Characterization of Functionalized Side-Chain Liquid CrystalMethacrylates Containing Nonmesogenic Units by DielectricSpectroscopy

Abstract

[EN] 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 degrees C. The relaxation spectra of these polymers have been studied in terms of the complex dielectric permittivity (epsilon' and epsilon '') and the dielectric loss tangent, tan(delta). The electric modulus, M*, has been also calculated. It is possible to distinguish two relaxations zones, one at low temperatures (including gamma and beta relaxations) and another at higher temperatures (including the alpha and beta 1 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 alpha and beta 1 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 (gamma and beta) 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 beta 1 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 beta 1 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.