Synthesis and microwave dielectric properties of polyphenylene liquid crystal compounds with lateral substitution by methyl and fluorine

Abstract

ABSTRACT Nematic liquid crystals are promising materials for microwave communication owing to their continuous tunability as dielectrics and wide frequency range selection. However,conventional liquid crystals exhibit large dielectric loss and high freezing point, hindering their research and application in microwave communication. in this paper,14 novel liquid crystal polyphenylene compounds with lateral methyl substitute were synthesised. Their structures were characterized by IR, NMR,MS and the elemental analyses. Their mesogenic properties were analyzed by POM and DSC. The dielectric properties in microwave frequencies(10-35 GHz)of eight target compounds (6a,7b,8a~8e) with LC matrix were obtained by extrapolation and perturbation techniques. The results show that i. fluorinated polyphenylene compounds with lateral methyl generally exhibited low melting points (≤75°C), ii. compounds with isothiocyanate group yielded high birefringence 0.385-0.441, iii. compounds based on quaterphenyl displayed a wide temperature range of nematic phase, and iv. the maximum dielectric loss of compounds 6e,8c,8e are less than 6.51×10-3,which are better than those of using fluorine replace the methyl group derivative. Therefore, the proposed approach can effectively reduce dielectric loss,reduce the melting temperature, improve the dielectric properties of liquid crystal materials for microwave communication.