Sustainable Poly(lactide-b-butadiene) Multiblock Copolymers with Enhanced Mechanical Properties

To develop mechanically improved polylactide (PLA)-based sustainable polymers, a series of poly(lactide-b-butadiene) (PLA-PB) multiblock copolymers were synthesized in a two-step procedure: PLA-PB-PLA (LBL) triblock copolymers were prepared using ring-opening polymerization of D,L-lactide, followed by chain extension of LBL triblock polymers with toluene-2,4-diisocyanate (TDI) and terephthaloyl chloride (TCl). Molecular characterization revealed that the synthetic procedures yielded the desired triblock and multiblock copolymers with a composition range of 0.5 <= f(PLA) <= 0.9. Differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) demonstrated nearly identical, well-ordered, morphologies in the homologous triblock and multiblock copolymer materials, in sharp contrast with the findings of a recent study involving poly(styrene-b-butadiene) (PS-PB) multiblock polymers. These results indicate a transition from classically ordered morphologies to a state of bicontinuous disorder for multiblocks containing < n'> >= 10, where < n'> is the average total number of blocks. Lamellae (f(PLA) = 0.6) and cylinder (f(PLA), = 0.7 and 0.8) forming PLA-PB multiblock copolymers exhibited dramatically enhanced mechanical properties compared to the corresponding LBL triblock copolymers. However, this toughening effect was not evident in samples containing a spherical morphology (f(PLA) = 0.9). These findings demonstrate a commercially viable approach to preparing sustainable plastics with competitive mechanical properties.