New techniques in diffusion ordered nuclear magnetic resonance spectroscopy and their applications to synthetic polymers

A new labeling mechanism has been introduced in nuclear magnetic resonance (NMR) with diffusion ordered spectroscopy (DOSY). By this principle it is possible to disperse NMR signals of different molecules in a mixture by their diffusion behavior. The results are two-dimensional spectra with one axis representing the chemical shift and the other the diffusion coefficient. In this thesis the principles of diffusion ordered NMR spectroscopy are reviewed, and the experimental and numerical difficulties are pointed out. A concise formalism for the treatment of NMR diffusion pulse sequences and their simulation is presented. The application to polymeric systems with the aim of providing information about molecular weight distributions and polymer blends is shown by examples. A pulse sequence for three-dimensional spectra, a diffusion ordered TOCSY, which is well suited for the characterization of the molecular structure of polymer fractions of different molecular weights, is introduced. The occurrence of convection currents in the samples, especially at elevated temperatures, is a severe problem that leads to erroneous results. A technique which allows the measurement of diffusion in the presence of convection is presented. (author)