NMRI Studies of Diffusion in Polymers

The principal advantage of NMR imaging is the possibility of making spatially localized diffusion measurements [30]. One can examine by NMR imaging the concentration and location of a permeating liquid in a solid sample. A true diffusion parameter image is obtained, where calculated diffusion coefficients are encoded into an intensity scale. 

The NMR imaging technique also allows the system to be studied dynamically, as measurements can be made on the solid sample immersed in the penetrant. The measurements are rapid. Using the FLASH techniques [22], an image can be obtained in a few minutes. In this fashion, it is possible to study the dynamics of 

NMR imaging techniques have been used for the study of sorption, diffusion and chemical reactions as well as the desorption of chemical substances in polymeric materials [23]. NMR imaging can directly provide the diffusion coefficient as a characteristic quantity of the fluidity of a component in a sample, making it possible to map molecular migration on a microscopic scale. 

NMR relaxation parameters are useful probes of molecular motions in polymers. Each correlation time represents the average value of the system, with some distribution around that average. NMR imaging permits the determination of the spatial distribution of NMR relaxation times. This distribution provides information concerning the local motions of the system. In this case, the polymer is partiaUy swollen with solvent, and the spatial distributions of relaxation times reveal the interactions between the solvent and the polymer in the diffusion process. 

Acetone swells PMMA to a greater extent than methanol, and the self-diffusion coefficients of the system are about two orders of magnitude greater than those of the methanol/PMMA system. This is apparently due to the increased volume available to the acetone molecules. The self-diffusion coefficients decrease by 35% from equilibrium in the outer regions to the region near the glassy core. The decreasing motions of the polymer chains as the core is 

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