Unveiling the uncommon crystallization features of 3,4-PEF, a thermal and DFT study

Abstract

The development of furandicarboxylic acid (FDCA) based polymers and materials is a rapidly growing research field in both academia and industry, driven by the need to replace fossil-based polymers with more sustainable alternatives. Despite the unequivocal potential of poly(ethylene 2,5-furandicarboxylate) (2,5-PEF), many other furanic polyesters, such as poly(ethylene 3,4-furandicarboxylate) (3,4-PEF), synthetized from the 3,4-FDCA isomer, remain underexplored. This study is the first to explore the conformational preferences of 3,4-PEF polyester using vibrational spectroscopy and density functional theory calculations. Additionally, a comprehensive thermal characterization of 3,4-PEF addresses current gaps in the literature.The results suggest that, in crystalline domains, 3,4-PEF chains adopt the ss-t conformation, where the 3,4-FDCA segment exhibits a syn-syn motif and the ethylene glycol (EG) segment is in the trans conformation (ss-t). In amorphous regions, however, multiple conformations coexist, with syn-syn-gauche (ss-g) and anti-syn-gauche (as-g) segments accounting for the bulk of the population distribution. As previously observed for 2,5-PEF, the formation of C-H···O interactions in the crystalline domain is the main driver for the crystallization preferences of 3,4-PEF. The energetic gain from interchain C-H···O bond formation compensates for the energy penalty associated with the ss-g/as-g to ss-t conformational transition.Differential scanning calorimetry (DSC) analysis revealed that 3,4-PEF has a glass transition temperature (T_g) of 39 °C and a melting temperature (T_m) of 155 °C. Kinetic studies showed that the fastest crystallization rate for 3,4-PEF occurs at 110 °C, with a half crystallization time of 12 min. Interestingly, 3,4-PEF crystallizes faster than 2,5-PEF at its optimal crystallization temperature (170 °C), though still more slowly than poly(ethylene terephthalate). These findings suggest that 3,4-PEF holds promise as a renewable polymer with fast crystallization behaviour.