Phase transitions in two polyesters containing a smectic diol

This work reports the phase behavior of the polyesters made from the condensation of isophthalic acid (IA) and terephthalic acid (TA) with the mesogenic diol 4,4′‐bis (6‐hydroxy hexoxy) biphenyl (BHHBP). The latter monomer forms a smectic mesophase in the interval 98‐179°C. Fischer et al., who investigated the room temperature x‐ray patterns of the oriented polyesters of BHHBP and IA, as well as BHHBP and TA, claimed that both polymers also showed smectic mesophases. However, they showed no other evidence to support their interpretation. In the present work, examination of these polyesters under the optical microscope revealed focal conic textures for the IA polyester, but spherulitic textures for the TA polyester. DSC studies revealed thermograms with two peaks in the cooling scan for the IA polyester, at different cooling rates, which supported the view that there were two phase transitions. The heating scans of the TA polyester showed multiple peaks that depended on heating rate whereas on cooling, only a single exothermic peak was observed at all cooling rates. Thus, the current results definitely support the case for a smectic A mesophase in the IA polyester but, contrary to Fischer et al.'s opinion, do not suggest any mesophase formation in the TA polyester. © 1995 John Wiley & Sons, Inc.