Dipole moments and the mechanism of molecular motion in a cyanobiphenyl-containing liquid-crystal monomer in a solution and in the bulk of the material in the absence of external orienting fields

Abstract

The dipole moments and dielectric polarization relaxation of a low-molecular liquid-crystal monomer, namely, 4-[ω-(2-propenoyloxy)pentyloxy]-4′-cyanobiphenyl (CBO-5A), are investigated in a mesomorphic phase, an isotropic melt, and a chloroform solution in the absence of external orienting fields. Analysis of the dipole polarization of the CBO-5A monomer in the chloroform solution (x ₂ ∼ 0.0566 mol/mol) demonstrates that the temperature dependences of the permittivity and the dipole moment measured with decreasing temperature are characteristic of molecular association with the Kirkwood factor g = 0.77. It is found that the Kirkwood factor g for the isotropic melt is 0.55. The values of g < 1 indicate the presence of associates with a tendency toward antiparallel orientation of the CBO-5A molecules in both the solution and the melt. Dielectric absorption of relaxation nature is observed in the chloroform solution at temperatures in the range from −50 to −20°C. Two relaxation processes of dipole polarization are observed in the isotropic and liquid-crystal phases of the CBO-5A monomer in the bulk of the material. Examination of the temperature dependences of the dielectric relaxation time revealed that molecules of the CBO-5A liquid-crystal monomer in the bulk can be involved in several types of motion occurring through either the local mechanism or the cooperative mechanism.