Crystallinity-Independent Toughness in Renewable Poly(l-lactide) Triblock Plastics

Poly(l-lactide) (PLLA)’s broad applicability is hindered by its brittleness and slow crystallization kinetics. Among the strategies for developing tough, thermally resilient PLLA-based materials, the utilization of neat PLLA block polymers has received comparatively little attention, despite its attractive technological merits. In this work, we comprehensively describe the microstructural, thermal, and mechanical properties of two compositional libraries of PLLA-rich PLLA-b-poly(γ-methyl-ϵ-caprolactone) (PγMCL)-b-PLLA (“LML”) triblock copolymers. Rubbery PγMCL domains microphase separate from the matrix in the melt and intercalate between PLLA crystal lamellae on cooling. Despite the mobility constraints associated with midblock tethering, the PLLA end-blocks crystallize as rapidly as a PLLA homopolymer control of similar molar mass. Independent of their degree of crystallinity, LML triblocks exhibit vastly improved tensile toughnesses (63-113 MJ m^-3) over that of PLLA homopolymer (1.3-2 MJ m^-3), with crystallinities of up to 55% and heat distortion temperatures (HDTs) as high as 148 °C. We investigated the microstructural origins of this appealing performance using X-ray scattering and microscopy. In the case of a largely amorphous PLLA matrix, the PγMCL domains cavitate to enable concurrent PLLA shear yielding and strain-induced crystallization. In highly crystalline PLLA matrices, PγMCL facilitates a lamellar-to-fibrillar transition during tensile deformation, the first such transition reported for PLLA drawn at room temperature. These results highlight the unique attributes of PLLA block polymers and prompt future architectural and processing optimizations to achieve ultratough, high-HDT PLLA block polymer plastics after a simple thermal history on economical time scales.