SURFACE MOLECULAR-MOTION OF POLYMERIC SOL ID FILMS

Surface dynamic storage modulus, E', and surface loss tangent, tan del ta, of monodisperse polystyrene (PS) films with various molecular weig hts were evaluated at 293 K on the basis of scanning viscoelasticity m icroscope (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. 3 0 k, the magnitudes of surface E' and surface tan delta were smaller a nd larger than the magnitudes showing a glassy state, respectively. Th us, 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 t han ca. 40 k is in a glass-rubber transition state at 293 K. The LFM r esults agreed well with the SVM results if the scanning rate of cantil ever 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) dib lock copolymer film was also investigated on the basis of the combinat ion 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 rema rkable depression of T-g at the polymeric surface was explained by the excess free volume induced due to the surface localization of chain e nd groups. The surface enrichment of chain end groups was confirmed by dynamic secondary ion mass spectroscopic measurement.