WO1989009419A1

WO1989009419A1 - Liquid crystal elastomer components

Info

Publication number: WO1989009419A1
Application number: PCT/EP1989/000313
Authority: WO
Inventors: Ludwig Pohl; Bernhard Rieger; Heino Finkelmann
Original assignee: MERCK Patent Gesellschaft mit beschränkter Haftung
Priority date: 1988-04-02
Filing date: 1989-03-22
Publication date: 1989-10-05
Also published as: DE3811334A1; EP0366741A1; JPH02504556A

Abstract

Optical components composed essentially of liquid crystal elastomers, process for producing them and their use in integrated optics.

Description

Liquid crystalline elastomer components

The invention relates to optical components which essentially consist of liquid-crystalline elastomers, a process for their production and their use in integrated optics.

The rapid growth of integrated optics makes it necessary to develop new optical components such as switches, modulators, couplers and polarizers. Such elements are used in optical data transmission and optical sensor technology.

For their production, materials are necessary in which optical structures can be produced in the simplest possible way. ■ = - ■ ^' -

In this context, liquid crystal materials have found an increasingly widespread technical application due to their diverse suitable physical and chemical properties. Under the influence of electrical fields, for example, light absorption, light scattering, birefringence, reflectivity or the color can be changed. Some time ago it was now possible to produce polymers in which the structural principles of liquid-crystalline compounds were implemented. In the meantime, linear and lateral liquid crystal main chains and side chains - polymers have been synthesized and investigated [H. Finkelmann, Angew. Chem. 9 £, 840 (1987)].

Liquid-crystalline side chain polymers can be oriented in electric fields by exposing the polymer to an alternating electric field above the glass transition temperature [V.P. Shibaev et al., Polymer Communications 24, 364 (1983)], which leads to alignment of the mesogenic groups.

This field-induced orientation can be frozen by cooling the polymer in the field below the glass temperature. The orientation then remains stable even after the electrical field has been switched off.

Aligned liquid-crystalline polymer films are distinguished from the non-aligned environment by their optical transparency and different birefringence.

In M. Piskunov, et al., Makromol. Chem. , Rapid Communications 3_, 443 (1982) describes a method for aligning nematic liquid-crystalline side chain polymers in magnetic fields.

H. Finkelmann et al. in Mol.Cryst.Liq.Cryst. 9_2, 49 (1983) describe the orientation of thin films of nematic liquid-crystalline side chain polymers on surfaces analogous to the surface orientation of low molecular weight liquid crystals. This surface orientation is only successful at temperatures just below the clearing point and requires a very long tempering period.

H.J. Coles and R. Simon [Polymer 2_6, 1801 (1985)] describe the use of liquid-crystalline side chain polymers as optical storage media.

Dipolar oriented polymeric liquid crystals for the production of nonlinear optical components are of great interest. Meredith et al. [Macromolecules 15_, 1385 (1982)] describe a process for producing dipolar-oriented films of a liquid-crystalline polymer, doped with the NLO chromophore DANS.

The functional integration of nonlinear optical components with optical waveguide structures for applications in optical communications technology and optical signal processing and storage is of particular interest in terms of application technology. The international publication WO 87/06019 describes the use of smectic low molecular weight liquid crystals for the production of optical components. By aligning a part of the liquid crystal film, refractive index patterns can be inscribed in the liquid crystal which are used as optical waveguides and diffraction gratings.

If the polymer chains in a liquid-crystalline polymer are linked to one another by bifunctional molecules, polymer networks with mesogenic side groups, which are called liquid-crystalline elastomers, are obtained. With these materials, the chain segments and mesogenic groups are movable above the glass transition temperature, but the material as such retains as a result of the crosslinking its dimensional stability. The particular interest in these liquid crystalline elastomers is their orientation by means of mechanical action. If, for example, a tensile stress is applied to an elastomer film above the glass transition temperature in the liquid-crystalline state, the longitudinal axes of the mesogenic side groups are oriented parallel to a preferred direction. If the film is cooled below the glass transition temperature and then relieved of the load, the orientation is permanently maintained. [J. Schätzle, H. Finkelmann, Mol.Cryst.Liq.Cryst. 142, 85 (1987)]. The mechanical deformation of the elastomers has thus on the orientation of the liquid crystalline molecules the same influence as electrical or mag¬ netic fields on the orientation of low molecular weight liquid crystals or not ^{"cross-linked} polymers flüssigkristal¬ liner.

It has now been found that liquid-crystalline elastomers can also be selectively oriented in localized areas by mechanical action. A liquid-crystalline elastomer with locally changed birefringence is obtained in this way. Because of the selectively achieved birefringence change in selectable ranges, the materials obtained in this way are therefore particularly suitable for the production of optical components.

The properties typical of such elastomers, anisotropic phase behavior and, at the same time, dimensional stability and rubber elasticity, are of importance from an application point of view. The invention therefore relates to a liquid-crystalline elastomer with locally changed birefringence.

The invention also relates to an optical component containing a liquid-crystalline elastomer.

The invention further relates to a process for the production of liquid-crystalline elastomers, suitable for use in optical components, characterized in that the elastomer in the liquid-crystalline state is locally subjected to a change in layer thickness, a local change in the Birefringence occurs.

The invention ^'finally elements using liquid crystalline elastomer in optical Bauele¬ and the use of such components in integrated optics.

Formula I shows schematically the structure of a liquid crystalline elastomer for better understanding:

The mesogenic side groups (2) are bound to a polymer backbone (1) via flexible spacers (3) terminally or laterally (shown here terminally). (4) represents the cross-linking unit between two neighboring polymer chains. In principle, all systems are suitable for the present invention in which mesogenic groups are bound as side groups to a polymer network and which lead to a liquid-crystalline phase state of the elastomer. Suitable elastomers are, for example, polysiloxanes [H. Finkelmann, et al. in Makromol.Chem. Rapid Commun. 5_, 287 (1984) and J. Schätzle, H. Finkelmann, Mol. Cryst. Liq.Cryst. 142, 85 (1987)]. However, other liquid-crystalline elastomers known to the person skilled in the art can also be used. Copolymers which also carry non-liquid-crystalline chromophores as side groups are also preferred.

Of the elastomers that are in the glass state at room temperature, those in which the polymer backbone is polymethacrylate, chloroacrylate or a polystyrene derivative are particularly preferred. Preferred spacers are alkylene groups -fCH-. n is preferably 1 to 6, in particular 3 to 6. In general, when the spacer length is shortened, the polymer backbone is stiffened and thus the glass transition temperature increases. Of the mesogenic groups, those are preferred which lead to a high clearing point, for example groups with 3 aromatics.

Preferred elastomers which are in the liquid-crystalline state at room temperature are those with polyacrylate, polysiloxane, polyphosphazene as the polymer backbone and copolymers with non-liquid-crystalline chromophores which lower the clearing point. In this group of elastomers, the preferred spacer length n is greater than 6, preferably n is in the range from 6 to 18. Preferred mesogenic groups are those with long-chain residues in the longitudinal axis of the mesogenic unit, for example alkyl, alkoxy and oxaalkyl groups with up to 15 links that lead to the lowering of the clearing point. Of the elastomers which have smectic or nematic phases, preference is given to those in which the director is oriented perpendicular to the deformation axis when the material is stretched, or parallel to the axis of deformation when compressed.

The elastomers are usually produced by the methods customary in macromolecular chemistry, for example by simple, random copolymerization or by statistical polymer-analogous addition reactions with polyfunctional crosslinker molecules.

Another simple method is the copolymerization of a mesogenic monomer with a functional comonomer into a liquid-crystalline copolymer, which is converted into the network by a crosslinker in a second reaction step [R. Zentel, Liq.Cryst. 1_, 589 (1986)].

Copolymers in which non-liquid-crystalline chromophores are bound as side groups are important from an application point of view, for example in order to lower the birefringence too high or to vary other specific material properties.

Copolymers with side groups having nonlinear optical properties are suitable as materials for nonlinear optics. Alternatively, liquid-crystalline elastomers can also be used as matrix polymers with low-molecular NLO chromophores dissolved therein. Also preferred are elastomers doped with low molecular weight liquid crystals for the production of pressure sensors. In a preferred method according to the invention, a 1st c. Elastomer film (Ie = liquid crystalline), preferably a macroscopically ordered film, is at a temperature above its glass temperature and below its liquid crystal isotropic phase transition temperature (T, _.) brought onto a carrier which carries mechanical elevations in the form of a stamp. The mechanical elevations can be in the μm range and reflect the contours of a waveguide. When contacting the elastomer / carrier, the elevations of the carrier are transferred into the originally flat surface of the elastomer. In. The elastomer is locally deformed in areas of the change in layer thickness of the elastomer produced in this way. Depending on the position of the optical axis relative to the interface of the elastomer, these local mechanical changes in layer thickness lead to local deformation of the optical axis and thus to a local change in the birefringence of the elastomer. These local changes in the optical axis are permanently retained at temperatures below T _lc and are frozen permanently and mechanically stably in the solid state at temperatures below the glass temperature of the elastomer.

The macroscopic orientation of the optical axis (s) of the elastomer can take place spontaneously by mechanical deformation of the elastomer film. The position of the optical axis (s) in relation to the interface of an elastomer film is determined both by the direction of the mechanical deformation of the film and by the phase structure (nematic, smectic) and the chemical constitution of the elastomer.

The carrier can be made of any material on whose surface the elevations necessary for the local deformation of the elastomer are applied. Preferred carrier materials are glass or polymers such as polymethyl methacrylate, polystyrene or polycarbonate. It can however, other materials known to those skilled in the art can also be used as supports. The elevations (or depressions) are generally in the μm range, for example 1-3 μm. However, they can also be larger. This depends, among other things, on the layer thickness of the elastomer. In addition, in the case of non-electrically conductive carriers, electrically conductive areas can be applied by conventional techniques.

Depending on the nature of the surface of the elastomer or the carrier, the contact between the elastomer and carrier can result in the two materials being bonded, or else a bond can be avoided. If bonding is not desired, the structure of the support surface can be inserted into the elastomer and, after the temperature has dropped below the glass transition temperature, can be removed. The surface structure is then permanently retained in the elastomer.

Examples of applications for this are the production of compacts, chips, plates and foils.

The optical waveguides produced by the described method according to the invention are characterized by low attenuation; values of approximately 1 dB cm are sometimes achieved. They can therefore advantageously be used in passive components of the integrated optics, for example in polarizers.

Optical waveguides, which consist of non-centrosymmetrically aligned elastomers and non-linear optical components with high second-order susceptibilities, are suitable for use in optical isolators, modulators, couplers, optical switches and light valves for optical communications technology and information storage. Optical waveguides, consisting of elastomers and nonlinear optical components with high 3rd order susceptibility, can be used in components for optical communication technology and information processing.

Furthermore, optical memories and holographic gratings can also be produced by the method according to the invention.

The following examples are intended to illustrate the invention without limiting it. The spacer lengths given mean the number n of the spacer fCH-. The onomeric ratios are given in mole percent.

example 1

elastomer used:

Monomer ratio

Polymer backbone: polymethacrylate

Mesogenic group: 93

Comonomer: _f O- (CH ₂ ) ₂ -OH o

Spacer length: 6

Crosslinker:

Glass temperature: 45 ° C, clearing point: 120 ° C A macroscopically pre-oriented film of the elastomer with a layer thickness of 50 μm is heated to 70 ° C and placed on a support made of glass, which bears a mechanical elevation of 3 μm that reflects the contours of a waveguide. It is then cooled to room temperature. An elastomer film with locally changed birefringence is obtained, specifically in the boundary regions of the change in layer thickness. Structures are thus deliberately inscribed in the selected areas of the film. This material shows excellent properties as an optical fiber.

Analogously to Example 1, the following elastomer structures are inscribed.

Example 2

4 elastomers as described in Example 1, but with spacer lengths 7, 8, 10 and 12.

Example 3

4 elastomers of the following structure with spacer lengths 6 and 11:

Monomer ratio

Polymer backbone: polymethacrylate

mesogenic group:

93

Comonomer

Crosslinker 2

m: 6.8

Example 4

4 elastomers of the following structure with spacer lengths 6 and 11:

Monomer ratio

Polymer backbone: polymethacrylate

mesogenic group:

₉ 3

^C omonomer:

Crosslinker: O

m: 6.8 Example 5

4 elastomers of the following structure with spacer lengths 6 and 11:

Monomer ratio

Polymer backbone: polymethacrylate

mesogenic group:

Comonomer:

Crosslinker:

m: 6.8

Example 6

4 elastomers of the following structure with spacer lengths 6 and 11:

Monomer ratio

Polymer backbone: polymethacrylate mesogenic group:

93

Comonomer:

Crosslinker:

m_ 6.8

Example 7

5 elastomers of the following structure [monomer ratios according to H. Finkelmann et al. Macromol. Chem., Rapid Commun. 5_, 287-293 (1984)] with the spacer lengths

3 (nematic, glass temperature: 12 ° C, clearing point: 59 ° C),

6 (smectic, glass temperature: -5 ° C, clearing point: 80 ° C), 8, 10 and 12.

Polymer backbone poly (hydrogenmethylsiloxane)

mesogenic group:

O CH ₃ CH ₃

I I I I

Crosslinker: CH ₃ -Si-CH ₂ -CH ₂ - (Si-0) ₉ -Si-CH ₂ -CH ₂ -Si-CH ₃

! III CH ₃ CH ₃ O

The materials according to Examples 2 to 7 also show good to excellent properties as optical waveguides.

Claims

1. Liquid crystalline elastomer, characterized in that it has areas with locally changed birefringence.

2. Optical component containing a liquid-crystalline elastomer.

3. A process for the production of liquid-crystalline elastomers, suitable for use in optical components, characterized in that the elastomer is locally subjected to a change in layer thickness in the liquid-crystalline state, a local change in birefringence taking place in these areas.

4. Use of liquid crystalline elastomers in optical components.

5. Use of optical components according to claim 2 in the integrated optics.