Toughening of in situ polymerized cyclic butylene terephthalate by chain extension with a bifunctional epoxy resin

Cyclic butylene terephthalate (CBT) was polymerized in the presence of a low molecular weight bifunctional epoxy resin. The resultant chain extended polymerized CBT (pCBT) showed an increased ductility compared to that of conventionally polymerized pCBT for all analyzed epoxy concentrations (1–4 wt.%). The best results were obtained with 2 wt.% of epoxy resin. Other mechanical properties remained relatively unaffected by the epoxy resin. ¹H NMR analysis suggested an esterification reaction of the carboxyl end groups of pCBT and the glycidyl functional groups of the diepoxide. With increasing epoxy content, the chain extended pCBT showed an increasing molecular weight and a decreasing glass transition. Crystallization and melting temperatures as well as crystallinity also decreased with increasing epoxy concentration.