Vertical Alignment Layer Research Articles

3D Printed MXene Aerogels with Truly 3D Macrostructure and Highly Engineered Microstructure for Enhanced Electrical and Electrochemical Performance.

Assembling 2D materials such as MXenes into functional 3D aerogels using 3D printing technologies gains attention due to simplicity of fabrication, customized geometry and physical properties, and improved performance. Also, the establishment of straightforward electrode fabrication methods with the aim to hinder the restack and/or aggregation of electrode materials, which limits the performance of the electrode, is of great significant. In this study, unidirectional freeze casting and inkjet-based 3D printing are combined to fabricate macroscopic porous aerogels with vertically aligned Ti3 C2 Tx sheets. The fabrication method is developed to easily control the aerogel microstructure and alignment of the MXene sheets. The aerogels show excellent electromechanical performance so that they can withstand almost 50% compression before recovering to the original shape and maintain their electrical conductivities during continuous compression cycles. To enhance the electrochemical performance, an inkjet-printed MXene current collector layer is added with horizontally aligned MXene sheets. This combines the superior electrical conductivity of the current collector layer with the improved ionic diffusion provided by the porous electrode. The cells fabricated with horizontal MXene sheets alignment as current collector with subsequent vertical MXene sheets alignment layers show the best electrochemical performance with thickness-independent capacitive behavior.

Tri-state switching of a high-order parameter, double-layered guest-host liquid-crystal shutter, doped with the mesogenic molecule 4HPB

ABSTRACT Dye-doped guest-host liquid crystal (GHLC) light shutters are widely used for privacy protection (windows) and sunlight blocking because of their low power consumption and fast response speed. The mesogenic molecule 4-(4-Heptylphenyl) benzoic acid (4HPB) can form hydrogen bonds with a vertical PI alignment layer to create a superhydrophobic vertical alignment (polyimide) PI layer, which can improve the order parameters of the dye. Because the initial transmittance and contrast of the GHLC are proportional to its own order parameters, the addition of the mesogenic molecule 4HPB can increase both the initial transmittance and contrast of the GHLC significantly. Moreover, the addition of 4HPB can also greatly increase the ion concentration of the GHLC, which introduces an electro-hydrodynamic effect. The GHLC can be switch states between transparent, haze-free dark, and high-haze dark by changing the strength of the applied voltage of appropriate frequency. Double-layered guest-host liquid crystal (DGHLC), doped with mesogenic molecules, are suitable for fast-moving applications (such as cars and airplanes) due to their short response time and high-order parameters.

Tunable pretilt angle based on gelator-doped planar liquid crystal cells

ABSTRACT This study demonstrates a simple approach for controlling the pretilt angles of liquid crystals (LCs) using gelator-doped cells fabricated with homogeneously aligned substrates. The competition between the gelator-induced spontaneous vertical alignment and planar alignment layer coated on the substrates results in various pretilt angles of LCs. The LC pretilt angle can thus be controlled from a homeotropic to planar state by adjusting the anchoring energy of the planar alignment layer and gelator concentration used for doping. A no-bias pi cell with multistable property is demonstrated by utilizing the proposed method.

Low voltage liquid crystal microlens array based on polyvinyl alcohol convex induced vertical alignment

ABSTRACT In this study, low driving voltage liquid crystal microlens array (LCMLA) based on polyvinyl alcohol (PVA) convex array was fabricated by facile method. The PVA convex array served as a vertical alignment layer led to formation of hybrid-aligned LC layer with concentric nonuniform distribution refractive index in planar LC cell, conducive to lens usage. The virtual focal length of LCMLA was short during the initial state. At voltages from 0 to 0.6 V rms, the focal length was tuned from – 4.5 cm to ∞. The low surface tension of the surface outside the lens unit reduces the anchoring force of LC which led to very low driving voltage. In sum, the proposed LCMLA possessed low power consumption, compact structure and simple fabrication, uniform structure in large scale, conducive to future development and applications of high-quality diffusers, image processing and displays.

Photo‐Tunable Supramolecular Ultrathin Surfaces for Simultaneous Homeotropic Anchoring and Superfast Switching of Liquid Crystals

AbstractPhoto‐tunable supramolecular ultrathin surfaces functionalized with a soft ultraviolet (UV) treatment of star‐shaped reactive amphiphiles are fabricated to simultaneously afford homeotropic anchoring of liquid crystals (LCs) and improve the LC switching performance. Photochemically functionalized ultrathin surfaces are generated by cross‐linking star‐shaped amphiphilic monomers in the self‐assembled monolayers with retention of cluster structure under electric field. Since these star‐shaped amphiphiles can construct the close‐packed self‐organized molecular layer with an alkyl brush at the interface between indium tin oxide and LCs through intermolecular hydrogen bonding, they might make elegant alignment surfaces for the homeotropic anchoring of LCs to provide a new type of vertical alignment layer. It is experimentally revealed that supramolecular ultrathin layers with vertical nanopillars form hydrophobic surface, and afford a stable and uniform homeotropic alignment of LCs even in high‐temperature environment. Furthermore, a soft UV irradiation on the self‐assembled ultrathin surfaces under electric field yields superfast responsive cross‐linked ultrathin surfaces with low power consumption compared to conventional polyimide layer. Notably, this controlled methodology offers advantages of simplicity, low‐cost, and scalability in the fabrication of functional alignment surfaces without the need of complex manufacturing steps.

Fabrication of Pre-tilted Vertical Alignment Layers for High-Speed Liquid Crystal Display Using Bi-functional Photoreactive Monomers

To enable recent 3D and moving picture applications, liquid crystal displays (LCD) must exhibit fast response and wide viewing angle characteristics. Pre-tilted vertical alignment layers for high-speed LCD were fabricated using bi-functional photoreactive monomers. The monomers can be confined to the polyimide layer and photo-polymerized by UV exposure under voltage. Response characteristics of photo-controlled alignment films according to the structure were investigated. Vertical alignment properties were evaluated using a polarizing optical microscope, and electro-optical characteristics were compared through the voltage-transmittance curve and response time measurement. Faster response speed could be obtained by using photoreactive monomers having a long alkyl chain.

Enhancement of absorption and haze with hybrid anchoring of dye-doped cholesteric liquid crystals.

We demonstrated that hybrid anchoring of dye-doped cholesteric liquid crystals (ChLCs) could be used for the simultaneous control of haze and transmittance. Hybrid anchoring of ChLCs can be obtained by the vertical anchoring at one substrate surface and planar anchoring at the other substrate surface. In a ChLC cell with hybrid anchoring, the LCs near the planar alignment layer are in the planar state, while those near the vertical alignment layer are in the focal-conic state. In the initial opaque state, the incident light can be absorbed by the LC mixture in the planar state and scattered by the LCs in the focal-conic state. The ChLC cell with hybrid anchoring exhibited lower transmittance and haze value than those of a focal-conic state in a ChLC cell with vertical anchoring. The cell can be switched to the transparent state without use of a complicated drive scheme at a driving voltage significantly lower than that for a double-layered cell.

39.4: Invited Paper: Self‐alignment of liquid crystal for multi‐domain liquid‐crystal display

By introducing photopolymerizable amphiphile organic monomers(PAOM) into nematic liquid crystals (NLC) with controlled amount of UV‐curable reactive mesogen (RM), a amulti‐domain vertical‐alignment LC device was successfully demonstrated. The device, possessing a vertically aligned LC director in four different azimuthal directions, exhibited a fast response time,wide‐viewing‐angle characteristics and high voltage holding ratio(VHR), in the absence of conventional polyimides vertical‐alignment layers. The formation of automatic and robust vertical alignment layer was anlayzed by using scanning electron microscopy(SEM).

Self-assembled vertical alignment layer of bifunctional amphiphiles for fast electro-optical switching of nematic liquid crystals

ABSTRACTWe present a novel homeotropic alignment method that simultaneously improves the ability of vertical alignment and fast switching of liquid crystals (LCs) in the electro-optical performance. The proposed method utilizes dual self-assembly of 4-(4-heptylphenyl)benzoic acid and 2-carboxyethyl acrylate via hydrogen bonding with ITO substrate. The 4-(4-heptylphenyl)benzoic acid composed of aromatic moieties and alkyl chains induce the homeotropic alignment of the LC molecules. Photopolymerization of 2-carboxylethyl acrylate with a reactive acrylate group under UV irradiation gives the LC molecules a fast response. Our proposed alignment method allows for fabrication of LC devices exhibiting fast LC switching without a conventional polyimide layer.

Fabrication of new photoreactive vertical alignment thin layers for fast switching liquid crystal display

New photoreactive vertical alignment layers have been fabricated by spin coating the mixtures of polyimide (PI) and bi-functional monomers on the indium tin oxide substrate. Post ultraviolet (UV) surface treatment under electric field induced fast electro-optical switching of liquid crystal (LC) through formation of pre-tilted alignment layer. To investigate the effect of reactive monomers on the electro-optical properties of LC device, newly designed four types of monomers with different aromatic cores and photoreactive groups were applied to PI mixtures. Stable homeotropic alignment and fast response time in the LC device were achieved with a simple coating and subsequent UV process of monomer with both a phenyl core and methacrylic groups in the PI layer and it yields electro-optical performance superior to that achieved with the conventional PI alignment layer. This elegant approach provides a stable and fast switching LC alignment layer for high performance LC displays.

Fabrication of an initially-focal-conic cholesteric liquid crystal cell without polymer stabilization

A dye-doped cholesteric liquid crystal (LC) cell with the focal-conic initial state is demonstrated. Although dye-doped LC/polymer composites have been widely used for LC light shutters, dye-doped LC cells with polymer structures suffer from the degradation of dichroic dyes during the UV curing process. To avoid this problem, we propose a dye-doped cholesteric LC cell using vertical alignment layers instead of polymer structures. We have shown that the focal-conic initial state can be obtained by homeotropic anchoring without the polymer structure. The proposed device not only provides the black color but can also hide objects behind a display panel in the focal-conic initial state without the degradation of dichroic dyes.

Construction of Polymer-Stabilized Automatic MultiDomain Vertical Molecular Alignment Layers with Pretilt Angles by Photopolymerizing Dendritic Monomers under Electric Fields.

The synthesized itaconic acid-based dendritic amphiphile (Ita3C12) monomers and the methacryl polyhedral oligomeric silsesquioxane (MAPOSS) cross-linkers were directly introduced for the construction of automatic vertical alignment (auto-VA) layers in the host nematic liquid crystal (NLC) medium. The auto-VA layer can be stabilized by irradiating UV light. For the automatic fabrication of a polymer-stabilized multidomain VA (PS auto-MDVA) layer with a pretilt angle, Ita3C12 and MAPOSS were photopolymerized under the electric field by irradiating UV light on the multidomain electrode cell. Mainly because of the pretilted NLC at zero voltage, the electro-optic properties of the PS auto-MDVA cell were dramatically improved. From the morphological observations combined with surface chemical analyses, it was found that various sizes of protrusions on the solid substrates were automatically constructed by the two-step mechanisms. We demonstrated the PS auto-MDVA cell with the enhancement of electro-optic properties as a single-step process and investigated how the protrusions were automatically developed during the polymer stabilization.

Effects of pretilt angle and anchoring energy on alignment of uniformly lying helix mode

ABSTRACTWe investigated the effects of pretilt angle and anchoring energy on the formation of a uniformly lying helix (ULH) texture of cholesteric liquid crystals (CLCs). Pretilt angle was controlled by the thickness of a vertical alignment layer coated onto a planar alignment layer. In the given pretilt angle, the anchoring energy was enhanced by introducing reactive mesogen to the vertical alignment material. To characterise quantitatively the formation of the ULH texture we introduced reflectance, governed by areas of the ULH region and the planar-aligned CLC region. We found that the ULH texture was more widely formed under the condition of higher pretilt angle and weaker anchoring energy. Also, a more uniform alignment of the ULH texture was achieved with the higher pretilt angle even under the same anchoring energy condition.

Photopolymerization of Reactive Amphiphiles: Automatic and Robust Vertical Alignment Layers of Liquid Crystals with a Strong Surface Anchoring Energy

A photopolymerizable itaconic acid-based amphiphile (abbreviated as Ita3C12) consisting of a hydrophilic carboxylic acid, three alkyl tails, and a reactive vinyl function was newly designed and synthesized for the formation of automatic and robust vertical alignment (VA) layer of nematic liquid crystals (NLC). Since a hydrophilic carboxylic acid was chemically attached to the end of Ita3C12, the Ita3C12 amphiphiles initially dissolved in the host NLC medium were migrated toward the substrates for the construction of VA layer of NLC. The alkyl tails of Ita3C12 in the VA layer directly interacted with host NLC molecules and made them to automatically align vertically. Because of the reactive vinyl functions of Ita3C12 amphiphiles, it was possible to stabilize the automatic VA layer by the photopolymerization with methacryl polyhedral oligomeric silsesquioxane (MAPOSS) cross-linkers. The polymer-stabilized robust Ita3C12 VA layer exhibited a strong surface anchoring energy without generating any light scatte...

Hybrid anchoring for a color-reflective dual-frequency cholesteric liquid crystal device switched by low voltages

Cholesteric liquid crystal (CLC) materials used in electro-optical (EO) devices are characterized by high operating voltage and slow response speed, which hinders their further development in display applications. Dual-frequency CLCs (DFCLCs) can solve the problem of slow bistable transition, but the operating voltage is still high, especially in color-reflective DFCLC cells. Here we report a simple approach to lowering the switching voltage as well as to shortening the response time. This technique adopts hybrid surface treatment to modulate the structural arrangement of CLC molecules. Both planar- and vertical-alignment layers are employed and coated on one and the other substrates separately to improve the electro-optical properties of DFCLCs. We show that the threshold voltage for switching can be decreased to as low as 5 V and the shortest response time is measured to be 0.8 ms, which renders CLC EO devices including displays more practical for commercial purpose.

Synthesis of novel soluble rubbing-resistant polyimides used as liquid crystal vertical alignment layers

ABSTRACTA novel functional diamine N12, containing triphenylamine moiety, biphenyl, tert-butyl substituents and long alkyl chain, N,N-bis(4-aminophenyl)-p-(3,5-di-tert-butyl-4-dodecyloxy phenyl) aniline (N12), was synthesised and characterised. A series of polyimides (PIs) were prepared based on 2,2ʹ-bis(trifluoromethyl)benzidine, 4,4ʹ-oxydiphthalicanhydride and different contents of N12 via a conventional two-step procedure that included a ring-opening polyaddition to gain polyamic acids, followed by chemical cyclodehydration. The chemical structures of the intermediates, diamines and PIs were characterised by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. All of the PIs were amorphous and exhibited good solubility in both polar aprotic solvents and some low boiling point solvents. PIs containing different content of N12 could induce highly uniform vertical alignment of liquid crystals (LCs). More importantly, PIs derived from N12 showed a good rubbing resistance. Besides, all PI films showed high transmittance in the wavelength range 400–700 nm and good thermal stabilities. Such PIs could be suitable candidates for alignment layers used in the manufacture of high-performance vertical alignment mode LC displays.

Electro-Optical Properties of Robust Flexible VA LCDs Using Polymerization Induced Phase Separation

We developed flexible VA LCDs with high electro-optical performance and thermal reliability for flexible display applications. It was achieved by applying the precise control of phase separation, forming well defined cylindrical liquid crystal domains surrounded by polymer matrix. The LC domains have uniform size and the LC molecules inside the domains were well aligned through the contact to vertical alignment layers coated on plastic substrates, so that low driving voltage, high transmittance and high contrast ratio were achieved. We fabricated 5-inch flexible VA LCD panel, showing good electro-optical properties and high mechanical stability.

Photoresponsive carbohydrate-based giant surfactants: automatic vertical alignment of nematic liquid crystal for the remote-controllable optical device.

Photoresponsive carbohydrate-based giant surfactants (abbreviated as CELAnD-OH) were specifically designed and synthesized for the automatic vertical alignment (VA) layer of nematic (N) liquid crystal (LC), which can be applied for the fabrication of remote-controllable optical devices. Without the conventional polymer-based LC alignment process, a perfect VA layer was automatically constructed by directly adding the 0.1 wt % CELA1D-OH in the N-LC media. The programmed CELA1D-OH giant surfactants in the N-LC media gradually diffused onto the substrates of LC cell and self-assembled to the expanded monolayer structure, which can provide enough empty spaces for N-LC molecules to crawl into the empty zones for the construction of VA layer. On the other hand, the CELA3D-OH giant surfactants forming the condensed monolayer structure on the substrates exhibited a planar alignment (PA) rather than a VA. Upon tuning the wavelength of light, the N-LC alignments were reversibly switched between VA and PA in the remote-controllable LC optical devices. Based on the experimental results, it was realized that understanding the interactions between N-LC molecules and amphiphilic giant surfactants is critical to design the suitable materials for the automatic LC alignment.

Synthesis of a novel polyimide used as liquid crystal vertical alignment layers

A novel functional diamine containing a triphenylamine moiety and a biphenyl group, N,N-bis(4-aminophenyl)-4-(biphenyl)-4′-aminophenyl ether (N0), was successfully synthesized and characterized.

Synthesis and characterisation of novel soluble polyimides for the application as liquid crystal vertical alignment layers

A new triphenylamine-based functional diamine monomer named N, N-bis (4-aminophenyl)-4-(nonyloxy-biphenyl)-4′-aminophenyl ether (N9) was successfully synthesised from cesium fluoride-mediated N,N-diarylation of 4-(nonyloxy-biphenyl)-4′-aminophenyl ether with p-fluoronitrobenzene and subsequent reduction of the resultant dinitro compound. A series of polyimides (PIs) with triphenylamine (TPA) moieties were prepared from the newly synthesised diamine monomer, 3,3′-dimethyl-4,4′-methylene-dianiline and 4,4′-oxydiphthalic anhydride through a conventional two-step procedure that included a ring-opening polyaddition to give polyamic acids, followed by chemical cyclodehydration. The PIs were completely amorphous and exhibited good solubility in organic solvents. PI films attained from a casting method showed high transmittance above 90% in the wave length range of 400–700 nm and could align liquid crystals vertically before and after rubbing treatment. The PIs derived from N9 exhibited much higher thermal stability compared with the corresponding PIs from 4-dodecyloxy-biphenyl-3′,5′-diaminobenzoate (D12). The results demonstrated that the introduction of TPA units into the polymer backbone could largely enhance the thermal stability of PIs while sustaining its solubility.