The effect of voltage controlled orientation order of liquid crystals in non-acrylic polymer dispersed liquid crystals films

The nematic liquid crystals (LCs) are randomly dispersed material with random orientation order in polymer dispersed liquid crystal (PDLC) films. The LCs change their orientation from random to vertical as electric field is applied. This transformation of orientation order of nematic liquid crystals in the PDLC films is controlled by many factors operating simultaneously. For instance, some factors like the internal forces of attractions among the neighboring LC molecules, anchoring with polymeric matrix, ITO glass boundaries, and chemical structures of the materials are less studied. The learning of extent of vertical orientation of liquid crystal droplets in an electric field is essential to attain optimum electro optical properties of PDLCs. In this finding, bipolar and radial LCs droplets with random orientation have been observed in non-acrylic polymeric media. It is learned that with small increase of contents of external material, the extent of vertical orientation has been varied intensely. The extent of vertical orientation of LCs molecules increases as the contents of external non-acrylic polymeric material decreased. For this study, the orientations of LCs with respect to material type/contents, external applied force, and restoration of electric filed as hysteresis have been studied in details.     

Computer simulation of liquid crystals

Summary We review recent progress in the computer simulation of liquid crystals, with special emphasis on hard particle models. Surprisingly, the simplest molecular models, taking account only of molecular size and shape, are sufficient to generate a wide variety of liquid crystalline phases, closely analogous to those observed in real life. Thermodynamic stability of different phases is very sensitive to shape, and presumably will also be sensitive to further details of intermolecular interactions as they are incorporated into the model. Realistic atom-atom potential models of liquid crystals are available, but the associated simulations are quite expensive. Thus, while idealized models may be used to study quite general, fundamental properties of mesophases, the modelling of specific liquid crystal systems in a realistic way remains a great challenge. Progress continues to be made on both these fronts.     

偶氮型液晶高分子的研究及应用

偶氮液晶高分子由于其具有特殊的光致异构和光致变色等光学性质,在光信息贮存材料、光开关材料和非线性光学设备材料等领域具有巨大的应用潜力,近年来得到了国内外研究者的广泛关注。本文概述了偶氮型液晶高分子的结构特征,详细介绍了主链型偶氮液晶高分子、侧链型偶氮液晶高分子、树枝状偶氮液晶高分子以及星型偶氮液晶高分子的研究现状及在光储存、光开关及光调制偏振片等方面的应用进展,指出了存在的问题和研究方向。     

Switchable anti-peeping film for liquid crystal displays from polymer dispersed liquid crystals

Liquid crystals displays (LCDs) currently dominate the display market, wherein a wide viewing angle is considered as one of the most important characteristics. However, for LCDs with wide viewing angles, some private information inevitably becomes more visible; thus, an LCD with a switchable viewing angle has attracted greater interest. Here, we report a novel switchable viewing angle film that can make the viewing angle of an LCD electrically switchable between ±30° and ±60°, i.e. between an anti-peeping mode and a share mode, by 5.0 V is turned on and off, respectively. The response time necessary to change between the modes is in milliseconds. It is believed that it has potential applications in LCDs with high quality.     

Monte Carlo simulations of a liquid crystal copolymer in the solid state

The condensed phase of the alternating copolyester of p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) is investigated by studying the room temperature packing arrangement of the copolymer chains. A molecular modeling methodology is employed with a Monte Carlo sampling of the configurational phase space. Realistic poly(HBA-alt-HNA) polymer chains are represented by an explicit atom representation of the HBA/HNA dimers. States are sampled from the NVT ensemble using a sampling scheme consisting of (1) valence and torsional variations, (2) rigid body rotations of the chain about the chain axis, and (3) rigid body translations of the chain. The effect of chain packing on the conformation of chains, as well as the relative intra- and intermolecular orientations of aromatic rings, is investigated. Correlation of chain positioning along the chain axis is dominated by aromatic rings maintaining a center-to-center plane of registry. These layers of aromatic units pack with a preference for edge-to-face orientations in a herringbone-type pattern and have an intermolecular ring angle between the pairs of aromatic rings in the unit cell that is ca. 68°. The aromatic rings, on average, are rotated 38° out from the b–c plane. The phenylene rings of these copolyesters are less restricted in their relative orientation in comparison to the naphthalene rings. Intramolecular orientational probability density distributions indicate a preference for staggering the successive aromatic rings along the chain, with a staggering angle of ca. 66°. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 727–741, 1998     

Effects of the fluorinated liquid crystal molecules on the electro-optical properties of polymer dispersed liquid crystal films

The different fluorinated liquid crystal (LC) molecules doped to E8 were used as LC component to prepare polymer dispersed liquid crystal (PDLC) films. The mass fraction of the LC mixture is fixed 50.0 wt%. Results indicate that doping 8.0 wt% fluorinated LC molecule ME3CP to E8 significantly reduced the driving voltage of the PDLC films, and the driving voltage reduced with the rise of mass fraction of ME3CP. Besides, the terminal flexible chain length of the fluorinated LC molecule influenced the LC mixture properties based on E8, such as the dielectric anisotropy, birefringence and viscosity of the LC mixture, and the morphology and the electro-optical properties of PDLC films were controlled not only by the physical properties of the LC mixture, but also by the terminal flexible chain length of the fluorinated LC molecule .     

Exploitation of orientation of liquid crystals 5CB and DSCG near surfaces to detect low protein concentration

We report the alignment of 4-cyano-4-pentylbiphenyl (5CB), a thermotropic liquid crystal (LC), and disodium chromo glycate (DSCG), a lyotropic LC, on the hydrophobic head group containing dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP) and hydrophilic amine head group. Based on the texture observed using a polarised microscope for modified surfaces, we propose the alignment of the LCs on the modified surfaces. It is further supported by Fourier transform infra-red spectroscopy (FTIR) data of the corresponding system. Next, the texture under a polarised microscope for the LCs on protein bovine serum albumin adsorbed over the amine and DMOAP surfaces was observed. The difference in texture of LCs over the proteins depends on its concentration in solution. The difference is due to the amount of adsorbed proteins present on the surface. Above a critical amount of protein at the surface, the LCs show homeotropic arrangement.     

Restricted polymer-stabilised electrically suppressed helix ferroelectric liquid crystals

The electrically suppressed helix ferroelectric liquid crystal (ESHFLC) electro-optical mode is characterised by the high contrast and fast response time at low driving voltage. However, the ESHFLC demands several constraints on the alignment layer and the cell gap. However, due to the fundamental characteristics of the ferroelectric liquid crystals, the cone angle decreases at higher temperatures, which is the biggest hurdle for this electro-optical mode to find the real application. In this article, we disclose a restricted polymer stabilisation (with very small polymer concentration in the mixture) for the ESHFLC without pushing other vital parameters out of the range of the constraints for ESHFLCs. The restricted polymer-stabilised ESHFLC shows large temperature range with stable high contrast, fast response time and low driving voltage, thus, it could find application in a range of the modern display and photonic devices.     

Monocyclic rod-type liquid crystals; 2,5-dialkanoyloxytropones and 5-alkanoyloxy-2-alkoxytropones

Two kinds of monocyclic troponoid mesogens, 2,5-dialkanoyloxytropones (4) and 5-alkanoyloxy2-alkoxytropones (5), were prepared. The former showed monotropic smectic A phases and the virtual isotropic liquid-smectic A transitions of the latter were determined by extrapolation of results in a binary phase diagram. Comparing the mesogenic properties between the tropones 4 and the 2-alkanoyloxy-5-alkoxytropones (1), the alkanoyloxy group at C-5 enhances both the melting points and the transition temperatures of the smectic A phases. From the comparison between 5 and 1, the alkanoyloxy group at C-2 lowers the melting points.     

High-pressure phase transitions in liquid crystals

Simultaneous measurements of the rate of heat evolution and changes of the mechanical variable of a transformation such as volume or pressure, performed in a p-V-T controlled scanning calorimeter have been applied to investigations of phase transitions in liquid crystals. In the instrument, the phase transitions can be induced by a controlled change of pressure, volume or temperature under isothermal, isobaric or isochoric conditions respectively. The present investigations have ben performed on 4-n-penthyl-penthylthiol-4-decycloxybenzoate which demonstrates in the liquid crystal state a nematic and three smectic phases

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Zusammenfassung In einem p-V-T-kontrollierten Scanning Kalorimeter wurden zur Untersuchung der Phasenumwandlungen in Flüssigkeitskristallen simultane Messungen der Geschwindigkeit der Wärmeentwicklung und der Veränderungen von mechanischen Größen von Umwandlungen, wie z.B. von Volumen oder Druck durchgeführt. In diesem Gerät kann die Phasenumwandlung durch eine kontrollierte Veränderung von Druck, Volumen oder Temperatur unter isothermen, isobaren oder isochoren Bedingungen ausgelöst werden. Vorliegende Untersuchungen wurden an 4-n-Pentyl-phenylthiol-4-dezyloxybenzoat durchgeführt, welches im Flüssigkristallzustand eine nematische und drei smektische Phasen aufweist.

     

Chiral banana liquid crystals derived from sugars

This paper describes the synthesis and physical properties of a chiral bent-core liquid crystal containing the chiral non-aromatic central ring system 1,5-anhydro-2-desoxy-D-arabino-hexitol. The preparation involved a stepwise construction of the mesogenic side chains via a selective diesterification of glucal at positions three and six with 4-O-acetylbenzoyl chloride followed by selective deacetylation. Repeated esterification of the aromatic hydroxy group with a substituted benzoic acid and hydrogenation of the double bond resulted in the desired product. To our knowledge, this is the first banana shaped liquid crystal synthesized containing a sugar derivative as a chiral core. Investigation of the mesogenic properties and electro-optic experiments showed traditional calamitic phases along with banana phases.     

Coumarin-based emissive liquid crystals

Calamitic luminophores are presented based on an elongated aryl-substituted coumarin core. The substitution pattern at terminal and lateral positions were systematically modified. The new compounds exhibit smectic C and/or nematic liquid crystalline phases. The chromophores show fluorescence in the blue spectral region. The benzopyranones are miscible with acrylate-substituted reactive nematic mesogens. Uniaxial orientation of the mixed systems was achieved by surface-assisted alignment on polyimide orientation layers. The film anisotropy was stabilised by subsequent photocrosslinking. The crosslinked oriented films display linear polarised photoluminescence upon isotropic excitation.     

An electrically light-transmittance-switchable film with a low driving voltage based on liquid crystal/polymer composites

ABSTRACT

As a typical class of electrically light-transmittance-switchable (ELTS) composites materials, polymer dispersed liquid crystal (PDLC) films have been widely used in displays, smart windows, light shutters, etc. However, the commercialised PDLC film still requires a comparatively high voltage to maintain its transparent state, leading to huge power consumption and even a potential safety risk. In this regard, we proposed a ‘heat followed UV’ stepwise polymerisation strategy for preparing a kind of ELTS film with a low driving voltage (~20.7 V) through constructing a coexistent system of polymer dispersed and polymer stabilised liquid crystal (PD&SLC). In this new PD&SLC system, vertically orientated polymer networks were formed within LC domains to induce the vertical alignment of LC, thereby reducing the driving voltage. Also, the as-made PD&SLC film exhibited good flexibility due to the high content of polymer. Moreover, the effects of the liquid crystalline polymerisable monomers content on the polymer morphologies as well as the electro-optical properties of the as-made PD&SLC films were elaborately investigated.     

Fabrication of flexible light shutter using liquid crystals with polymer structure

We fabricated a light shutter using plastic substrates for high visibility of a flexible see-through display. To achieve a flexible light shutter using liquid crystals (LC), it is essential to maintain the cell gap when the light shutter is bent. We studied methods to fabricate flexible LC light shutters using plastic substrates. We demonstrated light shutters that are initially transparent and flexible with or without polymer walls. We have elucidated that polymer walls and networks provide mechanical stability against the bending of an LC light shutter without any degradation in the electro-optic characteristics. We predict that a flexible light shutter provides not only high visibility but also mechanical stability to a flexible see-through display by positioning it at the back of a flexible see-through display panel.     

Synthesis and properties of reactive liquid crystal monomers and side-chain liquid crystalline polymers

Two nematic liquid crystal (LC) monomers containing double bonds in the side chain were designed and synthesised. Length of the side groups varied from 1 to 2 methylene units. The side-chain polymers were synthesised by hydrosilylation reaction. The molecular structures of the intermediates and the LC monomers were characterised by Fourier transform infrared, elemental analysis and nuclear magnetic resonance spectroscopy. The thermal phase behaviour of the monomers and polymers were investigated by differential scanning calorimetry and polar optical microscopy coupled with hot stage. The LC monomers showed only one nematic mesophase in the cooling process. And, the two polymers exhibit an enantiotroppic nematic mesophase either in the heating or in the cooling process.     

New developments in the dye-doped polymer dispersed liquid crystals gratings: A review

In this study, the dye-doped polymer dispersed liquid crystals (PDLC) gratings techniques performed by the various research groups or being developed are briefly reviewed. Especially, the electrically switched diffraction and holographic gratings, have attained much attention by various research groups working in the PDLC-related display studies. The fabrication methodologies used for such grating texture, include like the conventional dye-doped PDLC grating, Azo-dye doped PDLC gratings, and lasing techniques etc., adopted for dye-doped PDLC gratings. The useful features and characteristics of their fabrication process of such gratings are discussed. Finally, some of the future perspectives on this particular research field are presented.     

Molecular composite. II. Novel block copolymer and semi-interpenetrating polymer network of rigid polyamide and flexible polyimide

2,2'-Dimethyl-4,4'-diamino-biphenyl was used to prepare a lyotropic amine-terminated polyamide (PBTA) prepolymer with terephthaloyl dichloride, and an amide oligomer with reactive endcaps of maleimide, respectively. The former was employed in an attempt to increase the solubility of PA/PI block copolymers, and the latter to produce a three-dimensional (isotropic) reinforced molecular composite through the technology of a semiinterpenetrating polymer network (semi-IPN). PBTA prepolymer was copolymerized with an amine-terminated polyimide via a coupling reaction using terephthaloyl dichloride. The resultant block copolymers were characterized by IR, NMR, η_inh and GPC to confirm that the copolymerization reaction was precisely accomplished, as well as to determine their composition and molecular weight. Thermal analysis and lyotropic behavior were also studied for these copolymers. Rigid-rod amide oligomer was crosslinked in the presence of another flexible polyimide. Five sets of semi-IPN films were prepared under varying compositions. The curing conditions were investigated by DSC; in addition, the cured semi-IPNs exhibited a single phase (1 T_g) according to examination by an instrument for thermally stimulated current (TSC). No large-scale phase separation was observed in SEM images of the fractured surfaces. © 1994 John Wiley & Sons, Inc.     

A study on the polymer structures and electro-optical properties of epoxy-mercaptan-based polymer dispersed liquid crystal films

ABSTRACT

In this paper, polymer dispersed liquid crystal (PDLC) films based on epoxy-mercaptan system were prepared by thermal-initiated polymerization. The effects of the liquid crystal (LC) content, the proportion and the functionality of epoxy monomers on the polymer structures and electro-optical properties of the as-made PDLC films were investigated systematically. It was found that the morphologies of the polymer matrix can be altered from polymer meshes to polymer balls by increasing the LC content as well as the functionality of epoxy monomers. Accordingly, the electro-optical properties could be regulated by the morphologies of polymer networks. Especially, the as-made PDLC films with homogeneous porous structures exhibited the optimal electro-optical properties. Consequently, this work offers a meaningful approach to control the microstructures and optimize the electro-optical properties of PDLC films, which indeed can form a wonderful footstone for the wide application of PDLC.     

The physical properties of alkene-terminated liquid crystal molecules/E8 mixture and the electro-optical properties as they doped in polymer-dispersed liquid crystal systems

Effects of the content of fluorinated alkene-terminated liquid crystal (LC) molecules on the physical properties of the fluorinated alkene-terminated LC/E8 mixture were studied. The morphology and electro-optical properties as they doped in polymer-dispersed liquid crystal (PDLC) films were investigated. The detailed discussion of the obtained results is given. As a result, comparing with the physical properties of the series of LC mixtures with the same content of the analogous fully saturated compounds doped with E8, we find that the birefringence is significantly larger for the LC mixture with the alkene-terminated materials. Both fluorinated alkene-terminated LC molecules and the analogous fully saturated compounds doped with E8 reduce the driving voltage of PDLC films. Moreover, PDLC films with the fluorinated alkene-terminated LC molecules possessed higher contrast ratio and faster response time than that of the PDLC films prepared by adding the same mass fraction of the analogous fully saturated compounds. Thus, the ability to manipulate physical properties of LC mixture and electro-optical properties of PDLC films by changing the LC molecular structures may have future relevance for new LC structures design and applications of PDLC films.     

Network morphology and electro-optical characterisations of epoxy-based polymer stabilised liquid crystals

ABSTRACT

In this paper, by utilising the photoinitiated cationic polymerisation of liquid crystalline epoxide in LC solvents, the epoxy-based polymer stabilised liquid crystals (EPSLCs) were fabricated. The effects of the liquid crystalline epoxide content, UV intensity, polymerisation temperature, and initiator content on the network morphology of liquid crystalline epoxy polymers (LCEPs) and electro-optical characterisations of EPSLC samples were studied. The results suggested that the preparation conditions had a dominant effect on the morphology of the LCEP and the void size in the EPSLC sample. The increase of liquid crystalline epoxide content, UV intensity, and initiator content induced that the density of LCEP network became larger and the void size decreased, which resulted in that the stabilising effect from the LCEP network on the mesogenic solvent molecules was strengthened. Conversely, the increase of polymerisation temperature induced that the stabilising effect from the LCEP network on the LC molecules was weakened.