Frequency and temperature dependence of the dielectric parameters of wide temperature range TGBA and TGBC* phases of unsymmetrical liquid crystal dimers

Abstract

In the present work we have carried out frequency and temperature dependent dielectric studies of an optically active dimeric compound, 4-n-undecyloxy-4′-(cholesteryloxycarbonyl-1-butyloxy)chalcone which shows wide temperature range twist grain boundary (TGB) phases namely TGBA and TGBC*. Dielectric permittivities and DC conductivities have been determined along and normal to the TGB helix axes. Two weak collective modes of dielectric relaxations have been detected for the planar oriented sample where permittivity is measured normal to the long axes of the molecules (but along the TGB helix axis). One mode, which exists in the MHz region, has behavior similar to those of the soft mode due to the amplitude fluctuation. The second one exists in the low frequency region (∼100 Hz) and appears due to phase fluctuation as happens in the case of Goldstone mode. Sample confined between electrodes treated for the homeotropic alignment (permittivity measured normal to the helix axis) does not show any mode of relaxation in the frequency range of 1 Hz to 1 MHz. Measured permittivities suggest negative dielectric anisotropy for the system.

Keywords:

• TGB phases
• dielectric relaxation
• Goldstone and soft modes
• dc conductivity

Acknowledgements

We wish to thank Department of Science and Technology (DST) New Delhi, for financial support under a project. The authors sincerely thank Prof. H. Prakash, Head of the Physics Department and Prof. I.M.L. Das, Physics Department, University of Allahabad, Allahabad for their support.