DIELECTRIC RELAXATIONS IN BILAYER STRUCTURES OF COMB-LIKE COPOLYMERS - PERFLUORALKYL AND LIQUID-CRYSTALLINE SIDE-CHAINS

A series of comb-like copolymers and homopolymers containing different liquid-crystalline and perfluoralkyl side chains was studied by frequ ency and temperature dependent dielectric measurements. The structure and the phase behaviour of the systems were characterized by X-ray sca ttering and differential scanning calorimetry. The dielectric relaxati on spectra of these polymers reflect the molecular mobility in bilayer structures formed by the mesogenic or crystalline side chains. By cha nging temperature it was possible to investigate the molecular motions in the different phases of the copolymers (smectic-crystalline, smect ic-isotropic, and isotropic). The homopolymer containing perfluoralkyl side chains and all copolymers show a beta-relaxation at low temperat ures, which is assigned to local molecular motions. Above the glass tr ansition temperature, all samples exhibit a segmental (alpha) relaxati on with WLF-like temperature dependence in the activation plots. For t he polymers forming LC structures only one relaxation process was dete cted in all phases, i.e. in the smectic, nematic, and isotropic struct ure or, in case of the copolymers, in the smectic-crystalline and smec tic-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 Del ta epsilon. The dipole reorientations of the mesogens seem to be more restricted by the crystalline layers in the copolymers as by the smect ic order of the LC homopolymers.