Probing the structure of wood pulps with NMR q-space imaging.

The diffusive motion of water in wood pulps has been studied as a function of time between gradient pulses (Delta) using the pulsed field gradient stimulated echo NMR method while varying the gradient magnitude (g). The basis of this experiment is the attenuation of the echo signal by thermal or diffusive motion of water. Two types of liquid water in a pulp wet to below saturation can be identified on the basis of the attenuation of this signal, water which has its diffusional motion restricted by the cell lumen and water which exists outside the fibres for which the attenuation is less pronounced. The echo amplitude is plotted versus the spatial wave number vector, q = Vg delta/2 Pi, where Y is the magnetogyric ratio of the nucleus under consideration, g is the field gradient vector and delta is the gradient pulse duration. The Fourier transform of the echo amplitude in inverse space (q-space) gives the probability distribution of molecules being displaced a distance R during the time between the gradient pulses (Delta). From this representation an average pore size of 50 mu m for pulp fibre lumens over the entire sample is measured from a Gaussian peak-width at half height at a diffusion time of 1200 ms. This result is discussed in terms of the orientation of fibre lumens.