Hierarchical thermal molecular motion in thin polymer films

Hierarchical thermal molecular motion was studied in thin films of polystyrene (PS). The segmental motion in the thin films was examined by dynamic mechanical analysis. Molecular motions with a relatively small scale in the thin films were discussed on the basis of the kinetics of photoinduced trans to cis isomerization for azobenzene probes tagged to PS. Apparent glass transition temperatures generally decreased with decreasing film thickness. Although relaxation temperatures of β and γ processes in the thin films would be comparable to those in the bulk, the photoisomerization reaction of azobenzene probes became faster with decreasing thickness. Finally, a model for hierarchical thermal molecular motions in thin films was proposed on the basis of the above-mentioned results.