Orientational order and tricritical behavior in the trans-p-n-alkoxy-alpha-methyl-p[prime]-cyanophenyl cinnamate (n-OMCPC) series of liquid crystals

Abstract

The temperature dependence of the orientational order parameter S has been evaluated from their infrared spectra for six homologues of the n-OMCPC (trans-p-n-alkoxy-alpha-methyl-p[prime]-cyanophenyl cinnamate) series of liquid crystals with n = 7–12, where n is the number of carbon atoms in the end alkyl chain. The even–odd effect of the S values in the nematic phase is rather weak for homologues 7–10. In homologues 10–12, the changes of S at the transitions have been correlated with their thermodynamic behavior. The smectic A–nematic (A–N) and nematic–isotropic (N–I) transitions in several binary mixtures of 11 with 8, 10, and 12 have been studied by differential scanning calorimetry. With decreasing average alkyl chain length in the mixtures, the nematic range becomes extended and the enthalpy of the A–N transition is found to decrease. We deduce that the tricritical point in the A–N transition occurs when the reduced temperature TR=TAN/TNI=0.94±0.005. The triple point between A, N, and I phases has also been observed in an 11–12 mixture at the mole percent ratio of ~27:73.