Characterization of Functionalized Side-Chain Liquid Crystal Methacrylates Containing Nonmesogenic Units by Dielectric Spectroscopy

ABSTRACT: The dielectric response of a series of side-chain liquid crystal copolymers, SCLCPs, the poly[6-(4′-methoxyazobenzene-4′-oxy)hexyl methacrylate]-co-poly[methyl methacrylate]s, MeOAzB/MMA copolymers, is presented in the frequency range f = 10−2 to 107 Hz and over the temperature interval T = −150 to 120 °C. The relaxation spectra of these
polymers have been studied in terms of the complex dielectric permittivity (ε′ and ε″) and the dielectric loss tangent, tan(δ). The electric modulus, M*, has been also calculated. It is possible to distinguish two relaxations zones, one at low temperatures (including γ and β relaxations) and another at higher temperatures (including the α and β1 relaxations), all of them reported for liquid crystalline poly(methacrylate)s. The individual relaxations have been analyzed using Havriliak−Negami (HN) functions and the effect of conductivity at high temperatures is subtracted. The thermal activation of the relaxations at low temperatures is studied using the Arrhenius equations as a function of copolymer composition, while the α and β1 relaxations are analyzed using Vogel−Tammann−Fulcher equations. The activation entropy has been also evaluated for all the relaxations through the Eyring equation. The temperature ranges, activation energies, and entropies of the relaxations at low temperatures (γ and β) are similar in the homopolymer and copolymers. However, the introduction of MMA units promotes variations in all the parameters related to the relaxations associated with the motions of the ester groups adjoining the polymer backbone. Specifically, a decrease is observed in the activation entropy values of the β1 relaxation, which suggests that the activation of the local motions of the side groups involves smaller cooperative regions in the copolymers with respect to the homopolymer. This fact may account for the extinction of the smectic behavior, together with the dilution of the anisotropic interactions between the mesogenic units on increasing MMA content. The study of this β1 relaxation can be then applied to anticipate the formation and stability of smectic phases in functionalized SCLCPs, by controlling the local mobility resulting in different mesogenic behavior.