Broadband dielectric relaxation study on a diluted solution of a ferroelectric liquid crystal by time domain reflectometry

Abstract

In this work, complex permittivity measurements on a diluted solution of the ferroelectric liquid crystal, 4-[(S,S)-2,3-epoxyhexyloxy]-phenyl 4-(decyloxy)-benzoate, which shows ferroelectric smectic C phase (S*_C), have been performed. Using time domain reflectometry, at frequencies between 10 MHz and 10 GHz, in the temperature range from 50°C down to 10°C for every 10°C, at 5, 10 and 30 wt.% in benzene we observe two independent relaxation processes around 150 MHz and 2 GHz. It is shown that the high frequency process is due to the internal molecular reorientations while the low frequency one is due to molecular orientation around the long molecular axis. It is concluded that the reorientation around the long axis is hindered and leads to a resultant macroscopic polarization.     

Broadband dielectric relaxation study on a diluted solution of a ferroelectric liquid crystal by time domain reflectometry

In this work, complex permittivity measurements on a diluted solution of the ferroelectric liquid crystal, 4-[(S,S)-2,3-epoxyhexyloxy]-phenyl 4-(decyloxy)-benzoate, which shows ferroelectric smectic C phase (S*_C), have been performed. Using time domain reflectometry, at frequencies between 10 MHz and 10 GHz, in the temperature range from 50°C down to 10°C for every 10°C, at 5, 10 and 30 wt.% in benzene we observe two independent relaxation processes around 150 MHz and 2 GHz. It is shown that the high frequency process is due to the internal molecular reorientations while the low frequency one is due to molecular orientation around the long molecular axis. It is concluded that the reorientation around the long axis is hindered and leads to a resultant macroscopic polarization.     

Modelling a nematic liquid crystal

The bulk phase liquid crystalline behaviour of a cyclic siloxane with a pentamethylcyclosiloxane core and biphenyl-4-allyloxybenzoate mesogens (BCS) was studied using molecular dynamics (MD) and wide angle X-ray analysis. This material exhibits partial crystallinity at room temperature and liquid crystalline behaviour above 120° C. For the MD simulations an ensemble of 27 molecules with 135 mesogenic units was simulated and a molecular mechanics force field was used to model the structural anisotropy of the siloxane molecules. Simulations were carried out both at room temperature and at an elevated temperature (425 K). Room temperature simulations showed that, contrary to our initial assumptions, the low energy molecular conformations were not cylindrical but splayed in shape. During the simulation a smectic-like, tilted layer structure was found to evolve for the cluster when full atom potentials were used, while no such development was observed when electrostatic interactions were neglected. The presence of a tilted layered structure was also suggested by the X-ray data. These results indicate that long range electrostatic interactions are significant for the molecular system under study. In order to calculate the orientational order parameter, the orientation of the molecular axis had to be determined. This was achieved by describing the mesogen shapes to be ellipsoidal and defining the principal axis of the ellipsoids to be the molecular directors. By sampling over 200 ps of simulation at 425 K, the time averaged order parameter (S) was calculated. The calculated S of 0.36 was comparable to the value of 0.4-0.45 found from the experimental data. Apart from providing insight into the relative importance of the various competing forces in the formation of the liquid crystalline phase, these simulations are also expected to be useful in predicting the mesophase behaviour of liquid crystalline systems.     

Alignment of a nematic liquid crystal using substituted calixarene Langmuir-Blodgett films

The alignment of a nematic liquid crystal (5CB) induced by several substituted calixarene-based films is reported. Calixarene molecules consisting of four or six moieties and different substituents (acyl and azobenzene groups) were synthesized. Films of such molecules were deposited using a Langmuir-Blodgett technique onto glass plates and were characterized using atomic force microscopy. These treated plates were used to prepare liquid crystal cells, and the overall alignment studied. In the case of photosensitive molecules, photoinduced reorientation experiments were undertaken and are reported. It is shown that it is not important to have a large number of interacting sites on the surface to induce a reorientation of the liquid crystal.     

Alignment of a nematic liquid crystal using substituted calixarene Langmuir-Blodgett films

The alignment of a nematic liquid crystal (5CB) induced by several substituted calixarene-based films is reported. Calixarene molecules consisting of four or six moieties and different substituents (acyl and azobenzene groups) were synthesized. Films of such molecules were deposited using a Langmuir-Blodgett technique onto glass plates and were characterized using atomic force microscopy. These treated plates were used to prepare liquid crystal cells, and the overall alignment studied. In the case of photosensitive molecules, photoinduced reorientation experiments were undertaken and are reported. It is shown that it is not important to have a large number of interacting sites on the surface to induce a reorientation of the liquid crystal.     

Achromatic polarization switch using a film-compensated twisted nematic liquid crystal cell

A broadband polarization switch consisting of an active twisted nematic liquid crystal cell and two passive uniaxial compensation films is analysed. The conjugate gradient method was used for optimizing the device parameters. Simulation results indicate that the polarization switch exhibits a broad bandwidth and high contrast ratio. The manufacturing tolerances on cell gap, twist angle and compensation films thickness are reasonably large.     

Achromatic polarization switch using a film-compensated twisted nematic liquid crystal cell

Two liquid crystal materials, 4,4'-bis(ω-hydroxyalkoxy)-α-methylstilbenes, with butyloxy and octyloxy terminal chains and denoted HAMS-4 and HAMS-8, respectively, were synthesized. The corresponding compounds, 4,4'-bis(ω-alkoxy)-α-methylstilbenes (AMS-n) (n = 2 to 7, 9, 10 and 11), which do not hydrogen bond, were also synthesized and used to study the influence of hydrogen bonding on phase behaviour. DSC, polarizing optical microscopy and miscibility studies were used for determination of the structure and property relationships of the various liquid crystalline phases exhibited by the two homologues and the AMS-n homologues. The FTIR spectra were measured at various temperatures on going from the crystalline phase to the isotropic phase through the intermediate phases. The characteristics of the OH stretch band, the CH₂ stretching, bending and rocking progression modes, as well as the ring C C stretch and the ring skeletal vibrations were studied to elucidate the modes and geometry of the hydrogen bonding of the terminal OH group, the conformation of the terminal chains and the intermolecular interaction between the aromatic mesogenic cores in the various phases.     

Practical aspects of time domain reflectometry

Dielectric relaxation data for n-propanol are obtained in the time-domain and in the frequency-domain (via Fourier transformation) using the method of time-domain reflectometry. The frequency-dependent dielectric permittivity was deduced numerically using an exact solution and various approximate methods. The accuracy of the available procedures is assessed in the frequency and time domains.     

Mass transport in a chiral nematic/nematic liquid crystal system

The optical microscopic mass transport technique has been used to study diffusion phenomenon in a chiral nematic/nematic solute/solvent mixture. Analysis of the concentration-distance, concentration-time and distance-time of the diffusion profile gave the diffusion coefficient of the system as a function of time, distance and concentration, respectively. The mutual diffusion coefficient of the system was independent of the distance and time, showing an average value of 2.65 × 10^-7 cm² s^-1. In non-steady state diffusion, the diffusion coefficient was dependent on both distance and time. The diffusion coefficient exhibited an inverse relation with the local concentration of the chiral solute. The self-diffusion coefficient of the nematic solvent gave a value of 3.4 × 10^-7 cm² s^-1 via extrapolation to zero concentration of the solute.     

Active control of terahertz radiation using a metamaterial loaded with a nematic liquid crystal

We experimentally demonstrate an effective method of manipulating terahertz radiation using a metamaterial loaded with a liquid crystal. Active control of the terahertz beam was performed by in-plane electrical switching of the nematic liquid crystal in the metamaterial device. By changing the magnitude of the AC bias voltage from 0 to 100 V, a reversible transmittance shift of up to 10% was achieved for an incident beam with horizontal electric field polarisation and 27% for vertical polarisation with frequency at around 0.7 THz. Metamaterials with electrically tunable characteristics have potential applications in transmission, modulation and switching components and devices for controlling the intensity and phase of terahertz radiation.     

Instabilities of nematic liquid crystal films

We discuss instabilities exhibited by free surface nematic liquid crystal (NLC) films of nanoscale thickness deposited on solid substrates, with a focus on surface instabilities that lead to dewetting. Such instabilities have been discussed extensively; however, there is still no consensus regarding the interpretation of experimental results, appropriate modeling approaches, or instability mechanisms. Instabilities of thin NLC free surface films are related to a wider class of problems involving dewetting of non-Newtonian fluids. For nanoscale films, the substrate–film interaction, often modeled by a suitable disjoining pressure, becomes relevant. For NLCs, one can extend the formulation to include the elastic energy of the NLC film, leading to an ‘effective’ disjoining pressure, playing an important role in instability development. Focusing on thin film modeling within the framework of the long-wave asymptotic model, we discuss various instability mechanisms and outline problems where new research is needed.     

Crystallisation of nematic liquid crystal mixtures

In this paper, we propose a method to accelerate the crystallisation of nematic liquid crystal mixtures based on crystallisation theory. This method is to hold a nematic liquid crystal sample at a temperature suitable for crystal growth after aging it at a temperature suitable for nucleation. After we specified these temperatures of a nematic liquid crystal mixture using differential scanning calorimetry, we demonstrate that the two-temperature aging method is effective for the crystallisation of other nematic liquid crystal mixtures in which the crystal-liquid crystal transition temperature has so far been undetectable.     

Ising wall instability in a nematic liquid crystal

A new instability for a splay-bend Ising wall was found in a 5CB nematic liquid crystal layer. This instability, which occurs in the presence of an external horizontal magnetic field, is driven by the elastic anisotropy of the liquid crystal material. Depending on the homogeneity of the magnetic field, the unstable straight interface evolves towards a new steady state or undergoes a spinodal decomposition into facets. Energy arguments are given in order to explain these physical phenomena.     

Electric field-induced structural transition of domain walls in nanoconfined nematic liquid crystal systems

The electric field-induced structural transition of domain walls in nanoconfined nematic liquid crystal systems was investigated on the basis of Landau–de Gennes theory. Two models of nanoconfined domain wall systems were established as splay–bend and twist wall systems under Fréedericksz transitions with two different rotation directions under the effect of electric field E/E₀. Results showed that two structural transition processes occur in both models. Pincement transition occurs under a critical external field E_c1/E₀. In pincement transition, walls change into two squeezed symmetric surface defects with opposite charges. Surface defects spread along the direction of the substrates and attain surface order reconstruction states as E/E₀ is enhanced to E_c2/E₀. The increment in cell gap d from the nanoscale to the microscale will not affect E_c2/E₀ in both models.     

Ising wall instability in a nematic liquid crystal

A new instability for a splay-bend Ising wall was found in a 5CB nematic liquid crystal layer. This instability, which occurs in the presence of an external horizontal magnetic field, is driven by the elastic anisotropy of the liquid crystal material. Depending on the homogeneity of the magnetic field, the unstable straight interface evolves towards a new steady state or undergoes a spinodal decomposition into facets. Energy arguments are given in order to explain these physical phenomena.     

NMR polarization echoes in a nematic liquid crystal

We have modified the polarization echo (PE) sequence through the incorporation of Lee-Goldburg cross polarization steps to quench the 1H-1H dipolar dynamics. In this way, the 13C becomes an ideal local probe to inject and detect polarization in the proton system. This improvement made possible the observation of the local polarization P(00)(t) and polarization echoes in the interphenyl proton of the liquid crystal N-(4-methoxybenzylidene)-4-butylaniline. The decay of P(00)(t) was well fitted to an exponential law with a characteristic time tau(C) approximately 310 micros. The hierarchy of the intramolecular dipolar couplings determines a dynamical bottleneck that justifies the use of the Fermi Golden Rule to obtain a spectral density consistent with the structural parameters. The time evolution of P(00)(t) was reversed by the PE sequence generating echoes at the time expected by the scaling of the dipolar Hamiltonian. This indicates that the reversible 1H-1H dipolar interaction is the main contribution to the local polarization decrease and that the exponential decay for P(00)(t) does not imply irreversibility. The attenuation of the echoes follows a Gaussian law with a characteristic time tau(phi) approximately 527 micros. The shape and magnitude of the characteristic time of the PE decay suggest that it is dominated by the unperturbed homonuclear dipolar Hamiltonian. This means that tau(phi) is an intrinsic property of the dipolar coupled network and not of other degrees of freedom. In this case, one cannot unambiguously identify the mechanism that produces the decoherence of the dipolar order. This is because even weak interactions are able to break the fragile multiple coherences originated on the dipolar evolution, hindering its reversal. Other schemes to investigate these underlying mechanisms are proposed.     

Dielectric relaxation of butyl acrylate—alcohol mixtures using time domain reflectometry

Dielectric relaxation measurements of butyl acrylate—alcohol mixtures at different concentrations and temperatures within the frequency range of 10 MHz to 10 GHz have been carried out using time domain reflectometry. Parameters such as the static permittivity, dielectric relaxation time, the Kirkwood correlation factor, the excess inverse relaxation time, and thermodynamic functions were determined and discussed to yield information on the molecular structure and dynamics of the mixture. The value of the dielectric properties decreases with increasing butyl acrylate concentration in alcohol and systematically varies with the length of alcohol alkyl chain. Negative values of the excess inverse relaxation time found for all concentrations and at all temperatures studied may indicate that the effective dipoles rotate slowly.     

Dynamics of a nanowire in twisted nematic liquid crystal

The dynamics of a nanowire immersed in a nematic liquid crystal sandwiched between two parallel plates are studied by applying an equivalent capacitance approach used in electrostatics. A lower cut-off eigenfrequency for the oscillation of the nanowire is found and the lighter the mass, the smaller the critical cell separation at which the cut-off frequency occurs is needed. A simulation of the dynamical process of the metal wires shows that the relaxation time is proportional to η/m in the small mass region.     

Anchoring of a nematic liquid crystal on a wettability gradient

We have studied the anchoring of the nematic liquid crystal 5CB (4'-n-pentyl-4-cyanobiphenyl) as a function of the surface wettability, thickness of the liquid crystal layer, and temperature by measuring the birefringence of a hybrid aligned nematic cell where the nematic material was confined between octadecyltriethoxysilane-treated glass surfaces, with one surface linearly varying in its hydrophobicity. A homeotropic-to-tilted anchoring transition was observed as a function of the lateral distance along the hydrophobicity gradient, typically in a region corresponding to a water contact angle of approximately 64 degrees. The effect of the nematic layer thickness was measured simultaneously by preparing a wedge cell where the thickness varied along the direction perpendicular to the wettability. The detailed behavior of the onset of birefringence was found to be consistent with a dual-easy-axis model that predicts a discontinuous anchoring transition from homeotropic to planar. The anchoring was independent of temperature, except within 1 degrees C of the nematic-to-isotropic transition temperature (T(NI)). As the temperature approached T(NI), the tendency for planar anchoring gradually increased relative to that for homeotropic anchoring.