Surface rheology of polymeric solids

Surface molecular motions of amorphous polymeric solids have been directly measured on the basis of scanning viscoelasticity microscopic (SVM) and lat eral force microscopic (LFM) measurements. SVM and LFM measurements were ca rried out fur films of conventional monodisperse polystyrene (PS) with sec- butyl] and proton-terminated end groups at room temperature, In the case of the number-average molecular weight, M-n, less than ca. 4.0 x 10(4), the s urface was in a glass-rubber transition state even though the bulk glass tr ansition temperature, T-g was far above room temperature, meaning that the surface molecular motion was fairly active compared with that in the bulk. LFM measurements of the monodisperse PS films at various scanning rates and temperatures revealed that the time-temperature superposition was applicab le to the surface mechanical relaxation behavior and also that the surface glass transition temperature, T-g(sigma) was depressed in comparison with t he bulk one even though the magnitude of M-n was fairly high at 1.40 x 10(5 ). The surface molecular motion of monodisperse PS with various chain end g roups was investigated on the basis of temperature-dependent scanning visco elasticity microscopy (TDSVM). The T(g)(sigma)s for the PS films with M-n o f 4.9 x 10(3) to 1.45 x 10(6) measured by TDSVM were smaller than those for the bulk one, with corresponding M(n)s, and the T(g)(sigma)s for M(n)s sma ller than ca. 4.0 x 104 were lower than room temperature (293 K), The activ e thermal molecular motion at the polymeric solid surface can be interprete d in terms of an excess free volume near the surface region induced by the surface localization of chain end groups. In the case of M-n = ca. 5.0 x 10 (4), the T(g)(sigma)s for the alpha,omega-diamino-terminated PS (alpha,omeg a-PS(NH2)(2)) and alpha,omega-dicarboxy-terminated PS (alpha,omega-PS(COOH) (2)) films were higher than that of the PS film. The change of T(g)(sigma)s for the PS film with various chain end groups can be explained in terms of the depth distribution of chain end groups at the surface region depending on the relative hydrophobicity.