Noriaki Satomi

Surface glass transition temperatures of monodisperse polystyrene films by scanning force microscopy

Abstract Surface molecular motion of monodisperse polystyrene (PS) films was examined by scanning viscoelasticity microscopy (SVM) in conjunction with lateral force microscopy (LFM). The dynamic storage modulus, ƴ, and loss tangent, tan d , at a PS film surface with a smaller number-average molecular weight, Mn, than 40k were found to be smaller and larger than those for the bulk sample even at room temperature, meaning that the PS surface is in a glass–rubber transition state or a fully rubbery one at this temperature if the Mn, is small. In order to elucidate quantitatively how vigorous the molecular motion at the PS surface is, SVM and LFM measurements were made at various temperatures. The glass transition temperature, Tg, at the surface wasdiscerned to be markedly lower than its bulk Tg, and the discrepancy of Tg between surface and bulk becomes larger with the decreasing Mn. Such an intensive activation of thermal molecular motion at the PS surfaces can be explained in terms of an excess free volume in the vicinit of the film surfaceinduced by the preferential segregation of chain end groups.

Determination Factors on Surface Glass Transition Temperatures of Polymeric Solids

The surface molecular motion of monodisperse proton-terminated polystyrene (PS-H), α, ω-diamino-terminated PS (α, ω-PS(NH2)2) and α, ω-dicarboxy-terminated PS (α, ω-PS(COOH)2) films was studied by scanning viscoelasticity microscopy in conjunction with lateral force microscopy. The glass transition temperature T g, at the surface, T g s was found to be markedly lower than bulk T g, T g b, and the number-average molecular weight, M n, dependence of T g s more remarkable than that of T g b. Also, the magnitude of T g s was strongly dependent on the chain end chemistry. Hence, the activation of surface molecular motion was explained in terms of an excess free volume induced by the preferential surface segregation of chain end groups. The chain end segregation at the film surface was confirmed by dynamic secondary ion mass spectroscopic measurement. However, the T g s for the PS-H with quasi-infinite M n was lower than the corresponding T g b, even though the number density of chain ends was almost negligible. In addition, T g s for PS films with hydrophilic chain ends, which might be depleted at the film surface, were lower than the bulk values. The apparent activation energy for the surface micro-Brownian motion corresponding to the α a-relaxation process was approximately half of the bulk value. Finally, the depression of T g s in comparison with T g s is discussed on the basis of several factors, such as a decreased segment size of molecular motion for the surface α a-relaxation process due to the existence of the free space on the polymer surface and/or a reduced chain entanglement at the surface, in addition to the chain end effect.

Effect of end group chemistry on surface molecular motion of monodisperse polystyrene films

The surface molecular motion of monodisperse polystyrene (PS) with various chain end groups was investigated on the basis of temperature-dependent scanning viscoelasticity microscope (TDSVM). The surface glass transition temperatures, Tgss for the proton-terminated PS (PS-H) films with number-average molecular weight, Mn of 4.9k–1,450k measured by TDSVM measurement were smaller than those for the bulk one, with corresponding Mns, and the Tgss for Mn smaller than ca. 50k were lower than room temperature (293 K). In the case of Mn = ca. 50k, the Tgss 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-H film. On the other hand, the Tgs for the α,ω-perfluoroalkylsilyl-terminated PS (α,ω-PS(SiC2CF6)2) film with the same Mn was much lower than those for the PS films with all other chain ends. The change of Tgs 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.

Interfacial Characteristics of Amorphous Polystyrene and Binary Polymer Blend Thin Films Based on Scanning Force Microscopy

Surface viscoelastic functions for monodisperse PS film was evaluated by using scanning viscoelasticity microscope(SVM). In the case of number average molecular weight, Mn less than ca. 30,000, it was revealed that the film surface was in a glass-rubber transition state even at 293 K. A depression of surface Tg compared with that for the bulk sample was explained by the surface localization of chain end groups. By combining atomic force microscopy(AFM) and SVM observations, the surface aggregation structure of the polymer blend films can be evaluated with the scale of sub-nanometers. The phase separated structures of polystyrene/poly(vinyl methyl ether) (PS/PVME) and polystyrene/polyisoprene (PS/PI) were analyzed by AFM and SVM.