The nature of phase transitions in reentrant CBOBP: 4-cyanobenzoyloxy-4 '-octylbenzoyloxy-p-phenylene

The phase transitions of 4-cyanobenzoyloxy-4'-octylbenzoyloxy-p-phenylene ( CBOBP) were proposed to be doubly reentrant with the phase sequence: crysta l to S-Al to N to S-Ad to N to I. We have reinvestigated the thermal behavi or of CBOBP in light of the theory of Navard and Haudin. According to the t heory, the peak height of a differential scanning calorimetry DSC signal ob tained at different heating rates may be used to determine whether the orde r of the phase transition is 1(st) or 2(nd) order. Our calorimetric data co mbined with previous characterization of the S-A phases by X-ray and NMR ex periments show that the S-Al and the S-Ad are different phases which must n ot be considered thermodynamically the same phase. Thus, since only the N i s reentrant, the term doubly reentrant incorrectly describes the phase beha vior. We propose the phase behavior to be crystal to S-Al 1(st) order, S-Al to N 2(nd)order, N to S(Ad)1(st) Order, S-Ad to N 1(st) order, and N to I 1(st) order.