Effect of Molecular Ordering on the Nematic-Isotropic Transition in Reentrant Nematic Mixtures: A High Pressure Dta and Optical Study

The effect of pressure on the smectic A-nematic (A-N) and the nematicisotropic (N-I) phase boundaries has been studied as a function of mole fraction in mixtures of 4′-n-hexyloxy-4-cyanobiphenyl (60CB) and 4′-n-octyloxy-4-cyanobiphenyl (80CB) by DTA and optical microscopy. The maximum pressure of occurrence of the smectic A phase decreases with increasing 60CB mole fraction (x) until at x≈0.30, there is no smectic A phase at all. The plot of dT/dp at 1 bar for the N-I transition versus mole fraction exhibits an anomalous decrease in the mole fraction range of 0.10 < × < 0.28 implying that the changes in molecular ordering which accompany the formation of the re-entrant nematic phase could manifest at much higher temperatures and affect thereby the pressure behaviour of even the N-I transition.