Surface Molecular Motion of the Monodisperse Polystyrene Films

Forced modulation scanning force microscopic (SFM) and lateral force microscopic (LFM) measurements of the monodisperse polystyrene (PS) films were carried out at 293 K in order to reveal surface molecular motion. Surface dynamic storage modulus, E‘, and surface loss tangent, tan δ, of the monodisperse PS films were evaluated on the basis of forced modulation SFM measurement. It was revealed that the magnitudes of surface E‘ and surface tan δ were lower and higher than those for its bulk state, respectively, in the case of the number-average molecular weight (Mn) lower than 26.6k. Based on forced modulation SFM measurements, the surface of the PS film with Mn lower than 26.6k was in a glass−rubber transition state even at 293 K, in spite of that the bulk Tg was far above 293 K. LFM measurements for the PS films revealed that the magnitude of lateral force was dependent on the scanning rate of the cantilever tip in the case of Mn lower than 40.4k. The scanning rate dependence of lateral force appeared in the case that the surface of the PS film was in a glass−rubber transition state. LFM results agreed well with forced modulation SFM ones if the scanning rate of the cantilever tip for LFM measurement was converted to the measuring frequency for forced modulation SFM measurement. The active thermal molecular motion on 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.