WO1998016865A1

WO1998016865A1 - 'reverse-mode' electro-optical cell made of liquid crystals encapsulated in a polymeric layer and method for manufacturing it

Info

Publication number: WO1998016865A1
Application number: PCT/EP1997/005599
Authority: WO
Inventors: Giuseppe Chidichimo; Giovanni De Filpo
Original assignee: SPS SpA
Current assignee: SPS SpA
Priority date: 1996-10-14
Filing date: 1997-10-10
Publication date: 1998-04-23
Anticipated expiration: 1999-04-14
Also published as: ITPD960249A0; ITPD960249A1; AU4946797A; IT1287963B1

Abstract

A cell composed of a polymeric layer which encapsulates a nematic liquid crystal which is self-adherent to an electrically conducting glass or plastic sandwich-like support. The cell is transparent in its natural state and becomes opaque by applying an electrical field (reverse-mode operation). The method for manufacturing a cell according to the present invention consists in dispersing, in a polymerizable organic medium, microspheres of nematic liquid crystal, forming a fluid emulsion. The emulsion is placed between two rigid or flexible glass or plastic supports which are rendered electrically conducting; then, by means of the action of a magnetic field (or other force field), the liquid crystal molecules are orientated; this also causes an impression, at the interface of the orientated liquid crystal spheres, in the dispersing organic medium. In this condition in which a magnetic field or other force field is present, the organic medium is polymerized for example by ultraviolet radiation or by heating. Once polymerization has occurred, and once the magnetic field has been removed, the liquid crystal molecules maintain their orientation, which can be altered reversibly by applying an electrical field to the conducting support. Operation is of the 'reverse mode' type with high optical contrast.

Description

"REVERSE-MODE" ELECTRO-OPTICAL CELL MADE OF LIQUID CRYSTALS ENCAPSULATED IN A POLYMERIC LAYER AND METHOD FOR MANUFACTURING IT Technical Field

The present invention relates to a "reverse-mode" electro-optical cell made of liquid crystals encapsulated in a polymeric layer, which constitutes the supporting matrix, and to a method for manufacturing it. Background Art Many conventional composite materials constituted by polymers and liquid crystals are used in electro-optical applications such as the production of displays and variable-transmission devices having large surfaces.

Liquid crystals per se are in fact not suitable materials for these applications.

On the one hand, control of optical transmission through liquid-crystal layers requires the use of polarizing layers which, by allowing only light components polarized in very specific directions to pass through them, drastically reduce overall light transmission.

Another drawback is constituted by the fluid nature of liquid-crystal substances, which makes it very difficult to obtain active layers having large surfaces and constant thicknesses. Accordingly, considerable research has been devoted to obtaining composite materials which combine the optical properties of liquid crystals with the mechanical properties of polymers.

Encapsulated liquid crystals, their method of use in electro-optical devices and a method for manufacturing them are the subject of US patent 4,435,047 of 1984, in the name of J.L. FERGASON.

According to this solution, an encapsulated liquid crystal is constituted by discrete microdroplets of liquid crystals having dieletric positive anisotropy Δe contained in an encapsulating medium.

The example provided in the FERGASON patent relates to an encapsulated liquid crystal confined within polyvinyl alcohol (PVA). The encapsulated liquid crystal is of the Λe- positive kind.

The FERGASON process uses an emulsion in which the e ulsifier is water, which must be removed after a certain amount of polyvinyl alcohol, by being arranged at the interface between the water and the liquid crystal, has encapsulated the microdroplets of the latter in the emulsion.

Inevitably, part of the water remains trapped within the material, causing an undesirable high conductivity and plasticity of the active material, with consequent poor adhesion to supports.

As regards the electro-optical operation of the materials patented by FERGASON (US 4,616,903), this author states that the walls of the capsule induce a generally nonparallel alignment in the liquid crystal molecules, generating a configuration of molecules in which the light is scattered, more than transmitted, when no electrical fields are applied.

The application of an electrical field univocally orientates the liquid crystal molecules and makes the film transparent when the indices of refraction of the two materials, the polymeric material and the liquid crystal material, are appropriately adjusted.

US patent 4,688,900, of 1987, in the name of J.V. DOANE ET AL., claimed a material which was morphologically similar to the one provided by FERGASON, but this patent introduced a different method for manufacturing composite films.

The method consisted in mixing a polymer (or prepoly er) and a (nematic) with positive dieletric anisotropy ^e liquid crystal to provide a single-phase solution. The liquid crystal is of the Δe-positive kind.

The liquid-crystal droplets were thus formed by phase separation by means of:

— polymerization

By proceeding according to this patent, it became possible to use a wide range of polymeric materials to confine the liquid crystals in discrete microdroplets and obtain composite films which could be used to produce displays and large-surface variable-transmission devices.

In this solution, the orientation of the liquid crystal is not uniform inside the microdroplets and also varies in its overall distribution from droplet to droplet.

This configuration is highly opaque.

The orientation of the liquid crystal becomes uniform in the entire material in the presence of an electrical field, owing to the positive dieletric anisotropy of the liquid crystal.

This configuration allows light to pass and the cell appears to be transparent.

The drawback of this method is that in any case a significant amount of liquid crystal remains dispersed in the polymeric matrix without contributing to the electro- optical characteristics of the device.

Moreover, the liquid crystal dispersed in the matrix acts as a plasticizer, worsening the structural characteristics of the device.

The same patent provides an example in which a device is produced by phase separation in the presence of a magnetic field which is parallel to the conducting substrates, using liquid crystals with positive dielectric anisotropy.

In this case, the magnetic field produces its orientating effects only after phase separation has already started; accordingly, the degree of alignment of the liquid crystals along the field is limited. Said device is a polarizing filter.

The materials derived from these first patents constitute direct-mode electro-optical films.

A composite material termed Liquid Crystal Gel (LCG) was devised in 1992 (reference should be made to US- 5,560,864); said material is constituted by approximately 90% nematic liquid crystal and 8% cholesteric liquid crystal, while the remaining 2% is a polymer whose function is not to confine the liquid crystal mix but only to stabilize its orientations. When the above material is confined between surfaces treated beforehand with polyimides, a planar orientation of the liquid crystal is obtained, which rotates in a helical pattern in which the pitch is perpendicular to the confinement surfaces (high optical transmissivity state). The application of the electrical field breaks the uniformity of the planar alignment, generating a highly opaque state.

This apparatus has a reverse-mode electro-optical operating mode, since it is transparent in the absence of an electrical field but assumes an opaque state when the field is applied.

The drawbacks of this apparatus are in particular the fact that it works well only in small devices and also that the base material does not self-adhere to the confinement surfaces and is also highly sensitive to temperature variations and chemically unstable.

Another electro-optical material of the PDLC type has been devised and refined by researchers of POLYTRONICS (USA)

(reference should be made to US-5 , 056 , 898 ) by adding surfactants in mixtures normally used to obtain PDLCs with microdroplets in a bipolar configuration.

The presence of the surfactant generates microdroplets in which the liquid crystal molecules are orientated at right angles to the polymeric parts. By appropriately matching the indices of refraction it is possible to achieve reverse mode operation of the composite film, which however does not achieve optical qualities which make said material useful from the technological point of view (very low transparency/opacity ratio) .

Another reverse mode device is also known which uses the dependence of the two dielectric constants of the liquid crystal on the frequency of the applied electrical field.

One of two constants, termed e, decreases as the frequency increases, while the other one is practically independent thereof .

This causes dielectric anisotropy to change from positive at low frequency to negative at high frequencies.

This effect has been used to obtain a display known as RNPDLC by using a mix of prepolymers which UV-crosslink and a liquid crystal which is orientated at low frequency during the photopolymerization of the polymers.

In this manner, transparency is ensured without an electrical field. With a high-frequency electrical field, an opaque state due to light scattering is obtained, owing to the behavior of the liquid crystal with negative dielectric anisotropy (P. Nolan and D. Coates, Mol. Cryst. Liquid Crystal Letter, 1991, 8, 75). The resulting low-level contrast and problems linked to the high frequency, especially for large areas, considerably limits its applicability (T. Gotoh and H. Murai, Appl. Physics Lett. 1992, 60, 392). Disclosure of the Invention

The aim of the present invention is to solve and overcome the problems observed in currently available products as described above.

A first object of the present invention is to provide an electro-optical cell having reverse-mode operation.

A second object, in combination with the first one, is to provide an electro-optical cell having a high optical contrast.

A third object of the present invention is to provide an electro-optical cell formed on a glass or plastic support . A fourth object of the present invention is to provide a cell in which the material comprised between the supports adheres strongly to the supports themselves.

A fifth object of the present invention is to provide a cell having high structural rigidity characteristics.

Another object of the present invention is to provide a method for preparing an electro-optical cell which achieves the above objects and is particularly simple and suitable for industrialization. Another object of the present invention is to optimize use of the liquid crystal, with consequent low cost of the electro-optical cell.

This aim, these objects and others which will become apparent hereinafter are achieved by an electro-optical liquid crystal cell in a polymeric layer, characterized in that it comprises a uniform dispersion of microspheres or microagglomerates of molecules of nematic liquid crystal, with uniformly aligned molecules, in a polymeric matrix which is self-adherent to electrically conducting glass or plastic, rigid or flexible supports, said matrix being able to maintain indefinitely the alignment of the molecules of said liquid crystal set during the preparation of the cell by an external magnetic field (or by other force fields), said molecules being able to rotate in a spontaneously reversible manner by means of an electrical field applied across the conducting supports, the two states determining transparency (spontaneous orientation) or opacity (orientation forced by the electrical field) of the cell (reverse-mode operation). The present invention also relates to a method for producing an electro-optical cell made of liquid crystal encapsulated in a polymeric layer, characterized in that it comprises the following operating steps: a) preparing an organic microemulsion of nematic liquid crystal in a polymerizable organic medium; b) spreading said microemulsion on glass or plastic supports which have been rendered electrically conducting beforehand on the inner faces; c) creating a sandwich which contains the fluid microemulsion; d) applying, to said sandwich, a magnetic field or another force field whose intensity and direction is such that it orientates the molecules of the liquid crystal; e) polymerizing the emulsifying organic medium, maintaining the magnetic field until said polymerization ends.

Brief Description of the Drawings

Further characteristics and advantages of the present invention will become apparent from the following detailed description of the cell according to the invention and of its process, which are illustrated by way of non-limitative example in the accompanying drawings, wherein: figures la-lb and 2a-2b are views of two operating modes of the cell; figure 3 is a photomicrograph, taken in a cross- polarizing microscope, of the liquid crystal emulsion in the organic medium before polymerization. Ways of carrying out the Invention

With reference to the above, a first kind of cell according to the invention is illustrated in figures la and lb .

Said cell is composed of microspheres 1 made of nematic liquid crystal with uniformly aligned molecules dispersed in a polymeric matrix 2 which is self-adherent to conducting supports 3 and 4 which can conveniently be of the glass or plastic, rigid or flexible type.

Said first cell has a geometry which includes mesogenic molecules which are "spontaneously" orientated at right angles to the surface of the layer. In this case, the index of refraction "n " of the polymer is matched to the index of refraction "n₀" of the liquid crystal, which must have negative dielectric anisotropy.

In its natural state, the composite film appears perfectly transparent.

By applying an electrical field E of suitable strength, as shown in figure lb, the mesogenic molecules assume a direction which is perpendicular to the field.

Owing to the mismatch between the index of refraction "np" of the polymer and the index of refraction "ne" of the liquid crystal, the light that passes through the cell is scattered in all directions, rendering the device opaque.

A second type of cell, which operates according to the same principle, is shown in figures 2a and 2b. In this case, the index of refraction "n_p" of the polymer is matched to the index of refraction "n_e" of the liquid crystal, which must have a positive dielectric anisotropy.

In this case, the application of the electrical field causes the mesogenic molecules to become parallel to said field, making the cell assume an opaque state owing to the mismatch between the index of refraction of the polymer "n " and the index of refraction "n " of the liquid crystal.

In the case of this geometry, it is necessary to add a polarizer 5 in order to select a single component of the polarization vector (the one along the direction of the mesogenic molecules).

The process that allows to obtain a cell of this kind is composed of a sequence of steps which can be described briefly as the preparation of an organic emulsion of nematic liquid crystal in a monomer or in a mix of monomers which can be polymerized by means of any conventional process, for example by using ultraviolet radiation or by using heat.

This emulsion is spread on two glass or plastic, rigid or flexible supports which have been treated beforehand so as to become conducting on the contact faces, thus forming a sandwich.

This sandwich, which contains the fluid emulsion, is subjected to a magnetic field: at the same time, polymerization is performed by irradiation with ultraviolet radiation or, as an alternative, by thermal treatment.

The application of the magnetic field or of other fields in this context uniformly orientates the separate mesogenic molecules in microdroplets, but also orientates those of the fluid organic medium which are present at the interface or otherwise located in a more or less wide neighborhood of the mesogenic microdroplets.

The subsequent process for polymerizing the organic monomer produces a polymeric matrix which, in a more or less wide neighborhood of the polymer/liquid crystal interface. acquires a structure which preserves, by molecular interaction, the orientation of the mesogenic molecules confined in the microdroplets even in the absence of external fields. The composite film obtained after polymerization of the organic monomer can appear perfectly transparent when the indices of refraction of the components are matched.

The mesogenic molecules that are uniformly aligned in their natural state in all the microdroplets that are present inside the film can be rotated at right angles by an electric field applied across the conducting supports, causing said film to become opaque owing to the mismatch of the indices of refraction.

It should also be noted that since the liquid crystal is insoluble in the monomeric organic part, it is used in the amount strictly necessary to determine the electro- optical operation of the cell.

This entails a cost reduction, since the liquid crystal is used only in the minimum amount required for the operation of the cell.

The size of the spheres of the liquid crystal can be better adjusted by using ultrasound methods and also by adjusting concentrations and temperatures.

The monomer has adhesive characteristics, so that it fixes itself well to the glass or plastic, rigid or flexible support.

Examples of preparation of the cell are now given.

EXAMPLE 1 In this example, an electro-optical cell was produced which had mesogenic molecules orientated at right angles to the surface of the electro-optical cell.

The liquid crystal used was a ZLI 4788-000 by MERCK, which was mixed with hydroxypropyl methacrylate in a 1:1 ratio (50% liquid crystal and 50% hydroxypropyl methacrylate) .

1% of the radical initiator UV IRGACURE 651 by CIBA- GEIGY was added to this mix. The mix was sonicated with a probe sonicator so as to obtain a stable emulsion.

Said emulsion was deposited in the form of a thin layer on metallized glass with the addition of a small amount of inert spacers with a 10-micron diameter. The cell was then placed in a magnetic field with a strength of approximately 3 tesla at right angles thereto, and after a few seconds it was irradiated with UV light generated by a PHILIPS HPK 125 125-watt lamp.

The cell appeared perfectly transparent in its natural state and became opaque by applying external electrical fields.

Maximum contrast was observed with alternating voltages at 80 volts.

EXAMPLE 2

An electro-optical cell with mesogenic mo lecules , orientated so as to be paral lel to the surf ace of the electro-optical film, was produced in this example .

The l iquid crystal E 7 by MERCK was mixed with hydroxypropyl methacrylate in a 1:1 ratio and 1% of the radical initiator UV IRGACURE 651 by CIBA-GEIGY was added to the mix.

The mix was processed as in Example 1, but the magnetic field was arranged parallel to the cell.

A cell was thus obtained which, when coupled to a polarizer, appeared transparent in its natural state and became opaque under the action of an external electrical field of approximately 80 volts. The above description clearly shows that the intended aim and objects have been achieved.

In particular, the fact should be noted that the electro-optically active cell with naturally-aligned nematic spheres has been manufactured through the polymerization of an organic emulsion of nematic liquid crystal in a mix of acrylic monomers.

This method differs from conventional ones in that the liquid crystal microspheres are already present in the fluid phase when suitably selected organic monomers are mixed with the liquid crystal.

This process offers the advantage of great simplicity and furthermore eliminate the severe drawback of contamination on the part of water molecules, which worsen the electro-optical functionality of the film, drastically reducing its resistivity.

Since the liquid crystal is insoluble in the monomeric organic part, it is used in the amount that is strictly necessary to determine the electro-optical operation of the cell.

Claims

1. An electro-optical liquid crystal cell in a polymeric layer in the solid state, characterized in that it comprises a dispersion of microspheres of nematic liquid crystal, with uniformly aligned molecules, in a solid polymeric matrix which is self-adherent to electrically conducting glass or plastic supports, said matrix being able to maintain indefinitely the alignment of the molecules of said liquid crystal set during the preparation of the cell by an external magnetic field (or other force field), said molecules being able to rotate in a spontaneously reversible manner by means of an electrical field applied across the conducting supports, the two states determining transparency (spontaneous orientation) or opacity (orientation forced by the electrical field) of the cell (reverse-mode operation).

2. An electro-optical cell according to claim 1, characterized in that the electrically conducting support is rendered conducting by the presence of a single layer of metal oxide.

3. An electro-optical cell according to one or more of the preceding claims, characterized in that the electrically conducting support is rendered conducting by the presence of a multilayer of metal and/or ceramic oxides.

4. An electro-optical cell according to one or more of the preceding claims, characterized in that in the absence of an applied field the polymeric matrix keeps the local alignment of the molecules of the liquid crystal uniform with respect to an assigned direction.

5. An electro-optical cell according to one or more of the preceding claims , characterized in that it uses nematic or operatively nematic liquid crystals .

6 . An e l ect ro-optic al c e l l ac cording to c l aim 1 , characte ri z ed in that said matrix i s obtained by polymerization of an organic medium.

7. An electro-optical cell according to claims 1 and 6 , characterized in that said organic medium is a monomer or a monomer mix.

8. An electro-optical cell according to claims 1 and 6, characterized in that said organic medium is constituted by an isomer or mix of isomers with low relative molecular weight.

9. An electro-optical cell according to claims 1 and 6, characterized in that said organic medium is constituted by monomers or isomers with a low relative molecular weight of the acrylic type.

10. An electro-optical cell according to one or more of the preceding claims, wherein said liquid crystals and/or said organic medium are mixed with radical initiators and/or inert spacers and/or colors and/or substances of any other kind which can improve its characteristics or performance.

11. An electro-optical cell according to one or more of the preceding claims, wherein the layer of composite film has a thickness between 10 and 30 μm or another thickness deemed more suitable to meet particular optical contrast requirements.

12. An electro-optical cell according to one or more of the preceding claims, wherein the alignment induced by the polymeric coating of the liquid crystal molecules having positive dielectric anisotropy is parallel to the surfaces of the supports and the transparent state is achieved by appropriately adjusting the index of refraction of said polymeric matrix and by applying a polarizing film.

13. An electro-optical cell according to one or more of the preceding claims, wherein the alignment induced by the polymeric coating of the liquid crystal molecules, having negative dielectric anisotropy, is perpendicular to the surfaces of the supports and the transparent state is achieved by appropriately adjusting the index of refraction of said polymeric matrix.

14. An electro-optical cell according to one or more of the preceding claims, wherein additives are present in order to improve its performance, such as: promoters of the adhesion of the polymeric matrix to the supports, stabilizers, spacer particles, coloring substances, etcetera.

15. An optical apparatus, characterized in that it comprises, as a component, a liquid crystal electro-optical cell as defined in one or more of the preceding claims.

16. A method for manufacturing an electro-optical cell made of liquid crystal encapsulated in a polymeric film, characterized in that it comprises the following operating steps: a) preparing an emulsion of nematic liquid crystal in a polymerizable organic medium; b) spreading said emulsion on glass or plastic, rigid or flexible supports which have been rendered electrically conducting beforehand on the spreading faces; c) creating a sandwich which encapsulates the fluid emulsion; d) applying, to said sandwich, a magnetic field (or other force field) whose intensity is such that it orientates the molecules of the liquid crystal; e) simultaneously polymerizing the emulsifying organic medium, maintaining the magnetic field until said polymerization ends.

17. A method according to claim 16, wherein the organic emulsion is constituted by nematic liquid crystal in an organic medium and contains optional additives according to claim 14.

18. A method according to claim 16, wherein step d) comprises, in addition to the orientating action of the applied field, the auxiliary action, in terms of the obtainment of uniformly orientated liquid crystal domains, of chemical or physical additives or of mechanical actions.