Crystallization-Driven Quadrant-Specific Spherulitic Self-Assembly in Partially Miscible Biodegradable PBS/PCL/PBS-ran-PCL Blends

The inherent immiscibility of biodegradable aliphatic polyesters, such as poly(butylene succinate) (PBS) and poly(e-caprolactone) (PCL), hampers the development of strong yet degradable plastics. Here, we demonstrate that random isodimorphic copolyesters, poly(butylene succinate-ran-e-caprolactone) (BS<inf>x</inf>CL<inf>y</inf>), effectively compatibilize equimolar PBS/PCL blends through a matrix-driven crystallization mechanism that couples the two phases at the molecular level. Multiscale characterization, combining DSC, in situ and spatially resolved polarized FT-IR imaging, and nanobeam synchrotron WAXD/SAXS, reveals the first structural evidence of quadrant-specific spherulites, in which alternating quadrants exhibit distinct lamellar architectures: banded regions with continuous twisting and nonbanded regions with uniform orientation. This pronounced morphological anisotropy arises from the selective cocrystallization of BS-rich segments (within the random copolymer) with the PBS blend component and the formation of PBS ß-form crystals with looser molecular packing. These features promote interfacial coupling and enhance degradability. The concept of matrix-directed crystallization establishes a potentially general framework for compatibilizing immiscible biodegradable polyesters and for designing biobased plastics with tunable crystalline hierarchy, mechanical performance, and controlled biodegradation behavior.