A Study on Phase Transition Behavior of Low Molecular Weight Liquid Crystal and Its Blend with a Polymer using Altenating Current Impedance Spectroscopy

Abstract

Alternating Current Impedance Spectroscopy (ACIS) was used to characterize phase transition behabior of a low molecular weight liquid crystal, 4''-n-heptylbiphenyl (7CB) and its blend with an amorphous polymer, poly(methyl methacrylate) (PMMA). ACIS showed that pure 7CB has a single relaxation time due to a large permanent dipole moment associated with cyane (-CN) group in the 7CB molecule, which could be deduced from the Nyquist plot ot the impedance spectra of 7CB. The Nyquist plot of 7CB showed a remarkable change in scale near TNI and TCN, but almost perfect semicircles in complex plane. Changes of the electrical properties of PMMA/7CB blend were also utilized to investigate the relationship between the electrical properties and the phase behavior. The maximum resonance frequency, fmax at which -Z" of the blend becomes maximum, was considered to be a very sensitive probe to investigate the physico-chemical changes within the samples.