Toughening Polylactide with Graft-Block Polymers

Poly[(styrene-alt-N-hydroxyethylmaleimide)-ran-(styrene-alt-N-ethylmaleimide)]-graft-[poly(4-methylcaprolactone)-block-poly((±)-lactide)] (g-ML) graft-block polymers containing 50 vol % poly((±)-lactide) (PLA or L) were mixed with a commercial PLA homopolymer to modify the brittle mechanical behavior of this industrially compostable plastic. Various graft architectures, including linear, tri-arm, and tetra-arm polymer backbones, were prepared using a grafting-from method. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) revealed that the pure g-MLs form a lamellar morphology where the degree of long-range order is dictated by the polymer architecture. When melt-blended with PLA at low concentrations, the g-MLs formed well-dispersed nanoscale particles within the PLA matrix, yielding moldable plastics with high optical transparency. The tensile toughness of the PLA/g-ML blends was substantially enhanced over that of pure PLA using g-ML concentrations as low as 5 wt % and exhibited average strains at break of 280% following 2 days of aging at room temperature; pure PLA failed at a 7% strain. The elastic modulus, yield stress, and transparency of the toughened plastic were virtually unaffected by the low concentration of rubbery poly(4-methylcaprolactone) (M) domains and the formation of well-dispersed nanoscale particles. Graft-block polymers were shown to toughen PLA more efficiently than a linear triblock copolymer analogue LML, which produced a strain at break of 105% at a loading of 5 wt %. Blending g-ML into PLA significantly delays the onset of physical aging and the onset of the ductile-to-brittle (DTB) transition, which depends on the concentration of g-ML utilized.