Domain Sizes from Spin Diffusion Experiments

6 DOMAIN SIZES FROM SPIN DIFFUSION EXPERIMENTS 

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One particular model of a biological system is a set of barriers in an otherwise homogeneous medium. If diffusion is observed over a short enough time, very little of the substance experiences the effect of the barriers, and the observed motion is characteristic of the medium alone. As the time of observation is extended, more of the substance is reflected at barriers and thus, its total displacement is less than would have been the case without the barrier bringing about less attenuation of the PG spin-echo [48, 49]. From an analysis of such data one can, in principle, obtain information on the geometry of the domains in which restricted diffusion occurs information on cell size, droplet size distribution [50] etc. 

The spin-diffusion rate kc can be used to calculate the domain size by adopting a proper morphology [86, 97, 98]. For example, the domain size of PPO//-PS is calculated to be 5 nm by putting kc = 135 s and D = 5 x 10 m s into Equation (10.6). From the relaxation experiments, the value of kc can be obtained only when the characteristic nonsingle exponential decay curves are observed, that is, only when kc is comparable to the ATa and Kq values. Unfortunately, in most miscible blends, kc is so fast that the decay curves are identical or too slow to average the decays. 

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