Influence of catalyst on transesterification between poly(lactic acid) and polycarbonate under flow field

This paper investigated the effect of catalyst on transesterification and transesterification mechanism between poly(lactic acid) (PLA) and polycarbonate (PC) under flow field. Three catalysts (zinc borate, titanium pigment and tetrabutyl titanate) were evaluated. It is found that transesterification reaction can take place without any catalyst, while three catalysts can all promote the transesterification reaction between poly(lactic acid) and polycarbonate to a greater extent. 1H nuclear magnetic resonance spectroscopy, gel permeation chromatography and dynamic mechanical analysis revealed that structures of copolymers are not identical in the blends with and without catalyst. For pure blend, most of copolymers have relatively high molecular weight with low PC content, which implies that transesterification reaction most likely happens only once between a PLA chain and a PC chain during mixing process, and only a small amount of multiple reactions happen. However, for the catalyst systems, catalysts induce much more multiple reactions accompanying with the reducing molecular weight in copolymers and increasing PC content. Moreover, it is found that the catalysts not only affect the chain compositions of the product copolymers, but also influence the amount of polymers involved in the reaction. Tetrabutyl titanate is found to be the most effective catalyst in this study where the amount of reacted polycarbonate is more than 4 times of that in pure blend. It is found that PLA segments in copolymer are easily aligned on the interface due to its relatively high Deborah number, which increases the probability of its contact with more PC chains. Although the flow effect on the alignment of chain segment is similar in blends with and without catalysts, the acceleration of reaction due to catalyst makes it possible for multiple reactions. The match of the reaction time and contact time of chain segment of PC and PLA at interfaces is then of key importance in the interfacial transesterification reaction. The effect of flow field on the interfacial reaction is then not only from the interfacial update, but also from the change of chain conformation near the interface.