Surface glass transition temperatures of monodisperse polystyrene films by scanning force microscopy

Surface molecular motion of monodisperse polystyrene (PS) films was examined by scanning viscoelasticity microscopy (SVM) in conjunction with lateral force microscopy (LFM). The dynamic storage modulus, E¹, and loss tangent, tan δ, at a PS film surface with a smaller number-average molecular weight, M_n, than 40k were found to be smaller and larger than those for the bulk sample even at room temperature, meaning that the PS surface is in a glass-rubber transition state or a fully rubbery one at this temperature if the M_n, is small. In order to elucidate quantitatively how vigorous the molecular motion at the PS surface is, SVM and LFM measurements were made at various temperatures. The glass transition temperature, T_g, at the surface was discerned to be markedly lower than its bulk T_g, and the discrepancy of T_g between surface and bulk becomes larger with the decreasing M_n. Such an intensive activation of thermal molecular motion at the PS surfaces can be explained in terms of an excess free volume in the vicinity of the film surface induced by the preferential segregation of chain end groups.