Dielectric relaxation of liquid crystalline side-chain poly(vinyl ether)s

Abstract

Dielectric permittivity and loss have been measured over the frequency range 10−2 Hz–10 kHz between 100 K and 350 K for samples of two mesomorphic side-chain poly(vinyl ether)s, poly(4-(11-(vinyloxy)undecyloxy)-4-ethoxyphenylbenzoate) (P-EtO) and poly(4-(11-(vinyloxy)undecyloxy)-4-cyanophenylbenzoate) (P-CN) of different degree of mesogen group orientation. X-ray scattering showed that P-EtO was in a semicrystalline state below 345 K, whereas P-CN displayed a titled smectic structure at temperatures lower than 315 K. Four relaxation transitions were found in P-EtO: α, the glass-rubber transition occurring at 290–300 K, and three subglass processes referred to as β, γ and δ. P-CN exhibited only three dielectric processes, α, β and γ. The low temperature process, δ, was absent in P-CN and could be assigned to torsion about the pendent phenyl-carbon-ether-oxygen bond. Subglass processes β and γ exhibited an Arrhenius temperature dependence with activation energies of 60–105 and 32 ± 2 kJ mol−1, respectively. The activation energy of the γ process was insensitive to morphology and it was assigned to local motions in the spacer group. The activation energy of the β process varied considerably among the studied polymers with higher values for the highly ordered P-EtO than for P-CN. It is suggested that the β process leads to reorientation of the carboxylic group in the phenyl benzoate moiety.