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.
A synthetic strategy to produce graft block copolymers (BCPs) with controlled grafting densities using both graftingthrough and grafting-from methods is reported. For graftingthrough, poly(4-methylcaprolactone-block-D,L-lactide) macromonomers were synthesized with a polymerizable maleimide end group. These macromonomers were copolymerized using reversible addition-fragmentation chain-transfer (RAFT) polymerization with styrene and various amounts of N-ethylmaleimide to control grafting density. A kinetic study showed that the macromonomers (>10 kDa) polymerized at the same rate as N-ethylmaleimide under RAFT conditions, suggesting that uniform grafting density could be achieved. However, incorporating more than 5-10 grafts per chain was found to be challenging and potentially limited by kinetics. A higher number of grafts per chain with controlled densities was achieved using a grafting-from technique. Macroinitiators of styrene, N-ethylmaleimide, and N-(2-hydroxyethyl)maleimide were copolymerized with a fixed density of grafting sites. Subsequently, BCPs were grown off of the macroinitiator using ring-opening transesterification polymerization (ROTEP).
The order−disorder transitions of diblock copolymers grafted to a common backbone were examined by oscillatory shear rheology and small-angle X-ray scattering. The effect of grafting density, graft molecular weight, number of grafts, and backbone dispersity were studied using poly[(styrene-alt-N-hydroxyethylmaleimide)-random-(styrene-alt-N-ethylmaleimide)]-graf t-poly(4-methylcaprolactone-block-D,Llactide) [PSHE-g-(P4MCL-PLA)] as a model graft block copolymer. At high grafting densities (25−50%), the order−disorder temperature (T ODT ) of the graft polymers was nearly identical to the analogous linear diblock. At lower grafting densities (<25%), the T ODT was found to systematically decrease. The number of grafts did not significantly change the value of the T ODT ; however, increasing the number of grafts resulted in broad, ill-defined transitions. Backbone dispersity was found to have little impact on the T ODT . Ordered morphologies were imaged by transmission electron microscopy. Long-range order was observed in polymers with at least 10 grafts/chain.
Poly[(styrene-alt-N-hydroxyethylmaleimide)-ran-(styrene-alt-N-ethylmaleimide)]-graft-[poly(4-methylcaprolactone)-block-poly((±)-lactide)] (g-ML) graft-block polymers containing 50 vol % poly((±)-lactide) (PLA or L) were mixed with a commercial PLA homopolymer to modify the brittle mechanical behavior of this industrially compostable plastic. Various graft architectures, including linear, tri-arm, and tetra-arm polymer backbones, were prepared using a grafting-from method. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) revealed that the pure g-MLs form a lamellar morphology where the degree of long-range order is dictated by the polymer architecture. When melt-blended with PLA at low concentrations, the g-MLs formed well-dispersed nanoscale particles within the PLA matrix, yielding moldable plastics with high optical transparency. The tensile toughness of the PLA/g-ML blends was substantially enhanced over that of pure PLA using g-ML concentrations as low as 5 wt % and exhibited average strains at break of 280% following 2 days of aging at room temperature; pure PLA failed at a 7% strain. The elastic modulus, yield stress, and transparency of the toughened plastic were virtually unaffected by the low concentration of rubbery poly(4-methylcaprolactone) (M) domains and the formation of well-dispersed nanoscale particles. Graft-block polymers were shown to toughen PLA more efficiently than a linear triblock copolymer analogue LML, which produced a strain at break of 105% at a loading of 5 wt %. Blending g-ML into PLA significantly delays the onset of physical aging and the onset of the ductile-to-brittle (DTB) transition, which depends on the concentration of g-ML utilized.
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