Persistent Toughness and Heat Triggered Plasticization in Polylactide Modified with Poly(ethylene oxide)-block-poly(butylene oxide)

Abstract

Poly(lactide) (PLA) is a promising biodegradable polymer with potential applications in single-use packaging. However, its use is limited by brittleness, and its biodegradability is restricted to industrial compost conditions due in part to an elevated glass transition temperature (T_g). We previously showed that addition of a poly(ethylene-oxide)-block-poly(butylene oxide) diblock copolymer (PEO–PBO) forms macrophase-separated rubbery domains in PLA that can impart significant toughness at only 5 wt %. This work demonstrates that PEO–PBO/PLA blends exhibit substantial toughness for at least nine months, beyond the average lifetime of single-use packaging, even amidst oxidative degradation of PEO–PBO into oligomeric products. Due to the glassy nature of the PLA matrix, these degradation products are confined to macrophase-separated domains, and the blend morphology is preserved. However, modest thermal annealing (∼60 °C) causes these domains to rapidly reduce in area fraction and size from migration and solubilization of the PEO–PBO degradation products into PLA, which plasticizes PLA and reduces the blend T_g. As a result, aged PEO–PBO/PLA blends degrade in just under half the time of similarly aged neat PLA when submerged in artificial seawater at 50 °C. This surprising combination of properties addresses two of PLA’s most significant limitations with a single additive by (1) toughening the PLA during its useful lifetime and then (2) accelerating its degradation rate by heat-triggered plasticization when exposed to elevated temperatures at end-of-life, such as those of industrial (or even home) compost.