Quantitative Cross Polarization NMR And Its Application To The Structural Characterization Of Bulk Materials

The quantitative cross polarization (QCP) nuclear magnetic resonance (NMR) based on the reciprocity relation between cross polarization (CP) and cross depolarization (CDP) can be used to determine CP enhancement factor directly. The quantitative results can be obtained from conventional cross polarization/magic angle spinning (CP/MAS) NMR spectrum with the correction of CP enhancement factor. The method has been applied in quantitative analysis for small molecules and their mixtures, but not for polymer phase structure and29Si/1H systems. Valuable information may be obtained from the new applications of QCP NMR method in these systems. The physico-mechanical properties of ultrahigh molecular weight polyethylene (UHMW-PE) are significantly dependent on its phase structure, However, direct determination of CP enhancement factors for carbon in its different phases, and further characterization of its phase composition from CP/MAS NMR are still a great challenge. Compared to13C/1H CP,29Si/1H CP can obtain a higher theoretic enhancement factor due to lower gyromagnetic ratio of|γsi|. In this dissertation, QCP NMR method is applied to UHMW-PE and29Si/1H systems, and the influence of CP contact time and MAS rate on the quantitative results are studied in detail. The main results obtained are as follow:(1) The QCP NMR method has been applied firstly to quantitatively characterize the phase structure of polymer. The crystallinity of melt-quenched UHMW-PE (MQ-UHMW-PE) and phase composition of high-strength high-modulus UHMW-PE fiber have been determined. The CP enhancement factors for carbon in crystalline and amorphous phase of MQ-UHMW-PE are directly obtained from CDP NMR with reciprocity relation. Then the crystallinity is derived from conventional CP/MAS NMR spectrum given their CP enhancement factor. The influence of CP contact time and MAS rate on CP signal enhancement is studied. It is found that MAS rate have more effect on CP enhancement factor for carbon in amorphous phase than that in crystalline component. The phase composition of UHMW-PE fiber is also obtained from QCP NMR method. It is found that monoclinic crystal phase, all-trans interphase and orthorhombic crystal phase have almost the same CP enhancement factor during0.1-0.5ms contact time, which is higher than that of amorphous phase. In addition, the method reported in reference is modified to determine the crystallinity without spectral deconvolution, which combines direct excitation (DE)/MAS with CP/T1C (13C spin-lattice relaxation time) filter NMR (compound experiment). The crystallinity of MQ-UHMW-PE and amorphous proportion of UHMW-PE fiber are also determined by modified compound experiment method. The results match well with that from QCP NMR method. Both DE/MAS and compound experiment NMR scheme are very time-consuming due to extremely long T1C in crystalline phase, but QCP NMR method saves considerable experimental time.(2) The QCP NMR method is firstly used in29Si/'H NMR. The ratio between two29Si atoms with different chemical environment in chloromethylphenyl isobutyl polyhedral oligomeric silsesquioxane (POSS-C1) is determined. For a29Si/1H spin system which has weak heteronuclear dipolar coupling, CP and CDP together with the reciprocity relation are performed with optimized experimental conditions. The quantitative results may also be obtained from the variable contact times (VCT) experiment. However, the method is extremely time-consuming with sufficient numbers of dynamics. While three experiments are sufficient in QCP NMR method.29Si/1H cross ralaxation time (TSiH) is governed by heteronuclear dipolar coupling and dependent on molecular motion. Based on CDP dynamics, a new scheme is proposed to determine TSiH eliminating the effect of T1pSi, where T1pSi is29Si spin-lattice relaxation time in the rotating frame. (3) Based on the QCP NMR and DE/MAS solid-state29Si NMR techniques, a novel scheme for quantitative characterization of silica structures is developed. The QCP NMR technique is used to determine the molar ratio between geminal silanol (Q2) and single silanol (Q3), while the DE technique with a small number of scans is used to determine the molar ratio of Q3and siloxane (Q4). As a result, the proportions of Q2, Q3and Q4groups in mesoporous SBA-15silica are determined and the scheme can save experimental time significantly. Its results are, in general, more reliable than that by VCT NMR. The number of surface hydroxyl groups per square nanometer is obtained combining with the specific area of SBA-15. The results are consisted with that reported in the literatures.(4) Octavinyl-POSS (OvPOSS) has eight reactively functional groups, the average numbers of reacted vinyl groups can reflect the degree of reaction straightforwardly. In this section, the29Si/1H QCP scheme is further applied to octavinyl-POSS (OvPOSS) nanocomposites containing perfluoropolyether (PFPE) for deriving directly and accurately the average number of reacted vinyl groups. The results from QCP NMR scheme are consisted with that of DE/MAS and VCT NMR method, but QCP NMR scheme requires shorter experimental time. The molar percentage of OvPOSS in nanocomposite is obtained combining with1H NMR, which may not be derived from FTIR because of the overlapped signals.