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Semicrystalline poly(l-lactide) (PLLA) is a leading biosourced, compostable alternative to conventional plastics but lacks sufficient toughness for many applications. Chain alignment via uniaxial stretching may be used to toughen PLLA but often creates anisotropic materials that are tough in the machine direction (MD) but brittle in the transverse direction (TD). This work reports uniaxially stretched films of PLLA blended with 3 wt % poly(ethylene oxide)-block-poly(butylene oxide) (PEO-PBO), which exhibit as much as a 5-fold increase in toughness in the TD compared to similarly stretched neat PLLA films─and elucidates the impact of PEO–PBO particles on the relationship between stretching, crystallization behavior, and resultant mechanical properties. Faster stretching rates were correlated with higher yield stress and a greater degree of crystallite alignment in the PEO–PBO/PLLA blends. This trend highlights the synergistic relationship between crystallinity and chain alignment and suggests a competing mechanism of heterogeneous crystallite nucleation around PEO–PBO particles. Importantly, PEO–PBO/PLLA exhibited a TD elongation at break of 36%, five times greater than the value of similarly stretched neat PLLA and even greater than the corresponding MD value of either material. Taken together, these findings demonstrate that uniaxial stretching of PEO–PBO/PLLA blends produces biaxially tough films, with the fastest stretching conditions producing the greatest enhancement in TD toughness. 

Chain orientation, a natural consequence of polymer film processing, often leads to enhanced mechanical properties parallel to the machine extrusion direction (MD), while leaving the properties in the transverse direction (TD) unaffected or diminished, as compared to the unoriented material. Here, we report that mixing poly(ethylene oxide)-block-poly(butylene oxide) (PEO-PBO) diblock copolymer that forms dispersed particles in an amorphous polylactide (PLA) matrix produces uniaxially stretched blend films with enhanced toughness in both the MD and TD. Small-angle X-ray scattering experiments and visual observations revealed that the dominant deformation mechanism for blend films transitions from crazing to shear yielding in the MD as the stretching ratio increases, while crazing is the primary deformation mechanism in the TD at all stretching ratios investigated. As the films age at room temperature, crazing becomes more prevalent in the MD without compromising the improved toughness. The stretched blend films were susceptible to some degree of mechanical aging in the TD but remained fivefold tougher than stretched neat PLA films for up to 150 days. This work presents a feasible route to produce uniaxially stretched PEO–PBO/PLA films that are mechanically tough, which provides a more sustainable plastic alternative.