A THERMODYNAMIC ANALYSIS OF REENTRANT CYANOESTER LIQUID-CRYSTAL SYSTEMS

The phase diagrams of 4-n-alkoxyphenyl 4'-cyanobenzoate nOPCB, n = 6, 8, and 12, and 4-cyanophenyl 4'-n-octyloxybenzoate CP8OB binary systems studied by Goodbye et al. (Goodby, J. W.; Leslie, T. M.; Cladis, P. E.; Finn, P. L. Liquid Crystals 6 Ordered Fluids; Griffin, A. C., Johnson, J. F., Eds.; Plenum: New York, 1984; Vol. 4 pp 89-110, 203-231) are analyzed thermodynamically. For the systems 6OPCB/8OPCB, 6OPCB/12OPCB, CP8OB/12OPCB, and CP8OB/8OPCB, the change in Gibbs excess energy for the reentrant nematic-smectic A transition extracted as a function of composition and temperature is highly temperature-dependent. For the same systems, the change in excess energy for the nematic-isotropic transitions is fit with a third-order Redlich-Kister expression. The compound 12OPCB is further analyzed to obtain transition enthalpies and predict transition temperatures for all relevant stable and metastable phase transitions. From these results, a semiquantitative chemical potential-temperature plot for 12OPCB was generated. A calculated phase diagram was prepared for the 6OPCB/12OPCB system.