Understanding Polymer Crystallization at Molecular Levels: A Solid-State NMR Study

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NON-TECHNICAL SUMMARY:About 70 % of industrial plastics are made of semicrystalline polymers, which have the ability to crystallize when processed industrially. The inherent nature of polymer molecules, consisting of very long molecular chains, leads to a highly complex hierarchy of structures in the solid state. The ability to control these hierarchical structures significantly influences mechanical and thermal properties of the resulting materials. Thus, semicrystalline polymers are used in a wide variety of applications including packaging, textiles, automobile, airplane parts, energy, biocompatible materials, and others. To further improve properties of semicrystalline polymers, it is necessary to understand their detailed structures in the solid state as well as the mechanisms of structural formation at the molecular level. This project focuses on fundamental understanding of molecular-level structures of semicrystalline polymers in samples crystallized in various controlled ways, and may lead to predictions of improved properties for commercially available polymeric materials. The PI will use novel magnetic resonance techniques to probe the molecular trajectory of polymer chains in the solid state of such polymer materials. This information will provide unprecedented detail about the molecular morphology of polymers and the interconnections among crystals, and would have implications on their mechanical properties. This project will also include advanced scientific training of graduate students in technologically important areas. They will experience a wide range of polymer research covering polymer synthesis, crystallization, advanced instrumentation, and molecular dynamics simulations. The research results will be reported in scientific journals and presented by the PI and students at national scientific meetings.TECHNICAL SUMMARY: Polymer crystallization induces structural changes of polymer molecules from random coils to folded chains in thin crystals. The mechanism of crystallization has been theoretically and experimentally studied in many publications over the past several decades. However, no decisive conclusion has been reached regarding the crystallization mechanism and chain-level structure. The PI has developed advanced techniques for 13C-13C double quantum (DQ) NMR combined with selective isotopic 13C labeling. Using these together with spin-dynamics simulation enables determination of the re-entrance sites of folded chains, the adjacent re-entry fraction, the successive folding number of semicrystalline polymers in the bulk and in solution-grown single crystals as a function of crystallization temperature. These experiments shed light on the extent of adjacent re-entry of polymer chains in solution-grown and bulk materials, the formation of three-dimensional nanostructures, and the effect of crystallization kinetics on the extent of chain folding. These aspects can be correlated with bundle and aggregation models and recent molecular dynamics simulation results. The planned experiments will further investigate the effects on chain trajectory in crystalline regions for a variety of important parameters including i) molecular weight, ii) polymer concentration, iii) solvent-polymer interactions, iv) confined spaces, v) entanglements, and vi) melt-memory. Such studies will provide detailed crystallization mechanisms of flexible semicrystalline polymers while contributing to the advanced training of students in technologically important areas.