Crystallization of isodimorphic aliphatic random copolyesters: Pseudo-eutectic behavior and double-crystalline materials

Isodimorphic random copolyesters are intriguing polymeric materials that can crystallize in their entire composition range, despite the random distribution of comonomer units along their chains. This behavior stems from the relatively similar chemical repeating units of the parent homopolymers. In this feature article, we review our recent works on isodimorphic aliphatic copolyesters, and extract general trends in the framework of the literature. Isodimorphic behavior is a complex phenomenon driven by comonomer partition within the crystalline unit cells formed. These copolyesters crystallize in the entire composition range displaying a pseudo-eutectic behavior when their melting points are plotted as a function of composition. Two crystalline phases, which resembled the crystalline structures of the parent homopolymers, are formed, depending on the considered composition range. The unit cell dimensions of the parent homopolymers change, as a consequence of the inclusion of co-units. At the pseudo eutectic point or pseudo-eutectic region, two crystalline phases can co-exist and their formation strongly depends on thermal history. In this case, double crystalline random copolyesters with two melting points and mixed double-crystalline spherulites can be obtained. The exact composition of the pseudo-eutectic point, the level of comonomer inclusion and the crystallinity degrees cannot be easily predicted by the copolyester chemical structure and composition. These are important issues for further future studies, as well as the quantitative determination of comonomer inclusion in the generated crystalline phases. The extraordinary variation of thermal properties, morphology and crystallinity that isodimorphic random copolyesters display as a function of composition, allows to conveniently tailor their biodegradation, permeability to gases and mechanical properties

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 \u3b2-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 \u3b2-polymorph, instead of the commonly observed monoclinic \u3b1-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