Peculiarities of δ- and α-relaxations in thermotropic side chain liquid crystalline polymers with and without nematic reentrant phase

We have performed a dielectric spectroscopy study of four homologous cyanobiphenyl polyacrylates with long side chains. The α- and δ-relaxation times were found to be sensitive to the sequential transformations between mesophases. The τ^δ in the isotropic phase exhibits the characteristics that obeys VFT relation and depend strongly on spacer length. The relaxation times, τ^δ, for the crossover from short range intermolecular interactions to long range LC ordering, decreases with increasing side chain length, implying that the cooperative motions of mesogenic dipoles arrange long range order at shorter time scales, as the spacer length is increased. In the SmA mesophases of CBPAn compounds with n = 8, 9 and 11 α-relaxation times were found to be nearly temperature independent at high temperatures. Thus, segmental motions take place in the state of diminished dynamical constraints of backbones, which can be attributed to the plasticization effects of polymeric layers in the case of long methylene spacers. In CBPA6 and CBPA8, with even numbers of methylene groups, anomalies of δ-relaxation processes were observed at the nematic reentrant (N_re) transitions. The anomalies of α-relaxation processes in the SmA mesophases were found to be precursors of N_re transitions as temperature decreases. The changes in backbone conformations of the SmA layers with decreasing temperature create the conditions for molecular corrugations of the side chains leading to the formation of the nematic order of N_re phases.