The linear viscoelastic behavior of poly­(norbornene)-graft-poly­(±-lactide) was investigated as a function of grafting density and overall molar mass. Eight sets of polymers with grafting densities ranging from 0 to 100% were synthesized by living ring-opening metathesis copolymerization. Within each set, the graft chain molar mass and spacing between grafts were fixed, while the total backbone length was varied. Dynamic master curves reveal that these polymers display Rouse and reptation dynamics with a sharp transition in the zero-shear viscosity data, demonstrating that grafting density strongly impacts the entanglement molar mass. The entanglement modulus (G e) scales with inverse grafting density (n g) as G e ∼ n g 1.2 and G e ∼ n g 0 in accordance with scaling theory in the high and low grafting density limits, respectively. However, a sharp transition between these limiting behaviors occurs, which does not conform to existing theoretical models for graft polymers. A molecular interpretation based on thin flexible chains at low grafting density and thick semiflexible chains at high grafting density anticipates the sharp transition between the limiting dynamical regimes.

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.

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