Crystal structure of poly(trimethylene 2,5-furandicarboxylate) redux - a new model supported by computational spectroscopy

Poly(trimethylene 2,5-furandicarboxylate) (PTF) is an emergent biobased polymer potentially able to outperform the fossil-based poly(ethylene terephthalate) counterpart. In this work, computational chemistry and vibrational spectroscopy tools are combined to elucidate the conformational preferences of PTF in both crystalline and amorphous regions. This approach departs from previous studies and leads to a new proposal for the crystal structure of this significant biobased polymer. In crystalline domains, PTF chains take on a helical conformation due to the gauche - gauche kinks present in 1,3-propanediol (PDO) segments, while 2,5-furandicarboxylate (FDCA) moieties adopt the syn - syn motif. Similarly to its counterparts, poly(ethylene 2,5-furandicarboxylate) (PEF) and poly(butylene 2,5-furandicarboxylate) (PBF), syn - syn FDCA units allow the formation of a vast array of C-H O contacts between furanic hydrogens and adjacent carbonyl moieties. The proposed crystal structure of PTF consists of two-dimensional sheets of chains connected by C-H O bonds, which are stacked upon one another forming π-π interactions among furanic rings. A thorough vibrational analysis of PTF's infrared and inelastic neutron scattering intensity profiles, with identification of vibrational modes sensitive to conformation and degree of crystallinity, sets a blueprint for future studies employing vibrational spectroscopy techniques. The crystal structure of biobased poly(trimethylene 2,5-furandicarboxylate) is revisited following new evidence from vibrational spectroscopy and computational chemistry tools.