Polymer-stabilized Ferroelectric Liquid Crystals Research Articles

A Review of Developments in Polymer Stabilized Liquid Crystals.

Polymer-stabilized liquid crystals (PSLCs) are multi-functional materials consisting of polymer networks in a continuous phase of liquid crystals (LCs), of which polymer networks provide anchoring energy to align the LCs. A number of improvements are detailed, including polymer-stabilized nematic liquid crystals (PSNLCs), polymer-stabilized cholesteric liquid crystals (PSCLCs), polymer-stabilized blue phase liquid crystals (PSBPLCs), polymer-stabilized smectic liquid crystals (PSSLCs), polymer-stabilized ferroelectric liquid crystals (PSFLCs), and polymer-stabilized antiferroelectric liquid crystals (PSAFLCs) in this review. Polymer stabilization has achieved multiple functionalities for LCs; in smart windows, a sufficiently strong electric field allows the LCs to reorient and enables switching from a scattering (transparent) state to a transparent (scattering) state. For broadband reflectors, the reflection bandwidth of LCs is manually tuned by electric fields, light, magnetic fields, or temperature. PSBPLCs open a new way for next-generation displays, spatial light modulators, sensors, lasers, lenses, and photonics applications. Polymer networks in PSFLCs or PSAFLCs enhance their grayscale memories utilized in flexible displays and energy-saving smart cards. At the end, the remaining challenges and research opportunities of PSLCs are discussed.

Surface effects in the model of polymer-stabilized ferroelectric liquid crystal cells

The interplay between volume and surface interactions in polymer-stabilized ferroelectric liquid crystals often results in the so-called quasi-bookshelf or tilted layer structure. Unambiguous description of the most probable orientation of the long molecular axes within a ferroelectric liquid crystal cell (director profile) stabilized by polymer network implies the consideration of the confined volume effects. The model of polymer-stabilized ferroelectric liquid crystal cell with a quasi-bookshelf layer structure was investigated by using the liquid crystal continuum theory. Promising applications of the biaxial surface potential for preparation of ferroelectric liquid crystal cells with the desired surface parameters has motivated us to embark upon the proposed model. To fully consider the surface effects, the proposed model accounts splay deformation of the spontaneous polarization. This enables us to estimate the effective applied voltage across the cell. The effect of polymer stabilization on the director orientation profiles across the cell was examined. We have found that the director-polymer network interaction coefficient induces insignificant difference between the director orientation profiles. We believe that this theoretical model can be useful for fabrication of experimental ferroelectric liquid crystal cells.

Biaxial surface potential effects in polymer-stabilized ferroelectric liquid crystal cells

Biaxial surface potential effects in polymer-stabilized ferroelectric liquid crystal cells

A Review of Polymer-Stabilized Ferroelectric Liquid Crystals.

The polymer stabilized state of ferroelectric liquid crystals (FLC) is reviewed; and the effect of a dispersed polymer network in an FLC outlined and discussed. All fundamental material aspects are demonstrated; such as director tilt angle; spontaneous polarization; response time and viscosity; as well as the dielectric modes. It was found that the data can largely be explained by assuming an elastic interaction between the polymer network strands and the liquid crystal molecules. The elastic interaction parameter was determined; and increases linearly with increasing polymer concentration.

Influence of Fabrication Conditions on Monostability of Polymer-Stabilized Ferroelectric Liquid Crystals

Polymer-stabilized ferroelectric liquid crystals (PS-FLCs) photocured in the SmA phase can show monostable V-shaped electrooptical characteristics. In this study, we have researched the influence of the fabrication conditions such as polymer concentration, cell gap, and alignment film materials on the monostable electrooptical characteristics of PS-FLCs. As a result, it is confirmed that the monostable capability strongly depends on the polymer concentration. However, the driving voltage increases as the polymer concentration increases. Therefore, to fabricate practicable PS-FLC devices, it is important to investigate the effects of other factors on the monostable capability. From the investigation of the influence of the cell gap and the alignment film materials on the monostable capability of PS-FLCs, it is concluded that the surface anchoring of the alignment film strongly influences the monostable characteristics of PS-FLCs photocured in the SmA phase.

Impact of the concentration in polymer on the dynamic behavior of Polymer Stabilized Ferroelectric Liquid Crystal using Snap-shot Mueller Matrix Polarimetry

Experimental results are presented related to the dynamic behaviour of Polymer Stabilized Ferro-electric Liquid Crystal (PSFLC) samples under external applied electric field, using Snap-shot Mueller Matrix Polarimetry (SMMP) and Mueller Matrix (MM) formalism. Different polarimetric coefficients are simultaneously extracted from each channeled spectrum measured with this full-optical SMMP technique. The impact of the concentration of polymer present into the liquid crystal cell on this dynamic behaviour is studied, permitting a direct and quick characterisation of the material. The results obtained for PSFLC are compared with those already measured for pure Surface Stabilized Ferro-electric Liquid Crystal (SSFLC) samples, which correspond to a 0% concentration in polymer.

The effect of cross-linked chains of polymer network on the memory states of polymer stabilized ferroelectric molecules

We proposed a microscopical model by suitably considering the interaction between ferroelectric ordering and polymer cross-links. Based on the proposed model, we obtained the reduced behavior of the tilt angle, rotational viscosity and the spontaneous polarization with the increase of polymer concentration in the composite system. A perfect surface stabilization is obtained at the cost of a little reduction of tilt angle for polymer stabilized ferroelectric liquid crystal (PSFLC). It was observed that the interacting cross-linked polymer chains played an important role in promoting multistability and resolution of the memory state in the composite system.

Dependence of Polymer-Stabilized Ferroelectric Liquid Crystals on Photocuring Condition

Polymer-stabilized ferroelectric liquid crystals may show monostable and V-shaped electrooptical characteristics. We have investigated the dependence of their characteristics on the photocuring condition, especially ultraviolet light intensity. Furthermore, we offer a new scheme of the photocure being done under an electroclinic effect. The bistability of ferroelectric liquid crystals remains partially in the case of a weak ultraviolet light intensity and the saturation voltage increases as the ultraviolet light intensity increases. The tilt angle decreases as the ultraviolet light intensity increases. The decrease of tilt angle cannot be suppressed even in polymer-stabilized ferroelectric liquid crystals fabricated using the electroclinic effect.

Effect of the Polymer Network Morphology Created Under Electric Field in the Short Pitch FLC on the Dielectric Responses

Polymer-stabilized ferroelectric liquid crystals (PSFLCs) are made by photo polymerizing of 7% reactive mesogenic monomer in a highly twisted ferroelectric liquid crystal (FLC). This study reveals that both the dielectric intensity and the relaxation frequencies of the collective relaxations mechanisms namely soft and the Goldstone modes are strongly affected not only by the presence of the polymer network but also by the polymerization conditions. It is shown that, for sample polymerized with the presence of electric field, the dielectric strengths decrease and the relaxation frequencies are shifted to high frequencies compared to the sample polymerized without the presence of the electric field. This behavior was found strongly dependent of the structure of the polymer fibers. A simple phenomenological model is proposed to explain the behavior of the soft mode dielectric intensity.

Theoretical approach to study the effect of free volumes on the physical behavior of polymer stabilized ferroelectric liquid crystal molecules

It was clearly indicative that the polymer chains make a tremendous interaction with the tilt angle in case of a polymer stabilized ferroelectric liquid crystal (PSFLC). After suitable consideration of such interaction, we expanded the Landau free energy for a PSFLC system. We theoretically demonstrated the effect of free volumes, which expected to create bulk self-energy, on the physical functionalities of a PSFLC system. Then we obtained spontaneous polarization, tilt angle, rotational viscosity and dielectric constant strongly correlated with the assumed interactions. We also observed a shift of transition temperature highly influenced by this interaction between polymer network and liquid crystal molecules. A microscopical picture of this polymer-liquid crystal interaction is provided in view of the free volume charge density present in the composite system.

Measurement of Polymer Stabilization Strength in Polymer-Stabilized Ferroelectric Liquid Crystals

We have analyzed the effect of polymer stabilization on the helical structure of FLCs, such as polymer stabilization strength and molecular-alignment-structural deformation, by the conoscopic figures of polymer-stabilized ferroelectric liquid crystals (PSFLCs) observed from computer simulation and experiment. In this study, we research the conoscopic figures and the polymer stabilization strength of PSFLCs fabricated using a new FLC material and several types of monomer. In the case using a photocurable monoacrylate LC-1, the elastic constant of PSFLC is as large as that of pure FLC. On the other hand, in the case of diacrylate 2A363, the elastic constant is one hundred times larger. Therefore, the polymer stabilization strength and the characteristics of PSFLC may be widely controlled by mixing several types of monomer.

Dependence of Characteristics of Polymer-Stabilized Ferroelectric Liquid Crystals on Monomer Materials

Polymer-stabilized ferroelectric liquid crystals (PSFLCs), in which a photocurable mesogenic monomer is doped into a ferroelectric liquid crystal and a UV photocure is carried out in the SmC* phase under the application of a monopolar DC electric field, show the monostable half-V-shaped electrooptical characteristics. In this research, we have investigated the dependence of the electrooptical characteristics of PSFLC on the type of monomer such as the number of functional group and the presence of chirality. As a result, it is found that the response time of PSFLC with relatively soft polymer is shorter than that with hard polymer.

Reorientational Motion of Polymer-Stabilized Ferroelectric Liquid Crystals

A polymer-stabilized ferroelectric liquid crystals (PSFLCs), in which photocurable mesogenic monomers are doped into an FLC and then a UV photocure is carried out in the SmC* phase under the application of a bipolar AC electric field, can exhibits monostable and V-shaped electrooptical characteristics. In this research, the time-resolved ellipsometry is applied to the PSFLCs, and we investigate the dependence of the molecular reorientational motion of the PSFLC on the photocuring condition such as the tilt angle which FLC molecules take in the photocure stage. The measurement results show that the dynamic behavior of PSFLC-molecular reorientation strongly depends on the photocuring condition of the tilt angle in addition to the concentration of polymer. Furthermore, the molecular reorientation of the PSFLC is 2-steps motion originated in the difference of the threshold voltage between near and far from polymer.

Polymer-Stabilized Ferroelectric Liquid Crystals Photocured at Nematic Phase

Polymer-stabilized ferroelectric liquid crystals (PSFLCs), in which a photocurable mesogenic monomer is doped into a ferroelectric liquid crystal and a UV photocure is carried out in the SmA or the SmC* phase under the application of a bipolar AC electric field, show the monostable and V-shaped electrooptical characteristics, and thus are expected to be a newly developed materials for LC devices. In this study, in order to increase the thermal reliability of PSFLCs, we fabricate a new PSFLC photocured at the nematic (N) phase, and then investigate its characteristics in terms of the electrooptical effect and the X-ray diffraction. Results show that the PSFLC photocured at the N phase does not become monostable because the chevron angle does not coincide with the tilt angle.

The Influence of Alkyl Spacer on Helical Structure of Polymer-Stabilized Ferroelectric Liquid Crystals

Polymer stabilization is a technique for stabilizing a liquid crystal molecular orientation. The effect of polymer stabilization on the helical structure of FLCs can be analyzed by the observation of the conoscopic figures of polymer-stabilized ferroelectric liquid crystals (PSFLCs) obtained from the computer simulation and the experiment. In this study, we investigate the conoscopic figures of PSFLCs fabricated using a new acrylate polymer having an alkyl spacer part between main- and mesogenic side-chain. The strength of polymer stabilization originated in the side-chain becomes lower due to the alkyl spacer. Although the polymer stabilization usually increases the threshold electric field, the increase may be controlled by utilizing a suitable spacer.

Photorefractive Effect of Photoconductive-Polymer-Stabilized Ferroelectric Liquid Crystals

The photorefractive effect of photoconductive-polymer-stabilized ferroelectric liquid crystals (FLCs) was investigated. A photoconductive acrylate monomer and an electron acceptor compound were mixed with an FLC, and the mixtures were photopolymerized in the ferroelectric phase. A photoconductive polymer network was formed in the FLC medium, and a polymer-stabilized FLC was obtained. The photorefractive effect was evaluated by a two-beam coupling experiment. A large gain coefficient was obtained in the polymer-stabilized FLCs.

Photorefractive Effect of Polymer-Stabilized Ferroelectric Liquid Crystals

The photorefractive effect of polymer-stabilized ferroelectric liquid crystals (FLC) was investigated. Two-types of monomers, a liquid crystalline monomer and a photoconductive monomer, were mixed with an FLC and the mixtures were photopolymerized in the ferroelectric phase. The polymer-network was formed in the FLC medium and a polymer-stabilized FLC was obtained. The polymer-stabilized FLC exhibited monostable switching properties. The photorefractive effect was evaluated by a two-beam coupling experiment. It was found that the photorefractive effect was strongly dominated by the preparation condition of the polymer-stabilized FLCs.

Electro-optic properties of polymer-stabilized ferroelectric liquid crystals before, during and after photo-polymerization

The electro-optic behaviour of polymer-stabilized polar smectic liquid crystals is studied in detail before and after photo-polymerization of a bifunctional monomer in the fluid SmA* and SmC* phases. We investigate the effect of a stabilizing, phase-separated polymer network on the electro-optic performance of smectic liquid crystals in relation to the electroclinic as well as the ferroelectric electro-optic effects. For the first time we also report time-dependent data obtained during the polymerization process, which allow the study of the dynamics of photo-polymerization in a liquid crystal host and provide evidence for how this polymerization process is accomplished and why the favourable electro-optic properties of ferroelectric liquid crystals are retained in polymer-stabilized FLCs.

Control of Helical Structure in Ferroelectric Liquid Crystals utilizing Polymer Stabilization

Ferroelectric liquid crystals (FLCs) are attractive mesogenic phases because of their unique characteristics such as high speed response. Since the helical structure of the FLC depends strongly on temperature, it is important to control it for the device application especially using the helical structure. In this study, we investigate the temperature dependence of the helical structure in polymer-stabilized FLCs (PS-FLCs) in terms of optical rotatory dispersion (ORD) and circular dichroism (CD). It is found that the helical structure of PS-FLC can be controlled over a wide range of temperature by polymer stabilization.

Landau model for polymer-stabilized ferroelectric liquid crystals: Experiment and theory

The interaction between a phase separated polymer network and a liquid crystal was studied across the smectic-A* (Sm-A*) to smectic-C* (Sm-C*) phase transition of a polymer-stabilized ferroelectric liquid crystal polymerized in the Sm-A* phase. Using precise measurements of the tilt angle and the spontaneous polarization as functions of the external electric field and polymer concentration, the effective coefficients of the Landau expansion of the free energy of the Sm-C* phase have been determined experimentally. The observed polymer concentration dependence of the Landau expansion coefficients is explained using a more general theoretical model which incorporates the effect of polymer networks on the local liquid crystal director configuration. In particular, using experimental estimates of the penetration depth of the polymer network into the liquid crystal, it is shown that the b coefficient calculated from the Landau model increases with polymer concentration, evidencing the relationship determined experimentally.