Solid-state NMR Characterization of Polymeric and Inorganic Materials

Download or read book Solid-state NMR Characterization of Polymeric and Inorganic Materials written by Jessi A. Baughman and published by . This book was released on 2015 with total page 173 pages. Available in PDF, EPUB and Kindle. Book excerpt: Multiple systems were studied to advance the understanding of the chemical composition of the materials. These materials contained various structures or structures within different physical phases. Solid-state NMR techniques were used to probe effects of different chemical processes and environmental conditions on the chemical structures and phase composition of these materials. Much of the high thermal and chemical resistance of poly(vinylidene-cohexafluoropropylene) is gained from cross-linking. The insolubility of the cross-linked fluoroelastomer has prevented the characterization of the structure at the cross-link site by NMR. Samples from each of the four stages of the cross-linking of poly(vinylideneco-hexafluoropropylene) were analyzed with solid-state NMR to determine the chemical structure at the cross-linking site and the effects of cross-linking on the mobility of the elastomer chains. Spectral overlap from chemical shift dispersion hindered the use of simple 1D techniques to assign structural components to peaks in the NMR spectra. Relaxation studies that measured T1, T2, and T1[subscript p] relaxation times were used to assign new peaks in the NMR spectra to the fluoride salts that are produced during cross-linking. The NMR relaxation data also indicated no reduction in the mobility of the fluoroelastomer from cross-linking. The chemical structure of the cross-link site was partially characterized by 2D-NMR. However, the amorphous nature of the polymer inhibited a full characterization of this location with 2D-NMR techniques. The structures that were identified at the cross-link site supported proposed structures. Solutions of NaCl and dextrose used in the preservation of premixed drugs were analyzed to distinguish the solid and liquid phases over a temperature range of -60 to 20 °C. The large chemical shift dispersion in the NMR spectra made analysis of the frequency domain data difficult. The time domain data of the single pulse NMR experiments were analyzed using a logarithmic fitting technique and the inverse LaPlace transform. The logarithmic fit technique was able to distinguish a single solid and a single liquid phase for the NaCl and dextrose solutions. The inverse LaPlace transform indicated the presence of three phases near the center of the temperature range. The additional phase was assigned to a liquid phase of concentrated solution based on the T2 time of the phase found by the inverse LaPlace transform. The two methods gave similar results and were used to identify the percentage of each phase present throughout the temperature range. 33S solid-state NMR was used to characterize the discharge products of a lithium-sulfur battery. A single pulse experiment with a solid echo was used to help minimize the broadening effects of the quadrupolar interactions in 33S. Li2S was detected in the cathode of the fully discharged battery. Longer lithium sulfide polymer chains expected at higher discharge potentials were not detected due to large line widths in the NMR spectrum and small sample size that severely reduced the signal-to-noise. A reduced signal-to-noise spectrum of natural abundance, elemental sulfur suggested an isotropic chemical shift at -50 ppm.