Surface Molecular Motion of Amorphous Polymeric Solids
Authors: Kajiyama Tisato , Tanaka Keiji , Takahara Atsushi 
Published: June 2006

Surface molecular motions of monodisperse polystyrene (PS) films, binary and ternary PS blend films, and polydisperse PS films were investigated on the basis of scanning force microscopic (SFM) measurements at 293 K. The monodisperse PSs were synthesized by a living anionic polymerization. It was revealed that the magnitude of the surface dynamic storage modulus E′ was remarkably lower than that for its bulk state, whereas, the surface dynamic loss tangent tan δ value was fairly higher 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 scanning viscoelasticity microscopic (SVM) measurements showed that the surface of the monodisperse PS film with Mn lower than 26.6 k was in a glass–rubber transition state even at 293 K, even though the bulk Tg was far above 293 K. Lateral force microscopic (LFM) measurements for the monodisperse PS films also revealed that the magnitude of lateral force was dependent on the scanning rate of the cantilever tip in the case of Mn lower than 40.4 k. It is well accepted that the scanning rate dependence of lateral force appears in the case that the surface of the PS film is in a glass–rubber transition state. LFM results correspond well to SVM ones if the scanning rate of the cantilever tip for LFM measurement was converted to the measuring frequency for SVM measurement. Active thermal molecular motion on the polymeric solid 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 (DSIMS) measurement.

The binary and the ternary PS blends were prepared by mixing the monodisperse PSs with different molecular weights. The commercially 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 Mn lower than ca. 30 k existed. More active surface molecular motion compared with the bulk one for the binary and the ternary PS blend films can be explained by the surface segregation of the lower molecular weight component. The surface enrichment of lower molecular weight chains was confirmed on the basis of the DSIMS measurement by using the deuterated PS as the one component. In the case of the polydisperse PS film, even though the molecular weight distribution was broad and a somewhat lower molecular weight component was mixed, the active surface molecular motion showing a glass–rubber transition state was remarkably depressed at room temperature in comparison with the case for monodisperse PS film with the corresponding Mns. 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 not present in the system in spite of the broad molecular weight distribution, the surface molecular motion corresponding to the glass–rubber transition was not observed at room temperature.

Also, two-dimensional mapping of topography and surface E′ for the [PS/poly(methyl vinyl ether)] ultrathin blend film was carried out by using atomic force microscopy (AFM) and SVM, respectively. The combination of topographical and surface mechanical images could characterize the interfacial structure on nanometer scale.