Experimental search for dynamic heterogeneities in molecular glass formers

TL;DR

This study measures dielectric susceptibility in molecular glass formers near Tg to estimate the size of dynamically correlated molecular clusters, revealing a threefold increase in correlated molecules as temperature decreases.

Contribution

It provides experimental measurements of dynamic heterogeneities in molecular glasses using dielectric susceptibility and non-linear response, offering new insights into glass formation.

Findings

Correlated molecule number increases by a factor of 3 from 226 K to 195 K in glycerol.

Upper limit on non-linear susceptibility ratio was established.

Temperature scans achieved high resolution, enabling precise correlation size estimation.

Abstract

We have measured the linear dielectric susceptibility of two molecular glass formers close to Tg in order to estimate the size of the dynamically correlated clusters of molecules which are expected to govern the physics of glass formation. This size has been shown to be related to the dynamic dielectric susceptibility dEps(w)/dT (Eps : dielectric susceptibility, T : temperature, w: frequency). To allow for an accurate determination of the T derivative, we scanned the interval 192 < T < 232 K every 1 K for glycerol and 159 < T < 179 K every 0.5 K for propylene carbonate. The resolution on T variations was about 1 mK. The result for glycerol is that the number of correlated molecules increases by a factor 3 when T goes from 226 to 195 K. It has been shown that the non-linear susceptibility provides a direct measurement of dynamic correlations. To measure it, we used a standard Lockin technique yielding the third harmonic of the current flowing out of a capacitor. We obtained only an upper limit on the ratio of the third to the first harmonic, due to the non-linear response of standard electronics.