| Abstract
| - Forced modulation scanning force microscopic (SFM) and lateralforce microscopic (LFM)measurements of the monodisperse polystyrene (PS) films were carriedout at 293 K in order to revealsurface molecular motion. Surface dynamic storage modulus,E‘, and surface loss tangent, tan δ, of themonodisperse PS films were evaluated on the basis of forced modulationSFM measurement. It wasrevealed that the magnitudes of surface E‘ and surface tanδ were lower and higher than those for itsbulk state, respectively, in the case of the number-average molecularweight (Mn) lower than 26.6k.Basedon forced modulation SFM measurements, the surface of the PS film withMn lower than 26.6k was in aglass−rubber transition state even at 293 K, in spite of that thebulk Tg was far above 293 K.LFMmeasurements for the PS films revealed that the magnitude of lateralforce was dependent on the scanningrate of the cantilever tip in the case of Mnlower than 40.4k. The scanning rate dependence oflateralforce appeared in the case that the surface of the PS film was in aglass−rubber transition state. LFMresults agreed well with forced modulation SFM ones if the scanningrate of the cantilever tip for LFMmeasurement was converted to the measuring frequency for forcedmodulation SFM measurement. Theactive thermal molecular motion on the polymeric surface was explainedby the excess free volume induceddue to the surface localization of chain end groups. The surfaceenrichment of chain end groups wasconfirmed by dynamic secondary ion mass spectroscopicmeasurement.
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