Coarse-Grained Molecular Dynamics Simulations for Oxidative Aging of Polymers under Various O2 Concentration

TL;DR

This study extends mesoscale simulations of polymer oxidative aging by modeling oxygen addition rates to explore how varying O2 concentrations affect reaction kinetics, heterogeneity, and crosslinking in polymers.

Contribution

It introduces a model for oxygen addition rate based on O2 concentration, revealing its impact on aging dynamics and heterogeneity in polymer simulations.

Findings

Lower O2 levels slow radical conversion and H-abstraction reactions.

Reduced O2 concentration increases polymer radical concentration.

Decreased O2 accelerates crosslinking reactions.

Abstract

Modeling of polymer oxidative aging has been actively studied since the 1990s. Insights from these studies suggest that the transport of oxygen and radicals significantly influences aging heterogeneity, alongside chemical reaction kinetics. A recent simulation study [Ishida et al., Macromolecules, 56(21), 8474-8483, 2023] demonstrated that mesoscale heterogeneity arises when the H-abstraction reaction occurs faster than the relaxation times of polymer chains. In this study, the simulations were extended by modeling the rate of oxygen addition to polymer radicals (k_2) to reflect the effects of the O2 concentration. Three key aspects of oxidative aging behavior were found to be influenced by the O2 addition rate: (i) reaction kinetics, (ii) the degree of heterogeneity, and (iii) amount of crosslinking. Namely, reducing O2 concentration slows the conversion of polymer radicals into H-abstractable peroxyl radicals. This deceleration delays H-abstraction reactions, increases the number of polymer radicals, and promotes crosslinking reactions between two polymer radicals.