Parameter-free predictions of the viscoelastic response of glassy polymers from non-affine lattice dynamics

TL;DR

This paper presents a parameter-free theoretical approach, combined with simulations, to predict the viscoelastic behavior of glassy polymers, revealing insights into the nature of the glass transition and vibrational excitations.

Contribution

The study introduces a non-affine lattice dynamics theory that accurately predicts polymer viscoelasticity without adjustable parameters, linking vibrational modes to mechanical response.

Findings

Theoretical predictions match simulation data over five orders of magnitude in frequency.

The glass transition appears as a continuous crossover rather than a sharp phase change.

Proliferation of low-frequency vibrational modes explains the sharp drop in storage modulus.

Abstract

We study the viscoelastic response of amorphous polymers using theory and simulations. By accounting for internal stresses and considering instantaneous normal modes (INMs) within athermal non-affine theory, we make parameter-free predictions of the dynamic viscoelastic moduli obtained in coarse-grained simulations of polymer glasses at non-zero temperatures. The theoretical results show very good correspondence with rheology data collected from molecular dynamics simulations over five orders of magnitude in frequency, with some instabilities that accumulate in the low-frequency part on approach to the glass transition. These results provide evidence that the mechanical glass transition itself is continuous and thus represents a crossover rather than a true phase transition. The relatively sharp drop of the low-frequency storage modulus across the glass transition temperature can be explained mechanistically within the proposed theory: the proliferation of low-eigenfrequency vibrational excitations (boson peak and nearly-zero energy excitations) is directly responsible for the rapid growth of a negative non-affine contribution to the storage modulus.