Determination Factors on Surface Glass Transition Temperatures of Polymeric Solids

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 Tg, at the surface, Tgs was found to be markedly lower than bulk Tg, Tgb, and the number-average molecular weight, Mn, dependence of Tgs more remarkable than that of Tgb. Also, the magnitude of Tgs 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 Tgs for the PS-H with quasi-infinite Mn was lower than the corresponding Tgb, even though the number density of chain ends was almost negligible. In addition, Tgs 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 αa-relaxation process was approximately half of the bulk value. Finally, the depression of Tgs in comparison with Tgs is discussed on the basis of several factors, such as a decreased segment size of molecular motion for the surface αa-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.