Mesophase relaxation measurements

Proton, deuteron and carbon spin relaxation measurements of liquid crystals have provided detailed information about the molecular motions of such anisotropic liquids (anisotropic rotation and translation diffusion of individual molecules), and about a peculiar feature of liquid crystalline phases, namely collective molecular reorientations or order fluctuations. Spin relaxation in liquid crystalline mesophases has challenged NMR groups since the early 1970s, shortly after the publication of theoretical predictions that order fluctuations of the director (OFD, OF), i.e. thermal excitations of the long-range orientational molecular alignment (director), may play an important unusual role in nuclear spin relaxation of ordered liquids. Unique to these materials, which are composed of rod-like or disc-like (i.e. strongly anisotropic molecules), it was predicted that such thermal fluctuations of the director should, at the frequencies of these fluctuation modes, produce rather peculiar Ti(p) dispersion profiles. For example in the case of uniaxial nematic... 

Relaxation measurements in mesophase solvents should provide insight into molecular dynamics in these systems, but as yet this aspect has been neglected. Calculations of Ti for a pair of protons on one molecule ° suggest a longitudinal relaxation of about 1 s, resulting from the modulation of the dipolar coupling. They also predict a dependence of Ti on the nuclear-resonant frequency applied. 

Relaxation measurements yield information on the size and shape of micelles. Hoffmann et al. [77] observed that below a certain temperature, the amplitudes of the two relaxation processes decreased rapidly and amplitudes of new relaxation processes appeared. The new processes were attributed to the relaxation effects of another type of micelle, which appeared to be emulsion dropletlike giant molecules. The residence time of the surfactant molecules in the new micelle was unusually long, explained by the incorporation of ion pairs, formed by the surfactant and its counterion, in the micelle. The existence of giant micelles has been disputed by Fontell and Lindman [82]. Subsequent studies by Hoffmann et al. have indicated that the giant aggregates are probably dispersions of liquid crystalline mesophases (see Section 7.1). 

Measurement of spin lattice relaxation times in a smectic A mesophase,... 

NMR can, in principle, provide complementary information on motional processes in liquid crystals. The dipole-dipole interaction between a C-H pair and the quadrupolar interaction when the proton is replaced with a deuteron share the same principal interaction axis. In the case where the carbon is not directly bonded to a proton, there is still dipole-dipole relaxation by nearby protons, but it is also necessary to include an additional relaxation mechanism, the modulation of the chemical shift anisotropy. Proton spin decoupling is necessary to give well-resolved chemically shifted lines in the mesophase of liquid crystals. Furthermore, it is not practical to determine individual spectral density parameters from measured relaxation rates. Proton-proton dipolar interactions may not be ignored even when observation is exclusively confined to the resonant spin [5.31]. This is because proton relaxation causes population flow among the proton spin levels through dipolar (or scalar) coupling. As a consequence, cross-... 

Overall molecular reorientations and internal motions take place in the 10 10 s time window and information about them can be accessed using relaxation rate measurements. By far the best approach is to use NMR experiments where deuterium Zeeman and quadrupolar spin-lattice relaxation times are measured on selectively deuteriated mesogens or on deuteriated probes dissolved in the mesophase. Frequency, orientation and temperature dependence of the spectral densities obtained in these relaxation studies give indications about the various motional modes and are extremely useful for testing orientational diffusion and chain dynamics models. Different deuterium relaxation experiments can be employed to extend the observable dynamic range the quadrupolar echo sequence gives spin-spin relaxation times sensitive to motions in the range s, while extremely slow motions... 

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