Sustainable Poly(lactide-<i>b</i>-butadiene) Multiblock Copolymers with Enhanced Mechanical Properties

Abstract

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 ≤ fPLA ≤ 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 (fPLA = 0.6) and cylinder (fPLA = 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 (fPLA = 0.9). These findings demonstrate a commercially viable approach to preparing sustainable plastics with competitive mechanical properties.