Cooperativity and the freezing of molecular motion at the glass transition

TL;DR

This paper investigates the dramatic slowdown of molecular motion near the glass transition, linking it to increasing cooperative molecular regions and a temperature-dependent energy barrier.

Contribution

It demonstrates a direct correlation between the activation energy and the number of correlated molecules using nonlinear dielectric susceptibility measurements.

Findings

Increase in correlated molecules Ncorr near glass transition

Strong correlation between E(T) and Ncorr(T)

Evidence for cooperative molecular motion causing dynamics slowdown

Abstract

The slowing down of molecular dynamics when approaching the glass transition generally proceeds much stronger than expected for thermally activated motions. This strange phenomenon can be formally ascribed to a temperature-dependent activation energy E(T). In the present work, via measurements of the third-order nonlinear dielectric susceptibility, we deduce the increase of the number of correlated molecules Ncorr when approaching the glass transition and find a surprisingly simple correlation of E(T) and Ncorr(T). This provides strong evidence that the non-canonical temperature development of glassy dynamics is caused by a temperature-dependent energy barrier arising from the cooperative motion of ever larger numbers of molecules at low temperatures.