Smectic–smectic phase transitions in binary liquid-crystalline mixtures of DB5–T8

Abstract

We have used high-resolution x-ray diffraction and ac calorimetry to study the liquid–crystalline smectic-A-smectic-A phase transitions in binary mixtures of polar mesogenic compounds DB5 [benzoic acid, (4-cyano)-4-[(4-pentylphenoxy) carbonyl] phenyl ester] and T8 [(E)-benzoic acid, 4-octyloxy-4-[2-(4-cyanophenyl) ethenyl] phenyl ester]. A detailed synthesis for each of these compounds is presented. The smectic A1 phase in this system shows two collinear diffuse x-ray maxima at incommensurate wave vectors, indicating a tendency to form the predicted Ai1 phase with two strongly coupled density modulations having incommensurate wavelengths. The Ad peak position, while both temperature and concentration dependent, is remarkably constant along the N–Ad phase boundary. In the vicinity of 16 mol % T8, both the x-ray and heat-capacity results are reproducibly history dependent. The simultaneous observation of sharp A2 and Ad peaks most likely results from phase coexistence, but a condensed incommensurate Ai2 phase cannot be ruled out.