Surface molecular motion of polymeric solid films

Surface dynamic storage modulus, E′, and surface loss tangent, tan δ, of monodisperse polystyrene (PS) films with various molecular weights were evaluated at 293 K on the basis of scanning viscoelasticity microscope (SVM) which was designed by the authors. In the case of the PS film with a number-average molecular weight, M_n, lower than ca. 30 k, the magnitudes of surface E′ and surface tan δ were smaller and larger than the magnitudes showing a glassy state, respectively. Thus, it seems reasonable to conclude that the PS film surface with M_n less than ca. 30 k is in a glass-rubber transition state even at 293 K. The scanning rate dependence of lateral force for the monodisperse PS films was investigated at 293 K by lateral force microscopy (LFM). Since the magnitude of lateral force was apparently dependent on the scanning rate, especially in the case of M_n lower than ca. 40 k, it seems reasonable to conclude that the PS film surface with M_n less than ca. 40 k is in a glass-rubber transition state at 293 K. The LFM results agreed well with the SVM results if the scanning rate of cantilever tip for LFM measurements corresponded to the measuring frequency for SVM measurements. The depth dependence of surface glass transition temperature, T_g, of the poly-(styrene-block-methyl methacrylate) diblock copolymer film was also investigated on the basis of the combination of temperature-dependent and angular-dependent X-ray photoelectron spectroscopic (TDXPS and ADXPS) measurements. It was revealed that T_g at the film surface was much lower than that of the bulk sample and its magnitude increased with the depth from the free surface. The remarkable depression of T_g at the polymeric surface was explained by the excess free volume induced due to the surface localization of chain end groups. The surface enrichment of chain end groups was confirmed by dynamic secondary ion mass spectroscopic measurement.