The frequency dependence (dispersion) of the spin-lattice relaxation rate, R1(w), provides the fluctuation spectrum of molecular dynamics in complex fluids. Employing a commercial and a home-built field-cycling (FC) relaxometer a broad frequency range of 100 Hz ≤ w/2p ≤ 30 MHz (1H) is covered. In polymer melts master curve construction assuming frequency-temperature superposition allows us to extend the effective frequency range even further. Effectively, up to ten decades in frequency/time are accessed, covering the entire mode spectrum ranging from the local (α-)relaxation to chain modes and ultimately terminal relaxation. Concerning protons, relaxation is composed by an intra- and an inter-molecular contribution along R1(ω) = R1intra(ω) + R1inter(ω) which we separated by isotope dilution. In the time domain R1intra and R1inter provide the reorientational correlation function C2(t) and the segmental mean square displacement 〈r2(t)〉 respectively (Fig. 1). Those quantities are used to scrutinize polymer models. Unlike shear rheology FC NMR relaxometry allows to resolve the transition from Rouse and entanglement dynamics (Fig. 1) promoting the method as an alternative toward molecular rheology applicable to study particularly slow relaxation processes in soft matter.

Field-cycling NMR as a tool of molecular rheology

255th National Meeting (2018)

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