Rheological analysis of surface relaxation process of monodisperse polystyrene films

Surface glass transition temperature, T_g^s, of monodisperse polystyrene (PS) films was determined from the temperature dependence of lateral force at a given scanning rate. The end groups of the PS were composed of sec-butyl group and a repeating unit terminated by proton. T_g^s of the PS films was discerned to be markedly lower than its bulk T_g, T_g^b, in the entire number-average molecular weight, M_n, range, 4.9 K to 1460 K. The M_n dependence of T_g^s was analyzed on the basis of a simple power law. The exponent of M_n related to T_g^s was -0.60±0.03 in contrast to the power of -0.5 from the Mayes scaling argument that is based on both chain ends being perfectly localized at the surface. This result implies that the surface localization of end groups of the PS used here is not complete. The T_g^s extrapolated to the infinite M_n was found to be lower than the corresponding T_g^b by approximately 20 K. The apparent activation energy of the surface α_a-relaxation process for the PS obtained from Arrhenius plot was 230±10 kJ·mol^-1, significantly smaller than that for the bulk PS. Also, it was shown on the basis of Ngai's coupling model that the cooperative motion at the surface can be achieved much easier than its internal bulk phase. Thus, the difference between the T_g^s and the T_g^b for the infinite M_n PS was explained in terms of the size and/or energy barrier reduction of the cooperative movement for the surface α_a-relaxation process, which might be arisen from the existence of the free space presented to polymer segments at the surface.