Tailoring the Structure, Morphology, and Crystallization of Isodimorphic Poly(butylene succinate-ran-butylene adipate) Random Copolymers by Changing Composition and Thermal History

Poly(butylene succinate-ran-butylene adipate) random copolymers (PBSA) were prepared by melt polycondensation in a wide composition range. Polarized light optical microscopy (PLOM) was employed to observe their superstructural morphology while their thermal and structural properties were studied by differential scanning calorimetry (DSC) and in situ synchrotron Xray diffraction at wide and small angles (WAXS and SAXS): The morphological study revealed negative spherulitic superstructures with (PBS-rich) and without (PBA-rich) ring band patterns depending on composition. The crystallization temperature, melting temperature, and related enthalpies display a pseudoeutectic behavior as a function of composition. WAXS studies demonstrated that these random copolymers are isodimorphic, as their unit cell parameters are composition dependent and switch from PBS-like unit cells to beta-PBA-like unit cells around the pseudoeutectic point. For PBA-rich compositions, the inclusion of butylene succinate units in the copolymer selectively promotes the formation of the orthorhombic beta-polymorph, instead of the commonly observed monoclinic alpha-structure. The pseudoeutectic point is located around the 50:50 and 40:60 compositions and is characterized by a remarkable rate-dependent cocrystallization. Parallel DSC, SAXS, and WAXS results for these intermediate compositions show that depending on the cooling rate employed, the materials can exhibit single- or double-crystalline character either upon cooling or during subsequent heating. The structure, morphology, and properties of these versatile random copolymers can be tailored by composition and thermal history.