How Composition Determines the Properties of Isodimorphic Poly(butylene succinate-ran-butylene azelate) Random Biobased Copolymers: From Single to Double Crystalline Random Copolymers

The structure, morphology, and thermal properties of biobased and biodegradable poly(butylene succinate-ran-butylene azelate) random copolyesters with a wide composition range were studied. These PBS-ran-PBAz copolymers can crystallize in the entire composition range despite being random, displaying a eutectic point when their melting point is plotted as a function of composition. Wide angle X-ray scattering (WAXS) studies confirmed isodimorphic behavior where PBS-rich copolymers crystallize with PBS type unit cells with some PBAz repeating unit inclusions and vice versa for PBAz-rich copolymers. Away from eutectic compositions the copolymers exhibit only one crystalline phase (PBS-rich or PBAz-rich crystalline phases) while at the eutectic point both phases can crystallize. The only double crystalline copolymer among those prepared had a composition close to the eutectic point of 45 mol % PBS (and 55 mol % PBAz). The crystallization of the two phases occurred in the same temperature range upon cooling from the melt at 10 degrees C/min in a DSC (i.e., coincident crystallization). Self-nucleation (SN) studies of the PBS-rich phase were performed. After SN, the separate crystallization of each phase was clearly observed during cooling from the self-nucleation temperature (i.e., PBS and PBAz-rich phases). Small angle X-ray scattering (SAXS) experiments were performed for the first time for these types of isodimorphic copolymers. The results show that the lamellar long period is a strong function of composition. While limited inclusions of PBAz units within the crystal lattice only cause a slight expansion of the PBS component unit cell, the increase of comonomer content produces an unexpected synergistic increase in long periods and lamellar thickness values. In the case of the only double crystalline copolymer examined, the PBS-rich phase forms space filling spherulites (observed by polarized light optical microscopy, PLOM) at higher temperatures that template the superstructural morphology of the copolymer. These PBS-rich phase spherulites contain radial lamellar stacks whose long period was determined by SAXS. Upon further cooling, the PBAz-rich phase crystallizes in the intraspherulitic amorphous regions with newly formed lamellae that have their own distinct long period according to SAXS results. AFM observations of the PBS-rich crystalline lamellae confirmed the lamellar thickness and long spacings determined by SAXS. A schematic morphological model of the mixed spherulites produced by this double crystalline diblock copolymer is proposed based on the experimental evidence collected by SAXS, PLOM, and AFM.