Dielectric Properties of Polymer-Liquid Crystal Composites

Abstract

Dielectric properties of two polymer-liquid crystal composites, comprised of the polymer, poly (methyl methacrylate) (PMMA), and liquid crystal, octyl cyanobiphenyl (K-24) and a biphenyl-terphenyl mixture (E-8), are reported as function of frequency (100 Hz-1.0 MHz) and temperature (20-80°C). Two distinct dispersion regions in dielectric permittivity are observed; one at ∼500 Hz due to space charge relaxation at the liquid crystal droplet-polymer interface and the other at ∼50 kHz due to dipolar relaxation. Correspondingly, dissipation factor `D' peaks are also seen in the low- and high frequency regions. Temperature variation studies show that the low-frequency dispersion becomes more and more prominent with increasing temperature, attains a maximum at ∼55°C and then disappears completely beyond 72°C. Correspondingly, `D' also shows a broad peak at ∼55°C. The above results are explained in terms of the Maxwell-Wagner interfacial polarization which arises due to the presence of liquid crystal or liquid microdroplets of higher conductivity in a resistive polymer matrix. Dielectric behavior of polymer-liquid crystal composites is not so much affected by the mesophase transitions, as by the glass transition temperature of the polymer.