Dipolar and ionic relaxations of polymers containing polar conformationally versatile side chains

摘要

This work reports a comparative study of the response of poly(2,3-dimethoxybenzyl methacrylate), poly(2,5-dimethoxybenzyl methacrylate), and poly(3,4-dimethoxybenzyl methacrylate) to electrical perturbation fields over wide frequency and temperature windows with the aim of investigating the influence of the location of the dimethoxy substituentsinthe phenyl moietiesonthe relaxation behavior of the polymers. The dielectric loss isotherms above T_g exhibita blurred relaxation resulting from the overlapping of secondary relaxations with the glass-rubber or α relaxation. At high temperatures and low frequencies, the α relaxation is hidden by the ionic conductive contribution to the dielectric loss. Asusual, the real component of the complex dielectric permittivity in the frequency domain increases with decreasing frequency until a plateau is reached corresponding to the glass-rubber (α) relaxation. However, at high temperatures, the real permittivity starts to increase again with decreasing frequency until a second plateau is reached, a process that presumably reflects a distributed Maxwell-Wagner-Sillars relaxation or α' absorption. The α and α' processes appear respectively as asymmetric and symmetric relaxations in the loss electrical modulus isotherms in the frequency domain. To facilitate the deconvolution of the overlapping absorptions, the time retardation spectra of the polymers were computed from the complex dielectric permittivity in the frequency domain using linear programming regularization parameter techniques. The spectra exhibit three secondary absorptions named, in increasing order of time Γ', Γ, and β followed by the α relaxation. At long times and well separated from the α absorption the α' relaxation appears. The replacement of the hydrogen of the phenyl group in position 2 by the oxymethyl moiety enhances the dielectric activity of the poly-(dimethoxybenzyl methacrylate)s. The temperature dependence of the relaxation times associated with the different relaxations is studied, and the molecular origin of the secondary relaxations is qualitatively discussed.