Crystallization and Polymorphism of Poly(ethylene furanoate)

In this work we report the observation of two different crystalline patterns of poly(ethylene furanoate) (PEF), corresponding to bulk and solvent induced crystallization. The crystal form generated by bulk crystallization is the a-form, while that observed after solvent crystallization is the beta-form. Crystal transition upon heating was not evidenced. However, in the case of bulk crystallization, the defective (alpha') crystal structure generated under large supercoolings (at temperatures well below 180 degrees C), was reorganized to a more perfect structure of the same form (alpha) when heated above 190 degrees C. Three new peaks at 13.8 degrees, 18.1 degrees, and 26.7 degrees were recorded after annealing at such elevated temperatures. Moreover, the melting temperature of PEF steadily increased, with increasing temperature of isothermal crystallization. Temperature modulated differential scanning calorimetry studies showed enhanced recrystallization upon heating for the samples crystallized under large supercoolings. However, for samples crystallized at 170 degrees C or above, two nonreversing melting peaks were observed and these were attributed to the melting of crystals of different stabilities. Recrystallization was evidenced after these melting peaks. For samples crystallized at 200 degrees C, only a weak recrystallization peak was recorded, after the double nonreversing melting. No indication for recrystallization/reorganization was observed for the solvent-crystallized samples. Finally, it was realized that fresh PEF samples always crystallized faster than those that suffered repeating melting. This was also observed for other furanoates, but is in contrast to what is observed for their terephthalate or napthalate counterparts.