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.