(552c) Design Principles for Tough, Highly Crystalline, Aging-Resistant Poly(lactide) Block Polymers

Poly(lactide) (PLA) is a promising candidate for replacing petroleum-derived plastics owing to its sourcing from natural sugars and bio-degradability under industrial composting. However, PLA embrittles rapidly after processing due to physical aging, and the crystallization of its isotactic forms, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), is prohibitively slow for most modern melt processing operations. PLA therefore requires upgrading to expand its market potential to compete with tough, highly crystalline commodity plastics. To address this challenge, we produced rigid thermoplastics by incorporating PLLA as a majority component in block polymers featuring poly(γ-methyl-ε-caprolactone) (PγMCL), a lignin-derivable and similarly compostable rubber, as a minority block. We employed well-established techniques in ring-opening transesterification polymerization to synthesize block polymers with highly controlled molar masses and compositions. In our first effort, we produced ABA PLLA-b-PγMCL-b-PLLA ("LML") triblock polymers using a 20 kDa PγMCL midblock and varying the PLLA composition from 60 to 90 vol %. These LML triblocks exhibited an unprecedented combination of high crystallinity (>40%) and toughness (63-113 MJ m^-3) while retaining Young's moduli in excess of 1 GPa. We used in-situ tensile synchrotron X-ray scattering and ex-situ microscopy to specify the deformation mechanism as the classic "lamellar to fibrillar transition" typical of semicrystalline polyolefins, the first documentation of this mechanism for PLLA at room temperature. Next, to ensure these properties could persist over long post-processing aging periods, we investigated the macromolecular design features supportive of mechanical longevity. We expanded the architectural scope to n-arm (PγMCL-b-PLLA)_n star-block polymers, separately fixing (1) the total molar mass, (2) the arm molar mass, and (3) the number of star arms. Diblock polymers (n = 1) were uniformly brittle. For n > 1, arm molar mass was most influential over aging resistance. Star-blocks with smaller arm molar masses better retained toughness from 3 to 80 days of ambient aging, on average. A high state of initial crystallinity also supported toughness retention, which we hypothesize resulted from the lamellar to fibrillar transition bypassing the increased difficulty of shear yielding in the amorphous PLLA region due to physical aging. We also studied structural relaxation kinetics through calorimetry, surprisingly finding that PγMCL inclusion accelerated physical aging relative to PLLA homopolymers despite enabling high toughness. In a final effort to accelerate crystallization while preserving long-lived toughness and high crystallinity, we incorporated PDLA into LML triblocks to leverage PLLA-PDLA stereocomplexes as crystal nucleators. "LDMDL" stereopentablocks with a 1:1 L:D ratio exhibited high toughness and thermal resilience but incurred solvent processing due to the high stereocomplex melting temperature. Shortening the PDLA block length restored typical melt processability, and we observed a strong dependence of crystallization kinetics on the PDLA block's length and position in the pentablock architecture. Physically blending PDLA with an LML triblock, as opposed to covalent incorporation, most significantly accelerated crystallization rate, though at the expense of ductility. These efforts have greatly specified the design space for renewable, high-performance PLLA-based block polymer thermoplastics, providing useful starting points for macromolecular design in future works. At the same time, we have uncovered new challenges for the neat PLLA block polymer approach, including a combination of both high impact and tensile toughness and efficient, mechanically innocuous crystal nucleation strategies.