Rotational Diffusion of Liquid Crystals in the Nematic Phase

R507 R. Righini, Rotational Diffusion of Liquid Crystals in the Nematic Phases , EMIS Datarev. Ser., 2000,25, 464... 

An overview of the apphcation of NMR to the field of liquid crystals over the past 50 years has been presented and the important aspects of proton and deuterium NMR have been delineated. The use of liquid crystals as solvents in NMR spectroscopy has been reviewed." Reviews on NMR studies of orientational order with 20 references, rotational diffusion of liquid crystals in the nematic phase and NMR spectroscopy in liquid crystals and membranes are available. A review on the application of spectroscopic methods to liquid crystalline phases has appeared during the period under report and it includes a discussion of the NMR methods. NMR and other methods used to determine the order parameters of nematics have been reviewed. ... 

Rotational diffusion of liquid crystals in the nematic phase... 

Rotational diffusion of liquid crystals in the nematic phase TABLE I Rotational relaxation times f - 7CB and 8CB. 

The structures typically obtained as a consequence of shift non-uniqueness are analogous to nematic phase liquid crystals in the sense that there exists a unique chain direction, but the monomer repeat units in neighboring chains are not well correlated in longitudinal position. As a consequence, the x-ray rotation photographs about the chain axis direction (c) display either (1) sharp hkO reflections and progressively more diffuse hk reflection with increasing Z (2) diffuse reflection for all layers combined with a few sharp hkO reflections or (3) diffuse reflections on... 

Incoherent quasi-elastic neutron scattering (IQENS) has been used to investigate the dynamics of nematic phases. When a monochromatic beam of neutrons is scattered from a nematic liquid crystal the shape of the elastic line in the energy-transfer spectra gives information on the molecular dynamics. A quasi-elastic neutron scattering measurement may be used to investigate the diffusive motions of the molecules in the nematic phase. The translational diffusion coefficients may be determined and localised motions such as molecular rotation and internal rotations may also be characterised. 

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... 

Fenchenko studied free induction decays and transverse relaxation in entangled polymer melts. He considered both the effects of the dipolar interactions between spins in different polymer chains and within an isolated segment along s single chain. Sebastiao and co-workers presented a unifying model for molecular dynamics and NMR relaxation for chiral and non-chiral nematic liquid crystals. The model included molecular rotations/ reorientations, translational self-diffusion as well as collective motions. For the chiral nematic phase, an additional relaxation mechanism was proposed, associated with rotations induced by translational diffusion along the helical axis. The model was applied to interpret experimental data, to which we return below. 

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