The Role of Intramolecular Barriers on the Glass Transition of Polymers: Computer Simulations vs. Mode Coupling Theory

TL;DR

This study uses computer simulations and Mode Coupling Theory to explore how intramolecular barriers influence the glass transition in polymers, revealing complex interactions between packing effects and chain stiffness.

Contribution

It provides a detailed comparison between simulation results and MCT predictions for polymers with varying intramolecular barriers, highlighting the interplay of different dynamic arrest mechanisms.

Findings

Increasing barrier strength raises the critical temperature and localization length.

MCT accurately predicts trends for flexible to moderately stiff chains.

Discrepancies occur for very stiff chains, indicating limits of current theoretical models.

Abstract

We present computer simulations of a simple bead-spring model for polymer melts with intramolecular barriers. By systematically tuning the strength of the barriers, we investigate their role on the glass transition. Dynamic observables are analyzed within the framework of the Mode Coupling Theory (MCT). Critical nonergodicity parameters, critical temperatures and dynamic exponents are obtained from consistent fits of simulation data to MCT asymptotic laws. The so-obtained MCT $\lambda$-exponent increases from standard values for fully-flexible chains to values close to the upper limit for stiff chains. In analogy with systems exhibiting higher-order MCT transitions, we suggest that the observed large $\lambda$-values arise form the interplay between two distinct mechanisms for dynamic arrest: general packing effects and polymer-specific intramolecular barriers. We compare simulation results with numerical solutions of the MCT equations for polymer systems, within the polymer reference interaction site model (PRISM) for static correlations. We verify that the approximations introduced by the PRISM are fulfilled by simulations, with the same quality for all the range of investigated barrier strength. The numerical solutions reproduce the qualitative trends of simulations for the dependence of the nonergodicity parameters and critical temperatures on the barrier strength. In particular, the increase of the barrier strength at fixed density increases the localization length and the critical temperature. However the qualitative agreement between theory and simulation breaks in the limit of stiff chains. We discuss the possible origin of this feature.