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Nanoscale sliding friction involving a polystyrene melt near its glass transition temperature Tg (373 K) exhibited dissipation phenomena that provide insight into the underlying molecular relaxation processes. A dissipative length scale that shows significant parallelism with the size of cooperatively rearranging regions (CRRs) could be experimentally deduced from friction-velocity isotherms, combined with dielectric loss analysis. Upon cooling to ∼10 K above Tg, the dissipation length Xd grew from a segmental scale of ∼3 Å to 2.1 nm, following a power-law relationship with the reduced temperature Xd∼T R-[lowercase_phi_synonym]. The resulting [lowercase_phi_synonym]=1.89±0.08 is consistent with growth predictions for the length scale of CRRs in the heterogeneous regime of fragile glass formers. Deviations from the power-law behavior closer to Tg suggest that long-range processes, e.g., the normal mode or ultraslow Fischer modes, may couple with the α relaxation, leading to energy dissipation in domains of tens of nanometers. [ABSTRACT FROM AUTHOR]

Interfacial glass transition temperature (T[sub g]) profiles in spin cast, ultrathin films of polystyrene and derivatives were investigated using shear-modulated scanning force microscopy. The transitions were measured as a function of film thickness (δ), molecular weight, and crosslinking density. The T[sub g](δ) profiles were nonmonotonic and exhibited two regimes: (a) a sublayer extending about 10 nm from the substrate, with T[sub g] values lowered up to ∼10 °C below the bulk value, and (b) an intermediate regime extending over 200 nm beyond the sublayer, with T[sub g] values exceeding the bulk value by up to 10 °C. Increasing the molecular weight was found to shift the T[sub g](δ) profiles further from the substrate interface, on the order of 10 nm/kDa. Crosslinking the precast films elevated the absolute T[sub g] values, but had no effect on the spatial length scale of the T[sub g](δ) profiles. These results are explained in the context of film preparation history and its influence on molecular mobility. Specifically, the observed rheological anisotropy is interpreted based on the combined effects of shear-induced structuring and thermally activated interdiffusion. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]