Experimental evidence of a crossover between cooperative relaxation and liquid growth dynamics

TL;DR

This study provides experimental evidence that the glass transition mechanism shifts from cooperative relaxation to liquid growth dynamics depending on the ratio of relaxation times, revealing a crossover in the transition process.

Contribution

It demonstrates a temperature-dependent crossover between cooperative relaxation and liquid growth dynamics during the glass transition, challenging conventional views.

Findings

High mobility regions nucleate and grow before cooperative relaxation at large relaxation time ratios.

The transition mechanism depends on the ratio of relaxation times, not the preparation protocol.

The observed behavior is consistent across different experimental procedures.

Abstract

In stark contrast with the conventional understanding of the glass transition, where the transition from glass to liquid appears as a dynamic process where atoms/molecules cooperatively relax into the equilibrium phase, we experimentally show that the nature of the glass transition depends at a given temperature on the ratio between the relaxation time of the glass, {\tau}_glass, taken as its transformation time, and the alpha relaxation time, {\tau}_{\alpha}. Although the relaxation of liquid-cooled glasses is not totally synchronous, due to the existence of a distribution of relaxation times, there has been no clear observation of phase separation. However, at temperatures at which {\tau}_glass/{\tau}_{\alpha} is large, high mobility regions nucleate into the liquid phase that subsequently grow by dynamic facilitation before, or while, cooperative glass relaxation sets into play. On the contrary, at temperatures associated to smaller {\tau}_glass/{\tau}_{\alpha} the glass transition proceeds by cooperative relaxation dynamics all across the material. This behavior is independent of the experimental procedure or protocol to produce the glass.