, validating key theoretical predictions of mode-coupling theory through detailed scattering function analysis.

Contribution

It provides the first simulation-based evidence supporting the space-time factorization and time-temperature superposition principles in polymer melts near the glass transition.

Findings

Evidence for the space-time factorization theorem in beta-relaxation.

Validation of the time-temperature superposition principle in alpha-regime.

Qualitative agreement of wave-vector dependence with hard sphere calculations.

Abstract

We present simulation results for a model polymer melt, consisting of short, nonentangled chains, in the supercooled state. The analysis focuses on the monomer dynamics, which is monitored by the incoherent intermediate scattering function. The scattering function is recorded over six decades in time and for many different wave-vectors. The lowest temperatures studied are slightly above the critical temperature of mode-coupling theory (MCT), which was determined from a quantitative analysis of the beta- and alpha-relaxations. We find evidence for the space-time factorization theorem in the beta-relaxation regime, and for the time-temperature superposition principle in the alpha-regime, if the temperature is not too close to the critical temperature. The wave-vector dependence of the nonergodicity parameter, of the critical amplitude, and the alpha-relaxation time are in qualitative agreement with calculations for hard spheres. For wave-vectors larger than the maximum of the structure factor the alpha-relaxation time already agrees fairly well with the asymptotic MCT-prediction. The behavior of the relaxation time at small wave-vectors can be rationalized by the validity of the Gaussian approximation and the value of the Kohlrausch stretching exponent.