SURFACE MOLECULAR-MOTION OF AMORPHOUS POLYMERIC SOLIDS

Surface molecular motions of monodisperse polystyrene (PS) films, bina ry and ternary PS blend films, and polydisperse PS films were investig ated on the basis of scanning force microscopic (SFM) measurements at 293 K. The monodisperse PSs were synthesized by a living anionic polym erization. It was revealed that the magnitude of the surface dynamic s torage modulus E' was remarkably lower than that for its bulk state, w hereas, the surface dynamic loss tangent tan delta value was fairly hi gher than that for its bulk state, in the case of the monodisperse PS with number-average molecular weight (Mn) lower than 26.6 k. The scann ing viscoelasticity microscopic (SVM) measurements showed that the sur face of the monodisperse PS film with M-n lower than 26.6 k was in a g lass-rubber transition state even at 293 K, even though the bulk T-g w as far above 293 K. Lateral force microscopic (LFM) measurements for t he monodisperse PS films also revealed that the magnitude of lateral f orce was dependent on the scanning rate of the cantilever tip in the c ase of M-n lower than 40.4 k. It is well accepted that the scanning ra te dependence of lateral force appears in the case that the surface of the PS film is in a glass-rubber transition state. LFM results corres pond well to SVM ones if the scanning rate of the cantilever tip for L FM measurement was converted to the measuring frequency for SVM measur ement. Active thermal molecular motion on the polymeric solid surface was explained by the excess free volume induced due to the surface loc alization of chain end groups. The surface enrichment of chain end gro ups was confirmed by dynamic secondary ion mass spectroscopic (DSIMS) measurement. The binary and the ternary PS blends were prepared by mix ing the monodisperse PSs with different molecular weights. The commerc ially available PSs were also used as the polydisperse PS samples. LFM and SVM measurements revealed that the surface of the binary and the ternary PS blend films was in a glass-rubber transition state even at room temperature, when the component with M, lower than ca. 30 k exist ed. More active surface molecular motion compared with the bulk one fo r the binary and the ternary PS blend films can be explained by the su rface segregation of the lower molecular weight component. The surface enrichment of lower molecular weight chains was confirmed on the basi s of the DSIMS measurement by using the deuterated PS as the one compo nent. In the case of the polydisperse PS film, even though the molecul ar weight distribution was broad and a somewhat lower molecular weight component was mixed, the active surface molecular motion showing a gl ass-rubber transition state was remarkably depressed at room temperatu re in comparison with the case for monodisperse PS film with the corre sponding M(n)s. The difference on the surface thermal molecular motion between monodisperse and polydisperse PS films might be explained on the basis of the chemical structure of the chain end groups. Also, in the case that the molecular weight component lower than ca. 30 k was n ot present in the system in spite of the broad molecular weight distri bution, the surface molecular motion corresponding to the glass-rubber transition was not observed at room temperature. Also, two-dimensiona l mapping of topography and surface E' for the [PS/poly(methyl vinyl e ther)] ultrathin blend film was carried out by using atomic force micr oscopy (AFM) and SVM, respectively. The combination of topographical a nd surface mechanical images could characterize the interfacial struct ure on nanometer scale.