Chapter 8: Application of Field-cycling ¹H NMR Relaxometry to the Study of Translational and Rotational Dynamics in Liquids and Polymers

Abstract

© 2019 The Royal Society of Chemistry. With the availability of commercial field-cycling (FC) relaxometers together with progress in home-built instruments, NMR relaxometry has gained new momentum as a method of investigating the dynamics in complex liquids such as polymer melts or glass-forming systems. The method provides the dispersion of spin-lattice relaxation rate. Assuming frequencyerature superposition, master curves that extend up to 10 decades in amplitude and frequency are constructed from individual relaxation spectra measured at different temperatures. In the case of protons, due to the intra- and intermolecular origin of the dipolar interactions, relaxation dispersion reflects both rotational and translational dynamics. The latter displays a universal low-frequency dispersion law that allows the determination of the diffusion coefficient D in addition to the rotational time constant, i.e. FC-1H NMR becomes an alternative to field-gradient NMR. Sub-diffusive translation found in polymers is accessed by singling out the intermolecular relaxation via isotope dilution experiments. The mean square displacement was revealed to be a function of time, thus complementing neutron scattering experiments. Likewise, information on segmental reorientational dynamics is provided by the intramolecular relaxation. Thereby, thorough testing of current polymer theories becomes possible.