Conformational and Structural Relaxations of Poly(ethylene oxide) and Poly(propylene oxide) Melts: Molecular Dynamics Study of Spatial Heterogeneity, Cooperativity, and Correlated Forward-Backward Motion

TL;DR

This study uses molecular dynamics simulations to analyze the relaxation behaviors of poly(ethylene oxide) and poly(propylene oxide) melts, revealing temperature-dependent deviations from simple models and highlighting the importance of heterogeneity, cooperativity, and correlated motions in their dynamics.

Contribution

It provides a detailed molecular-level analysis of relaxation processes, including the role of spatial heterogeneity and correlated motions, which advances understanding beyond existing theories.

Findings

Relaxation times deviate from Arrhenius behavior with temperature.

Mode-coupling theory partially explains glassy slowdown.

Correlated forward-backward motions dominate at low temperatures.

Abstract

Performing molecular dynamics simulations for all-atom models, we characterize the conformational and structural relaxations of poly(ethylene oxide) and poly(propylene oxide) melts. The temperature dependence of these relaxation processes deviates from an Arrhenius law for both polymers. We demonstrate that mode-coupling theory captures some aspects of the glassy slowdown, but it does not enable a complete explanation of the dynamical behavior. When the temperature is decreased, spatially heterogeneous and cooperative translational dynamics are found to become more important for the structural relaxation. Moreover, the transitions between the conformational states cease to obey Poisson statistics. In particular, we show that, at sufficiently low temperatures, correlated forward-backward motion is an important aspect of the conformational relaxation, leading to strongly nonexponential distributions for the waiting times of the dihedrals in the various conformational states