Interfacial Dynamics Accelerate Aging Yet Sustain Toughness in Poly(l‑lactide) Block Polymer Plastics

Poly-(lactide) (PLA) homopolymer embrittles under ambient conditions within two days after melt processing through physical aging, which restricts its growth as a sustainable alternative to petroleum-derived, nondegradable plastics. Block polymers containing PLA and immiscible rubbery segments have shown promising mechanical longevity, though only at very high total molar masses. To elucidate the basic architectural and morphological features of such aging-resistant materials, we used a straightforward two-step synthetic route to generate a library of n-arm block polymer plastics (n = 1-4) with poly-(γ-methyl-ε-caprolactone) (PγMCL) as the rubbery core and poly-(l-lactide) (PLLA) as the outer block, fixing PLLA content at 80 wt %. Triblocks and star-blocks (i.e., n ≥ 2) exhibited high tensile toughness that persisted over long aging times even in samples with poorly entangled PLLA matrices. Crystallinity and architectural purity also promoted mechanical longevity. Calorimetry revealed that the most mechanically long-lived specimens, with M _PLLA < 35 kg mol^-1, exhibited the fastest physical aging, which we ascribed to a coupling of block dynamics near the segregated PγMCL domain interfaces. Our results broaden the scope of viable block polymer architectures for PLLA mechanical longevity to more synthetically accessible macromolecules that are practically advantageous for scalability and melt processing.