Helical unwinding in polymer-dispersed ferroelectric liquid crystals

We demonstrate that ferroelectric liquid crystals dispersed in a polymer matrix can form uniaxially aligned elliptical droplets. The alignment is controlled by mechanical shear during the polymerization of a UV-curable adhesive. The possibility of prealignment makes polymer-dispersed ferroelectric liquid crystals suitable for application in flexible electro-optical displays. In contrast to surface stabilized ferroelectric liquid crystal displays, the electro-optical effect in our system is due to the deformed-helix ferroelectric (DHF) effect.     

固化条件对聚合物分散液晶膜电光性能的影响

选用聚乙二醇二缩水甘油醚(EGDE)/季戊四醇缩水甘油醚(PERTGE)/1,8-二氨基-3,6-二氧杂辛烷(EDBEA)/向列相液晶(SLC1717)复合体系,在不同的固化条件下,通过热聚合诱导相分离方法制备了一系列电光性能不同的聚合物分散液晶(polymer dispersed liquid crystal,简称PDLC)膜.研究了固化温度和固化时间对制备的PDLC膜中聚合物网络的微观形貌和电光性能的影响.结果表明,随着固化温度的升高以及固化时间的缩短,PDLC膜的对比度、驱动电压和开态响应时间逐渐增大,而关态响应时间逐渐减小.在固化温度为363.2 K,固化时间为7 h时,所制备的PDLC膜具有较佳的电光性能.     

Novel liquid crystal mixtures for a surface-stabilized ferroelectric LCD

We synthesized novel fluorine-substituted chiral compounds having a fluorine atom at an asymmetric carbon and a difluorinated biphenyl ring as a core of a liquid crystal molecule by means of an original method. The ferroelectric mixtures were obtained by mixing the chiral compounds with the non-chiral liquid crystal mixture with a wide SmC temperature range between 29 C (crystallized) and 78 C. They show a large P s greater than 10 nC cm -2 at 5 wt% of the chiral compound. Two types of cell, called 'parallel' and 'antiparallel', were fabricated according to the relative direction of the rubbing direction on the substrates. The parallel cells filled with the FLC mixtures show the usual bistable SSFLC (surface-stabilized ferroelectric liquid crystal). The fast response time of 60mus (22V pp pulse width 250mus, at room temperature) was obtained. The apparent cone angle was 45.2 in the switching state and 40.5 in the memory state. On the other hand, the antiparallel cells show an unusual monostable behaviour, i.e. the director falls back to the original configuration when the applied voltage is switched off. The surface-stabilized monostable cells show very attractive characteristics for application for a TFT-active matrix LCD; a high contrast of 81 :1, a fast response time (of the order of 1ms), and an analogue-grey scale with excellent linearity within the low voltage range below 4 V.     

Linear optical switching in a FLC/waveguide composite device

A fast electrooptic modulation in a polymer waveguide using a ferroelectric liquid crystal has been proposed. In this device, the surface stabilized ferroelectric liquid crystal and the soft mode ferroelectric liquid crystal are used as an active material on the passive polymer waveguide, and electrooptic switching is realized by controlling the total reflection at the polymer waveguide-liquid crystal interface. The response time is of the order of several microseconds. The analogue electrooptic modulation in the waveguide is realized using the field induced linear molecular tilt of the electroclinic effect in the soft mode ferroelectric liquid crystal.     

Effects of the chain length of crosslinking agent and dye-doped amount on the electro-optical properties of polymer-dispersed liquid crystal films prepared by nucleophile-initiated thiol-ene click reaction

ABSTRACT

A series of crosslinking agents with different chain lengths were used to explore their effect on electro-optical properties of dye-doped polymer dispersed liquid crystals (PDLCs) film prepared by nucleophile-initiated thiol-ene click reaction. The influences of dye contents and temperatures on electro-optical properties were also investigated. It was found that the increase in chain length of crosslinking agent leads to the increase in driving voltage and the decrease in memory effect. The decrease in reaction temperature causes a decrease in driving voltage and an increase in transmittance. Particularly, the dye content could be optimised to obtain promising materials with minimum driving voltage and high contrast ratio for display applications.     

Cyano terminated tolane compounds for polymer dispersed liquid crystal application: relationship between cyano terminated tolane based molecular structures and electro-optical properties

ABSTRACT

The structures of the liquid crystal (LC) molecules have a key role in impacting the electro-optical performance of a polymer dispersed liquid crystal (PDLC) film. In this paper, the relationship between the LC molecular structures and the electro-optical properties of PDLC films is investigated based on an unexplored cyano-terminated tolane compounds (CTTCs) doped E8 LCs/UV polymers system. Due to the high polarity of CTTCs, LCs doped with the cyano-terminated tolane (CTT) molecules exhibit high birefringence and large positive dielectric anisotropy. On the whole, PDLC films doped with the CTT molecules exhibit a lower driving voltage than that doped with the pure E8. More excitingly, PDLC films based on CTT molecules with larger length-to-width ratio and longer conjugated system show higher contrast ratio (CR) and faster response time. Eventually, the mechanism of the effects of CTT-based molecular structures and the relationship between the electro-optical performance of PDLC films and CTT molecules are illustrated. This work paves a new way for optimising the electro-optical properties of PDLC films.     

Linear optical switching in a FLC/waveguide composite device

Abstract

A fast electrooptic modulation in a polymer waveguide using a ferroelectric liquid crystal has been proposed. In this device, the surface stabilized ferroelectric liquid crystal and the soft mode ferroelectric liquid crystal are used as an active material on the passive polymer waveguide, and electrooptic switching is realized by controlling the total reflection at the polymer waveguide-liquid crystal interface. The response time is of the order of several microseconds. The analogue electrooptic modulation in the waveguide is realized using the field induced linear molecular tilt of the electroclinic effect in the soft mode ferroelectric liquid crystal.     

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.     

Liquid crystal-polymer composites. Anomalous electro-optical curve

Liquid crystal-polymer composites containing a low proportion of polymer are of interest because of their low driving voltage, but they can show anomalous electro-optical curves, such as a minimum in the electro-optical curve. We explain the existence of this minimum, by a partial orientation of liquid crystal domains, together with the presence of mobile polymer fragments. This partial orientation is visible using a polarizing microscope, and the presence of polymer that is not linked to the main network is proved by size exclusion chromatography of the liquid crystal extracted from the composite.     

The electro-optical properties of 'charged' PDLCs

The application of high intensity electric fields to polymer dispersed liquid crystal (PDLC) films can induce changes in their electro-optical properties and morphology. In particular, a quasilinear electro-optical response to an external electric field can be achieved if an internal built-in d.c. field is induced. In this work, we show how the liquid crystal/polymer weight ratio influences the electro-optical response of 'charged' PDLCs, i.e. of PDLC films after the application of a high intensity electric field. We observed that a quasilinear electro-optical response can be achieved in a well determined range of composition. Larger liquid crystal concentrations are unable to maintain the built-in field, while PDLCs with lower liquid crystal loadings do not allow the onset of a built-in d.c. field.     

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.     

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.     

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.     

不同光滑度立方体氧化铁对液晶电光性能的改善

向列相液晶被广泛应用于液晶显示中,但是由于杂质的存在,会导致液晶的驱动电压变大,增加能耗。 为了降低阈值电压和饱和电压,通常向液晶中添加纳米颗粒来提高电光性能。 本文利用水热法制备了表面粗糙和光滑的两种立方体Fe₂O₃纳米颗粒,其形貌均匀,尺寸约550 nm。 将二者分别掺杂到向列相液晶E7中,结果表明,粗糙立方体Fe₂O₃/E7复合体系具有比光滑立方体Fe₂O₃/E7复合体系和向列相液晶E7更优的电光性能,且在掺杂质量分数为0.4%时,其电光性能达到最优,阈值电压和饱和电压分别降低9.9%和11.6%,对比度增大80%,响应时间降低至6.0 ms。 这归因于粗糙立方体Fe₂O₃具有足够的表面积和表面所带电荷更多,所以会更易吸附体系中的杂质离子和减弱杂质离子的屏蔽作用,从而提高了电光性能。     

UV tuning of the electro-optical and morphology properties in polymer-dispersed liquid crystals

Polymer-dispersed liquid crystal (PDLC) films operating in reverse mode are transparent electro-optical devices, which can be turned into an opaque state by application of a suitable electric field. The effect was investigated of different UV powers, used during the polymerization process, on the electro-optical and morphology properties of PDLCs, working in reverse mode operation. Films were obtained by UV polymerization of mixtures of a low molecular weight nematic liquid crystal and a photopolymerizable liquid crystal monomer, homeotropically aligned by rough conductive surfaces. The electro-optical and morphology properties of samples were related to the polymerization conditions. Samples polymerized by lower UV powers exhibited “polymer ball” morphology and an electro-optical response due to the liquid crystal director reorientation, whereas samples obtained at higher UV powers showed a “Swiss cheese” morphology and an electro-optical response due to dynamic scattering. In addition, we observed by conductivity and IR measurements that UV exposure induces a degradation of the nematic liquid crystal.     

The in-plane switching of homogeneously aligned nematic liquid crystals

We have investigated the electro-optical effects and physical switching principle of homogeneously aligned nematic liquid crystals when applying an in-plane electric field with interdigital electrodes. By using the in-plane switching (IPS) of the liquid crystals which is achieved by the in-plane electric field, the viewing angle characteristics of the electro-optical effects were confirmed to be far superior to those of the conventional twisted nematic mode in which the electric field is applied along the direction perpendicular to the substrates. The non-reversal region of grey scales was extremely wide in which a high contrast ratio was kept, even along quite an oblique direction in the IPS mode. In order to clarify the switching principle of the liquid crystals in the IPS mode, a simplified expression describing the threshold behaviour of the device was derived with the assumption that a uniform in-plane electric field was applied along a direction perpendicular to the director and parallel to the homogeneously aligned nematic slab, and found to be sufficiently able to explain the experimental results. First, a critical field at which the liquid crystals just began to twist, was found to be proportional to the reciprocal of the cell gap. Second, it was the electric field and not the voltage that drives the liquid crystals. This relationship was due to the independence of the electric field regarding the liquid crystal layer normal direction. So the threshold voltage in the IPS mode was strongly dependent on the variation of the cell gap. For the dynamical response mechanism of the liquid crystals to the in-plane electric field, the switching on and off processes of the liquid crystals were analysed quantitatively. The relaxation time of the liquid crystals when removing the electric field could be described as proportional to the square of the cell gap. A thinner cell gap also proved to be effective in obtaining a fast response time in the IPS mode. In contrast, the switching on time when applying the in-plane electric field was found to be inversely proportional to the difference between the square of the electric field strength and the square of the critical electric field strength at which the liquid crystals began to deform.     

Enhancement of frequency modulation response time for polymer-dispersed liquid crystal

Through the ferroelectric nanoparticles of BaTiO₃ (BTO) doping, the response time for the frequency modulation of the polymer-dispersed liquid crystal (PDLC) was improved. The BTO-doped PDLC cells were prepared by polymerisation induced phase separation (PIPS) process using UV light. The capacitance of the PDLC composites was measured with an impedance analyzer in the frequency range of 100 Hz–1 MHz at 1 V. The dynamic signal for the response time of the PDLC devices was monitored through a digital oscilloscope. The electro-optical properties of the PDLC were found to strongly depend on the doped BTO concentration. The BTO doping caused a large increase in the capacitance. The dielectric constants were drastically decreased in the samples with rather low BTO doping ratio at a high frequency. No outstanding difference in the rising time of the LC was observed in the BTO-doped PDLC device, but the falling time was significantly decreased from 0.334 to 0.094 s. The present results imply that the nanoparticle-doping technology could improve the electro-optical performance of the PDLC requiring fast response and frequency modulation, such as optical modulators and PDLC-hybrid electroluminescence device for flexible electronic devices.     

Ferroelectric liquid crystal composites based on the porous stretched polyethylene films

Electro-optically active polymer–liquid crystal composites based on ferroelectric liquid crystals and stretched porous polyethylene films were developed. The alignment of ferroelectric liquid crystals incorporated into the porous polyethylene films with average porous diameter of around 200 nm was observed and studied. It was shown experimentally that these samples containing ferroelectric liquid crystals are flexible electro-optical films exhibiting a saturation electric field near 2·10⁷ Vm^?1 and a response time of about 30 μs under the action of the saturation field. A simple theoretical model of ferroelectric liquid crystal molecules' complete reorientation in electric fields inside pores of the films has been proposed and confirmed experimentally.     

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.     

Fast-switching effect using viscous chiral-nematic materials

We report on chiral-nematic mixtures containing viscous chiral dopants, composed of rigid rod-like chiral molecules, in order to solve the problem of backflow in the middle layers of the liquid crystal cells. During the study, the viscoelastic properties of the liquid crystal compositions have been optimised, as well as the helical twisting power of the chiral compounds, the anchoring energy and the pre-tilt angle of alignment materials, the ratio between the cell gap and the helical pitch (d/P₀). After optimisation, we prepared test cells with low operating voltage – from 1.5 till 5.0 V, fast response time (τ_on + τ_off) – less than 1 ms, wide operating temperature range from ?40°C till +100°C, and good thermal stability. It should be noted that the response time of the new electro-optical mode does not strongly depend on the cell gap, which is in contrast to other known operating modes. We have achieved rather linear than quadratic dependence of the response time when changing the thickness of the cell.