Crystallization and Reorganization Pathway of Semicrystalline Polymers

NON-TECHNICAL SUMMARY: Two-thirds of polymers are semicrystalline, and these are among the most widely and extensively used materials in our lives. Controlling the crystalline and amorphous structures that make up all semicrystalline polymers remains a fundamental issue for developing and optimizing their properties and applications. It is therefore important to understand the crystalline and amorphous structures of semicrystalline polymers. Most previous work focused on analysis of the crystalline structures. In this project, the PI aims at understanding the amorphous structure and the crystalline-amorphous interface of semicrystalline polymers. Using advanced spectroscopic techniques to understand interfacial and amorphous structures together with their crystalline counterparts, and studying dynamical processes, the project is focused on elucidating the crystallization and molecular re-organization pathways of semicrystalline polymers. A successful project will contribute to controlling the structures and properties of semicrystalline polymers and developing novel energy-efficient processing approaches. TECHNICAL SUMMARY The crystallization pathways of semicrystalline polymers have been debated for many decades, partly as a result of lack of understanding of the amorphous structure in the initial state as well as during the crystallization process. In this project, the PI and his team aim at elucidating the amorphous structure of semicrystalline polymers, making use of structural information obtainable through advanced techniques. 13CH3-labeled poly(L-lactic acid) (PLLA) and 13CH-labeled poly(D-lactic acid) (PDLA) racemate, which form stereocomplex crystals, are chosen as a model system. The PI will analyze intermolecular distances between PLLA and PDLA and fractions of closely paired racemate in both amorphous and crystalline regions by applying state-of-the-art solid-state NMR spectroscopy. Direct comparisons of the amorphous and crystalline structures will be used to elucidate the crystallization pathway of PLA racemate. In a second thrust, molecular-level information about the reorganization process upon heating of solution- and melt-grown PLLA crystals will be obtained by quantitative analysis for chain-folded structures at the crystal-amorphous interface in addition to crystalline chain dynamics. In both topics, molecular weight effects will be investigated. Ultimately, fundamental understanding of amorphous and interfacial structures will contribute to revealing the crystallization and re-organization pathways of semicrystalline polymers. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.