Aliphatic copolyester with isomorphism in limited composition range

A series of random poly(hexamethylene succinate-co-hexamethylene fumarate) (PHSF) samples was synthesized through a two-step reaction process, and their properties were systematically studied using wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). The crystal modification of polyester changes from monoclinic to orthorhombic after substituting hexamethylene succinate (HS) units by hexamethylene fumarate (HF) units, and the chain conformation changes from all-trans to gauche. The copolyesters were found to be in a particular case as strict isomorphism in a limited composition range, adopting the same crystal modification when HF content ranged from 19 to 100 mol%. The melting point of the copolyester increases almost linearly with increasing molar content of HF, and the melting enthalpy hardly changes. Interestingly, FTIR results suggest that a special hydrogen-bonding interaction, in “CC–H···OC–CC” form, forms in the crystalline phase of PHSF and poly(hexamethylene fumarate) (PHF). The strong hydrogen-bonding interaction might be responsible for the transformation of conformation from all-trans to gauche in hexamethylene segments, and triggers the variation of crystal modification.