From caging to Rouse dynamics in polymer melts with intramolecular barriers: a critical test of the Mode Coupling Theory

TL;DR

This study combines simulations and Mode Coupling Theory to analyze how intramolecular barriers affect the dynamics of non-entangled polymer melts, revealing qualitative agreement but also limitations of the theory as chains become stiffer.

Contribution

It critically tests MCT predictions against simulations for polymers with intramolecular barriers, highlighting where the theory succeeds and where it requires improvement.

Findings

Qualitative MCT trends match simulations for weak/moderate barriers.

Discrepancies increase for stiff chains, indicating limitations of MCT.

MCT explains deviations from Rouse model with anomalous relaxation behaviors.

Abstract

By means of computer simulations and solution of the equations of the Mode Coupling Theory (MCT), we investigate the role of the intramolecular barriers on several dynamic aspects of non-entangled polymers. The investigated dynamic range extends from the caging regime characteristic of glass-formers to the relaxation of the chain Rouse modes. We review our recent work on this question, provide new results and critically discuss the limitations of the theory. Solutions of the MCT for the structural relaxation reproduce qualitative trends of simulations for weak and moderate barriers. However a progressive discrepancy is revealed as the limit of stiff chains is approached. This disagreement does not seem related with dynamic heterogeneities, which indeed are not enhanced by increasing barrier strength. It is not connected either with the breakdown of the convolution approximation for three-point static correlations, which retains its validity for stiff chains. These findings suggest the need of an improvement of the MCT equations for polymer melts. Concerning the relaxation of the chain degrees of freedom, MCT provides a microscopic basis for time scales from chain reorientation down to the caging regime. It rationalizes, from first principles, the observed devations from the Rouse model on increasing the barrier strength. These include anomalous scaling of relaxation times, long-time plateaux, and non-monotonous wavelength dependence of the mode correlators.