Advances in studying order and dynamics in condensed matter by NMR
Why NMR? Because it is so informative for so many different types of applications. Since the discovery of the basic phenomenon in 1945, the field of NMR was continuously growing. NMR spectroscopy and relaxometry are powerful analytical techniques in material science and biochemistry. Magnetic resonance imaging (MRI) has a huge impact on the medical field, mentioning the non-invasive behaviour and the achievement of high spatial resolution. This work tries to emphasise three aspects for which NMR is nowadays considered unique: molecular transport, molecular dynamics, and molecular order. There is no other technique that can provide information about the translational diffusion in liquids, the type and timescale of slow molecular motion in solids as well as about the molecular order, at the same time. In this thesis, molecular transport in liquid samples is studied in terms of susceptibility induced magnetic field inhomogeneities and spectral distortions for interdiffusion in binary mixtures. Molecular order and dynamics are topics for two different soft solids, natural rubber and polyurethane. Going from extremely high homogeneity of the field required in liquid state NMR, through the microscopic world of solid state NMR, this work comes to the end with some recent ideas of more homogeneous low field devices and new magnet designs. Diffusion of liquids is an intense field of research, due to the necessity to understand the separation processes and molecular interactions. While the self-diffusion coefficients can be measured with high precision, the concentration profiles and the interdiffusion coefficients in mixtures encounter some difficulties. Therefore, the influence of the mixture heterogeneity on the magnetic field homogeneity was investigated in terms of a spatial and time-dependent magnetic susceptibility. The effect of the heterogeneous distribution of magnetic susceptibility in liquid mixtures on the static and rf field homogeneity was simulated together with the corresponding spectral distortions. The results are promising and open the way to an elaborated method to correct the apparent concentration profile in multi-component mixtures. From another point of view, the problem of low magnetic field homogeneity provided by the Halbach type of magnets is discussed. The design of modified Halbach magnets is studied in order to increase the field homogeneity. The work was focused on two types of Halbach magnets, consisting of 16 and 24 magnet blocks, respectively. Different modifications were applied to these magnet designs, and the field homogeneity was significantly improved. The new magnets were built and are in use for different purposes such as diffusion, porosity determination, imaging, and measurements of relaxation dispersion. This relatively new generation of low field NMR magnets opens new perspectives for portable NMR. The changes induced in molecular dynamics and order in stretched elastomers was investigated using multispin moments edited by multiple-quantum NMR. The main purpose of this part is to investigate the changes in proton residual dipolar coupling and the sensitivity of multiple quantum coherences of higher order for cross-linked natural rubber under uniaxial deformation. The effect of uniaxial deformation of a natural rubber band was investigated by measurements of second van Vleck moments and fourth moments edited by double-quantum and triple-quantum coherences, respectively. These results can be applied to compare 1H residual moments edited by double-quantum and triple-quantum experiments with those measured by other NMR methods. A spin diffusion experiment was employed for the elucidation of the morphology and domain sizes of a series of polyurethane samples. A proton DQ dipolar filer was used to select the magnetization of the rigid phase. The most probable morphology is three-dimensional for the TPU samples with a high content in HS as was established by a combination between the equilibrium magnetizations and the domain size ratios obtained from simulations of the spin-diffusion process using 1D, 2D, and 3D solutions of the spin-diffusion equations. The correlation between macroscopic, mesoscopic and microscopic properties of the TPU samples was also discussed. A semi-quantitative model was developed to explain the functional dependence of the residual second van Vleck moment on the effective volume of the hard segments.
info:eu-repo/classification/ddc/530, Magnetfeld, Magnetische Suszeptibilität, Mehrquanten-NMR-Spektroskopie, Physik, Magnetfeldinhomogenitäten, Suszeptibilitätseffekte, Mehrquanten-NMR, Domänengröße, Halbach-Magnet, magnetic field inhomogeneities, magnetic susceptibility effects, multiple-quantum NMR, domain sizes, Halbach magnet
Networked Digital Library of Theses and Dissertations
Publikationsserver der RWTH Aachen University