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

Poly(hexamethylene succinate- co -hexamethylene fumarate)s (PHSF) with low hexamethylene fumarate content ( C HF , 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 C HF -dependent but also crystallization temperature ( T c ) dependent. Both high C HF and high T c benefit the formation of orthorhombic modification, while low C HF and low T c benefit the formation of monoclinic modification. More intriguingly, the specific “C C C O⋯H C C” hydrogen bonds formed between fumaryl units provide a direct way to regulate the polymorphism from perspective inside chain structure. When C HF is between 8 and 12 mol%, the crystal modification of PHSF is determined by formation of intersegmental hydrogen bonding interaction or not through varying T c . At high T c , the specific hydrogen bonds form and render the formation of orthorhombic crystal; while at low T c , 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.