The increasingly sluggish response of a supercooled liquid as it nears its glass transition (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 postulated to explain diverging relaxation times and broadened (non-exponential) response functions near the glass transition. Recently, cooperativity was observed and analysed in colloid glasses and in simulations of binary liquids well above the glass transition. But nanometre-scale studies of cooperativity at the molecular glass transition are lacking. Important issues to be resolved include the precise form of the cooperativity and its length scale, and whether the broadened response is intrinsic to individual cooperative regions, or arises only from heterogeneity 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.

Using non-contact scanning probe microscopy (SPM) techniques, dielectric properties were studied on 50 nanometer length scales in poly-vinyl-acetate (PVAc) films in the vicinity of the glass transition. Low frequency (1/f) noise observed in the measurements, was shown to arise from thermal fluctuations of the electric polarization. Anomalous variations observed in the noise spectrum provide direct evidence for cooperative nano-regions with heterogeneous kinetics. The cooperative length scale was determined. Heterogeneity was long-lived only well below the glass transition for faster than average processes.Comment: 4 pages, 4 embedded PS figures, RevTeX - To appear in Phys. Rev. Let

Using noncontact scanning probe microscopy techniques, dielectric properties were studied on 50-nm-length scales in poly-vinyl-acetate (PVAc) and poly-methyl-methacrylate films. Low-frequency (1/f ) fluctuations observed in the measurements, peaked in intensity near the glass transition temperature in PVAc. The noise is shown to arise from thermal dielectric polarization fluctuations. Analysis of this noise provides a noninvasive method of probing equilibrium nanometer-scale dynamical processes in dielectric materials and devices.

Using scanning-probe-microscopy techniques, we have measured dielectric relaxation on ultrasmall, mesoscopic, length scales in a glassy polymer ͑polyvinyl acetate͒ film near its glass transition. The effective length scale sampled by these techniques is ϳ50 nm. This is large enough to probe subsurface properties, but small enough to observe deviations from bulklike behavior in the form of fluctuations in the shape of the relaxation function near the glass transition. These fluctuations are argued to arise from cooperative dynamics and intrinsic heterogeneities on somewhat smaller length scales. ͓S0163-1829͑98͒50524-3͔ RAPID COMMUNICATIONSR15 114 57 L. E. WALTHER et al.

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