Surface thermal molecular motion of chain end-modified polystyrenes

Surface glass transition temperature, T_g^s, of three classes of monodisperse polystyrene, proton-terminated (PS-H), α,ω-diamino-terminated (α,ω-PS(NH₂)₂) and (α,ω-dicarboxy-terminated (α,ω-PS(COOH)₂), were studied by scanning force microscopy. T_g^s of all samples was discernibly lower than the corresponding bulk glass transition temperature, T_g^b. Also, T_g^s value was strongly dependent on what the chemical structure of chain end groups was. Based on the time-temperature superposition principle, apparent activation energy of the α_arelaxation process corresponding to surface segmental motion was evaluated to be approximately 230 kJ•mol^-1. This value was much smaller than the reported bulk ones, and was 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 due probably to an 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.