SURFACE MOLECULAR-MOTION OF THE MONODISPERSE POLYSTYRENE FILMS

Forced modulation scanning force microscopic (SFM) and lateral force m icroscopic (LFM) measurements of the monodisperse polystyrene (PS) fil ms were carried out at 293 K in order to reveal surface molecular moti on. Surface dynamic storage modulus, E', and surface loss tangent, tan delta, of the monodisperse PS films were evaluated on the basis of fo rced modulation SFM measurement. It was revealed that the magnitudes o f surface E' and surface tan delta were lower and higher than those fo r its bulk state, respectively, in the case of the number-average mole cular weight (M(n)) lower than 26.6k. Based on forced modulation SFM m easurements, the surface of the PS film with M(n) lower than 26.6k was in a glass-rubber transition state even at 293 K, in spite of that th e bulk T-g was far above 293 K. LFM measurements for the PS films reve aled that the magnitude of lateral force was dependent on the scanning rate of the cantilever tip in the case of M(n) lower than 40.4k. The scanning rate dependence of lateral force appeared in the case that th e surface of the PS film was in a glass-rubber transition state. LFM r esults agreed well with forced modulation SFM ones if the scanning rat e of the cantilever tip for LFM measurement was converted to the measu ring frequency for forced modulation SPM measurement. The active therm al molecular motion on the polymeric surface was explained by the exce ss free volume induced due to the surface localization of chain end gr oups. The surface enrichment of chain end groups was confirmed by dyna mic secondary ion mass spectroscopic measurement.