Abstract
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.
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