Coupling model analysis of interchain coupled chain dynamics of PEO in blends with PMMA

TL;DR

This paper analyzes the dynamics of PEO chains in PEO/PMMA blends, showing a crossover from Rouse-like to slower relaxation, and offers a simpler explanation for observed anomalies without assuming broad friction coefficient distributions.

Contribution

The study presents a unified explanation for chain relaxation crossover in polymer blends, challenging the need for the broad distribution assumption of monomer mobilities in the RRM model.

Findings

PEO chain dynamics follow Rouse model at short times

At longer times, dynamics slow down and deviate from Rouse predictions

The crossover time tc is key to understanding relaxation behavior

Abstract

Quasielastic neutron scattering and molecular dynamics simulation data from PEO/PMMA blends found that for short times the self-dynamics of PEO chain follows the Rouse model, but at longer times past tc=1 to 2 ns it becomes slower and departs from the Rouse model in dependences on time, momentum transfer, and temperature. To explain the anomalies, others had proposed the random Rouse model (RRM) in which each monomer has different mobility taken from a broad log-normal distribution. Despite the success of the RRM, Diddens, Brodeck and Heuer [EPL, 95, 56003 (2011)] extracted the distribution of friction coefficients from the MD simulations of a PEO/PMMA blend and found the distribution is much narrower than expected from the RRM. We propose a simpler alternative explanation of the data by utilizing alone the observed crossover of PEO chain dynamics at tc. The present problem is just a special case of a general property of relaxation in interacting systems, which is the crossover from independent relaxation to coupled many-body relaxation at some tc determined by the interaction potential. The generality is brought out vividly by pointing out that the crossover also had been observed by neutron scattering from entangled chains relaxation in monodisperse homo-polymers, and from the segmental \alpha-relaxation of PEO in blends with PMMA. The properties of all the relaxation processes in connection with the crossover are similar, despite the length-scales of the relaxation in these systems are widely different.