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The formation mechanism of the polymer crystals in the early stage is still an open debate since the first discovery of the single crystal of polyethylene in 1957. The entanglements of long chains lead to incomplete crystallization and individual chains could exist in ordered and amorphous domains. Kinetics can control crystalline morphology and crystallinity. With increasing supercooling, the nucleation density significantly increases and reduces the size of the crystalline regions. Deep quenching the liquid state into glassy state may freeze the system to the extent where primary nucleation is barely initiated or even prohibited.Several different crystallization mechanisms have been proposed such as Lauritzen-Hoffman kinetic theory, multistage model, aggregation model and bundle model etc. Despite numbers of efforts in determination of the mechanism of polymer crystallization by various characterization methods, the decisive evidence in regards to whether polymer chain folds in the early stage of crystallization, was absent until recent development of Double Quantum (DQ) NMR in Dr. Miyoshi`s group. Utilizing DQ NMR techniques, the pioneer results in isotactic polybutene-1 suggest polymer chain folds adjacently in both solution grown and melt grown crystalline polymers, and the experimentally available quenching rates do not play a significant role in altering the chain trajectory, which partially in agreement with the bundle model rather than other the proposed polymer crystallization mechanisms. However, the investigation in the previous work with small cooling rate was not possible to discriminate whether the chain trajectory is due to the required bundle mechanism (which does not correlate with quenching rate) or due to a lack of quenching rate.In this dissertation, we tested the metastable states induced by deep quenching available in the lab (quenching to icy water) for two polymer systems. Our hypothesis is the quenching process could not largely reduce the folding process which is in accordance with the bundle model’s prediction. Utilizing advanced solid-state NMR (SSNMR) techniques such as DQ, Center-band Only Detection of EXchange (CODEX), as well as site-specific 13C labeling molecules, we successfully detected the packing and folding structure in both mesomorphic isotactic PolyPropylene (iPP) and glassy Poly(L-Lactic Acid) (PLLA).Quenching into icy water introduces mesomorphic form iPP, which is known to possess some local order but the error and small domain size prohibited the detailed characterization until recently. Before investigating the chain-folding mechanism in the mesomorphic form iPP, the local packing structure was clarified by means of DQ and CODEX experiments. The molecular dynamics in alpha, beta and mesomorpic phase was alternatively used to investigate local packing structure of the mesomorphic form by applying CODEX experiments, it was demonstrated that iPP chains in the mesomorphic phase performs helical jump motions in the same and faster dynamic ranges respectively with those in the beta and alpha forms. With the help of 13 C methyl labeled iPP, dipolar interactions between methyl groups were detected through DQ NMR buildup curve, and by comparing with available models for mesomorphic iPP, the packing structure was for the first time determined to be very similar as beta form iPP.In addition to the packing structure, folding structure for the mesomorphic form was presented in Chapter VI. As the morphology and packing structure was significantly limited to a metastable state, it was believed that the majority of the chains does not take adjacent folding structure in such a quenched form according to both kinetic theory and the solidification models. DQ NMR combined with 13 C isotope labeled chains diluted by natural abundant polymers, a contrast where single molecule can be traced through DQ NMR experiments without altering the metastable states can be built. It was determined that majority of chains mesomorphic form take double layered adjacent reentry as chain folding trajectory with ensemble averaged folding number n = 4 within the nano-scaled mesomorphic domains. The observed folding cluster indicates that the primary nucleation could be initialized through intra-chain folding process.Last but not least, chain conformation, packing, and chain trajectory of quenched PLLA sample in a glassy state were investigated by solid-state NMR and Molecular dynamics (MD) simulation.By introducing adjacent 13 CH into the backbone, the conformation information of the PLLA chain in glassy state was captured by the evolution time dependence in the CODEX NMR experiments, it was demonstrated that glassy PLLA adopts helical conformation which is very close to 10 7 helix in thermodynamically stable a crystal.Using methyl 13 C labeled PLLA for packing structure analysis, it was found that the local packing structure of glassy PLLA is well consistent with that in a form. It was also demonstrated that glass PLLA chains adopts chain folding with n = 2, which is well consistent with that in the a crystals. Through this thesis, it was clearly demonstrated that available kinetics does not quench chain-folding process of long polymer chains even in the quenched glassy state. This result supports bundle model under a large supercooling. |
