Molecular dissipation phenomena of nanoscopic friction in the heterogeneous relaxation regime of a glass former

Nanoscale sliding friction involving a polystyrene melt near its glass transition temperature T-g (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 similar to 10 K above T-g, the dissipation length X-d grew from a segmental scale of similar to 3 angstrom to 2.1 nm, following a power-law relationship with the reduced temperature X(d)similar to T (-phi)(R). The resulting phi=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 T-g suggest that long-range processes, e.g., the normal mode or ultraslow Fischer modes, may couple with the alpha relaxation, leading to energy dissipation in domains of tens of nanometers. (c) 2005 American Institute of Physics.