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 α-form, while that observed after
solvent crystallization is the β-form. Crystal transition upon
heating was not evidenced. However, in the case of bulk crystallization,
the defective (α′) crystal structure generated under
large supercoolings (at temperatures well below 180 °C), was
reorganized to a more perfect structure of the same form (α)
when heated above 190 °C. Three new peaks at 13.8°, 18.1°,
and 26.7° 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 °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 °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.
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