Poly(Isoprene)-Block-Poly(Ethylene Oxide) Diblock Copolymer Toughened Polylactide

Poly(lactide) (PLA) is currently the most successful bioderived synthetic polymer. However, applications of PLA are limited due to brittleness, which develops after about 1 day of physical aging at room temperature following melt processing. In this study, we investigate the ability of low molecular weight poly(isoprene)-block-poly(ethylene oxide) (IO) diblock copolymers to toughen glassy PLA. Melt blending IO with PLA leads to submicron diameter macrophase separated IO domains in the PLA matrix, stabilized by surfactant-like behavior associated with compatibility between the poly(ethylene oxide) (O) blocks and PLA. IO molecular weights that bracket the room temperature order-to-disorder transition temperature, M_n = 1.7-3.3 kg/mol, produce outstanding toughness with strains at break ϵ_b > 150%, which persists through 9 days of aging at room temperature. Higher molecular weight IO diblocks do not perform as well. We hypothesize that this behavior reflects two unique characteristics: (1) optimal particle sizes leading to cavitation and craze initiation when subjected to triaxial strain, and (2) draining of IO into the advancing crazes thereby stabilizing the porous craze structures. Experiments with melt mixed poly(isoprene) (I) homopolymer in PLA containing controlled particle sizes support this mechanism. These results demonstrate a general strategy for designing block copolymer additives that toughen otherwise brittle plastics.