Monte Carlo and Molecular Dynamics Simulation of the Glass Transition of Polymers

TL;DR

This study uses computer simulations of two coarse-grained polymer models to investigate the glass transition, revealing that mode coupling theory explains the slowdown in dynamics and challenging the entropy catastrophe hypothesis.

Contribution

It compares lattice and off-lattice models to analyze the glass transition, testing mode coupling and entropy theories through simulations.

Findings

Both models exhibit similar mesoscopic behavior despite microscopic differences.

Mode coupling theory effectively describes the slowing dynamics near the glass transition.

No entropy catastrophe occurs, contradicting some theoretical predictions.

Abstract

Two coarse-grained models for polymer chains in dense glass-forming polymer melts are studied by computer simulation: the bond-fluctuation model on a simple cubic lattice, where a bond-length potential favors long bonds, is treated by dynamic Monte Carlo methods, and a bead-spring model in the continuum with a Lennard-Jones potential between the beads is treated by Molecular Dynamics. While the dynamics of both models differ for short length scales and associated time scales, on mesoscopic spatial and temporal scales both models behave similarly. In particular, the mode coupling theory of the glass transition can be used to interpret the slowing down of the undercooled polymer melt. For the off-lattice model, the approach to the critical point of mode coupling is both studied for constant pressure and for constant volume. The lattice model allows a test of the Gibbs-Di Marzio entropy theory of the glass transition, and our finding is that although the entropy does decrease significantly, there is no ``entropy catastrophe'', where the fluid entropy would turn negative. Finally, an outlook on confinement effects on the glass transition in thin film geometry is given.