The Nature of Phase Transitions in Reentrant CBOBP: 4-Cyanobenzoyloxy-4′-Octylbenzoyloxy-p-Phenylene

Abstract

The phase transitions of 4-cyanobenzoyloxy-4′-octylbenzoyloxy-p-phenylene (CBOBP) were proposed to be doubly reentrant with the phase sequence: crystal to S_A1 to N to S_Ad to N to I. We have reinvestigated the thermal behavior of CBOBP in light of the theory of Navard and Haudin. According to the theory, the peak height of a differential scanning calorimetry DSC signal obtained at different heating rates may be used to determine whether the order of the phase transition is 1^st or 2^nd order. Our calorimetric data combined with previous characterization of the S_A phases by X-ray and NMR experiments show that the S_A1 and the S_Ad are different phases which must not be considered thormodynamically the same phase. Thus, since only the N is reentrant, the term doubly reentrant incorrectly describes the phase behavior. We propose the phase behavior to be crystal to S_A1 1^st order, S_A1 to N 2^ndorder, N to S_Ad 1^st order, S_Ad to N 1^st order, and N to I 1^st order.

Key Words:

• reentrant
• phase transition
• DSC
• CBOBP
• nematic
• smectic