Surface molecular motion of monodisperse α,ω-diamino-terminated and α,ω-dicarboxy-terminated polystyrenes

Surface glass transition behaviors of monodisperse α,ω-diamino-terminated and α,ω-dicarboxy-terminated polystyrenes (α,ω-PS(NH₂)₂ and α,ω-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, α,ω-PS(NH₂)₂, and α,ω-PS(COOH)₂ were discernibly lower than each corresponding bulk glass transition temperature, 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 difference between the 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 dependence of lateral force as a function of temperature, the apparent activation energy, ΔH^‡, of the α_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 α_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 α_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 the magnitude of T_g^s were discussed.