From isodimorphism to total exclusion: composition-dependent mixed crystallization in poly (propylene adipate-ran-butylene adipate) copolyesters

Poly(propylene adipate- ran -butylene adipate) (PA x BA y ) random copolymers were investigated over a wide composition range to elucidate their crystallization behavior, previously classified as isodimorphic, under multiple crystallization conditions. Differential scanning calorimetry (non-isothermal and isothermal), Successive Self-Nucleation and Annealing (SSA), and synchrotron WAXS/SAXS analyses reveal a pronounced compositional asymmetry that cannot be resolved using a single crystallization protocol. Although crystallization and melting temperatures exhibit a pseudo-eutectic dependence on composition under slow cooling, the underlying crystallization mechanisms differ markedly across the composition range. On the PBA-rich side, copolymers crystallize in a PBA-like lattice up to 60 mol% PPA, showing progressive melting-point depression, reduced crystallization kinetics, shifts in SSA fractionation, and composition-dependent polymorphic selection. These features are consistent with partial comonomer inclusion and lattice distortion. In contrast, on the PPA-rich side, crystallization is strongly suppressed: crystallinity and lamellar organization decrease abruptly, and no detectable lattice distortion is observed, indicating dominant exclusion of PBA units from PPA crystals. Thermodynamic analysis using exclusion/inclusion models applied to SSA-derived melting temperatures (with uniform temperature shifts for copolymers and homopolymers) shows that the equilibrium degree of comonomer inclusion remains limited (≤5–9%) in the PBA-rich domain, while it approaches the total-exclusion limit in the PPA-rich domain. The combined calorimetric, kinetic, structural, and thermodynamic evidence establishes a composition-dependent transition from partial inclusion to total exclusion, revealing a mixed crystallization response only discernible under multiple crystallization conditions. These results demonstrate that a minimal structural difference, as a single methylene unit between even- and odd-methylene adipate polyesters, can induce strong asymmetry in comonomer accommodation, highlighting the decisive role of crystallization conditions in revealing hidden asymmetries within nominally isodimorphic copolyesters. • Mixed crystallization behavior is revealed in random copolyesters • Transition from isodimorphism to total comonomer exclusion • Small molecular differences induce strong crystallization asymmetry • X-ray scattering reveals lattice distortion and exclusion signatures • Pseudo-eutectic trends can mask distinct crystallization mechanisms