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Title: NMR studies of structure-transport relationships in porous media : liquid diffusion in polymers
Author: Harding, S. G.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1998
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The main motivation behind this work was to explore the potential of nuclear magnetic resonance (NMR) to study small molecule diffusion in porous materials and the relationship between structure and transport processes, with particular emphasis on liquid diffusion in polymers. The diffusion of small hydrocarbons into a range of semicrystalline polyethylene (PE) samples is studied. Under the penetration conditions employed the polymer crystallinity is retained, supporting the idea that the penetrant resides only in the amorphous region. Magnetic resonance imaging (MRI) is used to monitor the bulk transport of penetrating liquids into the polymers. Pulsed Gradient Spin Echo (PGSE) NMR is used to measure the self diffusivity of the penetrant. The transport diffusivity estimated from MRI shows good agreement with the self diffusivity when the degree of crystallinity is taken into account. The data presented show the importance of the phase and chain structure of the polymer on the diffusion and mobility of the penetrating species. Deuterium NMR relaxation time and line shape analysis is used to measure the correlation time of various motional process. By comparing the correlation time for translational motion with the PGSE self diffusivity it has been possible to estimate the penetrant jump distance to be in the order of a few nanometers. Molecular Dynamics (MD) simulations of hexane diffusion in amorphous PE are shown to give reasonable agreement with PGSE results. A mesoscopic lattice model is used to include the effect of the impenetrable crystallites on the diffusion process. Preliminary results using 31P NMR spectroscopy to study the mobility and molecular scale distribution of additives show that some additives intimately mixed with the polymer while some reside in a discrete domain of around 100 nm. Diffusion weighted imaging is used to study liquid diffusion into polyethylene oxide (PEO) hydrogel. Despite the diffusion kinetics showing Fickian behaviour the concentration dependence of the diffusivity calculated from the PGSE measurements and from the concentration profiles are significantly different, highlighting the importance of the polymer chains in controlling the diffusion process. Three dimensional imaging shows that structural heterogeneities in catalyst support pellets can be characterised by a fractal parameter. The importance of slice thickness in determining the distribution of T1 times from a 2D image is highlighted. NMR cryoporometry is used to measure the pore size distribution and the results compare well in nitrogen adsorption, and T1 measurements. NMR cryoporometry is combined with MRI and the spatial variations in the pore size distribution are shown to be similar to those previously observed through NMR spin density and relaxation imaging.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available