A pulsed NMR relaxation and diffusion study of water in treated and untreated waterlogged wood
Freezing curve, NMR relaxation data, and steady field gradient and pulsed field gradient experiments were conducted on samples of water-logged woods excavated from the Tudor warship, the Mary Rose and on similar samples impregnated with Polyethylene-Glycol solutions: the polymer used as a bulking agent to prevent decay. At least two distinguishable populations of water molecules are found in wood. Freezing curves indicate the presence of approximately 0.38 g/g of hydration water, close to that observed in fresh timbers. Relaxation measurements on pre-treated samples provides evidence of a very tightly bound fraction, present at water contents of below 0.14 g/g 0i hydration water, with a second population of hydration water being present up to 0.1.8 9/9. Above this value a third ‘free' population is observed. The relaxation decays in longitudinal and transverse direction have been analysed in terms of a sum of exponentials. These indicate the presence of two populations of water which do not correspond to populations observed in freezing curve analysis. Exchange mechanisms dominate the temperature dependency of the relaxation behaviour in pre-treated samples, which is similar to that observed in other fibrous materials such as meat. However, t he different components do not appear to correspond to the physical characteristics of the wood and the spin populations cannot be associated with a distribution between identifiable compartments within the system. In PEG impregnated samples the contribution to the signal from the polymer is not resolvable on the equipment used. Samples treated with PEGs for which the degree of polymerisation is greater than 1540 show a dependency of relaxation characteristics on the water content of the sample. At low water contents PEGs of low molecular weight impart a mobility to the "bound" water molecules which is not seen in untreated samples. . Belt diffusion coefficients for water molecules in wood are anisotropic, and are reduced from those observed in distilled water. This reduction is brought about because water molecules are both held in a hydration layer, and bounded by the cellular structure of the wood. In impregnated samples the diffusion rates are lowered by a factor of 10, though this is not reflected in the relaxation behaviour. The anisotropy is reduced, and proton exchange mechanisms are blocked.