Uniaxial Orientational Order Research Articles

Boost Up the Mechanical and Electrical Property of CNT Fibers by Governing Lyotropic Liquid Crystalline Mesophases with Aramid Polymers for Robust Lightweight Wiring Applications

Monofilament type of polyaromatic amide (PA) and carbon nanotube (CNT) composite fibers is presented. A concept of a lyotropic liquid crystal (LLC) constructed via a spontaneous self-assembly is introduced to mitigate the extremely low compatibility between PA and CNT. These approaches provide an effective co-processing route of PA and CNT simultaneously to fabricate the uniform, continuous, and reliable composite fibers through a wet-spinning. Interestingly, the addition of a small amount PA into the dope solution of CNT governs the LLC mesophase not only in a spinneret stage but also in a coagulant region. Thus, the developed PA/CNT composite fibers have the high uniaxial orientational order and the close interfacial packing compared to the pure CNT fibers. The PA/CNT composite fibers achieve the outstanding tensile strength, electrical conductivity, and electrochemical response, while maintaining a lightweight. They also exhibit the chemical, mechanical, and thermal robustness. All of these advantages can make flexible, sewable, and washable PA/CNT composite fibers ideal nanocomposite materials for use in next-generation information and energy transporting system by replacing conventional metal electrical conductors.Graphical The lyotropic liquid crystal self-assembly governed by doping the aramid polymers shows the ability to construct mechanically strong and continuous carbon nanotube-based composite fibers that can be used in the lightweight and robust electrical wiring for extreme environmental applications.

Electrical modification of order parameters and director fluctuations in a dielectrically negative nematic doped with a positive additive

Electric effects of nematic liquid crystals (NLCs) have enabled the revolution of the display industry. Recently, the electro-optic switching of NLCs has been accelerated by applying electric fields in such a way that the field does not change the director orientation, but modifies the optical tensors of NLCs due to three effects: induced biaxial orientational order, enhanced uniaxial orientational order, and quenching of director fluctuations. Till now, these three effects have been explored only in NLCs with negative dielectric anisotropy. Here, we experimentally study the electrical modification of order parameters and director fluctuations in a dielectrically negative/positive composite NLC. The dielectrically positive dopant causes the following changes: (a) a faster nanosecond switching for optical responses attributed to the change of order parameters, (b) an increase of the biaxial order parameter, (c) a reduction of the uniaxial order parameter, and (d) a slight reduction in the field-induced birefringence change. This work provides a basis for further studies in nanosecond electro-optics.

Quantifying alignment in carbon nanotube yarns and similar two‐dimensional anisotropic systems

AbstractThe uniaxial orientational order in a macromolecular system is usually specified using the Hermans factor which is equivalent to the second moment of the system's orientation distribution function (ODF) expanded in terms of Legendre polynomials. In this work, we show that for aligned materials that are two‐dimensional (2D) or have a measurable 2D intensity distribution, such as carbon nanotube (CNT) textiles, the Hermans factor is not appropriate. The ODF must be expanded in terms of Chebyshev polynomials and therefore, its second moment is a better measure of orientation in 2D. We also demonstrate that both orientation parameters (Hermans in three dimensional (3D) and Chebyshev in 2D) depend not only on the respective full‐width‐at‐half‐maximum of the peaks in the ODF but also on the shape of the fitted functions. Most importantly, we demonstrate a method to rapidly estimate the Chebyshev orientation parameter from a sample's 2D Fourier power spectrum, using an analysis program written in Python which is available for open access. As validation examples, we use digital photographs of dry spaghetti as well as scanning electron microscopy images of direct‐spun carbon nanotube fibers, proving the technique's applicability to a wide variety of fibers and images.

A chiral-racemic lyotropic chromonic liquid crystal system.

The two main classes of liquid crystals are thermotropic (containing no solvent) and lyotropic (containing solvent). Both of these classes possess the nematic phase, the most simple of liquid crystal phases with only uniaxial orientational order. For both of these classes, if the constituent molecules are chiral or if a chiral dopant is added, the preferred direction of orientation rotates in helical fashion in what is called the chiral nematic phase. Recent research has shown that because the ordering entities of the two classes are quite different (molecules versus molecular assemblies), important differences in the properties of the nematic phase can result. While thermotropic chiral nematics have been extensively examined, less is known about lyotropic chiral nematics, especially for the most ideal case, a chiral-racemic system. Furthermore, none of the lyotropic chiral-racemic studies has included lyotropic chromonic liquid crystals, which are solutions of dyes, drugs, and nucleic acids. Inverse pitch measurements are reported for a chiral-racemic system of a chromonic liquid crystal across the entire chiral fraction range and over a 30 °C temperature interval. The inverse pitch depends linearly on chiral fraction and decreases with increasing temperature, indicating that achiral and chiral molecules participate in the assembly structure similarly. The helical twisting power is significantly larger than for other chiral lyotropic liquid crystals due to the very high scission energy of the investigated system.

Topological transition in a two-dimensional three-vector model: A non-Boltzmann Monte Carlo study.

Two-dimensional three-vector (d=2,n=3) lattice model of a liquid crystal (LC) system with order parameter space (R) described by the fundamental group Π_{1}(R)=Z_{2} was recently investigated based on non-Boltzmann Monte Carlo simulations. Its results indicated that the system did not undergo a topological transition condensing to a low temperature critical state as was reported earlier. Instead, a crossover to a nematic phase was observed, induced by the onset of a competing relevant length scale. This mechanism is further probed here by assigning a more restrictive R symmetry with Π_{1}(R)=Q (the discrete and non-Abelian group of quaternions), thus engaging the three spin degrees in the formation of point topological defects (disclinations). The results reported here indicate that such a choice of symmetry of the Hamiltonian with suitable model parameters leads to a defect-mediated transition to a low-temperature phase with topological order. It is characterized by a line of critical points with quasi-long-range order of its three spin degrees. The associated temperature-dependent power-law exponent decreases progressively and vanishes linearly as temperature tends to zero. The high-temperature disordered phase shows exponential spin correlations and their temperature-dependent lengths exhibit an essential singular divergence as the system approaches the topological transition point. Biaxial LC models have the required R symmetry owing to their tensor orientational orders and are suggested to serve as prototype examples to exhibit topological transition in (d=2,n=3) lattice models.

Oriented Active Solids.

We present a complete analysis of the linearized dynamics of active solids with uniaxial orientational order, taking into account a hitherto overlooked consequence of rotation invariance. Our predictions include a purely active response of two-dimensional orientationally ordered solids to shear, the possibility of stable active solids with quasi-long-range order in two dimensions and long-range order in three dimensions, generic instability of the solid for one sign of active forcing, and the instability of the uniaxially ordered phase in momentum-conserved systems for large active forcing irrespective of its sign.

Kerr effect at high electric field in the isotropic phase of mesogenic materials.

The well-known Kerr effect in isotropic fluids consists in the appearance of uniaxial orientational order and birefringence that grows as the square of the applied electric field. We predict and observe that at a high electric field, the Kerr effect displays features caused by the nonlinear dependence of dielectric permittivity on the field-induced orientational order parameter. Namely, the field-induced birefringence grows faster than the square of the electric field and the dynamics of birefringence growth slows down as the field increases. As a function of temperature, the field-induced birefringence is inversely proportional to the departure from an asymptotic critical temperature, but this temperature is no longer a constant (corresponding to the lower limit of the supercooled isotropic phase) and increases proportionally to the square of the electric field.

Approximation of Smectic-A liquid crystals

In this paper, we present energy-stable numerical schemes for a Smectic-A liquid crystal model. This model involve the hydrodynamic velocity-pressure macroscopic variables $({\bf u},p)$ and the microscopic order parameter of Smectic-A liquid crystals, where its molecules have a uniaxial orientational order and a positional order by layers of normal and unitary vector ${\bf n}$. We start from the formulation given in \cite{E} by using the so-called layer variable $\phi$ such that ${\bf n}=\nabla \phi$ and the level sets of $\phi$ describe the layer structure of the Smectic-A liquid crystal. Then, a strongly non-linear parabolic system is derived coupling velocity and pressure unknowns of the Navier-Stokes equations $({\bf u},p)$ with a fourth order parabolic equation for $\phi$. We will give a reformulation as a mixed second order problem which let us to define some new energy-stable numerical schemes, by using second order finite differences in time and $C^0$-finite elements in space. Finally, numerical simulations are presented for $2D$-domains, showing the evolution of the system until it reaches an equilibrium configuration. Up to our knowledge, there is not any previous numerical analysis for this type of models.

Computer simulation study of a two-dimensional nematogenic lattice model based on the Gruhn–Hess interaction potential

We consider a nematogenic lattice model, associated with a 2-dimensional lattice; the nearest-neighbour interaction potential, proposed by Gruhn and Hess [Z. Naturforsch. A 51 (1996) 1], is defined via an approximate mapping from the elastic free-energy density, and has already been studied by simulation in 3 dimensions [S. Romano, Int. J. Mod. Phys. B 12 (1998) 2305]. The present simulation results show that the model produces homeotropic anchoring, and a weaker uniaxial orientational order, now surviving up to temperatures higher than the transition temperature of the 3-dimensional counterpart, possibly at all finite temperatures.

Uniaxial Orientational Order–Disorder Transitions in Diammine Magnesium Halides, Mg(ND3)2Cl2 and Mg (ND3)2Br2, Investigated by Neutron Diffraction

Neutron powder diffraction on Mg(ND3)2Cl2 and Mg(ND3)2Br2 revealed as a function of temperature uniaxial orientational order–disorder behavior of the ND3 groups. The crystal structures of both compounds are built up from chains of octahedra 1∞[Mg(NH3)2X4/2] with X=Cl and Br arranged in different ways relative to each other. At ambient temperatures (X=Cl) and 270 K (X=Br) the ND3 groups are disordered with respect to a rotation about the bond Mg–N. The D atom density is well described by a fourfold split position, each D “site” connecting an N with an X atom: Mg(ND3)2Cl2, Cmmm, a=8.1828(6) Å, b=8.2007(6) Å, c=3.7543(2) Å, R(F2)Bragg=5.9%; Mg(ND3)2Br2, Pbam, a=5.9714(2) Å, b=11.9175(3) Å, c=3.98477(8) Å, R(F2)Bragg=7.9%. In both cases the c axis corresponds to the direction of the chains 1∞[Mg(NH3)2X4/2]. At low temperatures (8 K (X=Cl) and 1.5 K (X=Br)) both compounds are ordered with respect to the ND3 groups: They are arranged antiferroelectrically on either side of the chains 1∞[Mg(NH3)2X4/2]. The symmetry is lowered compared to the situation at ambient temperatures and 270 K respectively, which involves in both cases a doubling of the orthorhombic c axis: Mg(ND3)2Cl2, Ibmm, a=8.1319(3) Å, b=8.1338(3) Å, c=7.4410(2) Å, R(F2)Bragg=5.9%; Mg(ND3)2Br2, Pnam, a= 5.92837(8) Å, b=11.8448(2) Å, c=7.9117(1) Å, R(F2)Bragg=5.0%. Detailed evaluation of neutron diffraction data of Mg(ND3)2Cl2 as a function of temperature (50 K<T<150 K) characterizes the phase transition as continuous with Tt≈135 K.

Deuteron NMR investigation of a photomechanical effect in a smectic-A liquid crystal

Deuteron quadrupole-perturbed NMR is used to study the perturbation of orientational order in a smectic-A liquid crystal (octylcyanobiphenyl or 8CB) caused by photoinduced trans-to-cis isomerization of a photosensitive dopant (diheptylazobenzene or 7AB). The time and temperature dependences of the orientational order were independently studied for 8CB, 7AB, and their mixtures. Upon UV irradiation that causes trans-to-cis isomerization of 7AB, the orientational order parameter of the smectic-A phase is reduced. Relaxation in the dark exponentially restores the equilibrium value of the order parameter. The characteristic time for this process closely matches the lifetimes of the 7AB excited state. While in the 8CB smectic-A matrix, the cis-isomerized 7AB molecules retain a uniaxial orientational order with the director oriented along the normal to the smectic layers. The highly bent 7AB cis molecules act as a disorienting factor, decreasing the orientational order in the layers and causing a small increase in layer spacing. This disorder-induced increase in layer spacing is much smaller than the actual increase as observed by in situ x-ray experiments on UV-irradiated mixtures of 8CB:7AB. Concomitant with the experimental observation that only a fraction of 7AB molecules are converted to the cis state, this work provides indirect evidence for a nanophase segregation with the 7AB cis-isomers arranged within the interlayer space, thus significantly increasing the smectic layer spacing.

Surface-Induced Order Parameter Profiles in a Nematic Liquid Crystal from Molecular Dynamics Simulations

The tensor order parameter of a model nematic liquid crystal close to a rough surface has been analyzed by means of molecular dynamics simulations. We show that the rough surface induces a smectic C layering. Moving towards the bulk, the smectic order disappears and a depression in the uniaxial orientational order below its bulk value occurs, accompanied by a considerable biaxiality. Correspondingly, the director orientation undergoes a strong distortion localized within a few molecular lengths.

Molecular dynamics simulations of surface-induced ordering in a nematic liquid crystal

By means of a molecular dynamics simulation, we show that a uniaxial nematic liquid crystal may exhibit an unusual ordering that appears within a few molecular lengths from a rough surface. In contact with the surface, a smectic-C layering is induced. Moving towards the bulk, the smectic order disappears and a decrease of the uniaxial orientational order below the bulk value occurs. The intermediate region between the surface-induced smectic-C ordering and the bulk nematic is characterized by a considerable degree of biaxiality. Correspondingly, the director orientation undergoes a strong distortion close to the surface.

On the refractive indices, polarizabilities and order parameter of a nematic discogenic mixture

Abstract The phase diagram of a mixture of the discogens triphenylene hexa-n-heptanoate (HET7) and triphenylene hexa-(2-methyl-4-n-decyloxy)benzoate is presented. This mixture provides a more accessible nematic temperature range than was previously possible. The refractive indices of one mixture composition are measured and a rather small, negative birefringence found. The two-parameter Haller extrapolation method is used to calculate the molecular polarizability ratio Δα/α. This has a rather small magnitude compared with calamitic mesogens, although the individual polarizabilities are much larger, due to the greater molecular mass of these compounds. The calculated value of Δα/α is used to give the uniaxial orientational order parameter S, whose value is comparable to those of calamitic examples. Tabulated bond polarizability data are used to predict a value for Δα/α, which is found to be similar to the experimental result.

Spatial Dimensionality of a Liquid Crystal Lattice

The principal importance of uniaxial orientational order of anisotropic molecules in forming bulk phases with an effectively reduced spatial dimensionality of the crystal lattice is shown. The conditions under which such phases are formed in liquid crystals are determined within the framework of the Landau theory of weak crystallization. The peculiarities of the nematic molecular orderic observed in liquid crystal mesophases are explained

Induced orientation of free chains in a strained rubber: A deuterium magnetic resonance study

AbstractLinear free polydimethylsiloxane (PDMS) chains are dissolved in a PDMS model network. When the system is uniaxially stretched, the free chains, as well as the network chains themselves, acquire a uniaxial orientational order, which is monitored by 2H NMR. The induced orders measured on both kinds of chains are comparable. The order is then investigated as functions of the free chain length and concentration.