M. Vilfan

Proton spin–lattice relaxation in smectic TBBA

The angular, temperature, and frequency dependences of the proton spin–lattice relaxation rate T−11 in the smectic A phase of TBBA have been determined. An analysis is made of the possible relaxation mechanisms in smectic A systems. The results seem to show that the angular dependence of T−11 is produced by the modulation of the intramolecular dipolar interactions due to the coupling between self‐diffusion and local ’’director’’ fluctuations. The low temperature smectic phases of TBBA have been as well investigated. The decrease in the self‐diffusion constant due to two‐dimensional translational ordering at the smectic C→smectic H transition is shown to produce a dramatic increase in T1 and decrease in T1D. The rotation of the chain segments abruptly slows down on going from smectic H to smectic VI and discontinuously freezes out on going from smectic VI to smectic VII.

Nuclear spin relaxation due to order director fluctuations in the smectic A phase

The frequency dispersion and anisotropy of nuclear spin relaxation caused by order director fluctuations in the smectic A phase are calculated. Dispersion of (T−11)OF at low frequencies, which departs from the nematic‐like behavior, is examined in detail and the influence of the wave vector dependent viscosity coefficients on relaxation is commented. The deviation from the nematic‐like relaxation behavior drastically changes the angular dependence of (T−11ρ)OF.

Spin–lattice relaxation mechanisms in the smectic phases of TBBA

The observed frequency dispersions of the proton spin–lattice relaxation rate T−11 in the smectic phases of TBBA in the 105–108 Hz region have been analyzed in terms of the order fluctuation, self‐diffusion, and rotational contributions to T−11. In the smectic A and smectic C phases the main rate determining contributions are order fluctuations and fast self‐diffusion, whereas in the smectic H and smectic VI phases fast rotations and slow translation self‐diffusion determine T−11.

Proton spin–lattice relaxation in nematic microdroplets

The frequency and temperature dependences of the proton spin–lattice relaxation rate have been studied in nematic droplets embedded in a solid polymer matrix. The comparison of these data to those for bulk nematic and pure polymer samples shows that cross‐relaxation between liquid crystal and polymer protons dominates the liquid crystal relaxation in the MHz frequency region, while at low frequencies the translationally induced molecular rotation becomes important. The cross‐relaxation rate is found to be ≈103 s−1 giving an order of magnitude estimate 10−4–10−3 s for the time for which a liquid crystal molecule is bonded to the surface.

Deuteron spin-lattice relaxation mechanisms in partially deuterated nematic MBBA

The frequency and temperature dependence of the spin-lattice relaxation time, T 1, of the various benzene ring deuterons in partially deuterated p-methoxybenzylidene-p-n-butylaniline have been measured. It was found that in the nematic phase the deuteron T 1 is independent of frequency between 4 and 41 MHz and thermally activated. The results seem to demonstrate that benzene ring rotation is the dominant deuteron relaxation mechanism in the MHz region and that the aniline and benzylidene rings reorient at different rates around the para axes.

On the nature of spin-lattice relaxation in nematic MBBA

The measurements of the frequency dependence of the proton T1 in nematic MBBA have been extended to 270 MHz, and the results have been compared with the predictions of the order fluctuation and self-diffusion relaxation theories. Both the frequency and the temperature dependences of T1 are shown to be characteristic of a mechanism controlled by molecular self-diffusion.

Proton NMR study of a discotic columnar mesophase

The molecular ordering and the dynamics of the isotropic and the discotic columnar liquid crystalline phase of hexapentoxy‐triphenylene have been studied by high resolution proton NMR, self‐diffusion measurements and spin–lattice relaxation in the laboratory (T1) and the rotating frame (T1ρ). The onset of the columnar ordering is accompanied by the gradual appearance of a short ’’liquid crystalline’’ component in the proton free induction decay (FID) in addition to the long component characteristic of the isotropic phase. The proton T1 is frequency dependent in the isotropic and in the columnar phase. It changes discontinuously at the clearing point at low frequencies where as it is continuous at 270 MHz. An analysis of the possible relaxation mechanisms is presented.

Nuclear magnetic resonance field‐cycling proton relaxation study of polymer dispersed liquid crystals

The frequency and temperature dependence of the longitudinal proton relaxation time T1 has been studied in liquid crystal droplets embedded in a solid polymer matrix in the nematic and isotropic phase over a broad Larmor frequency range (500 Hz≤νL≤84 MHz) employing the fast‐field‐cycling technique. The comparison of the droplet data, bulk 5CB data, and the pure polymer data show that T1 is dominated by the cross relaxation at the liquid crystal–polymer interface in the entire frequency range. In the low frequency range (νL≤1 MHz), an additional relaxation process determines T1 in both phases, namely reorientations mediated by translational replacements in the nematic phase and the exchange relaxation in the isotropic phase. The analysis of the cross relaxation rate k reveals that the simplified model of Vilfan is only applicable in the nematic phase and leads to an anchoring time τAS of the molecules at the surface which is ≊1.3×10−4 s. This model, however, cannot be applied in the isotropic phase, but mu...

Two‐dimensional deuteron nuclear magnetic resonance of a polymer dispersed nematic liquid crystal

The two‐dimensional nuclear magnetic resonance (NMR) spectra and relaxation rates of polymer dispersed 4’‐pentyl‐4‐cyanobiphenyl droplets deuterated in the β position of the hydrocarbon chain have been measured in the isotropic and nematic phases. A typical Pake power line shape pattern characteristic for a system of ‘‘bipolar’’ droplets with a random distribution of their symmetry axes in the external magnetic field has been found in the ω2 domain whereas the homogeneous linewidth was observed in the ω1 domain. The existence of a weakly orientationally ordered surface layer above the nematic–isotropic transition has been demonstrated. Whereas the deuteron T1/T2≂4.3 in the isotropic phase, T1/T2≂188 in the nematic phase for confined droplets. This effect, which is absent in the bulk, shows that T2 is here mainly determined by translationally induced rotation in the nonuniformly oriented director field inside the cavity. T1, on the other hand, is determined by the slowing down of local molecular reorientat...

Deuteron spin relaxation and molecular dynamics of a nematic liquid crystal (5CB) in cylindrical microcavities

Spin–lattice and spin–spin relaxation times of deuterium nuclei have been measured for a nematic liquid crystal confined in cylindrical channels of diameter 0.2 μm. In the isotropic phase the relaxation in the MHz regime is not affected by the confinement. The possibility of rotating the cylinders with respect to the magnetic field enables the first direct measurements of the anisotropy of deuteron relaxation of a monomeric liquid crystal in the nematic phase. The observed temperature dependence of the spin–spin relaxation rate in the isotropic phase roughly obeys the (T−T*)−1 law and allows the determination of the average time the molecules reside at the wall, which is 5×10−5 s for 5CB at the lecithin coated surfaces. In the nematic phase the spin–spin relaxation rate is affected by order director fluctuations and rotation induced by translational diffusion at orientations with a nonhomogeneous director distribution with respect to the magnetic field.