Bent Liquid Crystal Research Articles

Synthesis and mesomorphic properties of bent liquid crystals containing triazole mesogenic unit with terminal flexible alkyl chain and laterally ethoxy substituted Schiff base

ABSTRACT A new series of bent-shaped liquid crystals containing a triazole ring were synthesised and characterised at ambient and elevated temperatures. The triazole-cored and bent liquid crystals possess a flexible alkyl chain at one terminal, while the other side of the core is connected by a laterally ethoxy Schiff base consisting of a CN-biphenyl moiety. The evaluation on optical and thermal properties was carried out by polarising optical microscope and differential scanning microscope which revealed the length of the alkyl chain, –CnH2n + 1, that influenced the mesophase and the related clearing temperatures (Tc). Whilst the compounds with higher number of n possess lower Tc points, the presence of SmA phase seemed to be more favourable for the compounds in which the alkyl chain length was n ≥ 12. This observation can be associated with its bending angle and charge distributions along the two arms of which the interaction between the opposite counterparts of both arms in these bent-liquid crystals would promote the formation of SmA phase. The computational calculations based on density functional theory were performed to further support the transition from non-mesomorphic to SmA phases upon the increase in alkyl chain length (n), dipole moment and polarity which contribute to its stability.

Distinct differences in the nanoscale behaviors of the twist–bend liquid crystal phase of a flexible linear trimer and homologous dimer

We synthesized the liquid crystal dimer and trimer members of a series of flexible linear oligomers and characterized their microscopic and nanoscopic properties using resonant soft X-ray scattering and a number of other experimental techniques. On the microscopic scale, the twist-bend phases of the dimer and trimer appear essentially identical. However, while the liquid crystal dimer exhibits a temperature-dependent variation of its twist-bend helical pitch varying from 100 to 170 Å on heating, the trimer exhibits an essentially temperature-independent pitch of 66 Å, significantly shorter than those reported for other twist-bend forming materials in the literature. We attribute this to a specific combination of intrinsic conformational bend of the trimer molecules and a sterically favorable intercalation of the trimers over a commensurate fraction (two-thirds) of the molecular length. We develop a geometric model of the twist-bend phase for these materials with the molecules arranging into helical chain structures, and we fully determine their respective geometric parameters.

Control of liquid crystal alignment using surface-localized low-density polymer networks and its applications to electro-optical devices

We developed an alignment method for control of the polar pretilt angle at the boundaries of a liquid crystal (LC) layer in a continuous range 2–90° by introducing a low-density polymer network. The network was formed as a result of the electric field-induced segregation of a photo-reactive monomer added in concentrations of 0.5–1% to the LC host and subsequent in-situ UV-light-induced polymerization. LC director was “frozen” within a less than a micron-thick layer in the vicinity of the cell substrates at high average tilt angles anchoring the LC in the bulk similarly to a high-pretilt alignment layer. The resultant pretilt angle was determined by the voltage applied before and during polymerization and the duration of voltage application before UV-light exposure. The desired pretilt angle could be set over a small area of a sample which allowed for the fabrication of LC devices with spatially variable optical retardation. Using this method, we fabricated high-pretilt splay and bend LC cells, optically recorded converging lenses and bi-prisms with electrically controlled optical characteristics, and phase diffraction gratings with resolution better than 50lines/mm.

Performance enhancement using a stable low-pretilt molecular configuration and a novel driving method for optically compensated bend liquid crystal devices.

A stable low-pretilt molecular configuration (SLPMC) is successfully developed in optically compensated bend (OCB) liquid crystal (LC) devices by simultaneously employing the curing voltage and surface-anchored crosslinking monomer during the polymerization process. For the SLPMC OCB cell with the low-bend state, the warm-up voltage making the LC molecules reorient from the splay to the bend state is annihilated, and the transient twist state occurring as the driven LC molecules recover from the bend to the splay state is also eliminated. In addition, with the novel driving method selecting the specific driving point, the proposed SLPMC OCB cell not only exhibits a good response performance, but also outputs a higher light transmittance, which is superior to the conventional OCB and no-bias-bend cells. This Letter demonstrates an effective SLPMC fabrication method, and points out the significant contributions of SLPMC on the electro-optical properties, which will benefit and enhance the performance design in OCB-based applications.

Effects of Pretilt Angle on Electro-Optical Properties of Optically Compensated Bend Liquid Crystal Devices With a Mixed Polyimide Alignment Layer

The effects of pretilt angle on electro-optical properties of optically compensated bend liquid crystal cells with a mixed polyimide alignment layer are investigated. By adjusting the mixed polyimide concentration, several kinds of alignment layers are produced, and a wide range of pretilt angles (2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> -66 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> ) are achieved. Differing from the conventional addresses, the maximum-light-transmittance voltage, obtained from the transmittance-voltage curve of the cell undergoing the reverse measurement, is employed, making a ~ 14% enhancement in the output transmittance. Compared to the 0-wt% cell, the 0.6-wt% cell with 16 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> pretilt angle achieves ~ 38% and ~ 50% improvement in the rising-time and falling-time response, respectively.

Ferroelectric response in an achiral non-symmetric bent liquid crystal:C12C10

An achiral Non-Symmetric Bent Liquid Crystal (BLC) with a Oxadiazole based hetero cyclic central moiety, abbreviated as C12C10 viz., dodecyl[4-{5-(4′-decyloxy)biphenyl-4-yl}-1,2,4-oxadiazol-3-yl]benzoate, exhibiting FerroElectric (FE) response is reported. Product is confirmed by 1H NMR, 13C NMR and elemental analysis. Characterization of BLC phases is carried out by Polarized Optical Microscopy (POM), Differential Scanning Calorimetry (DSC), Spontaneous Polarization (PS) and Low Frequency (10Hz–10MHz) Dielectric Relaxation studies. C12C10 exhibits enantiotropic LC SmA, FE B2, SmG, SmE phase variance. I–SmA, B2–SmG and SmG–SmE transitions are of first order nature. FE B2 phases exhibits a moderate PS of ~80nCcm−2. B2 phase exhibits Curie–Weiss behavior to confirm FE nature. Off-centered low frequency (KHz) dispersion infers a scissor mode and a high frequency (MHz) mode to reflect the distinct time-scale response. Dielectric Dispersion is relatively susceptible in lower frequency KHz region. Arrhenius shift in Relaxation Frequency (fR) infers higher activation energy (Ea) in non-FE phases for HF mode and lower value for KHz mode. Trends of fR, dielectric strength Δε, α-parameter and Ea are discussed in view of the data reported in other LC compounds.

Ferroelectric phases in non-symmetric achiral bent liquid crystals towards ambient temperatures: a novel series with oxadiazol as central moiety

Extensive survey carried out in bent liquid crystals (BLCs) exhibiting ferroelectric (FE) phases suggested for the design of non-symmetric (NS) frame. Novel series of BLCs (C8Cm) with distinct lateral moieties and heterocyclic central (oxadiazole) moiety, viz., octyl 4-(5-(4´-(alkyloxy) biphenyl-4-yl)-1,2,4-oxadiazol-3-yl)benzoates, are reported. NS BLC frame is realised by incorporating differing number of aromatic cores and varied length of end chains in the lateral moieties. Lateral moiety with biphenyl core is prepared by Suzuki coupling. Purity of the BLC product is confirmed by 1H NMR and elemental analysis. LC phases exhibited by C8Cm series of BLCs for m = 8, 10, 12, 15 and 16 are characterised by polarisation optical microscopy (POM), differential scanning calorimetry (DSC) and spontaneous polarisation (PS) techniques. C8Cm series are found to exhibit SmA, CrystalB, B2, B5, SmG and SmE phase variance. SmG occurs as monotropic phase. Temperature variation of PS(T) studied in FE B2 and B5 phases by field reversal method infers a moderate PS value of ~80–100 nC · cm−2. Order parameter growth in FE B2 and B5 phases is analysed through PS(T). Normalised order parameter θN exhibits asymptotic behaviour with universal temperature TU. Influence of aromatic cores and length of end chains (in lateral moieties) on the thermal stability of FE phases is discussed in the wake of the data on other BLCs.

Voltage holding properties of bend liquid crystal cells with treatments for bend alignment stabilisation

We investigated the voltage holding property of cells with additional fabrication processes for the formation of high and low pretilt angle regions within a dot for achieving a smooth splay-to-bend transition and of π-cells with a polymer network in a liquid crystal (LC) layer to stabilise a bend state. The pretilt angle could be adjusted by irradiation of deep ultraviolet (UV) light to a vertical alignment polyimide layer. The voltage holding ratio (VHR) was above 97% even with irradiation with deep UV light of 10 J/cm2. On the contrary, the VHR was decreased with increasing concentration of photoinitiator, which was added to LC mixture to form the polymer network. The VHR was 90% at the concentration of 0.3 wt%. The results indicate that the formation of the high and low pretilt angle regions within a dot is very useful from the point of reliability of the π-cell.

Long memory retention time and high contrast ratio in a tristate liquid crystal display device

We propose a method to obtain long memory retention time and high contrast ratio in a tristate liquid crystal display device. The proposed device has three unique liquid crystal (LC) states that are known as splay, π twist, and bend, with different operating schemes for the two modes of operation, which are the memory (M) mode or dynamic (D) mode. A form of selective vertical or horizontal switching changes the optically compensated π twist LC as a common black state in both the M and D modes into a bend LC state for the D mode or into the splay LC state for the M mode, respectively.

48.2: Advanced VA Mode with Fast Gray Scale Response and Wide Viewing Angle in a Bend Liquid Crystal Configuration

AbstractWe have developed “Transverse Bend Alignment (TBA) mode”, an advanced VA mode where liquid crystal materials are driven by a combination of a lateral field and a vertical field. We have made TFT‐LCDs with different spacings of comb‐shaped electrodes to reduce the steepness of the V‐T characteristics. By adjusting cell parameters, the LCD has wide viewing angle without color wash‐out in oblique directions and fast response between all gray scale levels.

Determination of surface tilt angle of splay and bend liquid crystal cells

This study presents and demonstrates an approach for determining the surface tilt angles of parallel-aligned liquid crystal cells. The relationships between the incident angles and the corresponding phase retardations in the splay and the bend configurations are analyzed. The equations for the surface tilt angles are derived accordingly, and their accuracy is discussed. This method does not require knowledge of the cell thickness and is easily applied to assembled pi-cells.