Dielectric dispersion in methacrylate polymers and its correlation with mechanical properties

Abstract

The dielectric measurements of Strella on ethyl, n-butyl, n-hexyl, and n-octyl methacrylate polymers at Picatinny Arsenal have been further examined. The frequency and temperature dependence of ϵ′ and ϵ″ reveal combined characteristics of two different classes of polymer systems previously recognized: a decrease in (ϵ 0 − ϵ ∞) T ρ with increasing temperature; and an increase in ϵ″ m T ρ with temperature, representing a sharpening of the relaxation spectrum and attributed to overcoming steric barriers associated with the backbone methyl groups. Despite the latter effect, at high temperatures the shape of the dispersion approaches constancy and the real and imaginary components of the dielectric constant can be superposed at different temperatures in accordance with the method of reduced variables after normalization by ϵ 0 − ϵ ∞. The temperature shift factors follow the Arrhenius equation with apparent activation energies close to those obtained for the β mechanism in the dispersion of mechanical compliance. From the resulting composite curves, the normalized relaxation spectra Ψ n can be calculated with good agreement between the values from real and imaginary components. The shapes of Ψ n are very similar for the three highest homologs; that for the ethyl polymer is somewhat broader. At its maximum, Ψ n is 0.16 ± 0.02 for the four methacrylate polymers, not far from values found for dielectric α dispersions in other polymer systems, and only moderately lower than the 0.25 and 0.30 predicted by the theories of Kirkwood-Fuoss and Hammerle-Kirkwood. Nevertheless, the temperature dependence of the polymethacrylate dispersions and their relation to the data of Hoff and of de Brouckère clearly identify them as of the β type. At 100°C., the Ψ n maximum shifts to shorter times with increasing side chain length, but not as rapidly as the monomeric friction coefficient characterizing the mechanical α dispersion changes. The mechanical β retardation spectra lie at longer times than the electrical β relaxation spectra, but their relative positions and shapes are qualitatively similar in comparing the ethyl, butyl, and hexyl homologs.