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

摘要

Dynamics of thin films of poly(vinyl acetate) (PVAc) and poly(methylmethacrylate) (PMMA) have been investigated by dielectric relaxationspectroscopy in the frequency range from 0.1Hz to 1MHz at temperatures from263K to 423K. The $\alpha$-process, the key process of glass transition, isobserved for thin films of PVAc and PMMA as a dielectric loss peak at atemperature $T_{\alpha}$ in temperature domain with a fixed frequency. For PMMAthe $\beta$-process is also observed at a temperature $T_{\beta}$. For PVAc,$T_{\alpha}$ decreases gradually with decreasing thickness, and the thicknessdependence of $T_{\alpha}$ is almost independent of the molecular weight($M_{\rm w}\le2.4\times10^5$). For PMMA, $T_{\alpha}$ remains almost constantas thickness decreases down to a critical thickness $d_c$, at which point itbegins to decrease with decreasing thickness. Contrastingly, $T_{\beta}$decreases gradually as thickness decreases to $d_c$, and below $d_c$ itdecreases drastically. For both PVAc and PMMA, the broadening of thedistribution of the relaxation times in thinner films is observed and thisbroadening is more pronounced for the $\alpha$-process than for the$\beta$-process. It is also observed that the relaxation strength is depressedas the thickness decreases for both the polymers.