Surface dynamics in rubbed polymer thin films probed with optical birefringence measurements

Measurements of the optical birefringence were used to probe the relaxation of rubbed ultrathin polystyrene (PS) films on glass substrates. It was fou nd that the glass transition temperature, T-g, of the films dropped by 15-2 0 K. as the film thickness decreased from 10 mu m to 5.8 nm. Experiments on thick films (similar to 10 mu m) revealed that molecules closer to the pol ymer-air interface relax more quickly than molecules farther from the inter face. These results are explained in the context of past experimental studi es of thermal properties of similar ultrathin polymer films.