# Thermodynamic scaling of vibrational dynamics and relaxation

## TL;DR

This study uses molecular dynamics simulations to explore how vibrational dynamics and structural relaxation in polymer melts scale with thermodynamic variables, revealing a unified scaling behavior and providing a new predictive expression for relaxation times.

## Contribution

It introduces a new thermodynamic scaling expression for structural relaxation that fits experimental data with fewer parameters and links fast vibrational motion to slow relaxation processes.

## Key findings

- TS master curves of fast mobility are nearly linear when cage effects are significant.
- The derived TS expression accurately fits experimental data for 34 glassformers.
- Isochoric fragility can predict the TS master curve of fast mobility without adjustments.

## Abstract

We investigate by thorough Molecular Dynamics simulations the thermodynamic scaling (TS) of a polymer melt. Two distinct models, with strong and weak virial-energy correlations, are considered. Both evidence the joint TS with the same characteristic exponent $\gamma_{ts}$ of the fast mobility - the mean square amplitude of the picosecond rattling motion inside the cage -, and the much slower structural relaxation and chain reorientation. If the cage effect is appreciable, the TS master curves of the fast mobility are nearly linear, grouping in a bundle of approximately concurrent lines for different fragilities. An expression of the TS master curve of the structural relaxation with one adjustable parameter less than the available three-parameters alternatives is derived. The novel expression fits well with the experimental TS master curves of thirty-four glassformers and, in particular, their slope at the glass transition, i.e. the isochoric fragility. For the glassformer OTP the isochoric fragility allows to satisfactorily predict the TS master curve of the fast mobility with no adjustments.