Temporal evolution of relaxation in rubbed polystyrene thin films

Temporal relaxation of rubbed polystyrene (PS) thin films (50 nm thick) was studied by probing the decay of the optical anisotropy in the sample. Our results show that the relaxation process is characterized by two single-exponential decays plus a temperature-dependent constant. Both relaxations are orders of magnitude faster than the main chain (α) relaxation at temperatures well below (>10°C)T_g, with activation energies 3.0 kcal/mol and 5.1 kcal/mol, respectively, either of which are much smaller than that of the α relaxation (∼50 kcal/mol). The decay time constants are found to be independent of the sample molecular weights M_w (=13.7 K to 550 K Daltons, M_w/M_n<~1.1) at these temperatures. This shows the local nature of the relaxation modes of rubbed PS.