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

Surface glass transition behaviors of monodisperse alpha,omega -diamino-ter minated and alpha,omega -dicarboxy-terminated polystyrenes (alpha,omega -PS (NH2)(2) and alpha,omega -PS(COOH)(2)) were studied by scanning force micro scopy and were compared with the results of proton-terminated polystyrene ( PS-H). All surface glass transition temperatures, T-g(s), of PS-H, alpha,om ega -PS(NH2)(2), and alpha,omega -PS(COOH)(2) were discernibly lower than e ach corresponding bulk glass transition temperature, T-g(b). However, the m agnitude of T-g(s) was strongly dependent on the chemical structure of chai n end groups, because the surface concentration of chain ends varied with t he surface free energy difference between the main chain part and the chain end portion, via the surface segregation or surface depletion of chain end s. This result makes it clear that chain end chemistry is one of determinin g 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, DeltaH( double dagger), 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 indepen dent of the chemical structure of chain ends. This result implies that the cooperativity for the alpha (a)-relaxation process at the PS surface is red uced in comparison with the bulk, probably due to the existence of the free space presented to polymer segments at the surface. Hence, it was conclude d that the surface alpha (a)-relaxation process was activated by not only t he chain end effect but also the reduced cooperativity at the surface. Fina lly, possible other factors determining on the magnitude of T-g(s) were dis cussed.