Physical Aging of Polylactide-Based Graft Block Polymers

Graft block copolymers (BCPs) with poly(4-methyl caprolactone)-block-poly(±-lactide) (P4MCL-PLA) side chains containing 80-100% PLA content were synthesized with the aim of producing tough and sustainable plastics. These graft BCPs experience physical aging and become brittle over time. For short aging times, t_a, the samples are ductile and shear yielding is the primary deformation mechanism. A double-yield phenomenon emerges at intermediate t_a where the materials deform by stress whitening followed by shear yielding. At long t_a, the samples become brittle and fail after crazing. PLA content strongly governs the time to brittle failure, where a 100% PLA graft polymer embrittles in 1 day, an 86% PLA graft BCP embrittles in 35 days, and at 80% PLA, the material remains ductile after 210 days. Molecular architecture is also a factor in increasing the persistence of ductility with time; a linear triblock ages three times faster than a graft BCP with the same PLA content. Small-angle X-ray scattering and transmission electron microscopy analysis suggest that the rubbery P4MCL domains play a role in initiating crazing by cavitation. Prestraining the graft BCPs also significantly toughens these glassy materials. Physical aging-induced embrittlement is eliminated in all of the prestrained polymers, which remain ductile after aging 60 days. The prestrained graft BCPs also demonstrate shape memory properties. When heated above the glass-transition temperature (T_g), the stretched polymer within seconds returns to its original shape and recovers the original mechanical properties of the unstrained material. These results demonstrate that graft BCPs can be used to make tough, durable, and sustainable plastics and highlight the importance of understanding the mechanical performance of sustainable plastics over extended periods of time following processing.