(210d) Biaxial Toughening in Uniaxially Stretched Films of Block Polymer-Modified Semicrystalline Poly(L-lactide)

Semicrystalline poly(L-lactide) (PLLA) is a sustainably sourced and compostable alternative to conventional, nondegradable plastics. Though it possesses a relatively high yield stress and Young’s Modulus, PLA lacks the toughness required for many applications. Chain alignment via uniaxial stretching can impart toughness to PLA in the machine direction (MD), but the transverse direction (TD) remains brittle. While processing methods that produce a degree of biaxial stretching, such as blown film extrusion, can mitigate this anisotropy in other polymers, PLA’s low melt strength often precludes such processing methods. This work reports uniaxially stretched films of PLLA blended with 3 wt % poly((ethylene oxide)-b-(butylene oxide)) (PEO-PBO/PLLA) which exhibit biaxial toughness, with as much as a five-fold increase in toughness in the TD compared to similarly stretched neat PLLA films. Furthermore, the impact of PEO-PBO particles on the relationship between stretching, crystallization behavior, and resultant mechanical properties are elucidated. Faster stretching rates produced films with higher yield stress and a greater degree of crystallite alignment in PEO-PBO/PLLA blends, highlighting the synergistic relationship between crystallinity and chain alignment. This finding also suggests a competing mechanism of heterogeneous crystallite nucleation around PEO-PBO particles. Importantly, PEO-PBO/PLLA exhibited a TD elongation at break of 36%, a five-fold increase over the value of similarly stretched neat PLLA and greater even than the corresponding MD value of either material. These findings demonstrate that by blending PLLA with low loadings of PEO-PBO, biaxially tough films can be obtained by uniaxial stretching, with the fastest stretching conditions producing the greatest enhancement in TD toughness.