An optical study of phase transitions of poly (ethylene terephthalate‐co‐p‐hydroxybenzoic acid) liquid crystal

The copolyester containing 40 mol % ethylene terephthalate (PET) and 60 mol % p‐hydroxybenzoate (HB) units has been reported by several investigators to be biphasic in the solid and the liquid states. The reported thermal transitions in the two phases, however, are in part contradictory, perhaps partly due to different polymerization conditions. The present work is a study of the transitions in each of the two phases of this copolyester by polarized light microscopy and by light transmission measurements. By light transmission measurements, the two phases were found to have two different glass transition temperatures for the onset of segmental motion, consistent with two assignable temperatures (T.). Cold crystallization and melting in each of the two different phases was also detected. The results help clarify the nature of transitions and agree with the results of dynamic mechanical analysis on the same thermotropic liquid crystalline copolyester. Copyright © 1990 John Wiley & Sons, Inc.