Field-cycling NMR relaxometry: Perspectives for investigating supercooled liquids and polymers

Professor Ernst A Rössler from the University of Bayreuth is coming to Leeds to talk about his research.

With the availability of commercial field-cycling (FC) relaxometers together with the recent progress of home-built instruments, NMR relaxometry has gained new momentum investigating the dynamics in (viscous) liquids, polymers, dendrimers and plastic crystals. The method provides the frequency dependence of the spin-lattice relaxation rate R1(ω) which reflects the spectral density/susceptibility of the underlying motional processes. Most studies employ protons and a frequency range of 100 Hz – 40 MHz can be covered in favourable cases. Here, the relaxation is caused by fluctuations of the magnetic dipole-dipole interaction, and one has to distinguish intra- and intermolecular relaxation pathways. The intermolecular relaxation caused by translational diffusion dominates at low frequencies, whereas rotational dynamics at high frequencies, i.e., rotational and translational contributions are more or less separated in R1(ω), yet, can be accessed in a single experiment. Reorientational correlation times and diffusion coefficient are obtained for simple liquids and polymer melts and compared to results from field gradient (FG) NMR. In the case of polymers, sub-diffusive translation caused by Rouse and entanglement dynamics is accessed by singling out the full intermolecular relaxation rate R1, inter(ω) via isotope dilution experiments. By Fourier transformation, the segmental mean square displacement as a function of time is revealed, thus competing with neutron scattering experiments. For example, complementing the FC msd data by that from FG NMR, all four diffusion regimes forecast by the tube reptation model are revealed. Likewise, information on reorientational dynamics is provided by the intramolecular relaxation R1,intra(ω). Thereby, thorough testing of current polymer theories becomes possible.