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The alignment of the smectic a phase of 4-octyl-4'-cyanobiphenyl induced by an electric field. A time-resolved deuterium NMR study
No full textStudies of the field-induced alignment of the SmA phase using deuterium NMR spectroscopy have revealed a complex pattern of behaviour when the director is initially orthogonal to the aligning field. Here we report the electric field-induced alignment of the SmA director using time-resolved deuterium NMR when the aligning electric field E is at an angle with the magnetic field B of the spectrometer which is considerably smaller than 90degrees; here the director is initially aligned parallel to the magnetic field. The dynamics of the electric field-induced alignment of the director for the smectic phase of 4-alpha,alpha-d(2)-octyl-4'-cyanobiphenyl (8CB-d(2)) was investigated at two angles between B and E of roughly 45degrees and 54.5degrees the so-called magic angle, at different electric field strengths and also at two temperatures, 305.1 K and 302.6 K The dynamics of the SmA director alignment of 8CB-d(2) was monitored by measuring the deuterium NMR spectrum as a function of time. The results for the 45degrees and 54.5degrees geometries revealed, in contrast to the complex dynamics of alignment of the SmA director observed for the 90degrees geometry, a much simpler pattern of relaxation. Here the director appeared to be aligned almost as a monodomain. In general, for either of the two geometries employed here, the ultimate angle of alignment the director achieves relative to E depends on the electric field strength. Lowering the temperature by just 2.5degreesC from 305.1 K to 302.6 K has a dramatic effect on the rate of director relaxation presumably because of the large increase in the combined rotational viscosity and the elastic energy effects of the SmA phase with decreasing temperature. Furthermore for the 45 geometry at 302.6 K, the SmA sample separates on relaxation ultimately into two domains with different alignment angles
A deuterium nuclear magnetic resonance investigation of the director distribution in a thin nematic liquid crystal slab
No full textThe director distribution in a thin nematic liquid crystal (NLC), 4-pentyl-d(2)-4'-cyanobiphenyl (5CB-d(2)) deuteriated in the ex-position of the pentyl chain, confined between two glass plates, with untreated and treated anchoring conditions, has been investigated using a deuterium nuclear magnetic resonance (NMR). The NMR spectra have been measured as a function of the applied electric field. In the absence of surface forces it is found that the director aligns parallel to the magnetic field at relatively low values of the electric field as Delta is positive for 5CB. In the presence of surface forces with increasing electric field the quadrupolar splitting decreases, passes through zero and then increases again to a value which is essentially half of that at zero electric field. That is, the director orientation changes more or less continuously from being parallel to the magnetic field to being orthogonal to it, as the electric field grows
Electric field-induced alignment of the directors in the smectic A phase of 4-octyl-4'-cyanobiphenyl. A deuterium NMR study
No full textDeuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the electric field-induced alignment of the director of the smectic A phase of the liquid crystal, 4-?,?-d(2)-octyl-4'-cyanobiphenyl (8CB-d(2)), at 303.3K. The electric field is arranged to be orthogonal to the magnetic field. The alignment process has been investigated at different electric field strengths and the rate of director alignment was monitored by recording the deuterium NMR spectra as a function of time after the electric field was switched on. The results reveal a complex pattern of electric field-induced director alignment. At high electric field strengths a rapid process is observed in which the director switches from an orientation parallel to the magnetic field to one in which it is parallel to the electric field. An induction period is also observed in which no apparent change in director orientation occurs. This induction period becomes longer (hours in magnitude) as the electric field strength is lowered. Other, intermediate, director orientations are observed as the electric field strength is lowered further. The role of defects is invoked in trying to interpret some of the observed processes underlying the mechanism of the director alignment in the smectic A phase
Field-induced director alignment for 4-nonyl-4'-cyanobiphenyl near the smectic A - Nematic transition
No full textDeuterium NMR spectroscopy has been used to investigate the director dynamics of deuteriated 4-alpha,alpha-d(2)-nonyl-4'-cyanobiphenyl, where the director was aligned by an electric field. The electric field direction made an angle of 47 degrees with the magnetic field in order to provide a unique alignment pathway; accordingly the director is expected to rotate as a monodomain. The time dependence of the director orientation was investigated at different temperatures. At each temperature in the nematic phase, all of the deuterium NMR spectra indicate that the director was uniformly oriented as a monodomain. It was found that the relaxation time in the smectic A phase is about 1,000,000 times as large as that in the nematic phase. This also results in a strong pretransitional growth of the relaxation time as the transition to the smectic A phase is approached
A deuterium nuclear magnetic resonance investigation of field induced director dynamics in a nematic slab subject to magnetic and pulsed electric fields
No full textDeuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the field-induced director dynamics in a nematic liquid crystal, 4-pentyl-d(2)-4'-cyanobiphenyl (5CB-d(2)) deuteriated in the alpha -position of the pentyl chain, confined between two glass plates. The NMR spectra have been measured as a function of time after turning an electric field on and off It is demonstrated that the field-induced director dynamics in the nematic liquid crystal cells can be successfully time-resolved. In addition, it is found that the doubler NMR spectra become powder-like during the turn-on and rum-off processes. It is shown that the rotational viscosity and the diamagnetic anisotropy of 5CB-d(2) can be determined from the time-resolved NMR spectra by assuming uniform alignment of the director
The twist-bend nematic phase: translational self-diffusion and biaxiality studied by 1H nuclear magnetic resonance diffusometry
Full text linkRecently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (NTB) phase that is shown to be chiral even though formed by effectively achiral molecules. Although it now seems to be clear that the NTB phase in the bulk is formed by degenerate domains having opposite handedness, the presence of a supramolecular heliconical structure proposed in the Dozov model has been contradicted by the Hoffmann et al. model in which the heliconical arrangement is replaced by a polar nematic phase. The evidence in support of this is that the quadrupolar splitting tensor measured in various experiments is uniaxial and not biaxial as expected for the twist-bend nematic structure. In this debate, among other evidence, the molecular translational diffusion, and its magnitude with respect to that in the nematic phase above the NTB phase, has also been invoked to eliminate or to confirm one model or the other. We attempt to resolve this issue by reporting the first measurements of the translational self-diffusion coefficients in the nematic and twist-bend nematic phases formed 1′′,7′′-bis-4-(4′-cyanobiphenyl-4′-yl) heptane (CB7CB). Such measurements certainly appear to resolve the differences between the two models in favour of that for the classic twist-bend nematic phase
Field-induced director dynamics of nematic 4-octyl-4'-cyanobiphenyl: a study by deuterium NMR spectroscopy
No full textDeuterium NMR spectroscopy has been used to investigate the director dynamics in the nematic phase of perdeuteriated 4-octyl-4'-cyanobiphenyl-d(25) (8CB-d(25)) When the electric field is applied to the nematic film, the director moves from being parallel to the magnetic field to being at an angle with respect to it. After the electric field is switched off, the director relaxes back to being parallel to the magnetic field. Deuterium NMR spectra were recorded during the, turn-on and the turn-off alignment processes as a function of time. This particular technique was chosen because the spectral peaks associated with each rigid group in the molecule are clearly resolved and of comparable intensity. For all of the experiments at different temperatures in the nematic phase of 8CB-d(25) we find that the field-induced relaxation times are independent of the group used to determine the director orientation during the alignment process
The surface-induced static director distribution in thin nematic liquid crystal films: A deuterium nuclear magnetic resonance spectroscopy study
No full textWe have studied the static director distribution in thin nematic liquid crystal cells with different film thicknesses and different surface anchoring strengths using a combination of deuterium nuclear magnetic resonance (NMR) spectroscopy and continuum theory. A nematic liquid crystal, 4-pentyl-d(2)-4'-cyanobiphenyl (5CB-d(2)) deuteriated in the ct position of:the pentyl chain, was confined between two glass plates with both weak and strong anchoring conditions; the anchoring strengths were measured by using a saturation voltage method. A series of deuterium NMR spectra, obtained using a quadrupolar echo sequence, was acquired as a function of the applied electric field, which can be used to explore the:director deformation. The deuterium NMR spectra predicted by continuum theory involving the magnetic, electric, elastic, and unified surface anchoring energies are found to be in good agreement with experiment. The investigation also reveals that profiles of the director distribution for the case with a magnetic field have a top-hat shape different to the bowler-hat shape, which is well known as the director profile for the case with no magnetic field
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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