Thermal and Structural Study on Liquid-Crystalline Phase Transition in Hexakis(4-(4‘-alkyloxy)biphenoxy)cyclotriphosphazene

Abstract

A series of hexakis(4-(4‘-alkyloxy)biphenoxy)cyclotriphosphazene [PN(OC 6 H 4 C 6 H 4 OC) n H 2 n +1 ) 2 ] 3 (HACP, n = 6−12) compounds was synthesized from sodium salts of 4-alkyloxy-4‘-hydroxybiphenyl and hexachlorocyclotriphosphazene. The phase transition and mesogenicity of these substances were studied by FT-IR, X-ray diffraction, and DSC measurements and polarizing microscope observations. In the crystalline-to-smectic C phase transition of HACP with n = 8, the infrared bands of P N and P−O−(C) stretching vibrations were found to shift to the lower frequency, indicating the weakening of the cyclotriphosphazene ring in the liquid-crystalline phase. The X-ray analyzed interlayer spacing 36 Å is smaller than the value expected for the fully extended molecular model, indicating that the tilt angle of the molecules is about 35° from the normal to the layer plane. For HACP with n = 6, monotropic nematic and smectic C phases were found to appear only in the cooling process. Enantiotropic N and Sc phases were found for the n = 7 member. For the HACP with the n = 8−12, only an enantiotropic Sc phase was detected. In the series of HACP, the temperature range of the mesomorphic phase increased with an increase in n . The odd−even effect was not clear, originating from the peculiar shape of the hexakis(4-(4‘-alkyloxy)biphenoxy)cyclotriphosphazene with three side chains pointing three upward and three downward in both sides of the cyclotriphosphazene ring.