Low Molecular Liquid Crystals Research Articles

Orientation and Dynamics of ZnO Nanorod Liquid Crystals in Electric Fields

ZnO nanorod polymer hybrids (i.e., ZnO nanorods coated with a block copolymer with a short anchor block (dopamine) and a longer solubilizing block of polystyrene (PS)) form liquid crystalline (LC) phases if they are dispersed at high concentration e.g., in a PS oligomer matrix. Due to the high mobility of the low T(g) -matrix the nanorod polymer hybrids show a switching behavior under an applied AC electric field. Hence, the orientation of the nanorod mesogens can be changed from planar (parallel to the substrate) to homeotropic (perpendicular) in full analogy to the switching of low molecular liquid crystals in an electric field. Dielectric measurements show that such a switching is mainly due to the cooperative LC behavior, because the rods themselves exhibit only a very small effective dipole moment. The process can be investigated by polarizing microscopy. SEM images show the orientations of the individual nanorods, which correspond to the Fredericks transition well known for liquid crystals aligned in an electric field. This was the first time such a transition could be visualized by electron microscopy due to the large nanorod mesogens. The observation is interesting to orient nanorods perpendicular to an electrode and can help to improve optoelectronic devices.

Influence of alkylene spacer length on photoinduced molecular reorientation and LC alignment behavior in photo-cross-linkable liquid crystalline polymeric films with H-bonded cinnamic acid side groups

Photoinduced reorientation of liquid crystalline polymethacrylates comprised of various lengths of alkylene spacers terminated with 4-oxycinnamic acid in the side chain and low-molecular liquid crystal (LC) alignment on the resultant photoreacted films were investigated using linearly polarized UV light. As the length of the spacer increased, the photoinduced optical anisotropy (Δ A) of the thin films increased, and Δ A increased as the irradiating temperature increased. Exposing the polymeric films in the LC temperature range of the material generated an in-plane molecular reorientation because the small photoinduced Δ A was simultaneously amplified. The low-molecular LCs aligned homogeneously on the photoreacted polymeric films, but the LC alignment direction depended on the alkylene spacer length and the degree of the photoreaction.

Photoinduced Cooperative Reorientation in Photoreactive Hydrogen-Bonded Copolymer Films and LC Alignment Using the Resultant Films

Polymethacrylate copolymers, which have hexamethylene spacer groups terminated with 4-oxycinnamic acid, 4-oxybenzoic acid, and 4-(4′-oxyphenyl)benzoic acid in the side chain, were synthesized, and the photoinduced cooperative molecular reorientation of thin films was demonstrated using linearly polarized ultraviolet (LPUV) light and subsequent annealing. Adjusting the exposure doses can control the molecular reorientation direction both perpendicular and parallel to the polarization E of LPUV light. Tuning the copolymer composition adjusted the optical birefringence of homogeneously reoriented films between 0.095 and 0.178. Finally, adjusting the exposure doses achieved uniform alignment control of low-molecular liquid crystals (LCs) on the molecular reoriented copolymer films both perpendicular and parallel to the polarization E of LPUV light.

UV tuning of the electro‐optical and morphology properties in polymer‐dispersed liquid crystals

Polymer‐dispersed liquid crystal (PDLC) films operating in reverse mode are transparent electro‐optical devices, which can be turned into an opaque state by application of a suitable electric field. The effect was investigated of different UV powers, used during the polymerization process, on the electro‐optical and morphology properties of PDLCs, working in reverse mode operation. Films were obtained by UV polymerization of mixtures of a low molecular weight nematic liquid crystal and a photopolymerizable liquid crystal monomer, homeotropically aligned by rough conductive surfaces. The electro‐optical and morphology properties of samples were related to the polymerization conditions. Samples polymerized by lower UV powers exhibited “polymer ball” morphology and an electro‐optical response due to the liquid crystal director reorientation, whereas samples obtained at higher UV powers showed a “Swiss cheese” morphology and an electro‐optical response due to dynamic scattering. In addition, we observed by conductivity and IR measurements that UV exposure induces a degradation of the nematic liquid crystal.

ER EFFECT OF LOW MOLECULAR LIQUID CRYSTAL ON ONE-SIDED PATTERNED ELECTRODES

Several kinds of ER fluids (ERF) have been developed and have been applied to some mechatronics devices and processing technologies. In many conventional applications of ERFs, these devices consist of bilateral electrodes to apply electric field in ERF. However, the electric field of several kV/mm may be necessary to generate an ER effect sufficiently for practical purposes. The gap between a pair of electrodes should be, therefore, maintained narrowly and exactly for fears of short-circuit. At the same time, this electrode system also requires an interconnection on driving parts. To improve these disadvantages, we proposed "one-sided patterned electrode" (OSPE) systems in previous works. In this report, we confirmed the flow characteristics of low molecular liquid crystal (LMLC) on OSPE. Next, we also confirmed the different characteristics depending on the pattern type. Depending on results of electro-static analysis, we conclude that such a difference may results from the directors of LC molecules derived by electric field.

Monodomain liquid crystal elastomers and elastomeric gels: Improved thermomechanical responses and phase behaviour by addition of low molecular weight LCs

Large monodomain-liquid crystal elastomer films have been prepared using mechanical alignment, either as pure materials or as gels doped with low molecular weight chiral liquid crystals. The thermomechanical strain behaviour of the resulting materials has been studied over a range of temperatures. The largest thermal strains have been achieved when the mechanical alignment was carried out in the smectic A phase, just below the transition to the apparent isotropic liquid phase. The pure elastomers exhibit thermomechanical responses over broad temperature ranges (ca. 60 °C), however the elastomer gels doped with a low molecular weight LC exhibit significantly sharper thermomechanical responses over a narrower temperature range (ca. 15 °C).

The synthesis of new liquid–crystalline mesogens containing bicyclohexane units

The synthesis and mesomorphism characterization of new thermotropic derivatives of cis, trans and trans, trans-4,4′-bicyclohexyl-bis[4-(6-hydroxyhexan-1-oxy)benzoate] are described. This last intermediate produces a liquid crystalline diacrylate, with lower refractive indices than the well-known aromatic analogues, that allows to obtain polymeric network via UV photopolymerization. The acrylate is very useful to optimize the contrast of polymer dispersed liquid crystals (PDLCs). Moreover, the acylation of the intermediates previously mentioned, with alkanoic or perfluoro alkanoic acids give low-molecular liquid crystals that exhibit a broad temperature range of mesophases.

Relaxation of a liquid-crystalline order induced by an electric field

The influence of a strong electric field on isotropic melts of 6-cyanobiphenyl and comb-shaped polyacrylate with mesogenic groups in its side chains is investigated. It is established that the electric field induces the isotropic liquid-nematic liquid crystal phase transition in melts of these compounds. A relaxation process was discovered that destroys the nematic ordering induced in substances by electric fields. It was established that, for polymer liquid crystals, the time of transition from the ordered state to the isotropic phase is several orders longer then that for low-molecular liquid crystal.

New cellulose derivatives composites for electro-optical sensors

In this work we used hydroxypropylcellulose (HPC) and acetoxypropylcellulose (APC) to produce free standing solid films (∼60 μm) for electro-optical devices. Cellulose derivatives films were prepared from crosslinked both HPC and APC isotropic and liquid crystalline solutions without and with a low molecular weight nematic liquid crystal mixture (LC) E7. Films prepared from anisotropic solutions presented a band texture and E7 droplets of micron and submicron size were found to coexist with the band texture in the films prepared with E7. The optical cells were composed by the cellulose derivatives films covered on both free surfaces by a layer of the LC E7 and placed between two transparent conducting substrates. All the electro-optical cells prepared showed switching times in the range of some milliseconds to hundreds of milliseconds. The on-voltage ( V on) is lowered for cells prepared with the ester derivative film obtained from isotropic solutions. The maximum transmission values show a clearly tendency to decrease for the cellulosic matrix prepared from anisotropic solutions with and without nematic liquid crystal droplets.

Phase Separation in Binary Polymer/Liquid Crystal Mixtures: Network Breaking and Domain Growth by Coalescence-induced Coalescence

A small-angle light scattering (SALS) technique together with optical microscopy observation are used to investigate phase separation kinetics in films of low molecular weight thermotropic liquid crystal (4-cyano-4'-n-octyl-biphenyl, 8CB) with flexible polymer (polystyrene, PS). The growth of domains is studied as a function of time, film thickness, and film composition. The light scattering results are correlated with the images obtained by optical microscopy observation. In this paper, we study the breaking of a bicontinuous network of polymer in liquid crystal into droplets and their further growth via the coalescence-induced coalescence mechanism. The appearance of droplets in the system leads to a strong scattering at small wave vectors, while the bicontinuous network gives a peak at a nonzero wave vector. Superposition of these scattering intensities leads to the appearance of a second peak in the full scattering intensity signal, when the bicontinuous network starts to break up into disjointed elongated domains. Finally, both peaks merge into a single peak, which moves quickly toward zero wave vectors, indicating a complete transformation of elongated domains into spherical droplets of variable size. We found that the separation process does not depend on the size of the system. Irrespective of the sample thickness, the network breaks into fragments always at the same time after temperature quench. On the basis of morphological analysis, we found that the average size of the droplets which formed from the network grows with time, t, as t(alpha), alpha = 0.9 +/- 0.1, in the isotropic phase and in the nematic phase.

Molecular Dynamic Simulation of Side‐Chain Liquid Crystalline Elastomer

AbstractSummary: Molecular dynamic simulation of side chain liquid crystalline elastomer has been carried out. As an initial state a flexible polymer network in a low molecular liquid‐crystal (LC) solvent was used. The LC solvent comprises of anisotropic rod‐like semiflexible linear molecules (mesogens) composed of particles bonded into the chain by FENE potential. Rigidity of LC molecules was induced by a bending potential. All interactions between nonbonded particles are described by a repulsive Lennard‐Jones potential. For the systems with different values of density and order parameter obtained after sufficiently long trajectory the attachment of ends of mesogens to the polymer network was simulated. The kinetic of the process of mesogens attachment to network was studied as well as morphology of attachment. The structural and dynamical behaviour of side chain LC elastomer was studied and compared with systems of polymer network in low molecular LC solvent.

ER Effect of Low Molecular Liquid Crystal on One-sided Patterned Electrodes

ER fluid (Electro-rheological fluid) is a kind of functional fluids whose apparent viscosity changes under an electric field. Recently many researches on this material are developed for applications with clutches, brakes, dampers and other kind of mechatronics technologies. The ER fluids are classified into two types, particle-type ER fluid and homogeneous-type ER fluid. The homogeneous-type ER fluids include low molecular liquid crystal (LC). In this report, the flow characteristics of LC on one-sided patterned electrodes were confirmed. And next it was also confirmed that such effects have different characteristics depending on pattern type. According to a result of electro-static analysis and Miesowicz viscosity model, we conclude that such difference results from a director of LC molecules derived by electric field.

Temperature-Sensitive Permeation of Methimazole through Cyano-biphenyl Liquid Crystals Embedded in Cellulose Nitrate Membranes

ABSTRACT In this study, the application of thermotropic liquid crystals (TLCs) embedded in cellulose nitrate membranes as binary thermosensitive drug barriers in response to temperature changes is described. Liquid crystals are physically observed to flow like liquids, but they have some properties of crystalline solids. Two kinds of low-molecular nematic liquid crystals, n-pentyl-cyanobiphenyl (K15) and n-heptyl-cyanobiphenyl (K21), with nematic to isotropic phase-transition temperatures (T n-i) of 36.3°C and 43.3°C, respectively, were chosen to modulate drug release. Triple-layer membranes (TLMs), composed of K15 and K21 as TLCs sandwiched between two layers of cellulose-nitrate (CN) films, were prepared. No TLC leakage was observed through this hydrophilic membrane. TLMs successfully acted as rate-controlling systems, which may also be regarded as time-controlling systems. The transport of methimazole as a low-molecular-weight drug model through the membranes was examined. It was found that changing the environmental temperature below and above the T n-i of TLCs could modulate methimazole permeation through these composite membranes. These experiments were also repeated with thermal cycling between 30°C and 46°C. The permeation profiles were reversible and followed zero-order kinetics.

Transient Shear Stress Response After Flow Start-up and Flow Reversal in Low Molecular Weight Nematic Liquid Crystals

ABSTRACT Within the framework of the Leslie–Ericksen theory we develop a unified model for the interpretation the complex shear stress response of tumbling nematics after flow start-up and flow reversal. The model accounts quantitatively for: (i) the damping of the oscillations, the vanishing of the doublet structure and eventually a pronounced decay after flow start-up, and (ii) the progressive reappearance of the oscillations after flow reversal (echo phenomenon). Within this model the loss of director coherence, induced partially by noise and partially by a distribution of velocity gradients, is a significant additional cause of damping. A primary cause (but not sufficient) is the director drift towards the vorticity axis as pointed out by other authors. On the other hand, the reappearance of the oscillation is related to reversibility dictated by the mechanical Leslie–Ericksen equations combined with the velocity gradient distribution.

Photo-alignment Properties of Main-Chain Liquid-Crystalline Polyimides Containing Chalcone Derivative

We investigated photo-alignment properties of main-chain polyimides prepared from acid anhydrides [4,4′-(hexafluoro-isopropylidene) diphthalic anhydride (6FDA) or 4,4′-biphthalic anhydride (BPDA)] and diamine derivatives containing a chalcone group. The polyimides in the film state were irradiated with linearly polarized UV (LPUV) light under different conditions. The liquid-crystalline (LC) cell was fabricated from the polyimide film irradiated with LPUV light. A mixture of a low molecular liquid crystal (4-cyano-4′-n-pentylbiphenyl: 5CB) and a dichroic dye was filled in the LC cell. 5CB in the LC cell was easily aligned perpendicular to the electric vector of the incident LPUV light.

Molecular dynamics simulation of a flexible polymer network in a liquid crystalline solvent; dynamical properties

Molecular dynamics simulation was performed for the systems consisting of a flexible regular tetrafunctional polymer network and a low molecular liquid crystal (LC) solvent. The LC solvent comprises of anisotropic rod-like semiflexible linear molecules composed of beads bonded by a FENE potential. Rigidity was induced by a bending potential, proportional to the cosine of the angle between neighbouring valence bonds. All interactions between non-bonded beads are described by the repulsive part of the Lennard–Jones potential. For comparison the simulations of the system of flexible polymer chains in a low molecular LC solvent and a system of pure low molecular LC solvent were also carried out. The influence of the network and linear chain polymer on the translational and rotational mobility of low molecular LC solvent was studied. The influence of the LC solvent ordering on the local translational mobility of the polymer chains was also observed.

Correlation between photorefractivity, photoconductivity, and dark conductivity in dye-doped, low molecular mass nematic liquid crystal cells

Abstract The photoconductivity and photorefractivity of dye-doped low molecular mass nematic liquid crystal homeotropic cells exhibit a threshold at an applied voltage of the order of 1 V (in our samples). At about the same voltage, the dark conductivity of the cells exhibits a change from a cubic current/voltage characteristic to a linear one. We explain these observations by postulating that the cell's conductivity is due to ionic charge carriers generated near the electrodes by some electrochemical processes activated by the injection of electrons and holes. Below the noted threshold, the dark current is limited by a residual ionic space–charge distribution, and grows as the cube of the applied voltage. In this regime, photogenerated charge carriers are screened and are prevented from forming the static space–charge field needed to initiate an orientational photorefractive response. Above that threshold, the residual ionic space–charge disappears, the cell is a weak ohmic electrolyte, and the photogenerated charges contribute to the photoconductivity and photorefractivity of the cell.

Organization of a Polar Molecule at the Air−Water Interface

We have studied the behavior of a low molecular mass organosiloxane ferroelectric liquid crystal at the air−water interface. Data obtained with Langmuir isotherms, surface potential measurements, and Brewster angle microscopy are presented and discussed. The compound forms films on the Langmuir trough, with phase transitions to multilayers upon compression. As seen with Brewster angle microscopy, the monolayers are optically isotropic whereas striking birefringent structures develop in the multilayer films. These periodic structures result from a surface elasticity phenomenon.

Molecular dynamics simulation of a flexible polymer network in a liquid crystal solvent; structure and equilibrium properties

A flexible regular tetrafunctional polymer network containing a low molecular liquid crystal (LC) solvent was simulated with molecular dynamics. The LC solvent comprises of anisotropic rod-like semi-flexible linear molecules composed of beads bonded by a FENE potential. Flexibility was induced by a bending potential proportional to the cosine of the angle between neighbouring valence bonds. All interactions between non-bonded beads are described by the repulsive part of the Lennard–Jones potential. The average length of the network chain was chosen to be close to the length of a mesogen. The number of network cells was constant and the simulated systems differ from each other by the number of LC layers. The simulations of a system of flexible polymer chains in a low molecular LC solvent and a system of pure low molecular LC solvent were also carried out. Increasing the density of the composite system the LC solvent experiences the same phase transition as the pure LC: isotropic, nematic and smectic. The presence of the network shifts the isotropic–nematic transition to higher densities but does not significantly change the position of nematic–smectic transition. Transition of the LC solvent into the smectic state changes the morphology of the network. The periodicity of LC phase determines the number of network layers. The presence of linear chains in the LC solvent decreases the number of LC layers in the smectic phase. The LC order induces some stretching of the network chains along the direction of orientation and at the same time causes shrinkage in the perpendicular direction especially in the smectic phase.

Liquid Crystal Influenced Self-assembly in Mesogenic Polymer and Liquid Crystal Blends

ABSTRACTIn this work, we found that by performing photo-polymerization of the mesogenic monomer RM257 in liquid crystals, well organized polymer walls were formed through out the cell by self-assembly of the polymer within the anisotropic host – the liquid crystal. The polymerization conditions, which were parameterized as UV intensity, photo-reactivity (characterized by photo initiator concentration), curing temperature, monomer concentration and mesophase of the liquid crystal host, were systematically varied. Different liquid crystal host also raised some difference in polymer network. We believe this kind of morphology comes from spinodal decomposition and the anisotropic elastic property of the liquid crystal host. It is found that for RM257 and low molecular weight nematic liquid crystals a monomer concentration of 5% is sufficient to use liquid crystal hosts to work as templates for well structured polymer walls.