Effect of catalysts on thin-film polymerization of thermotropic liquid crystalline copolyester

Abstract

Using a novel thin-film polymerization technique, we have investigated in situ uncatalyzed and catalyzed polycondensation reaction systems for 73/27 (mol ratio) poly(p-oxybenzoate/2,6-oxynaphthoate) [P(OBA/ONA)] thermotropic liquid crystalline copolyester. We have also determined the effect of catalysts on the kinetics and morphological changes of the reactions. Because the thin-film polymerization is conducted on the heating stage and the morphology is observed in situ by a polarizing microscope, we can directly observe and determine the accurate onset time for LC phase generation. The number-average degree of polymerization (DP) at the onset of this morphological change decreases with decreasing reaction temperature in the range of 230-290 °C. The LC phase may form at a DP as low as 2 at 230 °C. Most importantly and surprisingly, the reaction rate constant obtained from the thin-film polymerization is much greater (20-30 times) than the previous reported value obtained from the bulk polymerization reaction because the release of acetic acid in the former is much easier and quicker than in the latter. Clearly, the thin-film polymerization may be a better and accurate technique to observe the approximately inherent properties of polymerization kinetics than the traditional bulk polymerization reaction. Three kinds of catalysts, namely, sodium acetate, calcium acetate, and antimony oxide, have been studied. Sodium acetate has obvious acceleration effect on the reaction. Reaction rate constant increases almost proportionally to the catalyst content in the low catalyst content range, and activation energy slightly decreases with an increase in sodium acetate percentage. Calcium acetate has a higher catalytic effect than sodium acetate when the catalyst content is high, but the trend reverses when the catalyst content is low. Polymerization with high content of calcium acetate produces LCP with undesirable morphology because it suppresses the coalescence process among LC domains. Antimony oxide is a polymerization inhibitor for this reaction. It slows down the reaction, but does not alter the sequence of the morphological changes during the polymerization.