Thermodynamic Features of Extended-Chain Crystals of Poly(butylene succinate) and Its Random Copolymers with Adipic Acid

Thermodynamic features of extended-chain crystals (ECCs) are the basis for determining crystallization driving force of polymers. However, accurate determination of melting points (Tm0) and enthalpy of fusion (ΔHm0) of ECCs is still a challenge due to the lack of corresponding physically fabricated structures, especially for a copolymer whose ECC has not been prepared or reported yet. Herein, we choose poly(butylene succinate) (PBS) and its random copolymers with adipic acid (PBSA) as model polymers and successfully fabricate their ECCs through a preparation pathway by forming inclusion complexes with urea. Consequently, the values of Tm0 and ΔHm0 have been directly determined, which are quite different from those extrapolated in previous literature. The Tm0 and ΔHm0 of PBS are respectively identified as 136 °C and 133.5 J/g. With the increase of cocrystallized butylene adipate (BA) units in the lattice, the as-measured Tm0 of the ECC declines more significantly than the extrapolated value by the Hoffman–Weeks equation based on a mismatched crystalline structure. The ΔHm0 of PBSA displays an almost linear downward dependence on BA composition, explicitly revealing a defect Gibbs free energy of 0.7 kJ/mol for the cocrystallization of BA units in PBS lattices. Our work sheds light on the accurate determination of properties of polymer ECCs and is essential to expand the understanding of ECCs from homopolymers to copolymers.