Relaxation Spectroscopy of the Dielectric β-Relaxation in Poly(n-alkyl methacrylate)s by Absorption–Current Measurements. I. Dielectric Relaxation Spectra for Atactic Polymers

Abstract

In order to determine the dielectric relaxation spectra for the β-processes in atactic poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), poly(n-propyl methacrylate) (PnPMA), and poly(n-butyl methacrylate) (PnBMA) which were sufficiently separated from the α-processes, absorption current was measured for 10 to 600 s after application of a step voltage at temperatures well below the glass transition temperatures. By applying the Hamon approximation, the data on time dependence of the absorption current were converted to those on the frequency dependence of loss permittivity, and the latter were reduced to master curves by the method of reduced variables. Shift factors gave an activation energy of ca. 19 kcal mol−1 irrespective of the side-chain length. Shapes of the β-relaxation spectra for these polymers were the same, but the relaxation strengths for PMMA, PEMA, PnPMA, and PnBMA were in the ratios 1:0.8:0.6:0.5, which correspond to the ratios of their dipolar densities. The β-relaxation spectrum began to sharpen with the increase in temperature above a critical temperature, which was estimated for PMMA and PEMA to be ca. 10 and 18°C respectively. A two-site model analysis suggests that the scale of the molecular process for the β-relaxation is as large as a 180°-rotation of the side group about the C-C bond connecting it to the main chain.