Polymorphism regulation in Poly(hexamethylene succinate-co-hexamethylene fumarate): Altering the hydrogen bonds in crystalline lattice

Abstract

Poly(hexamethylene succinate-co-hexamethylene fumarate)s (PHSF) with low hexamethylene fumarate content (CHF, between 0 and 18 mol%) were prepared and employed to explore the effect of intersegmental interaction in crystalline lattice on the polymorphism phenomenon. Wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) results demonstrate that the crystal modification of PHSF exhibits not only CHF-dependent but also crystallization temperature (Tc) dependent. Both high CHF and high Tc benefit the formation of orthorhombic modification, while low CHF and low Tc benefit the formation of monoclinic modification. More intriguingly, the specific “CCCO⋯HCC” hydrogen bonds formed between fumaryl units provide a direct way to regulate the polymorphism from perspective inside chain structure. When CHF is between 8 and 12 mol%, the crystal modification of PHSF is determined by formation of intersegmental hydrogen bonding interaction or not through varying Tc. At high Tc, the specific hydrogen bonds form and render the formation of orthorhombic crystal; while at low Tc, only van der Waals forces remain and PHSF chains crystallize into monoclinic crystal. The content of carbonyl group belonging to fumaryl in polyester chain is a key factor for the formation of hydrogen bonds and the selection of crystal structure during crystallization. Additionally, the orthorhombic crystal shows higher equilibrium melting point than monoclinic crystal; and the monoclinic crystal can transform to orthorhombic crystal by annealing, revealing that orthorhombic crystal is more thermally stable.