Direct observation of molecular cooperativity near the glass transition

Abstract

The increasingly sluggish response of a supercooled liquid as it nears its glass transition¹ (for example, refrigerated honey) is prototypical of glassy dynamics found in proteins, neural networks and superconductors. The notion that molecules rearrange cooperatively has long been postulated² to explain diverging relaxation times and broadened (non-exponential) response functions near the glass transition. Recently, cooperativity was observed and analysed in colloid glasses³ and in simulations of binary liquids well above the glass transition⁴. But nanometre-scale studies of cooperativity at the molecular glass transition are lacking⁵. Important issues to be resolved include the precise form of the cooperativity and its length scale⁶, and whether the broadened response is intrinsic to individual cooperative regions, or arises only from heterogeneity^7,8,9 in an ensemble of such regions. Here we describe direct observations of molecular cooperativity near the glass transition in polyvinylacetate (PVAc), using nanometre-scale probing of dielectric fluctuations. Molecular clusters switched spontaneously among two to four distinct configurations, producing random telegraph noise. Our analysis of these noise signals and their power spectra reveals that individual clusters exhibit transient dynamical heterogeneity and non-exponential kinetics.