Phase behavior in HIQ40 liquid crystalline polymers

The thermotropic copolyester consisting of 40 mol% p-hydroxybenzoic acid and 30 mol% isophthalic acid/30 mol% hydroquinone (HIQ40), prepared by two different synthetic routes ("acidolysis" or "phenylester") was studied by differential scanning calorimetry and X-ray diffraction. All these samples show a glass transition temperature in the range of 125-141°C, dependent on the sample thermal history. A crystal to nematic transition is observed between 300 and 375°C. The samples prepared by the acidolysis route, when compared to those produced via the phenylester reaction, have higher crystallinity which is due to the formation of "1A/HQ" crystals. The crystallinity can be increased significantly (as much as twice) by annealing the polymers at 300°C. The heat of fusion (X → N,crystal to nematic) of the annealed acidolysis samples is also higher than that of the polymers prepared by the phenylester process. The nematic to isotropic (N → I) transitions are broad, exhibiting no definite clearing points. Several mesogenic transitions as suggested by X-ray studies and two crystallization peaks were found for all samples studied. This may be an indication that the samples consist of two, distinct polymeric compositions or that there are two crystal forms. Furthermore, since the intensity ratio of the two crystallization peaks changes as a function of annealing, it is clear that the chemical/physical state of the polymer depends on the thermal history of the samples. Attempts to prepare amorphous HIQ40 and nematic HIQ40 glass by quenching from the isotropic state (480°C and 400°C, respectively) were unsuccessful, however, it can be prepared by precipitation of the acidolysis polymer from solution.