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.     

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.     

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.     

Photoinduced grafting polymerization onto the surface with the use of radiation of high-power ultraviolet light-emitting diodes

The photoinduced grafting polymerization of acrylic acid onto the surface of a polypropylene film under the action of radiation with a wavelength of 365 nm and an intensity of 8–193 mW/cm² emitted by high-power UV light-emitting diodes is studied. Grafting is performed from a thin layer of the aqueous solution of the monomer onto the surface containing a photoinitiator (benzophenone) deposited from a solution in a volatile solvent. The amount of graft polymer is determined with the use of ATR FTIR spectroscopy. At an intensity of UV radiation of 193 mW/cm², the time of attainment of grafting-polymerization saturation is decreased by a factor of 6 relative to this time at an intensity of 8 mW/cm², typical for emitters based on mercury lamps. The rate of grafting polymerization in the studied UV-radiation-intensity range grows in proportion to the square root of intensity.     

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.     

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.     

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.     

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.     

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.     

Polarization‐Driven Self‐Powered Photodetection in a Single‐Phase Biaxial Hybrid Perovskite Ferroelectric

Self‐powered photodetection driven by ferroelectric polarization has shown great potential in next‐generation optoelectronic devices. Hybrid perovskite ferroelectrics that combine polarization and semiconducting properties have a promising position within this portfolio. Herein, we demonstrate the realization of self‐powered photodetection in a new developed biaxial ferroelectric, (EA)₂(MA)₂Pb₃Br₁₀ ( 1 , EA is ethylammonium and MA is methylammonium), which displays high Curie temperature (375 K), superior spontaneous polarization (3.7 μC cm^?2), and unique semiconducting nature. Strikingly, without an external energy supply, 1 exhibits an direction‐selectable photocurrent with fascinating attributes including high photocurrent density (≈4.1 μA cm^?2), high on/off switching ratio (over 10⁶), and ultrafast response time (96/123 μs); such merits are superior to those of the most active ferroelectric oxide BiFeO₃. Further studies reveal that strong inversion symmetry breaking in 1 provides a desirable driving force for carrier separation, accounting for such electrically tunable self‐powered photoactive behaviors. This work sheds light on exploring new multifunctional hybrid perovskites and advancing the design of intelligent photoelectric devices.     

A coloured ferroelectric side chain polymer

Abstract

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.     

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.     

Electrically Switchable Persistent Spin Texture in a Two-Dimensional Hybrid Perovskite Ferroelectric

As a momentum-independent spin configuration, persistent spin texture (PST) could avoid spin relaxation and play an advantageous role in spin lifetime. Nevertheless, manipulation of PST is a challenge due to the limited materials and ambiguous structure–property relationships. Herein, we present electrically switchable PST in a new 2D perovskite ferroelectric, (PA)₂CsPb₂Br₇ (where PA is n-pentylammonium), which has a high Curie temperature of 349 K, evident spontaneous polarization (3.2 μC cm⁻²) and a low coercive electric field of 5.3 kV cm⁻¹. The combination of symmetry-breaking in ferroelectrics and effective spin-orbit field facilitates intrinsic PST in the bulk and monolayer structure models. Strikingly, the directions of spin texture are reversible by switching the spontaneous electric polarization. This electric switching behavior relates to the tilting of PbBr₆ octahedra and the reorientation of organic PA⁺ cations. Our studies on ferroelectric PST of 2D hybrid perovskites afford a platform for electrical spin texture manipulation.     

Polarization and viscosity measurements of ferroelectric liquid crystalline monomer polymer mixtures

Measurements of spontaneous polarization, tilt angle and response time on mixtures of a ferroelectric side chain polyacrylate with a low molecular mass liquid crystal of a very similar structure are reported. From the obtained values the rotational viscosity for the ferroelectric switching process is calculated. All mixtures exhibit an Arrhenius-like temperature dependence of the rotational viscosity, the values for the polymer being three orders of magnitude higher than for the low molecular mass compound.     

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.     

Comparative analysis of basic physical properties of a ferroelectric liquid crystal and a polymer dispersed ferroelectric liquid crystal

A polymer film of polyvinylbutyral with dispersed droplets of a ferroelectric liquid crystalline mixture (FLC309c) has been prepared and characterized. The collective processes have been studied by dielectric relaxation spectroscopy in the frequency range 10 Hz to 13 MHz. In comparison with the FLC309c mixture, the polymer dispersed ferroelectric liquid crystal (PDFLC309c) based on FLC309c exhibits a Goldstone-like mode relaxation with a much higher relaxation frequency, but a smaller dielectric strength than the Goldstone mode observed for the FLC309c mixture. The spontaneous polarization of PDFLC309c decreases by nearly one order of magnitude in comparison with FLC309c, while the tilt angle decreases by 20%. Considering these results, we believe that a non-switching region exists near the polymer boundaries and that significant deformations of the helical structure occur due to stronger anchoring.     

A novel ferroelectric liquid crystal having a trifluoromethyl group at the chiral centre

A novel ferroelectric liquid crystal (FLC), 4-(1,1,1-trifluoro-2-octyl)phenyl 4'-nonyloxybiphenyl-4-carboxylate has been synthesized using optically active 4,4,4-trifluoro3-(4-methoxyphenyl)butanoic acid. Although the FLC seemed to satisfy an empirical rule for a large spontaneous polarization (Ps) value, the observed maximum Ps value, c. 100 nCcm-2, was not as large as expected. However a relatively short response time of 68 mus at 50 C was observed.     

Hysteresis-free electro-optical switching in conductive ferroelectric liquid crystals: experiments and modelling

The hysteresis-free electro-optical switching, or so-called V-shaped, regime has been studied in a commercial ferroelectric liquid crystal (FLC) mixture having a smectic C* phase with a very small value of spontaneous polarization. The FLC was introduced into commercial EHC cells with thin aligning layers. In such cells V-shaped switching could be observed only at very low frequencies, less than 1 Hz. However, when the same material is strongly doped with a conductive impurity, its conductivity markedly increases and hysteresis-free switching is observed over a wide range of applied frequencies and voltages. Experimental results are in good agreement with computer modelling carried out as part of this work. The modelling takes into account all the important parameters of smectic C* FLC: non-polar anchoring conditions, possible bookshelf and chevron structures, the capacitance of the aligning layers and the conductivity of a FLC. The last two factors appear to be the most crucial for hysteresis-free switching in the smectic C* phase.     

Synthesis and physical properties of laterally fluoro substituted ferroelectric liquid crystals with a fluoro substituted chiral terminal chain

A new series of ferroelectric liquid crystals (FLCs) has been designed for active matrix displays based upon the chiral smectic C phase. The FLCs have been derived from optically active fluorinated alkanols and a laterally fluoro substituted biphenylyl-1,3-dioxan core. Their physical properties such as spontaneous polarization, current response time, and tilt angle have been determined. The FLC derived from 2-fluoro-octanol showed a very short current response time of 10μs at T _Sm*-N* - T= 10°C, while another FLC with the same core derived from 5-fluoro-octanol gave a value of 150μs.