Self-constructed stable liquid crystal alignment in a monomer-liquid crystal mixture system

Here, we demonstrate excellent liquid crystal (LC) vertical alignment without using an alignment layer printing process by introducing octadecyltrichlorosilane (OTS) into the LC mixture. Further, we investigated the alignment mechanism by analysing the surfaces of the substrates. The optimum concentration of OTS was found to be about 0.03 wt%, which is 1/100 of that in the previously reported polyhedral oligomeric silsesquioxane (POSS)–LC system. Moreover, the OTS–LC system exhibited a more stable LC alignment compared with the POSS–LC system. These differences may arise from the different strengths of surface–dopant interactions; that is, the covalent bond in the OTS–LC system and the van der Waals interactions in the POSS–LC system. We also demonstrated that the method can be used in a capillary tube, which may serve as a new method facilitating the application of LCs with curved surfaces.     

Fabrication of photoluminescent liquid crystal device using an in situ self-assembled molecular layer of a pyrene derivative

An in situ self-assembled molecular layer of 1-pyrenesulfonic acid sodium salt as an alignment agent was formed on indium tin oxide substrates for vertically aligning liquid crystals (LCs). The thus-aligned LCs exhibited uniform vertical alignment under crossed polarisers. The electro-optical characteristic of the LC cell fabricated using this method exhibited better performance than those of conventional LC cells with a polyimide alignment layer. Because the proposed alignment method is a simple one and involves low concentrations of the alignment agent (0.05 wt%), it is highly cost-effective. Further, the pyrene derivative, when mixed with LCs, exhibited photoluminescence (PL) under ultraviolet light. Given that the proposed method resulted in highly vertically aligned LCs and the alignment agent exhibited PL, the method should find wide use in the fabrication of colour-filter-free LC displays.     

The effect of surface polarity of glass on liquid crystal alignment

The effects of the surface polarity of a glass substrate on the orientation of nematic liquid crystals (LCs) were studied using the polarised optical microscope and Fourier-transform infrared spectroscopy. On the surface of oxygen plasma treated glass, a homeotropic alignment of LCs was induced for LCs with negative dielectric anisotropy. This suggests that vertical orientation of LCs could be induced on a polar glass substrate without using an LC alignment layer. Upon cooling towards the isotropic–nematic transition, E7 with positive dielectric anisotropy changes its LC arrangement to isotropic, homeotropic, planar orientations in order. The nematic LC anchoring transition of E7 was interpreted by considering the competition between van der Waals forces and dipole interactions that control the alignment of LC molecules on a polar glass surface.     

In situ self-assembled homeotropic alignment layer for fast-switching liquid crystal devices

We propose a novel method for homeotropic alignment of liquid crystals (LCs) utilising in situ self-assembly of a low concentration of 4-(4-heptylphenyl)benzoic acids that form hydrogen bond with the indium tin oxide (ITO) substrates. Stable homeotropic alignment in the LC device is achieved with a simple mixing process of benzoic acid derivative in LC media, and it yields electro-optical performance similar to that achieved with the conventional alignment method using polyimides. It is experimentally confirmed that an ultrathin self-assembled molecular layer of 4-(4-heptylphenyl)benzoic acid formed by hydrogen bonding on ITO substrate makes it possible to attain a reliable homeotropic alignment of LCs. Furthermore, this simple approach provides a cost-effective and stable LC alignment layer with fast response time and thermal stability.     

Liquid crystal alignment induced by controllable surface wettability of BiFeO3 bumps thin layer

The alignment of liquid crystals (LCs) in LC-based optical devices determines the visual quality of devices, and a simple pretreatment is necessary for LC alignment. Here, we present a BiFeO₃ (BFO) thin layer working as an alignment layer, homogeneously aligning LCs after a pre-thermal annealing treatment. Dramatic BFO bumps were observed during the thermal annealing process, and a corresponding enhancement in wettability was detected from measurement of tunable contact angles. LCs in the BFO bump thin layer sandwiched TN cell were found to be perfectly homogeneously aligned due to the enhanced anisotropy and wettability caused by the morphology change of the thin layer.     

UV-driven switching of chain orientation and liquid crystal alignment in nanoscale thin films of a novel polyimide bearing stilbene moieties in the backbone

A novel photosensitive polyimide, poly(4,4'-stilbenylene 4,4'-oxidiphthalimide) (ODPA-Stilbene PSPI) was newly synthesized. The most surprising feature of this PSPI is that the PSPI films irradiated with linear polarized ultraviolet light (LPUVL) can favorably induce a unidirectional alignment of liquid crystals (LCs) in contact with the film surface and further switch the director of the unidirectionally aligned LCs from a perpendicular direction to a parallel direction with respect to the polarization direction of LPUVL by simply controlling the exposure dose in the irradiation process. These LPUVL-irradiated films were found to provide high anchoring energy to LCs, always giving very stable, homogeneous cells with unidirectionally aligned LCs regardless of the LC alignment directions. In the films, the PSPI polymer chains were found to undergo favorably unidirectional orientation via a specific orientation sequence of the polymer chain segments led by the directionally selective trans-cis photoisomerization of the stilbene chromophore units in the backbone induced by LPUVL exposure. Such unidirectionally oriented polymer chains of the films induce alignment of the LCs along the orientation direction of the polymer chains via favorable anisotropic molecular interactions between the oriented polymer chain segments and the LC molecules. In addition, the PSPI has an excellent film formation processibility; good quality PSPI thin films with a smooth surface are easily produced by simple spin-coating of the soluble poly(amic acid) precursor and subsequent thermal imidization process. In summary, this new PSPI is the promising LC alignment layer candidate with rubbing-free processing for the production of advanced LC display devices, including LC display televisions with large display areas.     

Novel alignment mechanism of liquid crystal on a hydrogenated amorphous silicon oxide

The mechanism of liquid crystal (LC) alignment has been investigated during the last few decades for inorganic materials as well as for organic materials; however, it has not been clearly confirmed for some alignment materials. Inorganic alignment materials such as amorphous silicon oxide (a-SiOx) and hydrogenated amorphous silicon oxide (a-SiOx:H) are deposited on indium tin oxide (ITO) films on glass by reactive sputtering deposition. After deposition, the inorganic alignment materials are irradiated using an Ar+ ion beam (IB) for LC alignment. On the basis of the experimental results, a-SiOx films deposited by the sputtering do not align the LC, but a-SiOx:H films treated with varying IB energies, IB incident angles, IB doses, and IB irradiation times have excellent alignment properties and electrooptical properties, identical to those of polyimide (PI). These results imply that inorganic alignment layers irradiated by IB can be adopted as an LC alignment layer instead of rubbed PI. Additionally, hydrogen plays an important role in LC alignment because of the difference in alignment properties between a-SiOx films and a-SiOx:H films. We investigate the mechanism of IB-treated inorganic alignment layers and suggest that LCs are aligned by chemical effects, such as van der Waals interaction, more than by physical effects, such as morphology effects, in the inorganic alignment layer irradiated by IB.     

Effects of carbon nanotubes on electro-optic characteristics in vertically aligned liquid crystal display

Size- and aggregation-controlled dispersion of thin multiwalled carbon nanotube (t-MWCNT) in negative dielectric anisotropic liquid crystal (LC) material exhibits remarkable improvement in electro-optic response time in vertically aligned LC cells. The physical properties such as birefringence, dielectric anisotropy and clearing temperature of nanotube dispersed LC material appear to be almost invariant to that of pristine LC. Nevertheless, the response time shows noticeable improvement, especially in decaying time associated with transition from maximum to minimum transmission, hence important for faster switching LC devices. The effect is attributed to that vertically aligned t-MWCNTs along the field direction play role of vertical alignment layer between LCs, consequently resulting in increased bend elastic constant of LCs.     

Achieving a robust homogenously aligned liquid crystal layer with reactive mesogen for in-plane switching liquid crystal displays

Uniformly aligned liquid crystals (LCs) are of crucial importance in a practical application, such as displays, phase modulators and virtual reality devices. Although an alignment layer using polyimide-type polymers can almost perfectly align the LCs, polymer-stabilisation at the surface using pre-mixed monomers has been attempted to reduce the fabrication process, i.e. eliminating the alignment layer, thereby reducing the fabrication cost. Here, we propose an approach to achieve a homogeneous alignment of LCs by controlling the molecular structure of reactive monomers mixed in LCs without using conventional polyimide alignment layer. The result shows an excellent initial dark state and acceptable electro-optic performance which implies that the polymer stabilisation at the surface successfully anchors the homogenous orientation of LCs. We believe that the proposed fabrication method can contribute to cost-effective fabrication process by eliminating an alignment layer with no fabrication step of a surface treatment.     

Fabrication of homogenously self-alignment fringe-field switching mode liquid crystal cell without using a conventional alignment layer

We propose a novel method to fabricate a uniaxially homogeneous alignment of liquid crystal (LC) molecules without using a conventional alignment layer such as polyimide film. The method produces the polymer alignment layer (PAL) by polymerisation of the monomer including in the LC layer above the T_NI of the LC material. The fringe-field switching (FFS) mode LC cell with the PAL (FFS-PAL-LC cell) produced from the monomer 4,4?-di-mehacryloyl-oxy chalcone (4,4?-DMOCh) exhibited enough level of alignment state and electro-optical property compared with the FFS-LC cell having the conventional polyimide-type alignment layer. We can expect that the FFS-PAL-LC cell is useful for next-generation displays such as flexible liquid crystal displays (LCDs) because the method does not need high-temperature process of over 200°C.     

Polarised UV-induced homogeneous alignment for fringe-field switching liquid crystal display with a new azobenzene monomer

A novel preparation method of homogeneous alignment polymer film (HAPF) was proposed by polymerisation of the monomer, 4,4?-di-methacryloyl-oxy azobenzene (4,4?-DMOAz), being dissolved in the liquid crystal (LC) material of positive dielectric anisotropy. For obtaining the homogeneous alignment, exposure of the polarised UV light was carried out to the LC cell above the nematic to isotropic transition temperature of the LC material. The fringe-field switching (FFS) mode LC cell with the HAPF formed from the monomer 4,4?-DMOAz (FFS-HAPF-LC cell) exhibited enough level of alignment state, electro-optical and response properties compared with the FFS-LC cell carrying the conventional polyimide-type alignment layer. FFS-HAPF-LC cell can be expected to be useful for next-generation displays such as flexible LC displays.     

Synthesis of novel hyperbranched polyimide for liquid crystal alignment

A novel hyperbranched polyimide (HPPI) was synthesized from a new four‐functional amine and 1,2,3,4‐cyclobutanetetracarboxylic dianhydride. The polyimide with hyperbranched structure exhibited excellent alignment for nematic liquid crystal (LC) compared with traditional linear polyimides. The solution viscosities of the poly(amic acids) (PAAs) and the surface morphology of PI films were investigated using an Ubbelhode‐type viscometer and atomic force microscopy, respectively. The viscosities of the hyperbranched PAAs were much lower than that of the linear PAA solution at any monomer ratio (amine:dianhydride). Pretilt angles of LCs above 2.8° were achieved with HPPI.     

Nanocrystalline LaYSrO films for liquid-crystal alignment via a solution process

The alignment of liquid crystals (LCs) has been widely studied to enable their use in high-quality displays. In addition to the deposition of an alignment layer, an alignment process to orient the LCs is also required. To accomplish self-alignment and to simplify the fabrication process, many alternatives have been proposed, including the doping of nanoparticles or organic/inorganic composites into LCs or the use of special treatments, such as ultra-violet (UV) irradiation or imprinting. However, an additional treatment is required to achieve proper LC orientation. Herein, we demonstrate the fabrication of an aligned LaYSrO (LYSO) via a solution process. Using this technique, we have generated a simple manufacturing method that eliminates the alignment process, and we have found that the quality of the LC cell is comparable to that of polyimide-rubbed films. The mechanisms of the film formation and the LC alignment were elucidated with a nanocrystalline LYSO film. In comparison with LCs, the nanocrystal is sufficiently large so that order in the LCs is induced along the direction of the nanocrystal. This approach allows for the fabrication of LC displays that meet specific goals such as high performance with a simple fabrication process.     

Polyimide-free homogeneous photoalignment induced by polymerisable liquid crystal containing cinnamate moiety

We designed and synthesised a reactive mesogen (CRM) containing cinnamate moiety in the core and two acrylate groups at both ends. The structure was characterised by ¹H NMR and FT-IR, and its liquid crystalline property was investigated by DSC and POM. The compound showed a nematic phase between 175.2 – 97.5 ^oC, followed by a smectic C phase between 97.5 and 57.9 ^oC during the cooling cycle. We proposed a fabrication method to achieve uniaxial homogeneous alignment of liquid crystals by irradiating the CRM-doped LC mixture in a sandwich cell of in-plane switching mode (CRM-IPS) with linearly polarised ultraviolet light. The results indicated that the CRM-IPS cell showed excellent initial dark state with a good alignment state, electro-optical performance and durability. This confirmed that the photo-induced alignment of LCs by CRM possesses outstanding alignment capability compared to the conventional rubbed polyimide alignment layer. We expect that this fabrication method is a promising candidate for cost-effective, green-manufacturing, and high-quality alignment method for the manufacturing of high-resolution liquid crystal displays (LCDs).     

Homogeneous alignment of liquid crystals on low-temperature solution-derived gallium oxide films via IB irradiation method

In this paper, solution-derived gallium oxide (GaO) films are fabricated for the homogeneous alignment of liquid crystals (LCs) after an ion-beam (IB) irradiation process. GaO thin films are prepared under a variety of temperatures and different IB irradiation intensities, and the physicochemical performances of the fabricated GaO thin films are analysed using a UV-vis spectrometer, an atomic force microscope, and X-ray photoelectron spectroscopy. A higher transmittance of 85.40% from GaO thin film is obtained compared with that of polyimide (PI) film (83.52%), which indicates the feasibility for a GaO thin layer to substitute for a conventional PI layer as an alignment layer. LCs are found to align on the GaO thin film after pre-baking at 100°C and homogeneous and uniform low-IB intensity irradiation. We also determined the electro-optical (EO) characteristics of the twisted nematic (TN) cells fabricated with GaO thin layers and found them to be similar to those of cells fabricated with PI layers. Overall, GaO films achieved via the IB irradiation method are promising LC alignment layers due to the method’s low-temperature solution-derived process.     

Fabrication of vertically aligned liquid crystal cell without using a conventional alignment layer

We propose a novel method to fabricate a vertical alignment (VA) of liquid crystal (LC) molecules without using a conventional alignment layer such as polyimide film. The method produces the vertical alignment polymer layer (VAPL) by polymerisation of a monomer or mixed monomers including in the LC layer above T_NI of the LC material. The VA mode LC cell with the VAPL (VAPL-LC cell) produced from the mixed monomers of acrylic acid 4-(4?-octyloxy-biphenyl-4-yloxy)-butyl ester and 1,2-bis-(4-methacryloxy-phenyl)-2,2-dimethoxy-ethane-1-one exhibited enough level of alignment state and electro-optical property with high voltage holding ratio. We can expect that the VAPL-LC cell is useful for next-generation displays such as flexible liquid crystal displays because the method does not need the process including high temperature over 200°C.     

Photoaddressable polymers for liquid crystal alignment

We demonstrate reversible photoinduced in situ reorientation of low molecular mass liquid crystals (LCs) by means of photoaddressable polymers (PAPs). These polymers contain mesogenic azobenzene side chains optimized to reorient cooperatively and effectively upon illumination with polarized light. Various low molecular mass LCs were introduced between two PAP layers and these sandwich devices were tested with respect to stability and reversibility of photoinduced orientation. Dissolution of the PAP layer by the low molecular mass LC was observed for several material combinations and systematically investigated. Different anisotropic dyes were added as fluorescence markers in order to monitor the photoinduced LC orientation. With an optimized material combination, more than 10 reversible reorientation processes could be realized with polarized light of either 514 or 405 nm wavelength, without any reduction in alignment quality. Further, microscopic polarized fluorescence patterns could be produced and erased within short exposure times.     

Photoaddressable polymers for liquid crystal alignment

We demonstrate reversible photoinduced in situ reorientation of low molecular mass liquid crystals (LCs) by means of photoaddressable polymers (PAPs). These polymers contain mesogenic azobenzene side chains optimized to reorient cooperatively and effectively upon illumination with polarized light. Various low molecular mass LCs were introduced between two PAP layers and these sandwich devices were tested with respect to stability and reversibility of photoinduced orientation. Dissolution of the PAP layer by the low molecular mass LC was observed for several material combinations and systematically investigated. Different anisotropic dyes were added as fluorescence markers in order to monitor the photoinduced LC orientation. With an optimized material combination, more than 10 reversible reorientation processes could be realized with polarized light of either 514 or 405 nm wavelength, without any reduction in alignment quality. Further, microscopic polarized fluorescence patterns could be produced and erased within short exposure times.     

Dielectric properties of two high birefringence liquid crystal mixtures in the Sub-THz band

ABSTRACT

In order to produce liquid crystal (LC) with high birefringence (Δn) in the THz band, eight LC monomers were designed and synthesized. We formulate two mixtures (LC S200-2 and E7-2) by adding eight LC monomers to two commercial LCs (S200 and E7), respectively. The dielectric constants of the mixture LCs were measured using the frequency selective surface (FSS) in the region from 90 to 140 GHz. We compare the simulated results with the experimental measurements and determine the dielectric constants of the LCs. The dielectric anisotropy (Δε) of the nematic S200-2 and E7-2 LC mixtures are 16.4% and 16.3% higher than that of the commercial S200 and E7 LCs, respectively. The results show the frequency tunability of the two LC mixtures is greater than that of the commercial LCs.     

Size control of phase‐separated liquid crystal droplets in a polymer matrix based on the phase diagram

When a mixture of liquid crystal (LC) and photo reactive monomer is irradiated by UV light, polymerization occurs and LC droplets form through phase separation, producing polymer dispersed LCs (PDLCs). Although size control of LC droplets and reduced amounts of LC in PDLC films are important in applications, precise size control of LC droplets at a low LC fraction has not yet been accomplished. In this study, the phase diagrams of the LC/initial monomer and the LC/polymer during polymerization were used to control LC droplet size at various LC fractions. Both the relative position of the sample in the initial phase diagram and the shift of the phase separation line during polymerization were shown to be important in determining the size of LC droplets. Our results are expected to provide a new strategy for precise size control of LC droplets especially at a low LC fraction range, which would be a great help for PDLC applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012