Evolution of the switching current during the preparation of polymer network-ferroelectric liquid crystal microcomposites

The polymer network-ferroelectric liquid crystal (FLC) microcomposites are prepared by UV crosslinking of a chiral prepolymer diluted in a FLC. A fast cross-linking process involving 20 mW cm^-2 UV intensity produces material with ferroelectric properties which have strongly deteriorated compared with the pure FLC. By a slow process using a UV intensity of 2 mW cm^-2 the ferroelectric properties are less affected. It is supposed that the very dense polymer network, which arises during the fast process, is responsible for hindering the switching process and decreasing the spontaneous polarization. The photopolymerization kinetics are determined by measuring the switching current during the UV irradiation. The characteristic time of cross-linking is mainly reflected in the evolution of the spontaneous polarization and rotational viscosity with time.     

Photomechanically induced phase transitions in ferroelectric liquid crystals

We demonstrate that the addition of small amounts of a novel azo-dye to a ferroelectric liquid crystal and illumination with low intensity (∼ 0.8 mW cm^-2) UV light can result in reversible, isothermal phase transitions and dramatic changes in the properties of the system. In particular we examine light induced transitions between the S*₁, and S*_C phases and the consequent photomechanical regulation of spontaneous polarization.     

Fast switching ferroelectric side-chain liquid-crystalline polymer and copolymer

New ferroelectric side-chain liquid-crystalline polymers, a copolymer and a homopolymer, with siloxane backbone and a triaromatic mesogen as the side group have been synthesized. The materials exhibit a chiral smectic C phase over a large temperature range extending to room temperature. They possess high values of spontaneous polarization: 105 nC cm^-2 for the homopolymer and 180 nC cm^-2 for the copolymer. The electro-optic switching time in the chiral smectic C phase is extremely fast (150 μs). In the smectic A phase, an electroclinic effect with switching times less than 100 μs and with field induced tilt angles of 18° is observed.     

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.     

Photomechanically induced ferroelectricity in smectic liquid crystals

We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm^-2) UV light. In the case of a sample initially in the S*_c phase, this results in a fall in the magnitude of spontaneous electrical polarization (P_s) and changes in electro-optic switching characteristics. If the sample is illuminated in the S_A phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the S_A phase, a reversible, isothermal transition to a biphasic S*_c/isotropic state occurs. In this case, the P_s is seen to rise from zero in the S_A phase to a finite value(∼2 nC cm^-2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*_c phase and UV illuminated, a more complicated variation of P_s occurs with the sample again undergoing a transition to a biphasic S*_c/isotropic state. Possible mechanisms for the transition are discussed.     

A coloured ferroelectric side chain polymer

Coloured ferroelectric liquid-crystalline side chain copolymers containing 5 per cent and 15 per cent of an azo dye were synthesized and characterized by DSC, GPC and optical microscopy. Order parameters, S, of the azo compound exceeding 0·8 were measured in the frozen smectic phase for the 5 per cent copolymer. The copolymers exhibit fast electrooptic switching in the range of a few hundred microseconds to milliseconds in the S_c* phase. Both linear (i.e. electroclinic-like) and ferroelectric switching have been observed. Tilt angles of ∼19° and spontaneous polarization of ∼300nC cm^-2 have been recorded in the S_c* phase.     

New low molar mass organosiloxanes with unusual ferroelectric properties

A new low molar mass chiral organosiloxane mesogen and its racemic analogue have been synthesized and their mesomorphic and ferroelectric properties investigated. The chiral derivative, denoted A*B, exhibits one tilted enantiotropic ferroelectric smectic C mesophase over a broad temperature range, with very high tilt angles and moderate spontaneous polarization (36° and 19 nC cm^-2 at 20°C). The achiral siloxane derivative, denoted A*B, exhibits one broad enantiotropic smectic C phase. Preliminary electro-optic measurements indicate that the spontaneous polarization is weakly dependent on temperature between 10°C and 50°C, the latter being the S*_c to isotropic phase transition. The tilt angle and layer spacing are temperature independent, and current response times of less than 200 μs were measured at 25°C for fields of 10 V μ^-1. These results are discussed in comparison with those for side chain polymer liquid crystal structures and other low molar mass ferroelectric materials.     

Effect of lateral substitution by fluorine and bromine atoms in ferroelectric liquid crystalline materials containing a 2-alkoxypropanoate unit

Two homologous series of ferroelectric liquid crystalline compounds with 2-alkoxypropanoate chiral unit containing biphenyl benzoate core laterally substituted by fluorine and bromine have been synthesized and studied. All compounds possess the ferroelectric smectic C* phase over a broad temperature range. For bromine-substituted compounds values of spontaneous polarization reach high values up to 250 nC cm^-2. The effects of the lateral substitution on the phenyl ring far from the chiral centre by methyl and methoxy groups, fluorine, chlorine and bromine atoms on mesomorphic properties and on values of the spontaneous polarization are discussed.     

Photorefractive effect in composites of ferroelectric liquid crystal and photoconductive polymer

The photorefractive effect in composites of a ferroelectric liquid crystal (FLC) and several photoconductive polymers was investigated. The photorefractivity of mixtures of photoconductive polymers and an FLC (polymer/FLC), as well as that of photoconductive-polymer-stabilized ferroelectric liquid crystals (PPS-FLCs) was examined. The polymer/FLC samples exhibited two-beam coupling gain coefficients of about 6∼12 cm^-1 in a 5 µm gap cell. The photopolymerization of a methacrylate monomer in the FLC medium established a polymer-stabilized state in which the alignment of FLC molecules was mechanically stabilized. The noise in a two-beam coupling signal was reduced significantly in the PPS-FLC samples.     

Fluorescent ferroelectric liquid crystalline copolyacrylates

Fluorescent liquid crystalline side chain polymers were synthesized by copolymerization of a ferroelectric monomer and 5 per cent of various blue fluorescent naphthalic imide dye comonomers. Those copolymers were characterized by DSC, X-ray, GPC and optical microscopy. In favourable cases, fast switching fluorescent ferroelectric polymers resulted, exhibiting high tilt angles (up to ∼ 34°) and spontaneous polarization values (up to ∼ 115 nC cm^-2) in the S*_c phase. One fluorescent copolymer shows orthogonal smectic phases exclusively due to the structure of the incorporated fluorescent comonomer. In this case a strong electroclinic effect and high induced tilt angles (12° 10 V μm^-1) have been observed in the S_a phase. Order parameters, S, of the dye moieties up to 0.64 were measured in the room temperature S_b phase for the copolymers     

Fast-switching ferrolectric liquid crystalline polymers containing one or two centres of chirality in the side chain

In a liquid crystalline side chain polyacrylate containing one center of chirality in the terminal alkyl chain of the mesogenic part, switching times of 200–400 μs were measured in the SmC^* phase. Below this phase an unidentified phase exists, which shows electroclinic-like switching. The phase transition between those two phases can be shifted by applying an electric field. At higher molecular weights three subphases emerge in the SmC^* region. Variation of the spacer length revealed, for the first time, ferroelectric switching even at a spacer length of only two CH₂ groups. By shifting the centre of chirality into the spacer of the side group a polymer resulted, which shows electroclinic switching in the SmC^* phase, changing to ferroelectric switching when the voltage is increased. Incorporation of an oxirane ring as chiral building block into the spacer yielded a polymer that shows a sign inversion of the spontaneous polarization in the SmC^* phase. A polymer containing a dioxolane carbonic ester as chiral unit exhibits three switching states, with the third state existing at a low or zero electric field. This phenomenon is known for antiferroelectric liquid crystals. By doping a racemic LC polymer with a chiral monomeric LC we induced a spontaneous polarization. Colored FLC polymers were obtained by two different approaches. In an FLC–dye copolymer, increasing switching speed in three different chiral smectic phases was observed when increasing the dye concentration.     

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.     

Two ferroelectric phases of a columnar dibenzopyrene

The ferroelectric switching of columnar 1,2,5,6,8,9,12,13-octakis-((S)-2-heptyloxy-propanoyloxy)dibenzo[e,l]pyr-2-heptyloxy-propanoyloxy)dibenzo[e,1]pyrene was studied in detail between 60°C and the clearing point 115°C. The switching angle (optical tilt angle) is ±24.5° up to 10V μm^-1 and ±37° at higher field strengths. The electric polarization is 60nC cm^-2 in the low field phase and 180nC cm^-2 in the high field phase. The switching rate has apparent activation energies that increase with voltage from 3 × 10^-19 to 10^-18 J. It varies roughly with a power of the voltage, the exponent increasing with decreasing temperature from 2 to 5. At equal voltages, switching is faster in the low field than in the high field phase. We tentatively infer the structures of the two columnar lattices from the ratio of polarizations and other data. From the switching angles, we calculate a tilt angle of 44° for the aromatic cores of the disc-like molecules stacked in the columns. Finally, we point out possible advantages of ferroelectric columnar liquid crystals over their smectic counterparts in electro-optical displays.     

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.     

New ferroelectric liquid crystals with a trifluoromethyl group

New chiral dopants, possessing a trifluoromethyl group, have been synthesized. These dopants have different polar groups, located between the chiral centre and the diphenylacetylene core. The polar part is chosen from ester, methylene ether and ether linkages. Each new chiral dopant was mixed with a non-chiral liquid crystal mixture or a ferroelectric liquid crystal mixture, and their transition temperatures and electrooptic properties measured. The largest spontaneous polarization, 4·1 nC cm^-2, was obtained at 25°C for the ferroelectric liquid crystal mixture in which the dopant, with a methylene ether linkage, was incorporated. Molecular orbital calculations (MNDO method) for the dopants show that the calculated dipole moments are correlated with the measured spontaneous polarization, and the calculated results explain the experimental results quite well. In addition, it has been found that the conformation of the chiral part is an important factor which affects the magnitude of spontaneous polarization.     

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.     

On the relationship between electro-optic and dielectric parameters of a smectic C* ferroelectric liquid crystal

A relationship between the electro-optic switching time and dielectric parameters of a S*_c ferroelectric liquid crystal (FLC) is obtained. This relationship is derived in terms of spontaneous polarization P_s, relaxation time τ_G and dielectric strength Δε_G of the Goldstone mode. It shows clearly that the switching phenomenon in FLCs is governed by the dielectric behaviour of the Goldstone mode. Based on the Landau model, the switching time has also been related to the material parameters of the FLC.     

利用强度调制光电流/光电压谱研究碳点/KOH电解液界面的动力学行为

通过强度调制光电流谱(IMPS)和强度调制光电压谱(IMVS)技术研究在光电分解水制氢体系中碳点光阳极与KOH电解液界面的动力学行为。结果表明,光强在30–90 m W·cm~(-2)范围内,界面的电子传输时间(τ_d)、电子寿命(τ_n)、电子扩散系数(D_n)、电子扩散长度(L_n)均没有变化;当光强增加到110和130 m W·cm~(-2)时,τ_d和τ_n延长,而D_n减小。实验表明,不同于Ti O_2/电解液等界面,碳点光电极/电解液界面中碳点电极存在的缺陷少,因此电子主要以无陷阱限制扩散方式传输为主。且在30–130 m W·cm~(-2)的光强范围内,与τ_d和τ_n相关的载流子收集效率(η_(cc))相近。     

Structure-property relationships of 'diluted' ferroelectric polysiloxanes

A series of new ferroelectric copolysiloxanes with systematically varied comonomer content ('dilution') has been synthesized. Good planar alignment could be achieved for all copolysiloxanes and they were studied with respect to their mesomorphic and ferroelectric properties. Broad enantiotropic S*_C phases and spontaneous polarizations up to 286 nC cm^-2 are found. X-ray diffraction experiments show a linear increase of the smectic layer spacing by 'dilution'. This points to a microphase separated structure of mesogenic groups and siloxane chains. It is found that the fixation of a mesogen to homopolysiloxane leads to an increase of P_s, whereas the 'dilution' of the mesogens with dimethylsiloxane units decreases P_s again. Based on the microphase separated model, it can be shown that the decrease of P_s is not only due to the decrease of the vol % of mesogenic groups. The coupling between different mesogens mediated by the polymer chain, has additionally to be taken into consideration. A remarkable drop in the response times τ with decreasing mesogen content is confirmed and switching times less than 1 ms were measured.     

Measurement of the rotational viscosity of ferroelectric liquid crystals based on a simple dynamical model

Rotational viscosity and spontaneous polarization are the most important properties of a ferroelectric liquid crystal with regard to its switching time in surface stabilized or a.c. field stabilized displays. Whereas there is an abundant literature about spontaneous polarization, only a few attempts have been made to determine the rotational viscosity. We set up a model for the electric response of a ferroelectric liquid crystal cell on application of an electric field. For the application of a triangular wave voltage we derive a relation between the rotational viscosity, the spontaneous polarization, the tilt angle, the maximum induced polarization current and the electric field strength. Experiments are carried out on several ferroelectric liquid crystals and the derived relation was used to determine the rotational viscosity. The relation between the rotational viscosity and the polarization on the one hand and the optical switching time on the other hand is discussed in some detail.