Water diffusion in poly[2-hydroxyethyl-methacrylate] hywith increasing diffusion times and level off for long times. drogels has been measured by PFG NMR techniques, using both Time dependence of D is usually caused by restricted diffuspin-echo and stimulated-echo methods. It is found that the results sion, but this should affect both methods equally. Therefore, differ when evaluated using the standard equations for the echo an alternative explanation is needed.
attenuation. Furthermore, an apparent dependence of the diffu-In many macromolecular systems containing water, cross sion coefficient on the diffusion time was found in stimulated-echo relaxation is observed (1-4). Therefore, the effect of cross experiments. This effect is shown to be caused by cross relaxation relaxation on the stimulated-echo amplitude is evaluated, between the protons of the water and those on the polymer matrix.
although it is not immediately apparent that this effect can An equation is derived describing the attenuation of a stimulated echo in the presence of cross relaxation. This equation shows that depend on the gradient strength at constant timing of the the additional damping due to cross relaxation depends on the experiment. Therefore, a simplified description of the PFG gradient amplitude. If this equation is used with the measured stimulated-echo experiment in the presence of cross relaxcross-relaxation parameters, both methods are found to agree ation is useful.
within experimental error, and no diffusion-time dependence is
The first 90Њ pulse starts the preparation interval by creatfound for the stimulated-echo experiments. The equation predicts ing xy magnetization with constant phase along the sample. the effect of replacing part of the H 2 O by D 2 O; this has been After a gradient pulse with amplitude G and duration d, a experimentally verified. It is concluded that macromolecular sysvolume element at z has acquired a phase of gGdz. The tems should be checked for cross relaxation when stimulated-echo height h of a sample typically is 5 mm or more. Because methods will be used for self-diffusion measurements. ᭧ 1996 Academic Press, Inc.
gGdh ӷ 1, the phase is periodic along z with a spatial frequency q Å gGd which can be made larger than 10 6 m 01 with the present technology. For simplicity, the assumption