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 lateral force microscopic (LFM) measurements. SVM and LFM measurements were carried out for 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 × 10⁴, the surface was in a glass-rubber transition state even though the bulk glass transition 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 applicable to the surface mechanical relaxation behavior and also that the surface glass transition temperature, T_g^σ, was depressed in comparison with the bulk one even though the magnitude of M_n was fairly high at 1.40 × 10⁵. The surface molecular motion of monodisperse PS with various chain end groups was investigated on the basis of temperature-dependent scanning viscoelasticity microscopy (TDSVM). The T_g^σs for the PS films with M_n of 4.9 × 10³ to 1.45 × 10⁶ measured by TDSVM were smaller than those for the bulk one, with corresponding M_ns, and the T_g^σs for M_ns smaller than ca. 4.0 × 10⁴ were lower than room temperature (293 K). The active thermal molecular motion at the polymeric solid surface can be interpreted 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 × 10⁴, the T_g^σs for the α,ω-diamino-terminated PS (α,ω-PS(NH₂)₂) and α,ω-dicarboxy-terminated PS (α,ω-PS(COOH)₂) films were higher than that of the PS film. The change of T_g^σ 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.