Dynamics of Polymer Systems Studied by NMR Field-cycling Relaxometry

Abstract

With the availability of commercial field-cycling (FC) relaxometers together with progress of home-built instruments, NMR relaxometry has gained new impetus as a method of investigating the dynamics in polymer systems. The FC method provides the dispersion of the spin–lattice relaxation rate. Transforming the relaxation data to the susceptibility representation and assuming frequency-temperature superposition master curves are constructed from individual relaxation spectra measured at different temperatures. Thereby, relaxation spectra covering up to ten decades in amplitude and frequency are obtained, which allow transformation into time correlation functions. In the case of protons, due to the intra- and intermolecular origin of dipolar interactions, spin–lattice relaxation dispersion reflects segmental rotation as well as translation. The latter displays a universal low frequencies dispersion law, which allows determining the diffusion coefficient in addition to the reorientational time constant. By singling out the intermolecular relaxation via isotope dilution experiments, sub-diffusive translation in terms of the mean-square displacement as a function of time is accessed. Likewise, information on reorientational dynamics is provided by the intramolecular relaxation. The results complement those of neutron scattering and rheological experiments. All in all, thorough testing of current polymer theories becomes possible and FC NMR relaxometry may become a method of molecular rheology.