Porous structure of cellulose fibers studied by Q-space NMR imaging

The molecular translation displacement probability (displacement profile) characterizes the average pathway of liquid molecules imbibed in a microporous medium. Under the narrow pulsed field gradient approximation, displacement profiles can be obtained by Fourier transform of the spin-echo attenuation profiles measured as a function of the amplitude of the pulsed magnetic field gradient (Q-space NMR imaging). In the present study, the molecular displacement profiles of water molecules in cellulose fiber samples with different water contents were measured as a function of the diffusion observation time (5-1200 ms). The time course of the translational displacement profiles indicates the presence of both free diffusing and restricted water in the fiber samples oversaturated with water. This confirms the results from previous studies on the same system by using a population-weighted model. Water molecules in the appropriately saturated fiber samples exhibit essentially restricted diffusion behavior when the diffusion observation time is sufficiently long. The anisotropy of water diffusion in cellulose fibers was also investigated by studying the fiber sheet sample using a tetrahedral gradient combination pattern consisting of six different combinations of simultaneously applied orthogonal gradients (x, y, and z gradients).