Measurements of the optical birefringence were used to probe the relaxation of rubbed ultrathin polystyrene (PS) films on glass substrates. It was found that the glass transition temperature, T g, of the films dropped by 15-20 K as the film thickness decreased from 10 µm to 5.8 nm. Experiments on thick films (∼10 µm) revealed that molecules closer to the polymer-air interface relax more quickly than molecules farther from the interface. These results are explained in the context of past experimental studies of thermal properties of similar ultrathin polymer films.

We have measured the tracer diffusion coefficient in PS films oriented by surface rubbing. The results show that the diffusion perpendicular to the rubbed surface is slower in oriented than in nonoriented chains. The decrease of the diffusion coefficient was correlated with the total oriented volume as determined by birefringence measurements.

Abstract. -Optical retardation measurements were used to probe the chain relaxation dynamics in rubbed polystyrene films of varying thicknesses on glass substrates. A model based on Kohlrausch-Williams-Watts relaxation was developed and used to determine the glass transition temperature (Tg) of the films. Results showed reductions of 15-20 K in Tg for thin films of thicknesses comparable to the radius of gyration as well as for cast films rubbed with different strengths. These results provide evidence of a faster relaxation dynamics relative to the polymer-substrate interface for thinner films and enhanced chain mobility at the polymer-air interface.

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