Measuring Material Properties through Gradient Based NMR: Diffusion and Imaging
Abstract: In this work new methods based Nuclear Magnetic Resonance (NMR) have been developed to measure different kinds of structural properties in soft materials. The advantage of using NMR as a technique to measure material properties is that it is non-destructive, which makes it possible to measure on the actual piece of material that will be used for the application. One does not have to be concerned with whether a small sample represents the whole material or if each piece is similar. By applying a time-dependent gradient in the magnetic field the spins are labelled according to their position. This labelling can then be used either to make density images showing the local concentration or to monitor molecular displacements which carry information on sample structure. Both of these approaches have been used when developing the new NMR-methods, even if the focus has been on molecular displacement techniques. The homogeneous length-scale is obtained by monitoring the displacement of the molecules on different length-scales. For small length-scales the microheterogeneous structures can be observed while for long length-scales an averaging is taking place and the sample will appear homogeneous. The homogeneous length-scale thus indicates the scale of the structural features in the system. If different molecules can be followed independently further information of the structure can be obtained. One can identify obstacles and restrictions that effect a specific type of molecules. This can help to identify the structural features and what they are made up of. Existing experimental sequences were modified so that spectral resolution could be obtained. For biological tissue the intracellular diffusion and cell wall permeability are interesting properties, not the least within the content of medical applications. Methods to measure these two properties have been developed and tried out on yeast cell suspensions. Interbilayer forces were measured by centrifuging samples of lamellar phases with different concentration until equilibrium was reached. The concentration profile obtained at equilibrium and measured using magnetic resonance imaging was then used to obtain the bending rigidity. All methods are based on previously established techniques but are here combined or used in new ways.
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