Dynamics of alpha and beta processes in thin polymer films: poly(vinyl acetate) and poly(methyl methacrylate)

Dynamics of thin films of poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) have been investigated by dielectric relaxation spectroscopy in the frequency range from 0.1 Hz to 1 MHz at temperatures from 263 to 423 K. The alpha process, the key process of glass transition, is observed for thin films of PVAc and PMMA as a dielectric loss peak at a temperature T(alpha) in temperature domain with a fixed frequency. For PMMA, the beta process is also observed at a temperature T(beta). For PVAc, T(alpha) decreases gradually with decreasing thickness, and the thickness dependence of T(alpha) is almost independent of the molecular weight (Mw< or =2.4x10(5)). For PMMA, T(alpha) remains almost constant as thickness decreases down to a critical thickness dc, at which point it begins to decrease with decreasing thickness. Contrastingly, T(beta) decreases gradually as thickness decreases to dc, and below dc it decreases drastically. For both PVAc and PMMA, the broadening of the distribution of the relaxation times in thinner films is observed and this broadening is more pronounced for the alpha process than for the beta process. It is also observed that the relaxation strength is depressed as the thickness decreases for both the polymers.