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 was 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^b 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.