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Content archived on 2022-12-23

Fluid transport and molecular dynamics in disordered materials: a nuclear magnetic relaxation, diffusion and imaging study

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The cooperation involves 5 different institutions in Kazan (Russia/Tatarstan), Chernogolovka (Russia), Orsay (France), and Ulm (Germany). The goal was to combine the expertises on different experimental techniques and theoretical formalisms in molecular dynamics studies in disordered materials. The results were published in numerous common publications and conference reports. Among these, two quite spectacular achievements should be mentioned in particular. Chain dynamics in polymer melts have been studied using field-cycling NMR relaxometry and field-gradient NMR diffusometry. A number of pronounced power law limits have been discovered. The twice renormalized Rouse theory of polymer dynamics was developed. On this basis, the experimental data can be described in all details whereas the theoretical approaches commonly in use fail. The influence of immaterial spin diffusion by flip-flop processes on field-gradient NMR diffusometry of polymers was shown. This was the first direct measurement of immaterial spin diffusion. The proper anomalous segment diffusion could be derived and compared with the twice renormalized Rouse theory. The coincidence is very good whereas the known tube/reptation model predicts diffusivities too small. The translational and reorientational dynamics of adsorbate molecules in porous glasses was studied. It could be shown that the surface diffusion effectively taking place in the so-called strong-adsorption limit is governed by a Cauchy displacement distribution. That is, the surface diffusion process takes the form of Levy walks. This is the first time, that this sort of anomalous diffusion could be verified on a molecular length scale. A computer simulation was carried out reproducing the conclusions drawn from the experimental data.

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