Nano-rheological properties of polymeric solid surfaces

Surface rheological analyses of polymeric solids were presented on the basis of scanning viscoelasticity microscopy (SVM) and lateral force microscopy (LFM). SVM observation for phase-separated polymer blend films revealed two-dimensional distribution of elasticity. In order to study surface thermal molecular motion of monodisperse polystyrene (PS), SVM and LFM measurements were made at various temperatures. Glass transition temperature, T_g, at the surface was discerned to be markedly lower than the corresponding bulk T_g, and the discrepancy of T_g between surface and bulk became larger with decreasing molecular weight. Such an intensive activation of thermal molecular motion at the PS surface can be explained in terms of an excess free volume induced by the segregated chain ends and a reduced cooperativity at the surface. Bulk T_g of miscible binary blends can be well expressed by the Gordon-Taylor equation. Extending this notion to surface, the surface composition in blends would be obtained by measuring surface T_g of each constituent as well as their blend. The surface composition in blend films of two PSs with different molecular weights was experimentally and systematically elucidated. The surface enrichment of a smaller molecular weight component became more remarkable with increasing molecular weight disparity between the two components due probably to an entropic effect.