Poly(Isoprene)-Block-Poly(Ethylene\nOxide) Diblock\nCopolymer Toughened Polylactide

Poly(lactide) (PLA) is currently the most successful\nbioderived\nsynthetic polymer. However, applications of PLA are limited due to\nbrittleness, which develops after about 1 day of physical aging at\nroom temperature following melt processing. In this study, we investigate\nthe ability of low molecular weight poly(isoprene)-block-poly(ethylene\noxide) (IO) diblock copolymers to toughen glassy PLA. Melt blending\nIO with PLA leads to submicron diameter macrophase separated IO domains\nin the PLA matrix, stabilized by surfactant-like behavior associated\nwith compatibility between the poly(ethylene oxide) (O) blocks and\nPLA. IO molecular weights that bracket the room temperature order-to-disorder\ntransition temperature, <i>M</i><sub>n</sub> = 1.7–3.3\nkg/mol, produce outstanding toughness with strains at break ε<sub>b</sub> > 150%, which persists through 9 days of aging at room\ntemperature.\nHigher molecular weight IO diblocks do not perform as well. We hypothesize\nthat this behavior reflects two unique characteristics: (1) optimal\nparticle sizes leading to cavitation and craze initiation when subjected\nto triaxial strain, and (2) draining of IO into the advancing crazes\nthereby stabilizing the porous craze structures. Experiments with\nmelt mixed poly(isoprene) (I) homopolymer in PLA containing controlled\nparticle sizes support this mechanism. These results demonstrate a\ngeneral strategy for designing block copolymer additives that toughen\notherwise brittle plastics.