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. In our first effort, we produced ABA PLLA-b-PγMCL-b-PLLA ("LML") triblock polymers, 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 over 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. 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. Star-blocks with smaller arm molar masses better retained toughness from 3 to 80 days of ambient aging, on average. Surprisingly, PLLA blocks with molar masses as low as 27 kDa produced excellent toughness despite a relatively poor state of entanglement, advantageously for melt processing. A high state of initial crystallinity also supported mechanical longevity. We also performed accelerated aging studies through calorimetry, surprisingly finding that PγMCL inclusion accelerated physical aging relative to PLLA homopolymers despite enabling high toughness. In a final effort to speed crystallization while preserving long-lived toughness and high crystallinity, we incorporated PDLA ("D") 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 found that an LDMDL architecture was ideal for speeding crystallization. Physically blending PDLA with an LML triblock, as opposed to covalent incorporation, most significantly accelerated crystallization rate, though at the expense of ductility.
Through these efforts, we insisted on taking on the challenges of realistic melt processing timescales and gate-to-grave time frames in the single-use plastics market. We distinguished our studies through close attention to ambient aging time, measuring the crystallinity of as-tested specimens, and imposing controlled thermal histories during melt processing to explore a range of crystallinities and their influence on mechanical performance. Our evaluations of our block polymers are thus well-informed by the target market and provide concrete parameter bounds for future design of PLLA-based block polymers.
Research Interests
My graduate studies have motivated me to continue research on intrinsically circular, commercially competitive polymers. In addition to exploring new feedstocks and scalable chemistries, I am also interested in developing testing protocols that bridge the gap between the lab setting and the manufacturing or application setting. For instance, I am eager to work with customers to design custom/non-standard calorimetric thermomechanical, or rheological tests for product resilience or aging in major use settings. For examples of other opportunities I have described to make lab-based research more actionable, see the Outlook section of my attached Perspective, recently published in Biomacromolecules. Additionally, I am interested in doing fundamental polymer research to further enhance properties in high-value, lower-volume applications with lower (gross) pollution burdens than single-use plastics, including biomedical devices, engineering plastics, batteries, and flexible electronics.
