Surface molecular motion of monodisperse #alpha#,#omega#-diamino-terminated and #alpha#,#omega#-dicarboxy-terminated polystyrenes

摘要

Surface glass transition behavios of monodisperse #alpha#,#omega#-diamino-terminated and #alpha#,#omega#dicarboxy-terminated polystyrenes (#alpha#,#omega#-PS(NH_2)_2 and #alpha#,#omega#-PS(COOH)_2) were studied by scanning force microscopy and were compared with the results of proton-terminated polystyrene (PS-H). All surface glass transition temperatures, T_g~s, of PS-H, #alpha#,#omega#-PS(NH_2)_2, and #alpha#,#omega#-PS(COOH)_2 were discernibly lower than each corresponding bulk glass transition temperautre, T_g~b. However, the magnitude of T_g~s was strongly dependent on the chemical structure of chain end groups, because the surface concentration of chain ends varied with the surface free energy differnece betweet he main chain part and the chain end portion, via the surface segregation or surface depletion of chain ends. This result makes it clear that chain end chemistry is one of determining factors on the magnitude of T_g~s. On the basis of the time-temperature superposition principle applied to the scanning rate dependenc eof lateral force as a function of temperature, the apparent activation energy, #DELTA#H~(++), of the #alpha#_a-relaxation process corresponding to micro-Brownian motion at the surface was evaluated to be approximately 230 kJ mol~(-1). This value is much smaller than the reported bulk ones and is independent of the chemical structure of chain ends. This result implies that the cooperativity for the #alpha#_a-relaxation process at the PS surface is reduced in comparison with the bulk, probably due to the existence of the free space presented to polymer segments at the surface. Hence, it was concluded that the surface #alpha#_a-relaxation process was activated by not only the chain end effect but also the reduced cooperativity at the surface. Finally, possible other factors determining on themagnitude of T_g~s were discussed.