Development Of Novel Solid-State NMR Methodsto Study On Complex Condensed Structures And Dynamics Of Polymers

With the rapid development of solid-state NMR techniques in recent decades, it’s possible for us to extract information about polymer microstructures and dynamics by utilizing various different interactions among nuclei and electrons, mainly through the manipulation of the pulse sequences over nucleus spins. Although solid-state NMR has made great progress in the characterization of polymer materials, a lot of solid state NMR methods are still suffering from certain limitations due to the complexity of polymers. Therefore, a major part of this thesis focuses on the development of novel solid-state NMR methods for the characterization of polymers. On the other hand, self-healing materials are enjoying great popularity recently due to their excellent self-healing ability with the potential of extending service life as well as their remarkable mechanical properties. But so far, it is still not well understood about the relationship between the self-healing ability and the microstructure and dynamics of polymers. Therefore, in this thesis, we adopted various kinds of advanced solid-state NMR techniques for the investigation ofthe self-healing mechanism of the self-healing rubbers as well as the heterogeneous structure and dynamics, which we wish to be helpful for the chemists indesigning new self-healing materials. The thesis could be divided into four parts:1. Identification of different types of carbon signals utilizing PISEMA spectroscopyPISEMA spectroscopy correlates13C CP/MAS spectrum with13C-1H dipolar coupling interactions, and through theoretical simulations and experiments, we found that in polymers for different types of carbons, CH3, CH2, CH, and quaternary C, there would be different dipolar splitting spectra as well as dipolar splitting values. This method for distinguishing different types of carbons is demonstrated on more than ten polymers, and utilizing this method we successfully differentiated overlapped signals resulting from backbone quaternary carbon of poly(methyl methacrylate) and CH2of poly(4-vinylphenol). All the experimental results confirmed that PISEMA is an efficient method for distinguishing different types of carbon signals in solid state13C spectroscopy. 2. Accessing structure and dynamics of mobile phase by real-time Tic filter PISEMAWhen complicated structure and dynamics problems are involved in organic solids, there is still not a highly efficient method to directly extract structure and dynamic information of mobile phases. Therefore, on the basis of "inverse T1c filter", we proposed a real-time Tic filter PISEMA pulse sequence through the combination with PISEMA pulse sequence by adopting special phase cyclings. Our proposed method not only enjoys high signal sensitivity, but also overcomes the instability of magnetic field and radiofrequency in long-time experiments. Utilizing this real-time spectral-editing method, we could directly extract the structure and dynamics information of mobile phases, even to distinguish overlapped rigid and mobile signals.3. Investigation on the artifical exchange signals induced by RIDER effect in CODEX experimentsIn this chapter, we mainly concern on how far13C is away from14N so that we could neglect the RIDER effect induced by14N relaxation. Therefore, on the basis of this distance, people could determine whether CODEX experiment could be used to detect slow motions of14N-containing system. Through adopting two different14N-containing systems, hexadecyltrimethylammonium bromide (CTAB) and semi-crystalline polyamide-6(PA6), we find that under appropriate chemical anisotropy shift (CSA) recoupling time, if14N-13C distance is beyond two-bond distance, then the RIDER effect could be neglected. On the other hand, RIDER signal of remote13C atoms increased with increasing CSA recoupling time, but still no more than5%compared to the reference signal at long recoupling time. On the mean time, based on two-site jump model, through the simulation of CODEX exchange signals, we concluded that even the CSA reorientation angle was only20°, RIDER signals of remote13C atoms could still be neglected. Therefore, it’s well suggested to observe the13C exchange signals on the chemical groups far away from14N in order to extract dynamic information.4. Studies on themicrostructure and dynamic heterogeneity as well as the self-healing mechanism of self-healing rubbersVarious kinds of characterization techniques, including IR, SAXS,XRD, rheology and NMR, were utilized for a detailed investigation on the microstructure, dynamics, self-healing mechanism as well as the aging process of a self-healing rubber. The experimental results showthat the self-healing rubber is a microphase-separated system, in which the polar hydrogen bonding groups assemble into a rigid hydrogen bonding network and the apolar aliphatic chains aggregate into the mobile domain. The period length of phase separation is about3nm, both proved by the SAXS and NMR spin diffusion experiments. Low-field NMR also revealed the heterogeneity in the microstructures and dynamics. Moreover, ATR-IR is utilized to investigate the hydrogen bonding dynamics of the self-healing rubber with increasing temperature. CODEX experiments also revealed the importance of the time scale of molecular slow motions for the self-healing ability of rubbers. Finally, through rheology experiments, we observed the aging process of self-healing rubbers, and it was shown that the self-healing rubber would undergo chemical cross-link reaction at a temperature above110℃, resulting in severely loss of self-healing ability.