D. J. Pusiol

Order fluctuations of the director in nematic thermotropic liquid crystals studied by nuclear magnetic resonance dipolar relaxation

Larmor frequency dependent NMR studies of dipolar order relaxation in liquid crystals have seldom been tried in the past. Using conventional static magnetic field techniques, the experiment cannot be extended to the low Larmor frequency (νL) regime due to limitations in the signal-to-noise ratio of the dipolar echo. In this work, we present an experimental study of the dipolar relaxation time in the frequency range 103–7×107 Hz in nematic thermotropic liquid crystals. To extend the study to such low frequencies, we used the Jeener–Broekaert pulse sequence combined with fast field-cycling NMR technique. For frequencies higher than 105 Hz, the dipolar relaxation time T1D(νL) follows the νL1/2-law that is characteristic of order fluctuations of the director (OFD) in nematics. In contrast, the Zeeman relaxation is driven by faster and less correlated motions, specially in the MHz frequency range. The relaxation of dipolar energy was measured to be remarkably faster than the one predicted by the usual semiclas...

Echo-planar rotating-frame imaging

A new rotating-frame imaging method that produces a complete cross section of an object in a single experiment is reported. The echo planar rotating frame imaging (EPROFI) technique uses two perpendicular RF gradients for two-dimensional spatial encoding and fully exploits the formation of rotary echoes for fast sampling of spatial frequencies. The acquisition scheme yields the Fourier transform of the spin distribution on Cartesian coordinates for straightforward image reconstruction. Implementation of the technique on a low-field portable NMR probe is described and results are presented for test objects with different geometries.

Nuclear magnetic resonance proton dipolar order relaxation in the selectively deuterated nematic para-azoxyanizole

Larmor frequency dependent measurements of Zeeman (T1Z) and dipolar order (T1D) relaxation times provide experimental information on the molecular dynamics in liquid crystals. However, at present, a comprehensive theoretical expression relating T1D with the spectral densities of the molecular motions is not available for liquid crystals. In fact, recent relaxation studies in nematic thermotropic liquid crystals have shown that the traditional model of isolated phenyl proton pairs predicts a relaxation of the dipolar order noticeably slower than the observed one. In this work we show that the failure cannot be assigned exclusively to the assumption of isolated spin-pairs. With this aim, we study the dipolar order relaxation in the nematic PAAd6 (methyl deuterated para-azoxyanizole). After calculating a generalized expression for T1D valid for an arbitrary number of spins, we found that the contributions from multispin interactions and correlations are negligible. This means that, from the point of view of ...

Time scale of short range order fluctuation in the isotropic phase of butylcyano-phenylcyclohexane liquid crystal

Abstract The NMR spin-lattice relaxation time at three Larmor frequencies – ν L =28 MHz , 1.235 MHz and 48 kHz – in the isotropic phase of butylcyano-phenylcyclohexane has been measured in the temperature range close to the isotropic–nematic phase transition. Spin-lattice relaxation mechanisms provided by both individual reorientational fluctuations and fluctuations of the local director can be distinguished at the different Larmor frequency scales. We conclude that the Landau–De Gennes theory for the weak first order nematic–isotropic phase transition, together the low frequency approximation can be applied if the experiment is carried out at Larmor frequencies in the kHz range.

Phase diagram of a lyotropic mixture sodium bis (2 ethylhexyl) sulfosuccinate/dodecanol/water: Reverse micellar, cylindrical, lamellar, and sponge phases

A combined study of flow-induced optical birefringence, x-ray diffraction, optical observations, electrical measurements and 23Na NMR is reported with the aim to characterize the phase diagram of the ternary lyotropic mixture of sodium bis (2 ethylhexyl) sulfosuccinate, dodecanol, and water. One of the most interesting features of this phase diagram is the existence of a bicontinuous structure associated to an inverted sponge phase in the amphiphilic-rich region of it. This phase is surrounded by a micellar isotropic inverted phase. Besides these phases, a lamellar and a cylindrical (triclinic) phase were observed in the phase diagram.

Nuclear magnetic resonance study of the internal magnetic field distribution in water base ionic and surfacted ferrofluids

This work reports a nuclear magnetic resonance (NMR) study of both hydrogen and deuterium on two different kinds of ferrofluids. One of them is a colloidal suspension of surfacted magnetic grains dispersed in a mixture of light and heavy water. The second one is an ionic ferrofluid composed by positively charged magnetic grains of Co–Fe2O4 dispersed in the same solution of water. A model of the local magnetic field distribution in the interstitial (bulk) water is worked out in order to account for the broadening of the NMR spectra observed in both samples.

PHASE-MODULATED NUTATION NUCLEAR QUADRUPOLE RESONANCE SPECTROSCOPY

Nutation nuclear quadrupole resonance spectroscopy enables one to determine the asymmetry parameter on powder samples. The method relies on two-dimensional methodology: The first period is the duration of the radio frequency exciting pulse and the second dimension is the free-evolution period of the quadrupolar nucleus. Varying lengths of the radio frequency pulse induce an amplitude modulation of the free-induction decay signals which is characteristic of the anisotropic nutation frequency distribution. Fourier analysis along the first dimension provides a powder nutation line shape that allows determination of the electric field gradient tensor’s asymmetry parameter. We describe the first application of two-dimensional phase-modulated spectroscopy to a quadrupolar nucleus at zero field. It is shown that a phase-modulated variant of the nutation spectroscopy is feasible in the pure quadrupole regime and it provides a gain in the signal-to-noise ratio compared to the amplitude-modulated method.

Relaxation properties of proton magnetic spin quasi-invariants in liquid crystals

Abstract We present an experimental study of the thermodynamic properties of proton pairs in two thermotropic nematic liquid crystals: chain deuterated 4-n-pentyl-4′-cyanobyphenyl, 5CBd11, and normal 5CB. In the first sample, we find the existence of pure intra-pair and inter-pair magnetic quasi-invariants. In the second, the dipolar signal is more complex, due to the nonequivalence of protons pairs in the molecule, but it is still possible to prepare states for which only one kind of order strongly dominates. In both compounds, the dipolar quasi-invariants relax independently to thermal equilibrium with the lattice. Finally, we discuss the temperature dependence of the characteristic relaxation times in terms of local and long-range cooperative molecular dynamics.

NMR study of the director fluctuations coherence length in the nematic phase of butylcyano-phenylcyclohexane

Abstract The temperature behavior of the director order fluctuation coherence length in the nematic mesophase of butylcyano-phenylcyclohexane is measured from the NMR spin–lattice relaxometry profile. The physical significance of the plateau in the relaxation profiles at low Larmor frequencies is discussed.

Intramolecular character of the intrapair dipolar order relaxation in the methyl deuterated nematic para-azoxyanisole

Abstract The proton intra-pair dipolar order relaxation time ( T 1D ) was measured for nematic methyl-deuterated para -azoxyanisole (PAA d6 ) as a function of both the temperature and concentration in perdeuterated PAA (PAA d14 ), at 27 MHz. Since the results coincide for all measured concentrations in all the nematic temperature range, we conclude that the intermolecular contribution to the relaxation of the dipolar energy in this compound is negligible, at the studied frequency. The observed temperature dependence of T 1D is typical of the order fluctuations of the director (ODF), which clearly indicates that the ODF is the relevant mechanism producing dipolar order relaxation, in accordance with previous field cycling experiments.