Characterization of Phase Transitions and Dynamics in Reentrant Nematic Liquid Crystals by Xenon-129 Nuclear Magnetic Resonance

In this paper, the ability to detect phase transitions and characterize the dynamics in binary and tertiary reentrant nematic liquid crystal mixtures via 129Xe NMR chemical shifts and spin−spin (T 2) and spin−lattice (T 1) relaxation times is demonstrated. A comparison of the T 1 temperature dependence acquired at 9.40 T and 11.7 T supports the proposition that the relaxation of the dissolved xenon is dominated by dipolar fluctuations. The experimental results show that the spin relaxation times and the chemical shift are continuous functions at the nematic → smectic and smectic → reentrant nematic phase transitions. The spin relaxation times are found to be much more sensitive than the chemical shifts to these transitions in these particular liquid crystal systems. Upon entering the smectic phase, a reduction in the slopes of ln(T 1,2) versus 1/T indicates a decrease in the activation energy for the motions responsible for spin relaxation. The activation energies of the dominating relaxation process within the nematic and reentrant nematic phases are determined from |d(ln T 1,2)/d(1/T)|. The results for the binary and tertiary reentrant nematic liquid crystal mixtures are compared in terms of the molecular packing and the dynamical modes responsible for nuclear spin relaxation.