NMR field-cycling study of proton and deuteron spin relaxation in the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl

NMR field-cycling measurements of the deuteron spin relaxation dispersion T₁(v) for the fully deuteriated nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB-d₁₉) over a broader Larmor frequency range (v≈10 kHz to 30 MHz) than reported so far in the literature basically confirm the magnetic relaxation mechanisms previously observed by frequency dependent proton spin studies of various nematogenic molecules, namely collective nematic modes of the director field in the kilohertz regime, and anisotropic reorientations of individual molecules (mainly self-diffusion for the protons and mainly rotations about the long axis for the deuterons) in the megahertz range. Within the experimental error limits such a model allows a self-consistent interpretation of the available deuteron and proton T₁(v) results for deuteriated or protonated 5CB, respectively. In particular, the magnitudes of the measured order fluctuation contributions are in approximate accordance, i.e. within a factor of less than two, with theoretical estimates from NMR line splittings and the relevant material parameters. More exact and more extensive deuteron studies are needed to locate the origin of the observed minor inconsistency.     

The angle of twist between the two phenyl rings in the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl

The NMR spectrum of the protons in the biphenyl group of 4-n-pentyl-4'-cyanobiphenyl (5CB), was obtained by Sinton et al. (1984, Molec, Phys., 53, 333) and analysed by them to give an angle of twist, φ_min, between the normals to the two phenyl rings of 30 ± 2°. Their analysis made the assumption that V(φ), the potential for rotation about the inter ring bond, is such that only the structures with the minimum energy need be considered when calculating averaged dipolar couplings. Re-analysis of their data by a method which allows for the whole of V(φ) to be sampled when averaging the dipolar couplings yields a value for φ_min of 38.4 ± 0.1°.     

Dielectric study on the flow alignment in 4-n-pentyl-4'-cyanobiphenyl

Dielectric permittivity and loss are measured under steady shear flow as functions of temperature, shear rate, electric field frequency, and electric field strength in the nematic (N) and the isotropic (I) phases of 4-n-pentyl-4'-cyanobiphenyl. In the N phase, the dielectric permittivity in the quiescent state is largely modified if the steady shear flow is applied. These behaviors are discussed based on the Leslie-Ericksen theory [Q. J. Mech. Appl. Math. 19, 357 (1966); Arch. Ration. Mech. Anal. 4, 231 (1960)], showing that the dielectric properties under the shear flow are consistently interpreted in terms of the flow alignment of the director, a unit vector specifying the orientation of the liquid crystals. It is also suggested that the behaviors of dielectric permittivities are similar to those of the viscosities.     

A molecular dynamics study of the nematic phase of 4-n-pentyl-4'-cyanobiphenyl

Preliminary molecular dynamics simulations of the nematic phase of 4-n-pentyl-4'-cyanobiphenyl are described. The simulations include all molecular degrees of freedom. The influence of the molecular dipole moment is investigated by comparing simulations with and without a charge distribution on the molecules. Inclusion of the charge distribution is found to lead to a slight broadening of the orientational distribution function, in qualitative agreement with Raman measurements of the orientational order parameters.     

Influence of hydrophilic and hydrophobic aerosil particles on the molecular modes in the liquid crystal 4-n-pentyl-4'-cyanobiphenyl

Dielectric spectroscopy in the frequency range 10⁶-10⁹ Hz was applied to investigate the influence of hydrophilic and hydrophobic aerosil particles on molecular processes in the liquid crystal 4-n-pentyl-4'-cyanobiphenyl. The dynamics of the molecular process in the isotropic phase is non-Arrhenius; it weakly depends on the aerosil density and shows critical temperature dependence of the activation energy near the isotropic-nematic phase transition. The relaxation rate of the process related to the hindered rotation of the molecule around its molecular short axis (slower process) follows an Arrhenius law over about thirty degrees in the nematic range (except close to the phase transition) with an activation energy comparable to the bulk and is almost independent of the aerosil density. The relaxation rate of the process originating from the fluctuation of the molecular long axis around the director (librational mode, faster process), however, follows the Vogel-Fulcher-Tammann law. With increasing disorder achieved by increasing the aerosil density, the relative dielectric strength of the librational mode increases in comparison with the bulk. The relaxation frequency of the slower process increases but that of the faster process decreases with increasing aerosil density. Both these effects are less pronounced for hydrophobic than for hydrophilic aerosils.     

Dielectric studies of liquid crystals under high pressure II. Low frequency relaxation process in 4-n-pentyl-4'-cyanobiphenyl in relation to theories of the nematic state

The results of high pressure dielectric studies of 4-n-pentyl-4'-cyanobiphenyl (5CB) are analysed in terms of theories of the nematic state. The retardation factor g∥ = τ∥/τ₀ and the effective, single-particle potential of mean torque were calculated at the nematic-isotropic transition temperature T_NI and along the isothermal, isobaric and isochoric paths within the nematic phase of 5CB. The potential of mean torque is compared with the order parameter known for the same conditions. The values of parameter γ relating the potential to the volume is discussed.     

Dielectric relaxation in polymer dispersed nematic liquid crystal films

Dielectric absorption studies for polymer dispersed liquid crystal (PDLC) samples of different nematic (4-hexyl-4'-cyanobiphenyl, 6CB) contents (20, 30, and 40 wt%) have been carried out in the frequency range from 10kHz to 10MHz. A method for evaluation of the fraction of the molecules which are not affected by the polymer surface anchoring forces in the nematic droplets is proposed.     

Anchoring of 4-n-alkyl-4'-cyanobiphenyl liquid crystal molecules on rubbed polyimides

In the literature it has been reported that the rigid cyanobiphenyl core of 4-n-alkyl-4'-cyanobiphenyl molecules deposited onto polyimide-coated substrates makes an average angle of ≈ 78° with respect to the surface normal, independent of the rubbing conditions applied and independent of the polyimide materials studied. A simple anchoring model based on a combination of dipole-dipole, van der Waals and steric interactions explains this angle.     

Dielectric properties of a strongly polar nematic liquid crystal compound doped with gold nanoparticles

This study focuses on the electrical characteristics of a strongly polar nematic liquid crystal, Hexyloxy-cyanobiphenyl (6OCB), doped with a low concentration (2% by weight) of citrate buffer stabilised gold nanoparticles (GNPs) at low frequencies between 20 Hz and 35 MHz. The doped samples have lower values of nematic–isotropic transition temperature, permittivity (both parallel and perpendicular to the field direction) and dielectric anisotropy; however, relaxation time and activation energy were increased. The observed results could be explained on the basis of weakly anisotropic nature of GNPs and a local rearrangement of liquid crystal molecules surrounding the nanoparticles. Moreover, a complimentary suggestion on a possible change in the dipole–dipole correlation is made to explain the difference in changes (qualitative and quantitative) observed for permittivity of the host nematic liquid crystal doped with GNP. Temperature dependent dielectric relaxation studies indicate an increase in viscosity and potential barrier; and hence a change in strength of inter-molecular and intra-molecular interactions is suggested.     

Dielectric spectroscopy analysis in employing liquid crystal phthalonitrile derivative in nematic liquid crystals

Dielectric anizotropy and relaxation properties of 2,3-dicyano-1,4-di[3,4,5-tri(dodecyloxy)phenylcarbonyloxy] benzene (DCDPB)-doped E7 and E7 liquid crystal have been investigated by the dielectric spectroscopy method. Dielectric anisotropy property of the LCs changes from the positive type to negative type. Dielectric relaxation properties suggest that LCs exhibit a monodispersive dielectric property. The relaxation frequency of E7 and E7/DCDPB liquid crystals was calculated by means of Cole-Cole plots. Consequently, DCDPB dopant changes the dielectric anizotropy and relaxation parameters of E7 LC.     

Molecular alignment of the 4-octyl-4'-cyanobiphenyl liquid crystal filled with SiO2

The molecular dynamics and molecular alignment of the dispersed 4-octyl-4'-cyanobiphenyl (8CB) liquid crystal with 5 wt % of SiO₂ nanosphere particles have been studied using dielectric spectroscopy. The measurements were performed in the frequency range between 10²-10⁷ Hz in the SmA, N and I phases. The results show only one Debye relaxation process at high frequency (10⁵-10⁷ Hz). In the bulk material, activation energies of 40, 58 and 63 kJ mol^-1 were determined for the SmA, N and the I phases, respectively. In the dispersed sample, the activation energies were found to be 52, 76 and 81 kJ mol^-1, respectively. These results are compared with the available data in the literature. The reversible electromechanical response of the dispersed sample under the influence of an applied a.c. electric field was investigated in the SmA, N, and I phases.     

Distinctive dielectric properties of nematic liquid crystal dimers

ABSTRACT

We provide an overview of the effect of the molecular structure on the dielectric properties of dimers exhibiting nematic and twist-bend nematic phases with special focus on how the conformational distribution changes are reflected by the dielectric behaviour. Nematic dimers show distinctive dielectric properties which differ from those of archetypical nematic liquid crystals, as for example, unusual temperature dependence of the static permittivity or dielectric spectra characterised by two low-frequency relaxation processes with correlated strengths. The interpretation of such characteristic behaviour requires that account is taken of the effect of molecular flexibility on the energetically favoured molecular shapes. The anisotropic nematic interactions greatly influence the conformational distribution. Dielectric behaviour can be used to track those conformational changes due to dependence of the averaged molecular dipole moment on the averaged molecular shape. Results for a number of dimers are compared and analysed on the basis of the influence of details of the molecular structure, using a recently developed theory for the dielectric properties of dimers.     

Scanning tunnelling microscopic images of the surface ordering of the liquid crystal 4-n-octyl-4'-cyanobiphenyl

Images of the surface ordering of 4-n-octyl-4'-cyanobiphenyl, a room temperature smectic liquid crystal deposited upon graphite, have been obtained by scanning tunnelling microscopy. The microscope was operated in air using the constant-current mode. Under certain tunnelling conditions it has been possible to resolve both the aliphatic and aromatic parts of the molecule, and to observe individual benzene rings. Two previously unreported conformations have been observed: an overlapping bilayer structure with spacing 3.7 nm, and a monolayer structure with spacing 2.4 nm. The latter structure may represent the first visual evidence for a surface polar ordered structure.     

On the transitional properties of rigid oligomers. The orientational order of the monomer 4-n-pentyl-4'-cyanobiphenyl and its platinum dichloride linked dimer

We have studied the orientational order of the monomer 4-n-pentyl-4'-cyanobiphenyl (5CB) and of the dimer, [PtCl₂(5CB)₂], formed by linking two cyanobiphenyl units via a platinum dichloride bridge, dissolved in a common nematic solvent, using deuterium NMR spectroscopy. Analysis of the second rank order parameters, obtained from these experiments, in terms of a molecular field theory yields the anisotropic solute-solvent strength parameter responsible for the solute alignment. In the limit of low solvent order the strength parameters for the monomer and dimer differ significantly, in accord with the differing anisotropies of these two solutes. However, as the solvent order increases, so the relative difference in the strength parameters decreases, tending to zero. A possible explanation for this intriguing behaviour is proposed.     

Intra-cell ionic properties of twisted nematic liquid crystal cells

Under 1-kHz and low-frequency driving, we report our investigation of intra-cell ionic properties of two twisted nematic (TN) liquid crystal (LC) cells made of same LC mixture but different polyimide (PI) materials for LC alignments. A heterodyne interferometry system was used to measure the phase retardations of the TN cells versus applied voltages at 1 kHz. We also measured the phase retardations and currents of the TN cells versus time by applying a mid-grayscale voltage of 1.3 V at 0.1 Hz to the cells. Related to the above-measured data, we have developed equations to characterise the field-driven transports of mobile charge carriers within the PI films independently from that within the LC medium of same TN cell to obtain time-dependent in-cell voltages of mobile charge carries across the LC mixture and across two PI-alignment films, respectively. Our experimental methods can be used to optimise LC mixtures and PI materials for low-refresh-rate thin-film-transistor-driven liquid crystal displays (LCDs) for low power and provide bases for further investigations on mobile-charge-carrier generation and transport within thin in-cell PI-alignment films as well as within the LC mixture of same LC cell.     

Persistence of ordering in 4-n-pentyl-4'-cyanobiphenyl above the nematic-isotropic transition as detected by picosecond time-resolved fluorescence spectroscopy

The orientational ordering of 4-n-pentyl-4'-cyanobiphenyl (5CB) was explored with the aid of picosecond, time-resolved fluorescence spectroscopy by monitoring the time dependence of excimer formation of the mesogen at various temperatures. It was found that in the nematic phase the excimer is formed with high efficiency, while in the isotropic phase the excimer formation was less effective. Immediately after pulse excitation the time-resolved fluorescence spectra revealed that short range ordering persists just above the N-I transition, although the macroscopic order had disappeared.     

Investigation of dielectric and electro-optical properties of nematic liquid crystal with the suspension of biowaste-based porous carbon nanoparticles

ABSTRACT

The effect of biowaste porous carbon nanoparticles (PCNPs) on the dielectric and electro-optical properties of nematic liquid crystal (LC) mixture (1823A) of 4-(4-alkyl-cyclohexyl) benzene isothiocyanates and 4-(4-alkyl-cyclohexyl) biphenyl isothiocyanates has been studied. The dielectric permittivity of nematic LC has been increased with the dispersion of carbon NPs. The dielectric anisotropy has been calculated and found to be decreased with the dispersion of PCNPs into the pure nematic LC. The response time and birefringence have been also observed with the variation of temperature, frequency as well as the concentrations of carbon NPs. After the dispersion of PCNPs achieved better birefringence and faster response in the dispersed system, which is the significant application in display devices. Threshold voltage splay elastic coefficient and rotational viscosity have been calculated for both pure and NPs dispersed nematic system. Its value is increased with the dispersion of NPs. Additionally, photoluminescence and figure of merit have investigated as a comparative study of nematic matrix as well the dispersed system. The experimental results have been found to have good agreement with the theoretical data of nematic LC. An effort has been made to explain these experimental results on the basis of interaction between nematic molecules and carbon NPs.     

Tuning the physical properties of a nematic liquid crystal elastomer actuator

In this report we demonstrate the ability to tune the physical properties of a liquid crystal elastomer (LCE) by varying the amount and type of crosslinking within the elastomer network. LCE films composed of a single mesogenic compound were capable of uniaxial contraction when thermally actuated through the nematic to isotropic phase of the material. We probed the physical properties of the LCE films while varying the amount and concentration of two crosslinking agents and measured actuation strains of 10-35%, elastic moduli of 3-14 MPa, and transition temperatures ranging between 75 and 60°C. The viscous losses of the elastomers and the estimated work capable of being produced by the films were also evaluated. The ability to tune the physical properties of the LCE films allows for a wide range of applications including robotics, microelectromechanical systems (MEMS), shape-changing membranes, and/or microfluidics.     

Tuning the physical properties of a nematic liquid crystal elastomer actuator

In this report we demonstrate the ability to tune the physical properties of a liquid crystal elastomer (LCE) by varying the amount and type of crosslinking within the elastomer network. LCE films composed of a single mesogenic compound were capable of uniaxial contraction when thermally actuated through the nematic to isotropic phase of the material. We probed the physical properties of the LCE films while varying the amount and concentration of two crosslinking agents and measured actuation strains of 10–35%, elastic moduli of 3–14 MPa, and transition temperatures ranging between 75 and 60°C. The viscous losses of the elastomers and the estimated work capable of being produced by the films were also evaluated. The ability to tune the physical properties of the LCE films allows for a wide range of applications including robotics, microelectromechanical systems (MEMS), shape‐changing membranes, and/or microfluidics.     

Dielectric properties of some nematic liquid crystals for dynamic scattering displays

The dielectric constants of three room-temperature nematic mixtures suited for dynamic scattering displays are reported. The asymmetrically substituted azoxybenzenes involved exist as two isomers. One isomer has a positive dielectric anisotropy (Δ =

- ?_⊥ > 0), the other a negative one.