Influence of the External (Electric or Mechanic) Field on the Dielectric Relaxation in Liquid Crystalline Side Chain Polymers

Abstract

The effect of orienting fields on the relaxation of dipole polarization in siloxane and methacrylate liquid crystalline polymers with cyanobiphenyl groups in side chains has been investigated.

The temperature-frequency dependences of dielectric losses of comb-like polymers with the main siloxane chain were studied in the mesomorphic state (in bulk). Two relaxation processes, a and δ, were established. They are related to the reorientation of the transverse and longitudinal components of the dipole moment of the mesogenic group. Similarity of the temperatures of the α- and δ-processes and of the glass transition are due to the dependence of the main chain segment motion on the interaction between side mesogenes, with the segmental mobility occurring only when the mesogenes move. The effect of the external orienting field leading to homeotropic orientation of mesogenic fragments results in the increasing intensity of δ-transition and the suppression of α-transition. The relaxation times and the activation energies of the δ-transition remain unchanged. On the basis of dielectric data the order parameter of mesogenic groups was evaluated. Three relaxation regions of dipole polarization were detected on the temperature-frequency dependences of dielectric losses in a comb-like polymer with the methacrylate main chain in a chloroform solution. Two of them were related to intramolecular mobility of polar kinetic elements in side chains, whereas the third region was associated with the cooperative mobility of mesogenic groups in the macromolecular coil. Only the third process was shown to be influenced by the external electric field.

Key Words:

• Dielectric relaxation
• liquid crystalline polymer
• order parameter
• molecular alignment in the mechanic or electric field.