Intermolecular ═C–H···O═C Hydrogen Bonding and Conformational Preference in Crystalline Poly(butylene 2,5-furandicarboxylate)

Abstract

Besides maximizing packing efficiency, molecular interactions also play an indispensable role upon conformational ordering for crystallization of long-chain polymers. Herein, structural evolution upon heating of biobased poly(butylene 2,5-furandicarboxylate) (PBF) is followed by temperature-dependent infrared (IR) spectroscopy and wide-angle X-ray diffraction (WAXD) to elucidate the molecular interactions and conformational preference in its crystalline state. By estimating the group distances according to atomic coordinates of its crystal structure, the stronger intermolecular interaction than the van der Waals force is speculated to exist in crystalline PBF, which is fully confirmed by the changes of IR absorptions associated with ═C–H and C═O groups upon heating as an abnormal ═C–H···O═C hydrogen bonding. The combination of temperature-dependent ═C–H band position and crystal lattice expanding supports a good thermostability of this interaction up to melting, fully defining its nature as strong hydrogen bonding rather than the generally reported weak H···O═C bonding in biodegradable polyesters. By further analyzing the confirmational ordering in its crystalline state, it is believed that such intermolecular ═C–H···O═C hydrogen bonding stabilizes the energetically inaccessible syn–syn conformer of 2,5-furandicarboxylic moieties for PBF crystallization.