Dielectric relaxations in bilayer structures of comb‐like copolymers: Perfluoralkyl and liquid‐crystalline side chains

Abstract

AbstractAbstract: A series of comb‐like copolymers and homopolymers containing different liquid‐crystalline and perfluoralkyl side chains was studied by frequency and temperature dependent dielectric measurements. The structure and the phase behaviour of the systems were characterized by X‐ray scattering and differential scanning calorimetry. The dielectric relaxation spectra of these polymers reflect the molecular mobility in bilayer structures formed by the mesogenic or crystalline side chains. By changing temperature it was possible to investigate the molecular motions in the different phases of the copolymers (smectic‐crystalline, smectic‐isotropic, and isotropic). The homopolymer containing perfluoralkyl side chains and all copolymers show a β‐relaxation at low temperatures, which is assigned to local molecular motions. Above the glass transition temperature, all samples exhibit a segmental (α) relaxation with WLF‐like temperature dependence in the activation plots. For the polymers forming LC structures only one relaxation process was detected in all phases, i.e. in the smectic, nematic, and isotropic structure or, in case of the copolymers, in the smectic‐crystalline and smectic‐isotropic double structures. This process was attributed always to the segmental motions, assisted by tumbling motions of the mesogens. The phase transitions are clearly indicated by discontinuities in the dielectric relaxation times and changes in the relaxation strength Δϵ. The dipole reorientations of the mesogens seem to be more restricted by the crystalline layers in the copolymers as by the smectic order of the LC homopolymers.