Heat-Triggered Plasticization: A Design Strategy for Accelerated Degradation of Polylactide

Polylactide (PLA) is a promising biosourced alternative to conventional fossil-fuel-derived plastics, but its widespread adoption is limited by its brittleness and slow degradation rate in most environments. We recently demonstrated that a poly(ethylene oxide)-block-poly(butylene oxide) diblock polymer (PEO–PBO) significantly toughens PLA at 5 wt % loading, a benefit which remains unchanged after 9 months of aging despite oxidative degradation of PEO–PBO to oligomers. In this work, we demonstrate that PLA degradation in aged PEO–PBO/PLA blends submerged in seawater at 50 °C is nearly twice as fast as that in neat PLA under similar conditions. The molar mass of neat PLA exhibits a relatively slow decrease for the first 35 days of degradation under these conditions, at which point the PLA glass transition temperature (Tg) falls below 50 °C, and the rate of molar mass change accelerates. In aged PEO–PBO/PLA under the same conditions, the moderately elevated temperature results in the collapse of the macrophase-separated PEO–PBO domains and migration of the oligomeric polyether oxidative degradation products into the PLA matrix, which depresses the PLA Tg to below 50 °C in less than 1 day. As a result, aged PEO–PBO/PLA immediately exhibits a rate of molar mass change consistent with that observed in the late stages of neat PLA degradation, effectively bypassing the 35 days of relatively slow change. These results suggest that heat-triggered plasticization can expand the range of practical conditions for PLA degradation, potentially including home compost conditions, and demonstrate a design strategy for additives that both toughen PLA and accelerate its degradation.