Contrast ratio and switching of zigzag defect-free surface stabilized FLCD by photoinduced alignment

The contrast ratio and electro-optical switching behaviours of surface stabilized ferroelectric liquid crystal cells with alignment layers of a photodegradable polyimide prepared by polarized ultraviolet light irradiation have been investigated. The higher contrast ratio was obtained for a zigzag defect-free sample obtained by the photo-induced alignment method than by the rubbing method. The switching behaviours of the FLC, such as spontaneous polarization and response time, were measured. Higher spontaneous polarization and faster response times were obtained with cells prepared by the photoinduced alignment method. In addition, response times of the order of microseconds, which are fast enough to realize high quality moving images with video-frame rate, were obtained.     

Fast switching ferroelectric liquid-crystalline polymers

The principles of ferroelectricity, especially in liquid crystals (FLC) and polymers, are briefly presented. In a liquid-crystalline side chain polyacrylate electro-optical switching with response times of 200–400 μs was measured. Besides the ferroelectric switching an even shorter electroclinic switching process was detected. The influence of the spacer length and the molecular weight on the ferroelectric properties is discussed. The kind and number of chiral centers and their location in the side group strongly affects the electrooptical properties. In one polymer a change of the optical contrast caused by a sign reversal of the spontaneous polarization was found. In another FLC polymer three switching states (antiferroelectric behaviour) were detected. With coloured and fluorescent FLC polymers, obtained by mixing or by copolymerization new promising applications in electrooptical devices are expected.     

Photo-dimerization of a chalcone-based side chain polymer for the alignment of ferroelectric liquid crystals

The alignment and optical properties of ferroelectric liquid crystal cells, having alignment films of a chalcone-based side chain polymer treated by linearly polarized UV irradiation were investigated. The long absorption band of the UV/Vis spectra gradually decreased and the FTIR spectra shifted as the irradiation times increased, indicating that cyclo-addition and isomerization reactions of the chalcone-based side chains occurred. UV dichroism demonstrated anisotropic changes in the alignment films, with a maximum at low exposure energy (0.5 J cm^-2). Liquid crystal molecules were aligned perpendicular to the polarization direction of the linearly polarized UV radiation. The azimuthal anchoring energy of liquid crystal E7 on a chalcone-based side chain polymer surface increased with exposure energy. Well aligned defect-free cells and high contrast ratio were achieved with irradiation of longer than 5 min; the geometric conditions for a stable C2 structure may be satisfied at low temperature with slowly cooling.     

Morphology control of liquid crystalline composite gels based on molecular self-assembling kinetics

Liquid crystalline composite gels consisting of a low molecular mass gelator and a low molecular mass liquid crystal were prepared by two types of gelation method (continuous cooling and isothermal gelating), which provide different molecular self-assembling kinetics of the low molecular mass gelator as gelation proceeds. Optical microscopy and atomic force microscopy revealed that numerous fine strands of the one-dimensionally assembled low molecular mass gelators were formed in the composite gels for both the continuous cooling method and the isothermal gelating method. However, the thinner strands were more homogeneously dispersed in the isothermal gelation product at an appropriate temperature, than in the continuous cooling process. This difference in dispersion state of the strands was shown (by polarizing optical microscopy) to have a significant influence on the molecular alignment of the low molecular mass liquid crystal in the liquid crystalline composite gel. The electro-optical response and light scattering-transmitting switching, of the liquid crystalline composite gel in an applied electric field was extremely dependent on the morphology of the gelators. High contrast light switching was achieved for the composite prepared by isothermal gelation. The response time of electro-optical switching was less than 100 µs under 30 V_rms.     

Morphology control of liquid crystalline composite gels based on molecular self-assembling kinetics

Liquid crystalline composite gels consisting of a low molecular mass gelator and a low molecular mass liquid crystal were prepared by two types of gelation method (continuous cooling and isothermal gelating), which provide different molecular self-assembling kinetics of the low molecular mass gelator as gelation proceeds. Optical microscopy and atomic force microscopy revealed that numerous fine strands of the one-dimensionally assembled low molecular mass gelators were formed in the composite gels for both the continuous cooling method and the isothermal gelating method. However, the thinner strands were more homogeneously dispersed in the isothermal gelation product at an appropriate temperature, than in the continuous cooling process. This difference in dispersion state of the strands was shown (by polarizing optical microscopy) to have a significant influence on the molecular alignment of the low molecular mass liquid crystal in the liquid crystalline composite gel. The electro-optical response and light scattering–transmitting switching, of the liquid crystalline composite gel in an applied electric field was extremely dependent on the morphology of the gelators. High contrast light switching was achieved for the composite prepared by isothermal gelation. The response time of electro-optical switching was less than 100?µs under 30?V_rms.     

Light exposure dependence of molecular orientation of glassy polyfluorene layers formed on photo-aligned polyimide films

We have investigated the molecular orientation of glassy poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) layers formed on photo-aligned polyimide films with different in-plane anisotropy. The polyimide contains azobenzene in the backbone structure (Azo-PI), allowing us to control the in-plane anisotropy of the film by varying linearly polarized light (LP-L) exposure. The glassy PFO layers (30 nm thick) were obtained by annealing the samples at the liquid crystalline phase of PFO and then quenching them to room temperature. The degree of alignment of PFO was assessed by the polarization ratio of photoluminescence (PL). The PL polarization ratio increased rapidly with the LP-L exposure, and it reached 10 at 2.8 J/cm². Beyond this LP-L exposure, it became almost constant around 10.4. This PL polarization ratio was much higher than the absorption dichroic ratio of the underlying Azo-PI film. This result suggests that the degree of alignment of PFO is determined by its liquid crystalline nature. The saturation dependence of the degree of alignment is very useful for fabricating alignment patterns by a simple photo-mask exposure method. We have succeeded in fabricating 3 μm line-and-space alignment patterns of PFO.     

Liquid crystals alignment with PbS nanosculptured thin films

Lead sulphide (PbS) nano-sculptured thin films (nSTFs) are prepared for the first time using glancing angle deposition technique by physical vapour deposition process. An anisotropic morphology is obtained, which provides an orientational effect. Due to this anisotropic nature of PbS nSTF an attempt has been made to check its effect on the alignment of nematic liquid crystals (LCs). Two different LC devices were fabricated, that is, anti-parallel aligned or electrically-controlled birefringence and hybrid twisted nematic. It is noticed that in both types of devices, good alignment is obtained with high extinction and contrast. A significant effect of these alignments on the electro-optic properties of the LC is reported such as the threshold voltage, response time and contrast ratio.     

High contrast ratio of a vertically aligned liquid crystal cell using photocrosslinking alignment

A photo-induced alignment layer for LCDs has been successfully fabricated using the polarized UV photoreaction of a photo-crosslinkable polymer with the incident UV light at some angle with respect to the cell normal. The surface alignment and electro-optic properties were investigated for various UV exposure times. The homeotropic alignment layer showed a discrete anisotropic dichroic ratio, its surface morphology became smoother as the UV exposure time increased, and the LCs were arranged in a perpendicular direction to the PUVL direction with a proper pretilt angle. The cell showed no defects under cross-polarized microscopy and the contrast ratio was as high as 550:1 in transmittance. The contrast ratio pattern was found to be very similar to that of dichroic ratio as a function of UV exposure time and depended upon the frequency change to some degree. The response time was also investigated.     

High contrast ratio of a vertically aligned liquid crystal cell using photocrosslinking alignment

A photo-induced alignment layer for LCDs has been successfully fabricated using the polarized UV photoreaction of a photo-crosslinkable polymer with the incident UV light at some angle with respect to the cell normal. The surface alignment and electro-optic properties were investigated for various UV exposure times. The homeotropic alignment layer showed a discrete anisotropic dichroic ratio, its surface morphology became smoother as the UV exposure time increased, and the LCs were arranged in a perpendicular direction to the PUVL direction with a proper pretilt angle. The cell showed no defects under cross-polarized microscopy and the contrast ratio was as high as 550:1 in transmittance. The contrast ratio pattern was found to be very similar to that of dichroic ratio as a function of UV exposure time and depended upon the frequency change to some degree. The response time was also investigated.     

Using "internal free volume" to increase chromophore alignment

Triptycenes have general applicability for increasing the alignment of fluorescent and dichroic dyes in LC hosts. Dyes containing varying numbers of triptycenes were synthesized to study the effect of free-volume alignment of triptycenes on the alignment of dyes. These dyes were designed such that multiple triptycenes could be incorporated and the triptycene-free volume is coincident to the aspect ratio of the dye, allowing a cooperative effect to increase their overall average alignment. With increasing triptycene incorporation, a stepwise increase in the alignment parameters of each dye was seen. It was also found that the attachment of one triptycene group has a negligible effect on the optical switching response times of the dyes. This can be a powerful tool for designing dyes with higher alignments for a variety of applications including guest-host reflective LCDs and holographic data storage.     

Spontaneous polarization and electro-optic behaviour of a ferroelectric liquid crystal cell fabricated with polyimide Langmuir-Blodgett films

The spontaneous polarization and electro-optic response of ferroelectric liquid crystals (FLCs) were investigated in a cell fabricated with a polyimide alignment layer coated by the Langmuir-Blodgett method. The surface properties of the cured polyimide layers were monitored by contact angle measurement, and by FTIR spectroscopy and AFM for the orientation and surface roughness, respectively. The apparent spontaneous polarization of an FLC determined in a practical sandwich-cell depended on various conditions such as cell thickness, cooling rate from the smectic A to chiral smectic C phase, and deposition pressure. Electro-optic response and decay times of FLCs were also measured. Furthermore, the ions in the FLC mixture reduced the magnitude of the effective electric field, but had no effect at high frequency.     

Aggregate formation of Rhodamine 6G in anisotropic solvents

The aggregation of Rhodamine 6G in liquid crystalline solutions was studied using polarised spectroscopy and in a guest-host system based on homogeneous-homeotropic alignment. The orientation of the dye molecules (guest) was controlled using an electric field, and this enabled the contrast ratio R of the dye to be obtained by electrically switching. The excitonic treatment of the absorption spectra suggests a coplanar structure for the dimeric aggregates.     

Hysteresis and ferroelectric liquid crystal displays

One cause of the hysteresis in the voltage-transmittance curve is polarization in the alignment layers brought about by the spontaneous polarization of FLC molecules. The hysteresis was reduced by increasing the conductivity of the alignment layers. The structural changes on switching were examined by measuring time-resolved shifts of FT-IR peaks of selected groups. The shifts of selected groups in the IR peak can be attributed to changes in the local dielectric constants of the ambient environment during switching by molecular orbital calculations. The orientation on switching in the SmC* phase was found to be similar to that in the N* phase.     

Synthesis and mesomorphic properties of chiral liquid crystals derived from (S)-lactic acid with 3-pentanol

The chiral swallow-tailed liquid crystals, 1-ethylpropyl (R)-2-[4-(4'-alkoxybiphenylcarbonyloxy)-phenoxy]propionates, EPmPBPP (m = 8-12), were prepared by using chiral (S)-lactic acid with 3-pentanol as starting materials. Mesophases and their corresponding transition temperatures were determined by polarizing microscopic textures and DSC. The results showed that all the chiral materials exhibited enantiotropic BP, N*, TGBA*, SmA*, and SmC* phases. Spontaneous polarization, dielectric constant and electro-optical response for the materials in the ferroelectric SmC* phase were investigated. It was noted that the electro-optical response of transmittance versus applied voltage obtained from the ferroelectric phase of material EPmPBPP (m = 10) displayed V-shaped switching, while that of other materials displayed the typical characteristics of ferroelectric hysteresis switching or U-shaped switching.     

Improving the electro‐optical properties of bistable reflective cholesteric liquid crystal displays

We have improved the electro‐optical properties of a bistable cholesteric liquid crystal display (Ch‐LCD) that is driven by a 3+2 dynamic driving scheme (3+2 DDS). The best contrast ratio is achieved at the selection period of 1.2–1.5 ms/line and the temperature range 26–30°C. The suitable preparation period and evolution period for contrast ratio are 40 times and 20 times the selection time, respectively. In the 3+2 DDS, rubbing the homeotropic alignment layer increases reflectivity of the ON state and keeps the reflectivity of the OFF state at the same level, so reflectivity and contrast ratio alike increase as a result. However, in a delayed homeotropic reset driving method, when the homeotropic alignment layer is rubbed, the reflectivity of both the ON and OFF states increases, thus the contrast ratio decreases. The combination of driving method and aligned surface morphology influences the relaxation mechanism in the cholesteric texture. By optimizing panel condition, we have demonstrated an 8.4″ foldable VGA Ch‐LCD that exhibits high reflectivity and contrast ratio with an addressing speed of around 1.2 ms/line.     

Improving the electro-optical properties of bistable reflective cholesteric liquid crystal displays

We have improved the electro-optical properties of a bistable cholesteric liquid crystal display (Ch-LCD) that is driven by a 3+2 dynamic driving scheme (3+2 DDS). The best contrast ratio is achieved at the selection period of 1.2-1.5 ms/line and the temperature range 26-30°C. The suitable preparation period and evolution period for contrast ratio are 40 times and 20 times the selection time, respectively. In the 3+2 DDS, rubbing the homeotropic alignment layer increases reflectivity of the ON state and keeps the reflectivity of the OFF state at the same level, so reflectivity and contrast ratio alike increase as a result. However, in a delayed homeotropic reset driving method, when the homeotropic alignment layer is rubbed, the reflectivity of both the ON and OFF states increases, thus the contrast ratio decreases. The combination of driving method and aligned surface morphology influences the relaxation mechanism in the cholesteric texture. By optimizing panel condition, we have demonstrated an 8.4' foldable VGA Ch-LCD that exhibits high reflectivity and contrast ratio with an addressing speed of around 1.2 ms/line.     

Effect of alignment layer on V-shaped switching in a chiral smectic liquid crystal

The influence of the alignment layer on V-shaped switching in thin homogeneous cells of a chiral smectic liquid crystal were studied by means of electro-optic and switching current measurements.Severalpolyimides withdifferent chemicalstructures wereusedas thealignment layer in thin homogeneous cells; V-shaped switching was observed with some of them and W-shaped switching with others. It was also shown that the switching current constantly flows during the V-shaped transmittance change. In order to clarify this effect of the alignment layer on V-shaped switching, polyimide films with various polarity and thickness were examined. It is concluded that thick alignment layers assist liquid crystal materials in realizing V-shaped switching, even if the materials are not ideal for this process.     

Shear aligned polymer dispersed ferroelectric liquid crystal devices

Fast switching liquid crystal devices can be produced by forming a dispersion of ferroelectric liquid crystal droplets in a polymer film. Such PDFLCs have been fabricated using a polymerization-induced phase separation technique involving ultraviolet photopolymerization, during which the film was sheared to obtain a uniform orientation of the liquid crystal medium. These birefringence devices show fast response times (sub-millisecond), optimum tilt angle (22.5°), and good contrast (∼ 30:1) at room temperature, using ferroelectric switching. We studied the tilt angles, response times and contrast ratio as a function of voltage and temperature to determine the effects of the preparation parameters on the electro-optic behaviour of these devices. Using a ferroelectric liquid crystal with long helical pitch, such devices appear to be bistable.     

Synthesis and mesomorphic properties of chiral liquid crystals derived from ( S )-lactic acid with 3-pentanol

The chiral swallow-tailed liquid crystals, 1-ethylpropyl (R)-2-[4-(4′-alkoxybiphenylcarbonyloxy)-phenoxy]propionates, EPmPBPP (m = 8?12), were prepared by using chiral (S)-lactic acid with 3-pentanol as starting materials. Mesophases and their corresponding transition temperatures were determined by polarizing microscopic textures and DSC. The results showed that all the chiral materials exhibited enantiotropic BP, N*, TGBA*, SmA*, and SmC* phases. Spontaneous polarization, dielectric constant and electro-optical response for the materials in the ferroelectric SmC* phase were investigated. It was noted that the electro-optical response of transmittance versus applied voltage obtained from the ferroelectric phase of material EPmPBPP (m = 10) displayed V-shaped switching, while that of other materials displayed the typical characteristics of ferroelectric hysteresis switching or U-shaped switching.     

A novel liquid crystals photoalignment layer-by-layer self-assembled film fabricated with long side-chain cinnamate polyelectrolyte

A novel photoalignment film for liquid crystals (LC) was prepared based on layer-by-layer self-assembly of photosensitive long side-chain cinnamate polyelectrolyte. A series of self-assembled films with different methylene spacer groups was prepared and used as alignment film. The film became anisotropic, and could induce uniform alignment of LC after irradiation by linearly polarised ultraviolet light (LPUVL). The effects of spacer chain lengths of the cinnamoyl polycations on the structure and photoalignment properties of the self-assembled film were studied. The polycation films with longer spacer chain obtained a larger dichroic ratio after LPUVL irradiation. The contrast ratio (T _max/T _min) of the LC cell increased with spacer chain length increase. However, it was found that the thermal stability of PSS/PACPYn films decreased with increasing chain length of polycation.