Determination factors on surface glass transition temperatures of polymeric solids

The surface molecular motion of monodisperse proton-terminated polystyrene (PS-H), alpha,omega -diamino-terminated PS (alpha,omega -PS(NH2)(2)) and al pha,omega -dicarboxy-terminated PS (alpha,omega -PS(COOH)(2)) films was stu died by scanning viscoelasticity microscopy in conjunction with lateral for ce 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-aver age molecular weight, M-n, dependence of T-g(s) was more remarkable than th at of T-g(b). Also, the magnitude of T-g(s) was strongly dependent on the c hain end chemistry. Hence, the activation of surface molecular motion was e xplained in terms of an excess free volume induced by the preferential surf ace segregation of chain end groups. The chain end segregation at the film surface was confirmed by dynamic secondary ion mass spectroscopic measureme nt. 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)s for PS films with hydrophilic cha in ends, which might be depleted at the film surface, were lower than the b ulk values. The apparent activation energy for the surface micro-Brownian m otion corresponding to the alpha (a)-relaxation process was approximately h alf 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 s egment size of molecular motion for the surface alpha (a)-relaxation proces s due to the existence of the free space on the polymer surface and/or a re duced chain entanglement at the surface, in addition to the chain end effec t.