RU2428732C1 - Multistable electrooptic element - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: in an element, having two parallel transparent plates with transparent electrodes on inner sides between which there is a film of a capsular polymer of cholesteric liquid crystal, the cholesteric liquid crystal used is a mixture of an ionic surfactant and a cholesteric liquid crystal with arbitrary value of dielectric anisotropy. Components of the composition - cholesteric liquid crystal polymer - are chosen such that n_o≅n_p, where at n_o is ordinary refraction index in untwisted state of the mixture of cholesteric liquid crystal and the ionic surfactant; n_p is the refraction index of the polymer.

EFFECT: possibility of obtaining a transparent state, a light-scattering state and intermediate states of the element, which can be realised at any values of dielectric anisotropy of the cholesteric liquid crystal used.

2 dwg

Description

The invention relates to optoelectronic technology, in particular to devices and elements based on liquid crystals and designed to control the light intensity using an electric field.

Known optical shutter [V.Ya. Zyryanov, S.L.Smorgon, V.A. Zhuykov “Optical shutter”, PTE, 1991, No. 1, p.240], containing two parallel located transparent plates with transparent electrodes on the inside between which there is a film of a polymer-encapsulated nematic liquid crystal (CPLC) with positive dielectric anisotropy. The operation of the element is based on a change in the refractive index of light in droplets of a nematic liquid crystal (NLC) under the influence of an electric field, which makes it possible to control the scattering of light in the film at the interfaces of the "NLC drop - polymer matrix" and, therefore, control the intensity of transmitted light. Without the action of the field of the CPLC, the film intensively scatters the light, and under the influence of the field, it passes into a transparent state. After turning off the field, the film returns to its original light scattering state.

According to the mechanical properties of KPNZHK films are close to polymer films: they are durable, flexible, easy to manufacture, reliable in operation. This allows the use of KPNZHK films for the production of flexible displays using polymer films as transparent wafer substrates.

A disadvantage of the analogue is the lack of the ability to obtain multistable optical states of electro-optical devices based on CNPC films.

The closest analogous combination of essential features is a bistable electro-optical display element [US Patent No. 6061107, IPC G02F 1/1333, publ. 05/09/2000 (prototype)], containing two parallel transparent plates with transparent electrodes on the inner sides, between which is a film of polymer-encapsulated cholesteric liquid crystal (CPLC) with positive dielectric anisotropy. Here, drops of a cholesteric liquid crystal (CLC) encapsulated in a polymer film can have two stable orientational structures when the field is off: reflecting planar and light-scattering focal-conical structures. In the planar structure, the axis of the cholesteric helix in the droplets is directed perpendicular to the CPCLC plane of the film, and such a film reflects light incident on it with a certain wavelength. In the focal-conical state, domains intensively scattering light form in CLC droplets. Switching between the selectively reflecting and light scattering states of the film is provided by the electric field.

The disadvantage of the prototype is the inability to obtain stable optical states of the element: a transparent state, as well as intermediate states between transparent and light scattering.

The technical result of the invention is to obtain multistable optical states of an electro-optical element based on a polymer-encapsulated cholesteric liquid crystal: a transparent state, a light-scattering state and intermediate between them, which can be realized at any values of the dielectric anisotropy of the used CLCs.

The specified technical result in the implementation of the invention is achieved by the fact that in a multistable electro-optical element containing two parallel located transparent plates with transparent electrodes on the inner sides, between which there is a film of a polymer encapsulated cholesteric liquid crystal, the mixture is used as a cholesteric liquid crystal ionic surfactant and cholesteric liquid crystal with an arbitrary value of the dielectric anisot ropias, and the components for the composition “polymer - CLC" are selected so that n _o ≅n _p , where

n _o is the refractive index of an ordinary wave in the untwisted state of a mixture of CLC and ionic surfactant;

n _p is the refractive index of the polymer.

The differences of the claimed electro-optical element from the prototype are that a mixture of ionic surfactant and a cholesteric liquid crystal with any size and sign of dielectric anisotropy is used as a liquid crystal, and CLC droplets encapsulated in a polymer film can have stable orientational structures: swirling radial, uniform and intermediate between them. Moreover, the components for the polymer-CLC composition are selected so that the refractive index of an ordinary wave n _o in the untwisted state of the CLC mixture is approximately equal to the refractive index n _{p of the} polymer (n _o ≅n _p ).

These signs allow us to conclude that the proposed technical solution meets the criterion of "novelty."

When studying other well-known technical solutions in this technical field, the features that distinguish the claimed invention from the prototype are not identified and therefore they provide the claimed technical solution with the criterion of "inventive step".

The invention is illustrated by drawings, which schematically depict the cross section of the inventive electro-optical element with various stable orientational structures of drops of CLC, defining various conditions for the passage of light (figa, b, c).

The inventive element contains two parallel arranged transparent plates 1 with transparent electrodes 2 on the inner sides, between which is a film of polymer-encapsulated liquid crystal, which is a polymer film 3 with drops of liquid crystal 4 inside it. As a liquid crystal, a mixture of a cholesteric liquid crystal and an ionic surfactant is used, which in the initial state (Fig. 1a) provides homeotropic (perpendicular) adhesion of the liquid crystal to the polymer matrix. The director’s orientation — the direction of the preferred orientation of the LC molecules — is shown by thin lines inside the droplet. In the initial case, the director lines, perpendicular to the surface, converge together in the center of the droplets, forming a twisted radial orientation structure with a point defect in the center of the droplets. CLC droplets with such an orientation structure scatter the light incident on the film due to the gradient of the refractive index of the extraordinary wave n _{e of the} used LC mixture and the refractive index n _{p of the} polymer (n _e ≠ n _p ).

A multistable electro-optical element works as follows:

A bipolar electrical signal is supplied to the electrodes 2, the parameters of which (shape, amplitude, duration) are determined by the characteristics of the CPCLC film (composition, film thickness, droplet size CLC). Under the influence of an electric field, redistribution of the surfactant ions occurs so that a new orientation structure is formed inside the CLC drops (Fig. 1b), which remains stable after switching off the electric signal. In the center of such droplets, a region is formed with a uniform director orientation directed perpendicular to the film plane. The larger the amplitude of the applied electrical signal, the larger part of the drop is occupied by a homogeneous region and the less light is scattered. When a uniform director orientation fills the entire droplet volume, the CPHLC film becomes transparent (Fig. 1c).

Example:

Based on the proposed design, an electro-optical element was manufactured using the following operations.

1. Prepared a cholesteric liquid crystal in the form of a mixture of a liquid crystal 5CB (98.5%) and cholesteryl acetate (1.5%).

2. In a cholesteric liquid crystal, cetyltrimethylammonium bromide ionic surfactant was dissolved in a ratio of 10: 1 by weight. The refractive index n _{o of the} obtained CLC mixture is approximately equal to the refractive index n _{p of the} used polymer matrix (a mixture of polyvinyl alcohol and polyvinylpyrrolidone in a ratio of 2: 1 by weight).

3. Using a solution technology, a CPCLC film was made in which CLC molecules were initially oriented perpendicular to the surface of the droplets, forming a twisted radial structure. A typical view of such a film is shown in the micrograph (Fig. 2a), where in the initial state the drops appear in the form of dark circles on a lighter background of the polymer film. The borders of the droplets are visible as sharp dark lines. This means that there is a large gradient of the refractive index at the “LCD polymer” interface and such a structure intensely scatters light, as shown schematically in FIG. 1a.

4. An electro-optical element is assembled, that is, a CPHC film is placed between two parallel transparent plates with transparent electrodes on the inside and the device is glued together.

The parameters of the switching signals are shown to the left and to the right of the microphotographs of Figures 2a, b, c, and the directions of the corresponding switching are shown by black curved arrows.

The structural and optical states of the assembled LCD element were switched as follows. If a bipolar rectangular electric signal with an amplitude of 80 V and a frequency of 1 Hz for 5 seconds is applied to a CPLCLC film in its initial state (Fig. 2a), then the cholesteric drops switch to a stable intermediate state (Fig. 2b), approximately corresponding to the orientation Director shown in figb. CLC drops and the “LCD polymer” interface in the photograph of FIG. 2b become lighter than in FIG. 2a. This corresponds to the situation when part of the radiation passes in the forward direction, and the remaining part is scattered (see figb).

If a bipolar sinusoidal electric signal with an amplitude of about 140 V and a frequency of 1 kHz for 8 seconds is applied to a sample in an intermediate state (Fig.2b), then the cholesteric drops switch to a stable homogeneous structure (Fig.1c). In this case, the boundaries of the droplet are barely visible (figv). This means that the gradient of the refractive index at the “LCD polymer” interface is minimized and the light passes through the film with almost no scattering (Fig. 1c).

Both an intermediate and a homogeneous structure can be returned to their original state by acting successively with two bipolar electrical signals with a frequency of 2 Hz. After the first signal with an amplitude of 75 V for 5 seconds, the homogeneous structure (Fig. 2b) switches to the transition structure (Fig. 2b), and after the second signal with an amplitude of 40 V - into the original structure (Fig. 2a).

Studies of experimental samples showed that the LCD device of the proposed design in terms of mechanical characteristics is not inferior to the prototype. At the same time, a new quality was obtained, such as the ability of an electro-optical element to be in stable optical states intermediate between light-scattering and transparent.

The proposed KPHZhK element is promising for use in such devices and devices where it is necessary to have a compact, cheap, easy to manufacture, and reliable in operation element for controlling the intensity of optical radiation, which is able to switch to various optical states intermediate between light scattering and transparent, and remain stable in these states when the electric field is off. Such elements are promising for use in projection displays with non-volatile memory, electronic curtains, smart glasses, etc.

Claims (1)

A multistable electro-optical element containing two parallel-mounted transparent plates with transparent electrodes on the inner sides, between which is a film of a polymer-encapsulated cholesteric liquid crystal, characterized in that a mixture of cholesteric liquid crystal with an arbitrary value of dielectric anisotropy and ionic surfactant is used as a cholesteric liquid crystal moreover, the components for the composition of the polymer - cholesteric liquid crystal selected n so that n _о ≅n _р , where
n _o is the refractive index of an ordinary wave in the untwisted state of a mixture of a cholesteric liquid crystal and ionic surfactant;
n _p is the refractive index of the polymer.

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