Nuclear magnetic resonance studies of structure and dynamics in heterogeneous samples

Abstract

The aim of this work is to develop and implement methods for determining the local structure and dynamics of heterogeneous samples (e.g. rocks, rubber, porous materials, etc.). From the physical point of view, the heterogeneities are best described in terms of variations in the local susceptibility. The proposed methods are based on analyzing the modulation of spin dynamics by the Magic Angle Sample Spinning (MASS) method in the presence of both local variations in the bulk magnetic susceptibility and molecular diffusion. The correlations between the dipolar and susceptibility fields are used for extracting information on the detailed structure and composition of heterogeneous samples. In the first part of this dissertation, a new method for characterizing porous media, diffusive MASS, is presented. This method is combining MASS and molecular diffusion, providing thus a unique way to simultaneously obtain high resolution spectra and information on the geometry and the internal fields inherent to porous samples. The second part is concerned with obtaining detailed information about the structure and dynamics in rubber samples with the aim of understanding the elastomer-carbon black interactions. The processes that occur at the elastomer-carbon black interface are responsible for the special properties that make rubber so useful and these processes are not yet fully understood. We use the dipolar and susceptibility interactions to characterize the elastomer spatial distribution relative to the carbon black surface, the elastomer mobility and the local order. The last section presents a MASS study of the relaxation and wettability of actual rock samples.

(cont.) It is important to design experiments for characterizing wettability which are less time consuming than the current core-flooding and imbibition experiments. The combination of MASS and relaxation measurements permits the determination of the chemical composition and wetting fluid in core samples. We apply this method to the characterization of two preserved sandstone cores.