Performance Beyond Molecular Perfection: Scalable, Recyclable PLA-Based Thermoplastic Elastomers and Toughened Polylactide

ABA triblock copolyesters based on poly(l-lactide) hard end blocks and a poly(ε-caprolactone-stat-lactide) soft midblock were synthesized by sequential ring-opening polymerization using either controlled solution polymerization procedures with purified monomers or a simplified one-pot bulk process employing technical-grade monomers. All materials exhibited typical thermoplastic elastomeric behavior, with elongations at break ranging from approximately 300% to nearly 1000%, together with efficient elastic recovery and pronounced hysteretic energy dissipation during cyclic deformation. As neat materials, the solution-synthesized triblock copolymers displayed the highest elastomeric performance, whereas the bulk-synthesized copolymer showed lower extensibility and tensile strength, reflecting its more heterogeneous molecular architecture. Unexpectedly, the opposite trend was observed in poly(lactic acid) (PLA) toughening. Despite its less outstanding elastomeric properties, the structurally heterogeneous copolymer obtained by the one-pot bulk process proved to be a significantly more effective toughening agent for commercial PLA than the structurally well-defined triblock copolymers prepared in solution. At 25 wt % copolymer loading, the elongation at break of poly(lactic acid) increased from approximately 4% to over 100% while maintaining a Young’s modulus of about 1 GPa, whereas the more regular triblock copolymers produced only limited improvements in ductility despite their superior mechanical performance as neat elastomers. Cryofracture SEM revealed markedly different deformation mechanisms in the corresponding blends, while DSC suggested comparable amorphous-phase miscibility but substantially different thermal evolution during solidification. WAXD further showed that the superior toughening efficiency cannot be simply correlated with the overall degree of crystallinity but is accompanied by distinct changes in the organization of the solid state. The combined evidence suggests that the heterogeneous molecular architecture generated under industrially relevant bulk polymerization conditions promotes the development of a solid-state morphology more favorable for stable plastic deformation of the PLA matrix. Finally, bulk depolymerization enables the recovery of the constituent monomers from both the neat copolyesters and PLA-based blends, supporting the chemical recyclability of these materials. Overall, this study shows that molecular heterogeneity, resulting from a less controlled polymerization process, can instead provide a functional advantage, offering a simplified and potentially scalable route toward recyclable thermoplastic elastomers and efficient PLA toughening agents.