Aging Resistant Toughness in Poly(lactide)

Poly(lactide) is a leading sustainable polymer derived from corn. Both glassy, poly((+/-) lactide) (PLA), and semicrystalline (PLLA) versions of this material suffer from physical aging, whereby the plastic, which is ductile immediately following thermal processing, becomes mechanically brittle after approximately one day of annealing at room temperature. Blending 5 wt% of a disordered poly(ethylene oxide)-block-poly(butylene oxide) (PEO-PBO) diblock copolymer (M_n = 7.4 kg/mol; 35 vol% PEO) with commercially available PLA and PLLA results in a dramatic increase in ductility, which persists after annealing for more than 100 days at room temperature. This behavior is attributed to the formation of a stable morphology during melt blending consisting of ~0.8 micron diameter particles of PEO-PBO, which cavitate and initiate crazing during tensile deformation. We speculate that the relatively low molecular weight block copolymer rapidly diffuses from the liquid droplets into the crazes, plasticizing the surfaces and fibrils, thereby inhibiting brittle fracture. Similar property improvements were obtained with uniaxially strained films of these toughened sustainable plastics.