Tuning the Thermal Properties and Morphology of Isodimorphic Poly[(butylene succinate)-ran-(ε-caprolactone)] Copolyesters by Changing Composition, Molecular Weight, and Thermal History

High molecular weight poly[(butylene succinate)-ran-(epsilon-caprolactone)] copolyesters (PBS-ran-PCL) were synthesized in a wide composition range and compared with significantly lower molecular weight samples synthesized previously. DSC and WAXS showed that these copolyesters are isodimorphic (i.e., each crystalline phase contains a small amount of the second comonomer) as they able to crystallize in the entire composition range and display a pseudoeutectic point, and their unit cell dimensions are a function of composition. Copolymers close or away from the pseudoeutectic point exhibited a single crystalline phase, i.e., PBS-rich or PCL-rich crystalline phase. At the pseudoeutectic point, both phases can crystallize in double crystalline banded spherulites, as demonstrated by polarized light optical microscopy (PLUM) studies. An increase in molecular weight of the copolyester does not influence T-m and T-c significantly, as their values are determined by the randomness of the comonomer distribution. However, crystallinity values are higher for lower M-w copolymers because of their faster crystallization rate. Copolymers with higher M-w exhibited higher T-g values as expected for random copolymers that are characterized by a single phase in the amorphous regions. Therefore, changing composition and molecular weight, a remarkable separate control over T-g and T-m values can be achieved in these copolyesters. SAXS results revealed that the lamellar thickness l(c) decreases with composition at each side of the eutectic point. Comonomer exclusion limits the length of crystallizable sequences; as a result, the lamellar thickness values do not significantly vary with M-w in the range studied here. At the pseudoeutectic copolyester compositions, the cooling rate determines for both series of samples (low and high M-w) if one or two crystalline phases can develop: only the PCL-rich crystalline phase, only the PBS-rich crystalline phase, or both crystalline phases. This behavior was studied in detail by DSC, in situ WAXS/SAXS, and PLUM. Our studies demonstrate that these biodegradable copolymers are versatile materials whose properties can be tuned by composition, molecular weight, and thermal history to better target specific applications.