Heating-induced dimensional changes in crystal lattice and lamellar thickness of isothermally crystallized poly(butylene 2,5-furandicarboxylate) (PBF)

Poly(butylene 2,5-furandicarboxylate) (PBF), one of the most promising biobased polyesters, was isothermally crystallized from the melt at Tc= 70, 140 and 160 degrees C and subsequently quenched to room temperature. For the first time, the different structural evolution of these thermally treated samples during heating was investigated in situ by synchrotron Wide-Angle and Small-Angle X-ray Scattering (WAXS and SAXS) and interpreted as a function of the amorphous chain mobility at different temperatures. A structural change in the crystal lattice of the PBF crystals grown at Tc= 70 degrees C was detected by WAXS during heating beyond 110 degrees C. Coincidently, SAXS evidenced an increase in lamellar thickness during heating at temperatures close to 110 degrees C for all samples, regardless of their crystallization conditions. This crystal improvement could be due to the significant mobilization of the methylene sequences within the polymer chains at that limiting temperature. Thanks to less restricted and easier conformational rearrangements, the reorganization of the crystals through a melting/recrystallization mechanism could be favored. On the other hand, a different mechanism involving the rigid amorphous fraction (RAF), i. e. the constrained interphase located at the amorphous/crystal boundary, is proposed to explain the increase in the lamellar thickness observed at around 90 degrees C after crystallization at Tc= 70 degrees C. The temperature limit for the presence of the RAF in PBF was indeed identified around 85-90 degrees C. Complete mobilization of the amorphous chains at temperatures above 90 degrees C could favor the rearrangements necessary to form thicker lamellae.