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 t...

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