Why is Understanding Glassy Polymer Mechanics So Difficult?

TL;DR

This paper explores the complex mechanics of glassy polymers, proposing new dichotomies for understanding their behavior and highlighting how relaxation dynamics influence strain hardening, supported by molecular dynamics simulations.

Contribution

It introduces broader dichotomies (unary/binary and cooperative/noncooperative) for analyzing glassy polymer mechanics and links these to strain hardening phenomena.

Findings

Strain hardening correlates with a transition from unary to binary relaxation.

Densely entangled systems show a crossover from noncooperative to cooperative relaxation.

Molecular dynamics simulations reveal how chain stretching affects local plasticity.

Abstract

In this Perspective, I describe recent work on systems in which the traditional distinctions between (i) unentangled vs. well-entangled systems and (ii) melts vs. glasses seem least useful, and argue for the broader use in glassy polymer mechanics of two more dichotomies: systems which possess (iii) unary vs. binary and (iv) cooperative vs. nonccoperative relaxation dynamics. I discuss the applicability of (iii-iv) to understanding the functional form of glassy strain hardening. Results from molecular dynamics simulations show that the "dramatic" strain hardening observed in densely entangled systems is associated with a crossover from unary, noncooperative to binary, cooperative relaxation as strain increases; chains stretch between entanglement points, altering the character of local plasticity. Promising approaches for future research along these lines are discussed.