Molecular Liquids and Polymers Investigated by Field Cycling 1H NMR Relaxometry: Impact of Rotational and Translational Dynamics on Relaxation

In this thesis field-cycling nuclear magnetic resonance relaxometry (FC NMR) is applied to measure the spin-lattice relaxation time, T1, of proton spins in condensed matter not only at different temperatures but also at different Larmor frequencies, ω. The results are presented by means of six publications, from which the first four deal with viscous molecular liquids while the latter two address melts of linear polymers of various length or molecular mass, M. The spin relaxation rate, R1=1/T1, reflects the molecular dynamics via the fluctuation of a certain interaction. In the present case of protons the spins relax due to magnetic dipole-dipole coupling which may be intra- or intermolecular. Because of the latter the proton relaxation is not only governed by molecular reorientation but also by translation. Contrary to the common assumption that the intramolecular interactions are the most relevant for proton relaxation, it even turns out that especially at low frequencies the relaxation rate actually is dominated by the intermolecular contribution. The rate dispersion curves, R1(ω) obtained by FC 1H NMR can directly be compared to the results of other techniques like dielectric spectroscopy (DS) or depolarized light scattering (LS). With respect to DS and LS, both solely probing molecular reorientation, R1(ω) shows an enhanced intensity at low frequencies for most molecular liquids. This thesis shows that this feature is due to translational dynamics which are only seen by 1H NMR in combination with a large spectral separation between both types of dynamics. Furthermore it is demonstrated that, besides rotational time constants, τrot, one can extract self-diffusion coefficients, D, from the rate dispersion, R1(ω). This can be done in a simple, model-independent way exploiting the universal translationally driven behavior of R1(ω) at low frequencies, which is a consequence of the Fickian diffusion limit at long times. The extracted D show a good agreement with results from field gradient (FG) NMR up to now the most prominent technique to access translational diffusion. To reveal the contribution of intra- and intermolecular relaxation in liquids isotope dilution experiments were done which allow the separation of the total rate, R1(ω), into its two respective components, R1intra(ω) and R1inter(ω). It is shown that the intermolecular relaxation in 1H NMR is not negligible at all even at high frequencies, because, besides the purely translational contribution, R1inter(ω) also contains intermolecularly reflected rotation. This is due to spins placed off the molecules’ center, a phenomenon called ‘eccentricity effect’, and is demonstrated in neat liquids for the first time. Finally it is shown that from the universal translational low-frequency behavior it is also possible to extract in the case of polymer melts. Here self-diffusion data in agreement to FG NMR could be collected up to molecular masses where the entanglement of the polymer chains already is established. In addition FC NMR provides time constants, τs, on the segmental motion which, in combination with the self-diffusion data, give access to the collective polymer dynamics via an ‘iso-frictional’ quantity, Dτs, which can be checked against common theories. Depending on the molecular mass three regimes could be identified: the simple liquid behavior, the development of Rouse modes and the final onset of the entanglement regime. Thereby, the pure Rouse regime is only seen in a very small M interval as the Rouse modes slowly evolve with growing M and subsequently entanglement is established for M exceeding the entanglement molecular mass. In summary, this thesis shows that the intermolecular relaxation channel of the proton spin relaxation is not to be considered as a peculiarity which has to be overcome when collecting information on reorientational/segmental dynamics in condensed matter, but that this feature provides additional information which gives access to translational motion. Thus FC 1H NMR is a powerful tool for the examination of molecular dynamics in condensed matter and may become a serious competitor to FG NMR regarding monitoring of translational diffusion in neat systems.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Meier, Roman

Contributors dc:contributor

• Rößler, Ernst

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1691/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1691