Asymmetrical formation of isomorphism in the crystalline/crystalline blend of poly(butylene succinate) and poly(butylene fumarate)

Blending is a convenient method to regulate the crystallization behavior and thus the performance of polymer materials, but poor compatibility in crystalline phase for different components remarkably depresses the co-crystallization capability, which poses a great challenge to effective regulation of the properties for crystalline/crystalline blends. In this work, the co-crystallization between two kinds of polyester with similar chain structure, poly(butylene succinate) (PBS) and poly(butylene fumarate) (PBF) is investigated and found to be dependent on the blending ratio and crystallization condition. A new type of co-crystal is only discovered in the PBS-rich blends (i.e., PBS/PBF-6/4 and PBS/PBF-8/2), and, further, it is proved as the isomorphism between PBS and PBF by employing various characterizations of differential scanning calorimeter and Fourier transformation infrared spectrometer and X-ray diffractometer. Both the enhancing cooling rate and the increasing PBS content benefit the growth of isomorphic co-crystal, and, its fraction in the blend of PBS/PBF-8/2 can reach as high as ∼75% during a fast cooling rate of 40 °C/min. In addition, the changes of Gibbs free energy for including butylene fumarate unit into PBS-type crystal and including butylene succinate unit into PBF-type crystal are respectively calculated as −0.96 and 1.94 kJ/mol, revealing the intrinsic thermodynamic driving force for the forming of isomorphism and clarifying why isomorphism only occurs in the PBS-rich blends. Finally, a summary of the crystallization behavior of PBS/PBF blend is reasonably sketched, which will help design the crystalline/crystalline blends with desired structure and performance.