Polymer Dispersed Nematic Liquid Crystal Research Articles

Electro-optical effect of unpolarized light modulation in homeoplanar structures of ferroelectric and ferrielectric liquid crystals.

A clearly expressed effect of unpolarized light electro-optical modulation by homeoplanar structures of a smectic C* ferroelectric liquid crystal (FLC) and a ferrielectric liquid crystal (FerriLC) was discovered and investigated for the first time to our knowledge. This effect of electrically controlled light scattering is insensitive to the applied voltage sign, as for polymer-dispersed nematic liquid crystals (PDLCs), but the electro-optical modulation frequency reaches the kilohertz range. Occurrence conditions and essential features of the effect, as well as its physical origin, are discussed.

Formation of Polymer-Dispersed Nematic Liquid Crystals Doped by Nitrogen-Containing Carbon Nanotubes in Magnetic Field

Formation of Polymer-Dispersed Nematic Liquid Crystals Doped by Nitrogen-Containing Carbon Nanotubes in Magnetic Field

Polymer dispersed nematic liquid crystal films with conical boundary conditions for electrically controllable polarizers

Polymer dispersed nematic liquid crystal (PDNLC) films with conical boundary conditions at the LC-polymer interface are considered. The conical surface anchoring with tilt angle 40° initiates forming the axial-bipolar director configuration inside nematic droplets. This droplet structure exhibits the strong scattering of light polarized parallel to the bipolar axis. In the initial state, the bipolar axes in all droplets are oriented randomly, and therefore PDNLC film scatters a light of any polarization. Electric field applied along the film plane orients the bipolar axes unidirectionally in the whole droplet ensemble, that results in its high polarization-dependent transmittance. Such PDNLC films can be used in the electrically controllable linear polarizers characterized by 89% value of the transmittance for the perpendicular polarized light and high extinction ratio 590:1 at the electric field 0.34 V/μm.

Director configurations in nematic droplets with tilted surface anchoring

ABSTRACTThe polymer dispersed nematic liquid crystal (LC) with the tilted surface anchoring has been studied. The droplet orientational structures with two point surface defects – boojums and the surface ring defect – are formed within the films. The director tilt angle α = 40° ± 4° at the droplet interface and LC surface anchoring strength Ws ~ 10–6 (J m−2) have been estimated. The bipolar axes within the studied droplets of oblate ellipsoidal form can be randomly oriented are oriented randomly relatively to the ellipsoid axes as opposed to the droplets with homeotropic and tangential anchoring.

聚合物分散液晶的电场诱导定向聚合实验研究

The field induced stereo regular polymerization of polymer dispersed nematic liquid crystal both positive and negative dielectric character are experimentally studied,the picture in kind and polarizing microscope photograph of samples are given.The experiment results show that there are obvious distinguish with in different area of electric field applied or not,the electric field has the induced stereo regular polymerization action under the polymer dispersed liquid crystal sample preparation process.The experiment results are useful to understand the influence of polymer border condition to liquid crystal micro drop configuration,and are in a sense of gaining a good deal to design a new type liquid crystal display device.

Dichroic dye-dependent studies in guest–host polymer-dispersed liquid crystal films

Abstract Guest–host polymer-dispersed liquid crystal (GHPDLC) films were prepared using a nematic liquid crystal, photo-curable polymer and dichroic dye (anthraquinone blue) by polymerization-induced phase separation (PIPS) technique. Non-ionic dichroic dye (1%, 2% and 4% wt./wt. ratio) was taken as guest in PDLC host. Polarizing microscopy shows that in the absence of electric field, liquid crystal (LC) droplets in polymer matrix mainly exhibit bipolar configuration, however, relatively at higher field, maltese-type crosses were observed. Our results show that ∼1% dye-doped PDLC film shows better transmission and faster response times over pure polymer-dispersed nematic liquid crystal (PDNLC) and higher concentrated (2% and 4%) GHPDLC films.

Electrohydrodynamic Behaviors in the Multiwalled Carbon Nanotubes Doped Optically Compensated Bend Polymer-Dispersed Nematic Liquid Crystal Cell

We fabricated three optically compensated bend (OCB) polymer-dispersed nematic liquid crystal (PDLC) cells doped with a minute amount of multiwalled carbon nanotubes (MWCNTs) and observed the eletrohydrodynamic (EHD) behaviors of LCs in these three MWCNT-doped OCB PDLC cells at 5 or 12 V AC voltage with a frequency of 1 kHz or 60 Hz, respectively. Using the polarizing microscope, we discovered many kinds of domain patterns, including the fingerprint-like domain pattern, the uniform domain pattern, the bean-like domain pattern, the irregular big spot domain pattern, the hexagonal short period lattice domain pattern, and the rectangular period lattice domain pattern. This suggests that some domain patterns differ from the Kapustin–William's domain pattern while others were somewhat similar to the Kapustin–William's domain pattern.

Orientational structure transformations caused by the electric-field-induced ionic modification of the interface in nematic droplets

The films of a polymer-dispersed nematic liquid crystal doped with an ionic surfactant were studied. The surface-anchoring modification effect caused by the local increase in the concentration of surface-active ions was observed at the polymer-liquid crystal interface under the action of an electric field. The modification of the boundary conditions leads to the transformation of the orientational structure of the nematic droplets and, as a result, to an appreciable change in their texture patterns and light-scattering efficiency at the interfaces. The monopolar director configurations (normal and curved) arising in the process of orientational structure transformations are considered and their typical textures are demonstrated. The possibility that the monopolar structure can be formed is theoretically substantiated by the computer simulation of the director field in a nematic droplet with the boundary conditions corresponding to the experiment.

Temperature dependency of phase transitions in polymer-dispersed nematic liquid crystal mediums

In this work, electrical measurements were performed on a polymer-dispersed LC sample so that we could demonstrate the temperature dependent molecular behavior, determined according to the electrical response of the system. Experimental results are found to be in favorable accordance with the molecular theory in several aspects. Critical temperatures, order parameters and entropy of transition are acquired for the studied system.

Interference and ion effects in the electro‐optical response of PDNLC films

Abstract— We have considered the interference in polymer‐dispersed nematic liquid‐crystal (PDNLC) films caused by the superposition of light beams that pass through and between the LC droplets. The relative phase retardation of the beams depends on the applied voltage, and, as a consequence, interference oscillations can occur at transmittance‐voltage curves. Typical forms of the curves for composite films with various structures are presented. Interference oscillations were revealed not only in the static electro‐optical characteristics but in the dynamic ones also. For PDLC films based on commercially available components produced without thorough purification, the combination of interference and ionic effects is observed in the dynamics of the optical response. The relaxation of transmittance due to the depolarization ionic field is presented, depending on droplet size, temperature, pulse duration, and the concentration of the ionic admixture.

INFLUENCE OF POLARIZING ELECTRIC FIELD ON ELECTRICAL AND OPTICAL PROPERTIES OF PDLC FILMS

We obtained polymer dispersed nematic liquid crystal films E7/PMMA. The thermally stimulated depolarization current (TSDC) measurements indicate a space charge limited current. Conduction of electronic type was proven and we measured the value of the activation energy, specific to the conduction process. Optical measurements have been performed simultaneously with TSDC measurements and the phase transition nematic – isotrop was detected by the switch of the optical transmission.

Coherent transmittance of a polymer dispersed liquid crystal film in a strong field: Effect of correlation and polydispersity of droplets

A method for computing the coherent transmittance of a film of polymer dispersed nematic liquid crystals aligned by an external field in the case of an oblique incidence of light is developed, and a theoretical analysis of this quantity is performed. The effects of close packing are considered in terms of the interference approximation of the theory of multiple wave scattering. The correlation arising in the spatial distribution of polydisperse droplets is taken into account by introducing partial distribution functions. It is shown that an increase in the concentration of liquid crystal droplets is accompanied by an increase in the coherent transmittance of the film.

Interference oscillations in the dynamics of the optical response of polymer dispersed nematic liquid crystals

The dynamics of the optical response of a polymer-dispersed nematic liquid crystal under the action of electric field pulses was experimentally studied for film samples differing in the size of nematic droplets. The optical signal relaxation curve exhibits an oscillating character, with the number of oscillations determined by the transverse size of the nematic droplets. The interference character of the signal oscillations is confirmed by an analysis of the oscillating response within the framework of the anomalous diffraction approximation and by a comparison with the dependence of the transmitted light intensity on the applied voltage measured in a static regime.

High Contrast Light Modulator Based on PDNLC Monolayer

Interference effects in polymer dispersed nematic liquid crystals (PDNLC) films with monolayer arrangement of the LC droplets are investigated. Transmittance characteristics were measured both in dependence on the collection angle and incidence angle. The method of increasing of contrast of light modulators based on such films is discussed.

Influence of Polarizing Electric Field on Phase Transition Temperatures and Optical Transmission of PDLC Films

We obtained polymer dispersed nematic liquid crystal films E7/PMMA. Using the thermally stimulated depolarization currents method and registering the optical signal simultaneously we studied the dependence of the phase transitions temperatures on the polarizing electric field. Thermal switch behavior and its dependence on polarizing electric field is obtained.

Quasilinear electro-optical response in a polymer-dispersed nematic liquid crystal

Polymer-dispersed nematic liquid crystals are known to exhibit a quadratic electro-optic response. For practical applications, a linear dependence on the applied electric field is often required. In this letter, polymer-dispersed nematic liquid crystal films with a built-in dc electric field are shown to be good electro-optical devices with a linear response. The built-in electric field is obtained by energizing films with an external dc field. The electro-optical response dependence on the strength and frequency of the driving field is discussed.

Mechanically Induced Electric Polarization in Polymer Dispersed Nematic Liquid Crystal Films

We obtained polymer dispersed nematic liquid crystal films E7/PMMA. In order to induce anisotropy and consequently electrical polarization, the films have been hot stretched and pressed. We studied the dependence of the polarization upon temperature, frequency and amplitude of the applied electric field. High values of the order of μC/cm2, have been obtained. The frequency dependence of the polarization indicates relaxation times of the order of milliseconds.

Orientation and anchoring effects in stretched polymer dispersed nematic liquid crystals

The induction of liquid crystal orientation through mechanical stretching was investigated for polymer dispersed liquid crystals (PDLCs) by means of infrared dichroism. Using a nematic liquid crystal BL006 and polyacrylic acid as the polymer matrix, it was possible to stretch the PDLC films with BL006 in either the isotropic or the nematic phase. After cooling the films under strain to room temperature, the molecular orientation of BL006 was found to be much higher for films that contained isotropic liquid droplets of BL006 at the time of stretching than for films that had nematic droplets. Stretching PDLC films with isotropic droplets results in no molecular orientation, but the orientation is induced during the subsequent cooling when BL006 goes through the isotropic-to-nematic phase transition. Interestingly for PAA/BL006, the nematic director orients along the long axes of the elongated droplets despite liquid crystal anchoring perpendicular to the polymer interface.

Interference quenching of light transmitted through a monolayer film of polymer-dispersed nematic liquid crystal

A theoretical study of light transmission through a polymer with a monolayer ensemble of bipolar nematic droplets under applied electric film is performed. The possibility of interference quenching of monochromatic coherent light directly transmitted through the film is predicted theoretically and realized in the experiment.

Oriented polymer dispersed discotic liquid crystals

Polymer dispersed discotic liquid crystals (PDDLCs) were prepared using the hexa-n-octanoate of rufigallol (RHO) and three polymer matrices: polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA). The molecular orientation of RHO in stretched PDDLC films was characterized by means of infrared dichroism. It was found that the stretching of films that contain RHO in both the columnar D1 and crystalline phase can effectively align columns of RHO along the stretching direction, with the short axes of the rigid cores lying in the plane of the film. By contrast with stretched polymer dispersed nematic liquid crystals, no orientation of RHO is induced for films stretched with RHO in the isotropic phase, followed by rapid cooling to room temperature. However, if stretched films are cooled under strain into the columnar D1 phase, orientation of RHO can develop with time.