Field-cycling NMR relaxometry: the benefit of constructing master curves

ABSTRACT

The currently available field-cycling NMR relaxometers still suffer from a rather narrow frequency range; they facilitate measurements of the ¹H spin–lattice relaxation rate in the frequency of 10?kHz–30?MHz. This limit may be overcome by constructing master curves via exploiting frequency-temperature superposition, the latter being an intrinsic feature of the collective dynamics in many soft matter systems. It states that the shape of the motional time-correlation function is essentially temperature independent over a large temperature interval. As will be demonstrated, master curves may be built in the susceptibility or the spectral density representation of spin–lattice relaxation data. As a result, the effective frequency range is expanded up to ten decades, and the applied shift factors provide the temperature dependence of the corresponding correlation time. Three examples are presented: cyano adamantane in its plastically crystalline phase, the liquid glycerol, and the melt of poly(ethylene propylene). Advantages and limitations of the approach are discussed.