CROSS-RELAXATION EFFECTS IN PULSED-FIELD-GRADIENT STIMULATED-ECHO MEASUREMENTS ON WATER IN A MACROMOLECULAR MATRIX

Water diffusion in poly [2-hydroxpethyl-methacrylate] hydrogels has be en measured by PFG NMR techniques, using both spin-echo and stimulated -echo methods, It is found that the results differ when evaluated usin g the standard equations for the echo attenuation, Furthermore, an app arent dependence of the diffusion coefficient on the diffusion time wa s found in stimulated-echo experiments, This effect is shown to be cau sed by cross relaxation between the protons of the water and those on the polymer matrix, An equation is derived describing the attenuation of a stimulated echo in the presence of cross relaxation, This equatio n shows that the additional damping due to cross relaxation depends on the gradient amplitude. If this equation is used with the measured cr oss-relaxation parameters, both methods are found to agree within expe rimental error, and no diffusion-time dependence is found for the stim ulated-echo experiments, The equation predicts the effect of replacing part of the H2O by D2O; this has been experimentally verified, It is concluded that macromolecular systems should be checked for cross rela xation when stimulated-echo methods will be used for self-diffusion me asurements. (C) 1996 Academic Press, Inc.