Small-molecule Liquid Crystals Research Articles

Self-Folding Liquid Crystal Network Filaments Patterned with Vertically Aligned Mesogens.

Fibrous soft actuators with high molecular anisotropy are of interest for shape morphing from 1D to 2D and 3D in response to external stimuli with high actuation efficiency. Nevertheless, few have fabricated fibrous actuators with controlled molecular orientations and stiffness. Here, we fabricate filaments from liquid crystal networks (LCNs) with segmental crosslinking density and gradient porosity from a mixture of di-acrylate mesogenic monomers and small-molecule nematic or smectic liquid crystals (LCs) filled in a capillary. During photopolymerization, phase separation between the small-molecule LCs and LCN occurs, making one side of the filament considerably denser than the other side. To direct its folding mode (bending or twisting), we control the alignment of LC molecules within the capillary, either along or perpendicular to the filament long axis. We show that the direction of UV exposure can determine the direction of phase separation, which in turn direct the deformation of the filament after removal of the small-molecule LCs. We find that the vertical alignment of LCs within the filament is essential to efficiently direct bending deformation. By photopatterning the filament with segmental crosslinking density, we can induce a reversible folding/unfolding into 2D and 3D geometries triggered by deswelling/swelling in an organic solvent. Moreover, by taking advantage of the large elastic modulus of LCNs and large contrast of the modulus before and after swelling, we show that the self-folded LCP filament could act as a strong gripper.

Polymorphic Phase Formation of Liquid Crystals Distributed in Semicrystalline Polymers: An Indicator of Interlamellar and Interspherulitic Segregation.

Amorphous and melted components can segregate into the interlamellar or interspherulitic regions of polymer crystals in their blends/mixtures; this phase behavior strongly influences the physical properties and functions of materials. However, it is experimentally difficult to evaluate the spatial distributions of the other components in polymer crystals. Herein, we use a small-molecule liquid crystal (LC) as a probe and find that it forms different solid phases when mixed with the semicrystalline polymer poly(l-lactic acid) (PLLA). The LC can form the metastable phase at the lower PLLA crystallization temperature but the stable phase at the higher PLLA crystallization temperature in the PLLA/LC mixture. The formation of LC metastable and stable phases is attributed to the segregation of the LC material in the interlamellar and interspherulitic regions of polymer crystals, respectively. This study provides a potential way to evaluate the spatial segregation in the crystallization-induced microphase separation of polymer blends/mixtures.

Liquid crystalline cellulose-based nematogels.

Physical properties of composite materials can be pre-engineered by controlling their structure and composition at the mesoscale. However, approaches to achieving this are limited and rarely scalable. We introduce a new breed of self-assembled nematogels formed by an orientationally ordered network of thin cellulose nanofibers infiltrated with a thermotropic nematic fluid. The interplay between orientational ordering within the nematic network and that of the small-molecule liquid crystal around it yields a composite with highly tunable optical properties. By means of combining experimental characterization and modeling, we demonstrate submillisecond electric switching of transparency and facile responses of the composite to temperature changes. Finally, we discuss a host of potential technological uses of these self-assembled nematogel composites, ranging from smart and privacy windows to novel flexible displays.

基于液晶引流效应的上板游离态液晶盒数值计算研究

Based on the Leslie-Ericksen theory for small molecule liquid crystals, a calculation model was established for liquid crystal cells with free-state upper plates. Under the specified initial boundary conditions, the 2nd-order Runge-Kutta method and the central difference method were applied to conduct spatial-temporal discretization of the equation set. Additionally, a calculation program was compiled on MATLAB. Then, the calculation parameters were adjusted to obtain the influences of the liquid crystal cell thickness and the electric field parameters imposed at 2 ends of the cell on the liquid crystalline backflow. The results indicate that, the size of the liquid crystal director alternates with the alternation of the electric field imposed on the upper and lower plates of the liquid crystal cell. With the increment of the cell thickness, the displacement of the upper plate within a period also increases. The duty ratio of the electric field imposed at 2 ends of the cell has little impact on the upper plate speed, but has large influence on the occurring time point of the maximal upper plate speed. Compared with the experimental data, the calculated displacement values of the upper plate of the liquid crystal cell are of the same orders, and the movement loci are in good agreement.

Rigid ring-shaped particles that align in simple shear flow

AbstractMost rigid, torque-free, low-Reynolds-number, axisymmetric particles undergo a time-periodic tumbling motion in a simple shear flow, with their axes of symmetry following a set of closed Jeffery orbits. We have identified a class of rigid, ring-like particles whose axes of symmetry instead reach a permanent alignment near the velocity gradient direction with the plane of the particle aligning near the flow–vorticity plane. An asymptotic analysis for small particle aspect ratio (ratio of length parallel to the axis of symmetry to diameter perpendicular to the axis) shows that an appropriate asymmetry of the ring cross-section with a thinner outer edge and thicker inner edge leads to a tendency to rotate in a direction opposite to the vorticity; this tendency can balance the usual rotation rate associated with the finite thickness of the particle. Boundary integral computations for finite particle aspect ratios are used to determine the conditions of aspect ratio and degree of asymmetry that lead to the aligning behaviour and the final orientation of the axis of symmetry of the aligned particles. The aligning particle follows an equation of motion similar to the Leslie–Erickson equation for the director of a small-molecule nematic liquid crystal. However, whereas the alignment of the director arises from intermolecular interactions, the ring-like particle aligns solely due to its intrinsic rotational motion in a low-Reynolds-number flow.

Self-Assembly of dPS-Liquid Crystalline Diblock Copolymer in a Nematic Liquid Crystal Solvent

A diblock copolymer dPS(9.1K)-b-LCP(7.2K), the number in parentheses represents the number-averaged molecular weight) with a deuterated polystyrene (dPS) block connected to a side group liquid crystal polymer [poly(6-(4′-cyanobiphenyloxy)hexyl acrylate, LCP] was self-assembled into ∼78 Å spherical micelles with a dPS core and LCP corona in a small-molecule nematic liquid crystal solvent, 4-pentyl-4′-cyanobiphenyl (5CB). The micellar structures were examined by small angle neutron scattering (SANS) at various block copolymer concentrations and temperatures. The SANS data was analyzed further using a curve fitting method. The critical micelle concentration (CMC) was as low as ∼0.0027 wt % and the self-assembled micelles were dissociated reversibly into a unimer state at 33 °C, which is lower than the nematic to isotropic transition temperature (Tni) of 5CB (36 °C). The entropic penalty imposed on the dPS by the ordered nematic state of the LC solvent caused phase separation of the flexible dPS block to form micelles and vanished above the Tni of the LC solvent. A low CMC of ∼0.0027 wt % suggests that 5CB was a strong selective solvent for dPS-b-LCP in the nematic state. The global orientation of 5CB, produced by a magnetic field (MF, 1.2 T) revealed the structure of the dPS core of the micelle to be prolate (elongated sphere) oriented with the long axis along the applied MF direction.

A Simple Model for Non-Topological Defects in Sheared Nematic Polymer Monodomains

Nematic fluids have a propensity for orientational defects. In Leslie-Ericksen theory for small molecule liquid crystals, defects are topological in nature and easily visible in textural snapshots. However, kinetic and mesoscopic theories of nematic polymers possess a richer class of defects, including non-topological defects in which the second moment tensor of the orientational probability distribution fails to uniquely define a major director. These defects include the isotropic phase, where the major director lies anywhere on the unit sphere, and so-called oblate defect phases where the major director lies on a circle in the plane normal to a well-defined minor axis. These isotropic and oblate defect phases are independent of the physical space dimension, and while they have appeared in dynamics of heterogeneous attractors [5, 12, 13], a simple monodomain model of defect phases has not previously been developed. Here we present such a model by restricting to the in-plane monodomain response to an imposed shear flow. The orientational dynamics are described by a three-dimensional dynamical system on a compact domain. We show that the set of isotropic and oblate defect phases separates this compact orientational space into two sets of well-defined orientational distributions, one parallel and another orthogonal to the vorticity axis. We identify orbits that pass through the oblate defect set, and how often they do so. Finally, we observe intriguing behavior at the tumbling-wagging bifurcation where limit cycles pass infinitely often through the oblate defect set. 2000 Mathematics Subject Classification. 76A15, 82D60.

Liquid crystal fibers produced by using electrospinning technique

This paper investigates the electrospinning process of liquid crystalline polysiloxane with cholesterol as side chain (LCPC) and the influence on the morphology of the formed fibers by mixing LCPC solution with small-molecule liquid crystal, triethylamine, and poly(ethylene oxide)(PEO). The mechanical properties of single fibers were characterized by a novel approach. The results indicate that, under appropriate conditions, fine liquid crystal fibers can be obtained and the preferable mechanical properties can be achieved, especially after annealing. WXRD was used to investigate the orientation of polymer molecules in the formed fibers, suggesting that strong interaction exists between LCPC and PEO molecule in the resulting composite fibers, and polymer molecular tends to arrange regularly during electrospinning processing. This research work provides a new and facile method of using electrospinning to prepare liquid crystal fibers, which would be useful for designing the related high-performance materials.

Well-Defined Liquid Crystal Gels from Telechelic Polymers

Well-defined liquid crystal networks with controlled molecular weight between cross-links and cross-link functionality were prepared by "click" cross-linking of telechelic polymers produced by ring-opening metathesis polymerization (ROMP). The networks readily swell in a small molecule liquid crystal, 5CB, to form LC gels with high swelling ratios. These gels exhibit fast, reversible, and low-threshold optic switching under applied electric fields when they are unconstrained between electrodes. For a given electric field, the LC gels prepared from shorter telechelic polymers showed a reduced degree of switching than their counterparts made from longer polymer strands. The reported approach provides control over important parameters for LC networks, such as the length of the network strands between cross-links, cross-linker functionality, and mesogen density. Therefore, it allows a detailed study of relationships between molecular structure and macroscopic properties of these scientifically and technologically interesting networks.

Synergistic Ordering of Side‐Group Liquid Crystal Polymer and Small Molecule Liquid Crystal: Order and Phase Behavior of Nematic Polymer Solutions

AbstractAddition of a small‐molecule liquid crystal (5CB) to a cyanobiphenyl‐based side‐group liquid crystal polymer (SGLCP) stabilizes nematic order, increasing the isotropization temperature (TNI) more than 15 °C. Despite synergistic ordering at high concentration, small amounts of polymer destabilize nematic order. Even though TNI(SGLCP) is 27 °C greater than TNI(5CB), 2H NMR shows that the order parameter of the SGLCP is less than that of 5CB at concentrations for which monodomains were accessible (≤10 wt.‐%). The results imply that nematic order is frustrated in the bulk polymer and addition of small molecule LC relaxes this frustration by allowing greater configurational freedom. Conversely, adding small amounts of polymer to the bulk 5CB introduces frustration, resulting in the strong asymmetry of the phase diagram.magnified image

Rheology and Morphology of Smectic Liquid Crystal/Polymer Blends

Polymers are often blended with additives (e.g., polymers, clays) to produce composite materials having desirable physical properties that reflect synergy between the blend constituents. Though polymer blends with small-molecule nematic liquid crystal phases have been investigated (primarily for displays), related materials containing small-molecule smectic phases have not been reported. Guanidinium alkylbenzenesulfonates (GCnBS, where n is the number of carbons in the alkyl chain), which form stable smectic phases at elevated temperatures because of a robust two-dimensional hydrogen-bonding network of guanidinium ions and sulfonate moieties, were melt-blended with various polymers. Rheological characterization of the blends reveals a yielding behavior, characteristic of the pure GCnBS smectic phases, which becomes more pronounced with increasing GCnBS concentration. Two microstructuresone a continuous polymer phase embedded with GCnBS droplets and the other a continuous GCnBS phase with a discontinuous polymer phaseare observed, the latter at GCnBS concentrations as low as 2 wt %. The tendency of the blends to form a microstructure in which GCnBS is continuous requires the existence of a smectic phase (observed only for n ≥ 7) and is dependent on alkyl chain length and GCnBS concentration. Injection of molten blends containing smectic phases into a rectangular mold resulted in stratification of the rectangular composite plaque due to the formation of large two-dimensional crystalline lamellae upon cooling of the GCnBS smectic phase dispersed in the blend. This stratification, which can be attributed to large extensional stresses experienced by the blend during molding, depends on alkyl chain length, GCnBS concentration, and the magnitude of the stresses exerted on the molten blend. These results illustrate that smectic phases can promote unusual microstructures in polymer blends at very low concentrations, which may be useful in certain commercial applications.

Self-assembled liquid-crystalline gels designed from the bottom up.

Liquid crystals are often combined with polymers to influence the liquid crystals' orientation and mechanical properties, but at the expense of reorientation speed or uniformity of alignment. We introduce a new method to create self-assembled nematic liquid-crystal gels using an ABA triblock copolymer with a side-group liquid-crystalline midblock and liquid-crystal-phobic endblocks. In contrast to in situ polymerized networks, these physical gels are homogeneous systems with a solubilized polymer network giving them exceptional optical uniformity and well-defined crosslink density. Furthermore, the unusually high-molecular-weight polymers used allow gels to form at lower concentrations than previously accessible. This enables these gels to be aligned by surface anchoring, shear, or magnetic fields. The high content of small-molecule liquid crystal (>/=95%) allows access to a regime of fast reorientation dynamics.

Neutron reflectivity studies of the interface between a small molecule liquid crystal and a polymer

In this report, we elucidate the structure of a small molecule liquid crystal/polymer interface using specular neutron reflectivity. More specifically, we have examined the interfacial transition zone width of a small molecule liquid crystal/polymer sample as a function of increasing temperature. The interface between a thin film (≈ 1000 Å thick) of the liquid crystal 4′- n -octyl-4-cyanobiphenyl (8CB) and a thin film (≈ 800 Å thick) of deuterated poly(methyl methacrylate) (d-PMMA) is broad and broadens with increasing temperature; it was also observed that the thin film geometry influences the mixing behaviour of the 8CB/d-PMMA system. These results may have implications for current theories of liquid crystal display devices formed by the phase separation of liquid crystal polymer mixtures.

Methods for the exact construction of mesoscale spatial structures in liquid crystal polymers

We examine orientational patterns of liquid crystalline polymers (LCPs) using a mesoscale Doi theory that couples short-range, excluded-volume molecular interactions, rotary molecular diffusion, and the Marrucci–Greco potential for finite-range distortional elasticity. The model is a full tensor, polymeric generalization of small-molecule liquid crystal (Ericksen–Leslie) theory. The symmetric, traceless, rank 2 orientation tensor corresponds to micron-scale, averaged 3D microstructure through an orthogonal frame of eigenvectors (the directors, or principal optical axes) and corresponding eigenvalues (the order parameters) which convey the degrees of orientation with respect to the optical axes. These model quantities are directly related to intensity data from light scattering experiments. We focus on a classical method, separation of variables, to provide an exact construction of spatial patterns, both steady and time-dependent. Our constructions arise from posited tensor representations in spectral variables, separating spatial structure arising from optical axes variations versus order parameter variations. The reduced equations are solved to generate a variety of mesoscale structures, presented both in terms of the geometric content of the orientation tensor and in terms of the light scattering intensity patterns which would result from these exact macromolecular structures.

Flow-aligning and tumbling in small-molecule liquid crystals: pure components and mixtures

The results of an experimental study to measure the tumbling parameter, λ, for various small-molecule liquid crystals and their mixtures are presented. The methods used include textural observations (twist walls), a direct method, a rheological method, and the oscillatory method developed by Mather, Pearson, and Burghardt in 1995. The single-component results are compared with a molecular theory derived in 1995 by Archer and Larson as well as Kroger and Sellers, which predicts the temperature dependence of λ, while the results from the binary mixtures are compared to a continuum theory derived by Rey in 1996, giving the concentration dependence of λ. The results from the four experimental methods agree with each other for single-component liquid crystals, but not for mixtures. This suggests a failure of the single director Leslie-Ericksen theory to describe the rheology of liquid crystal mixtures.

Curvature driven relaxation of disclination loops in liquid crystals

Relaxation of disclination loops created during shear flow of a low molar mass and a polymer liquid crystal were monitored using a special shear stage with a videomicroscope. Loops in the polymer system generally displayed initially highly distorted contours. In the small molecule liquid crystal, the loop contours consistently exhibited very simple, generally convex shapes. In the polymer system, the complex line shape reflects the many prior loop–loop coalescence events due to the greater density of loops than in the small molecule system. Sequential images of loops were analysed to determine the velocity of the disclination loops as a function of the local curvature. Observations and simulations indicate that local disclination line curvature is a driving force in loop evolution. The reduction of regions of high loop curvature is inherently slower in the polymer liquid crystal due to the higher viscosity. In addition, the motion of the disclination contour at very high values of curvature in the polymer liquid crystal is slowed due to the presence of lower molecular weight components at the defect core which themselves must diffuse along with the line defect.

Director dynamics of uniformly aligned nematic liquid crystals in transient shear flow

We have developed a modular rheo-optical apparatus to study the flow properties of liquid crystals. Its main components are shearing device, strong magnetic field, and optical microscope. We performed experiments on well defined initial morphologies with uniform molecular alignment. The monodomains were achieved with strong magnetic fields (4.7T). Time-resolved conoscopy is the primary optical technique in our investigation. We propose a simple relation between the distribution of alignment angles over the sample thickness and the conoscopically measured angle, to quantitatively measure the alignment angle in shear flow. We followed the relaxation of a shear-induced splay deformation in small molecule model systems (N-(p-methoxybenzylidene) p-butylaniline (MBBA), pentyl-cyano-biphenyl (5 CB) and a commercially available mixture OM14244). We define a rotational director diffusivity $$D_R = \frac{{K_s }}{{\eta _s }}$$ (K s splay elastic constant. i7s splay viscosity) from the relaxation process and devised a model, based on the diffusion equation to determine their values. The director alignment behavior of the small molecule liquid crystals (SMLC's) in shear flow is well described by the two-dimensional Leslie-Ericksen model. The effect of director elasticity can clearly be seen in our experiments, resulting in a decrease of the steady state alignment angle at smaller Ericksen numbers. We found that there is no strain rate dependence of the director vorticity from 0.002/s to 2/s for poly-(γ-benzyl-D/L-glutamate) (PBG). We determined α2/α3 = −44 for a 2007o solution of 280000 molecular weight PBG in m-cresol at 20°C. The conoscopic interference pattern vanished after 8 strain units from an initially planar alignment and shearing could be reversed up to 10 strain units to completely recover the initial monodomain.

Characterization of sheared liquid crystalline polymers by light microscopy

It is widely observed that nematic and cholesteric liquid crystalline materials develop a one-dimensional periodic microstructure during and/or after a uniaxial draw or simple shear (Fig. 1). This property is common to lyotropic and thermotropic examples of both small-molecule and polymeric liquid crystals. The periodic microstructure gives rise to a banded texture between crossed polars (Fig 2).A material under load will extend more readily if the microstructure contains crimps that can be straightened, compared to the extension that is achieved if covalent backbone bonds are highly aligned along the direction of load. The microstructure in Fig. 1 therefore is regarded as a stiffness-reducing defect. Two classes of stiff polymer that are produced from lyotropic solutions do not exhibit banded textures: the highest modulus variant of poly(p-phenyleneterephthalamide) (Kevlar), and various natural silk fibers. However, a banded texture is present in the less stiff variants of Kevlar, and also in silk fibers that have been drawn by hand from natural secretions, which demonstrates that the defect is not intrinsic to liquid crystalline molecular order, but is related to processing.

Modelling of microstructure in mesophases

Abstract Small-molecule liquid crystals show textures which are readily studied at low magnification in the optical polarizing microscope. In polymeric liquid crystals, however, the textures are often much finer, taxing the microscope's resolution. Nevertheless, studies of microstructure in such polymers have been made and it is apparent that they can differ widely both from small-molecule liquid crystals and, indeed, from polymer to polymer. This paper sets out to account for these variations by exploring the effect on microstructure of the marked differences between the magnitudes of the splay, twist and bend elastic constants which are a characteristic of many liquid crystalline polymers. We report a computer model which simulates the development of microstructure for different ratios of the elastic constants. When these are approximately equal, textures characteristic of small-molecule liquid crystals result, such as those involving escape into the third dimension with the degeneration of line defects into points. When the splay energy is high in comparison with bend and twist, as is the case for many thermotropic liquid crystalline copolyesters, escape does not occur and half integral disclination lines predominate. For simulations involving planar boundary conditions, layered microstructures result, frequently with little matching of the orientation from layer to layer. Within the layers the trajectory of the orienting units is sinuous. This simulated microstructure resembles textures observed in thermotropic copolyesters, studied both in this laboratory and elsewhere. The computer model uses a lattice approach which is similar in some respects to that developed by Lebwohl and Lasher. It should not be thought of as a molecular scale model, however, but rather as one based on assemblies of molecules which share a common director.

Smectic A–cholesteric transition in a side chain cooligomer–CE1 blend. A particular confined geometry for the TGB phase?

Abstract The blend of an industrial cooligomer side chain cholesteric material with a polar small molecule liquid crystal of reversed chirality induces a low temperature smectic A phase [1]. We present here the change in the textural characteristics at the SA-Ch transition observed by optical microscopy between untreated glass slides: particular fingers, different from those recently described [2], are obtained for a slow increase of temperature, starting from an unperturbed spontaneous homeotropic SA domain. These fingers, coexisting with large SA domains, are understandable as 180° Bloch walls introducing progressively the helicity in the medium. Over a large temperature domain, these chiral fingers coexist in an apparent thermodynamic equilibrium with the normal SA phase, suggesting the occurrence of an analogue [3,4] of the Shubnikov superconducting phase: probably due to the presence of the glass slides, the geometry of the screw dislocations, comparable to the vortex of type 2 superconductors, is different from the recently described case [5] of the twisted grain boundary phases of the A type.