Untersuchung der Protonen‐Spin‐Gitter‐Relaxation in der smektisch‐C‐ und der nematischen Phase langkettiger Homologer von PAA

In order to elucidate the nature of spin relaxation in nematic liquid crystals in the vicinity of the nematic-isotropic phase transition temperature, we present extensive measurements and a quantitative analysis of the proton spin, T,, relaxation dispersion for some homologues of the PAA series (PAA, PAB, HAB). The studies were made for the nematic state over a broad range of Larmor frequencies. By a detailed inspection of the frequency, temperature and chain length dependences we conclude that in all cases at least three kinds of molecular reorientations with rather different relaxation dispersion contribute to the total TI, i.e., order fluctuations of the director (OFN), self-diffusion (SD) and an additional mechanism, which we attribute to order parameter fluctuations (OFS). The combined OFN-SD-OFS model extends existing interpretations and allows a quantitative description of the observed phase transition effects. The effects of both OFN and OFS on TI clearly increase with the molecular length and we deduce from this that not only the ring protons, but also some chain protons undergo these types of fluctuation. Due to the rather high temperature of the mesophases, the diffusional contribution is hard to evaluate exactly and hence there are no significant variations between the homologues within the limits of experimental error. However, a small systematic discrepancy with respect to independent diffusion measurements, available for PAA, indicates that this relaxation model is still incomplete and that a fourth mechanism should be included in the analysis. Possibly, this could be some kind of rotation as previously reported for lowtemperature nematics.

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Proton Spin Relaxation Dispersion for Some Nematic Homologues of the Liquid Crystal PAA

Nagel

Wölfel

Noack

1983

Israel Journal of Chemistry

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Nuclear magnetic and dielectric relaxation of a homologous series

Schifferdecker

Aichmann

Nimtz

1981

The Journal of Chemical Physics

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Nuclear magnetic and dielectric relaxation behavior of an asymmetric homologous series of the nematic liquid crystalline compound p-alkoxybenzal-p'-ethoxyaniline and of p-octylbenzal-p'propylaniline have been investigated. The nuclear T 1 measurements indicate to consider the different molecules of the series in the nematic phase as ellipsoids with dissimilar lengths of the long axis. Applying a 90°-1'-90"90" pulse different decays were observed for the even and the odd members of the series, the differences vanishing below the clear point. They were interpreted by a different chain configuration for the odd and the even members. The dielectric measurements in the microwave range show a strong dependence of the Debye relaxation time on the chain length. The role of the oxygen atom in the chain became evident.

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Molecular motion in thermotropic liquid crystals: Deuterium relaxation and spectral densities of motion for a highly ordered spin probe in 4-octyl-4′-cyanobiphenyl

Selwyn

Vold

1984

The Journal of Chemical Physics

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Articles you may be interested inMolecular motions and ordering of the interfacial, droplet and binder regions of polymer dispersed liquid crystal displays: A paramagnetic resonance spin probe study Metastable, ordered solids of rodlike molecules quenched from their liquids at farfromequilibrium conditions: Phonon spectra of 4noctyl and 4ndodecyl4'cyanobiphenyl J. Chem. Phys. 98, 3571 (1993); 10.1063/1.464081 Nuclear spin relaxation and molecular dynamics in ordered systems: Models for molecular reorientation in thermotropic liquid crystals J. Chem. Phys. 88, 1443 (1988); 10.1063/1.454214 Deuterium spin relaxation and spectral densities in the liquid crystal 4npentyloxybenzylidene4'nheptylaniline2, 3, 5, 6,d 4 J. Chem. Phys. 82, 4838 (1985); 10.1063/1.448652Deuterium spin lattice relaxation studies of chloroformd 1 dissolved in mixtures of two thermotropic liquid crystals of opposite diamagnetic anisotropies Deuterium relaxation data obtained at 9.2 and 38.4 MHz are reported for a solution of pdiethynyl-d,d '-benzene (DEB) in 4-n-octyl-4' -cyanobiphenyl (8CB). The temperature and frequency dependences of the spectral densities of motion for this highly ordered spin probe in 8CB are markedly different from those observed for DEB in nematics without underlying smectic phases. J 1 (m O ) andJ 2 (2m 0 ) are both frequency dependent and because of the high degree of ordering observed for the deuterons (Szz = 0.28-0.67) we have interpreted the relaxation data in terms of "rotational diffusion in a cone." Given the simplistic nature of this model, the semiquantitative agreement observed between theory and experiment is satisfactory.

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Untersuchung der Protonen‐Spin‐Gitter‐Relaxation in der smektisch‐C‐ und der nematischen Phase langkettiger Homologer von PAA

Cited by 3 publications

References 22 publications

Proton Spin Relaxation Dispersion for Some Nematic Homologues of the Liquid Crystal PAA

Proton Spin Relaxation Dispersion for Some Nematic Homologues of the Liquid Crystal PAA

Nuclear magnetic and dielectric relaxation of a homologous series

Molecular motion in thermotropic liquid crystals: Deuterium relaxation and spectral densities of motion for a highly ordered spin probe in 4-octyl-4′-cyanobiphenyl

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