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Proton magnetic resonance of wood and water in wood Araujo, Cynthia D.


Proton nuclear magnetic resonance (11-1 NMR ) was used to investigate protons in solid wood and compartmentalized water in the wood cell walls and lumens. A lineshape second moment study found the second moment of protons in oven dry wood to be about 23% lower than the rigid lattice calculation, indicating a rigid structure with some anisotropic molecular motion of the polymeric constituents. Above 5% moisture content, the second moment decreased by a further 13 to 16% implying a "loosening" of the molecules in the solid with the increased moisture content. The T2 of the cell wall water was found to be single exponential and increased with moisture content. The 11-1 NMR measured fibre saturation point of the cell wall water agreed with the value calculated from the moisture isotherm. Two T2 techniques for characterization of water in wood are demonstrated. First a technique for analysing multi-exponential relaxation in terms of a continuous distribution of relaxation times was applied to T2 analysis of lumen water in wood. The lumen water T2 times vary as a function of the wood cell radius and are therefore expected to reflect the cell size distribution, which is continuous. A technique of selectively imaging water environments on the basis of T2 was applied for a range of moisture contents. The moisture density profile of the hound water was found to be independent of moisture content above the fibre saturation point. Spin- spin relaxation measurements of lumen water in wood were interpreted using a diffusion theory which models the lumen waterT2 relaxation in terms of the cell radius distribution, the bulk water diffusion coefficient and a surface relaxation parameter. Agreement between theory and experiment was excellent. Evidence was found for the existence of higher order T2 relaxation modes predicted in the slow diffusion regime, using a sample with rather large cell lumens and at low temperatures. Using this diffusion model, T2 relaxation decay data were fitted to give a cell size distribution, comparable to scanning electron microscope results, when the bulk water diffusion coefficient and the surface relaxation parameter were known. A two region diffusion model was considered with free water in the cell lumens and water in the cell walls. The surface relaxation parameter was found to depend on the spin-spin relaxation time and diffusion coefficient of the cell wall water. Consequently, the cell wall water diffusion coefficient may be estimated from spin—spin relaxation times and the relative populations of lumen and cell wall water. The cell wall diffusion coefficient of maximum hydrated redwood sapwood was found to be 0.2 x 10' m2/sat room temperature, and from the temperature dependence the activation energy was found to be 6700 cal/mol, about 40% higher than the free water value. Numerical simulations of the two region diffusion model were developed. The lumen water T2 was found to be independent of the simulated cell wall thickness, simplifying to a surface relaxation as modeled with the surface relaxation parameter in the one region model. The simulated effect of exchange on the fibre saturation point measurement was found to be an over estimate compared to experimental results. Three techniques were used to investigate the spin-lattice relaxation of the solid wood and the water in wood. Separate T1 measurements of the solid and water, separate T1 measurements of water in the early wood and latewood regions, and separateT1 measurements of the cell wall water and lumen water were acquired. The results indicated that, on the T1 time scale of 100 ms, all proton environments are mixed by diffusion of the water. The T1 of the water in the lumen and the cell wall and the protons of the solid were found to have the same T1, which is an average of the T1 of the three environments. The T1 was found to be dependent on the proportion of cell wall to lumen volume. Thick walled latewood cells had a lower T1 than thin walled early wood cells. Lastly, the cross relaxation of the protons in solid wood and the cell wall water was found to be the dominant mechanism for 7'2 relaxation of the cell wall water.

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