LIQUID CRYSTAL ELASTOMER FILMS AND METHOD OF PRODUCTION THEREOF

Abstract: This invention relates to the production of free-standing liquid crystal elastomer films, in particular of large area for use in various devices. Example of devices include piezo and pyroelectric devices. The method of production involves the use of a silicon wafer which may be doped thus allowing for poling during production.

Description: LI Q UID CRYSTAL ELASTOMER FILMS AND METHOD OF PRODUCTION THEREOF This invention relates to methods for the fabrication of free standing liquid crystal elastomeric films and such films obtained therefrom. It also relates to free standing liquid crystal elastomeric films in general. In particular this invention relates to the use of liquid crystal elastomer (LCE) films as transducers. There is currently a need for the production of free standing polymer and elastomer films for use in, inter alia, piezo and pyroelectric devices. Other requirements are that such a film should be as flat as possible, as free as possible from defects and should be available over a range of sizes. WO 97/25649 discloses the use of liquid crystal elastomer films. It is believed that the production of a free-standing liquid crystal elastomeric film has not been successfully achieved, wherein free-standing means that the film is effectively able to support itself ie it does not require a mechanical support to keep it intact. According to this invention a method for the production of a free-standing liquid crystal elastomer film comprises the steps of: providing a mixture of monomer, initiator and cross-linker, loading the mixture between spaced plates, curing the mixture with radiation to form the liquid crystal elastomer film, removing the film from between the plates; wherein one of the plates is a silicon wafer and the other plate is sufficiently transparent to the incident radiation at an appropriate wavelength such that polymerisation will occur. Preferably the other plate is a quartz plate. The types of incident radiation include UV and thermal. Similarly the initiator includes thermal and photo initiators. Preferably the monomer is aligned after it is loaded between the plates. This may be done by using any of the conventional techniques. For example, poling may be used. If poling is used then one or both of the plates may have electrodes present eg an ellipsoidal electrode pattern. These electrodes have the additional advantage that they prevent voltage accumulation spikes that may occur with electrode patterns that have corners. The plates may be heated before the monomer is loaded, the temperature chosen is dependent upon in which particular phase it is desired to cure the film eg an isotropic or liquid crystal phase. It is preferred to heat to the isotropic phase and cool through the various liquid crystal phases (if present) to the desired liquid crystal phase - typically the Sc phase. The plates may be spaced by using a spacer such as a Mylar spacer. The primary advantage associated with using silicon and quartz wafers is that the resulting film is easily peeled off resulting in little or no damage to the film. Further advantages associated with using Si wafers are that the Si may be highly polished resulting in a uniform surface which gives rise to a substantially defect free film. In addition the silicon may be doped using conventional techniques allowing for poling of the material thus aiding alignment. A standard alignment layer used for liquid crystals can also be used on the silicon (doped or bulk) surface in order to further improve the alignment. Poling may be achieved by applying a DC and/or AC and/or thermal bias across the plates. The resulting film may be removed in any suitable way including peeling or so-called "float-off' wherein a fluid is used to separate the film from the wafer. Acetone is an example of a suitable fluid. The cooling rate may be varied in the method - such a cooling rate lies in the range 0.01 °C - 10°C/minute. Preferably the cooling rate is of a value of about 0.1°C/minute. A cooling rate as described above, may be applied in addition to applying a voltage. Preferably a triangular waveform is applied. Even more preferably the triangular waveform is in the region of 0.5Hz at 50V/micron. A high positive or negative bias may also be used. With respect to loading the mixture between the spaced plates the mixture can be poured or placed onto the lower plate which has a spacer placed or fixed on it. The upper plate can then be placed on top to complete the cell. The plates can be clamped together if necessary. Alternatively the plates can be fixed together, incorporating the spacer, and the material can be flowed into the cell. Usually the lower plate is silicon. Preferably when such a combination of cooling rate and waveforms are applied then the mixture is cooled down from the isotropic phase into the desired final phase. Preferably the resulting elastomer material is an aligned chiral smectic C material. Preferably the resulting elastomer material is ferroelectric. A further aspect of this invention includes a free-standing liquid crystal elastomer film obtainable by the process of the current invention. According to a further aspect of this invention a free-standing liquid crystal elastomeric film is provided. By free-standing is meant that the film does not require further mechanical support to keep it intact. According to a further aspect of this invention a free standing liquid crystal elastomer film is provided of area 10mm 2 -40, 000mm 2 . Preferably the area is 100mm 2 -20, 000mm 2 , even more preferably 500mm 2 - 20,000mm 2 . According to a further aspect of this invention a liquid crystal elastomer film suitable for use in transducer devices is provided. Preferably the area of the film is 10mm 2 -40, 000mm 2 , even more preferably 100mm 2 - 20,000mm 2 and even more preferably 500mm 2 -20,000mm 2 . The invention will now be described, by way of example, with reference to the following diagram: Fig 1 illustrates a pyroelectric device With reference to Fig 1 A pyroelectric detector consists of electrode plates 1 ,2 at least one of which may be pixellated. In operation the detector is exposed to radiation R, for example infrared radiation, which is absorbed by the electrode 1. This results in a rise in temperature which is transmitted to a layer of pyroelectric material 3 (provided by the current invention) by conduction, The change in temperature results in a thermal expansion and a charge is generated. This change in charge is usually small when compared with the charge output due to the change in the spontaneous polarisation, Ps with a change in temperature; this constitutes the primary pyroelectric effect. A change in charge results in a change in potential difference between the electrodes. The charge on each pixel may be read out and the resulting signal is used to modulate scanning circuits in, for example, a video monitor and for a visual image of the infra red scans. Example of pyroelectric devices include detectors, steering arrays and vidicon cameras. The films of the present invention may be used generally as transducers in piezo electric devices and in optical recording media. Any suitable monomer material may be used in order to form the polymer/elastomeric material of the present invention. Suitable materials are well known to those skilled in the art. They include and are hereby incorporated by reference, but are not limited to; those monomers disclosed and referred to in US 5866038 and PCT GB91/01074 which are a part of the same patent family. Preferred monomers are given by the following general formula CH 2 =CH-COO(CH 2 )n-R Formula I wherein n = 0-12; Preferably n = 3-12, even more preferably n = 6-11 ; R is a mesogenic group; Preferably R is given by the general formula II; Y- ■ Wi -H- -D-7— Z 1.2 0 , 1 , 2 Formula II A, B, D are selected from the following rings: the above rings may be subst tuted with one or more of the following substituents in at least one of the available substitution positions: F, Cl, Br, CH 3 , CN, OR, R and NCS where R is given by C 1-5 branched or straight chain alkyl; Z is selected from CN, F, Cl, N0 2 , R, OR, C0 2 R, CF 3 , OOCR, NCS, SCN, where R = straight chain or branched chain alkyl and may include from 1-16 carbon atoms and including where one or more non-adjacent CH 2 groups may be substituted by CH(CN), CH(CF 3 ), CH(CI), CH(CH 3 ) in chiral or non-chiral form; provided that the total number of rings present is not greater than 4; W T and W 2 are independently selected from COO, OCO, single bond, CH 2 CH 2 , CH-O, OCH 2 , O, S, CH=CH, C= C. Y = O or single bond or COO or OCO; provided that when n=0 then Y is a single bond. Synthetic procedures are carried out using known techniques and are described in US 5866038 and PCT GB91/01074 which are a part of the same patent family and references therein. Different monomer materials can be mixed together in the polymerisation process in any suitable ratio. The preferred ratio is one which gives the desired phase behaviour at the operating temperature of interest. For example in some circumstances the preferred ratio is in the region of 1 :1. Examples of suitable cross-linkers include pentaerythritol tetra-acrylate and related cross-linkers and liquid crystal based cross-linkers including diacrylate based proprietary materials such as RM1 from Merck. Any suitable cross-linker may be used. The cross-linker is usually present in the range 1-10wt%; preferably 1-3wt%. Any suitable initiator may be used, and preferably is present in the range 1-10wt% preferably 1-5wt%. Preferably the ratio of initiator to cross-linker is in the region of 2:1 by weight. An example of a method according to the present invention is described below. A mixture was formulated comprising monomer 145, monomer 138, a pentaerythritol tetra-acrylate cross-linker and a photo-initiator (eg Irgacure 184 available from CIBA GEIGY); the monomers are mixed in a 1 :1 weight ratio, the initiator to cross-linker ratio is 2:1 by weight, the overall weight % of initiator is 4wt% the overall weight % of cross-linker is 2wt%. Optionally a further monomer may be added (eg RM22) to aid in alignment - preferred is any monomer with the phase sequence of nematic/smectic A/smectic C. Clean (eg acetone, IPA, water), dry plates made of silicon and quartz are heated eg on hotplates - the temperature being determined by the temperature at which the isotropic phase is formed in the mixture to be used. The plates are heated until they are stable at the desired temperature eg in this case for about 30 minutes, and the mixture heated to a liquid - optionally can be mixed eg using a high speed vortex mixer. The mixture can be poured or placed onto the lower plate which has a spacer placed or fixed on it. The upper plate can then be placed on top to complete the cell. The plates can be clamped together if necessary. Alternatively the plates can be fixed together, incorporating the spacer, and the material can be flowed into the cell. The plates may have electrodes on the surface (eg elliptical) these may be used to aid in poling - AC or DC. The use of elliptical electrodes helps prevent voltage accumulation spikes that can occur with patterns that have corners. The electrodes are patterned on the surface, for example on the surface of the doped silicon and for example on the surface of an Indium Tin Oxide (ITO) layer on the radiation transparent plate - eg a thin layer of transparent film on the quartz. Optionaliy a cooling rate is applied, and/or Optionally a voltage is applied. The mixture is cured by exposing the quartz plate to UV radiation for about 20-100 seconds. The plates are undipped and allowed to cool. The film may be removed either by the use of a knife-like tool eg a razor blade or the float-off method in a fluid, eg acetone or water, may be used. The shape of the film obtained is defined by the shape of the spacer. Typically films of area 10mm 2 - 40,000mm 2 are obtained. It is particularly advantageous that larger high quality films may be obtained. The thickness of the film is also variable and preferably lies in the range 0.01 -0.5mm thick. CH 2 =CH-COO(CH 2 )n-R Monomer 145 n=10 R = Monomer 138 n=11 R = RM22 n=6 R=

Claims: Claims 1 A method for the production of a free-standing liquid crystal elastomer film comprising the steps of providing a mixture of monomer, initiator and cross-linker, loading the mixture between spaced plates, curing the mixture with radiation to form the liquid crystal elastomer film, removing the film from between the plates, wherein one of the plates is a silicon wafer and the other plate is sufficiently transparent to the incident radiation at an appropriate wavelength such that polymerisation will occur 2 A method according to claim 1 wherein the other plate is made from quartz 3 A method according to claim 1 or 2 wherein at least one of the plates has an electrode present 4 A method according to any of the preceding claims wherein the incident radiation comprises UV and/or thermal radiation 5 A method according to claim 4 wherein the initiators are chosen from thermal and photoiniators 6 A method according to any of the preceding claims wherein the monomer material is aligned 7 A method according to claim 6 wherein alignment is achieved by poling 8 A method according to claim 7 wherein the silicon is doped and wherein poling is carried out by applying an AC or DC voltage 9. A method according to any of the preceding claims wherein the mixture is heated to an isotropic phase prior to curing. 10. A method according to any of the preceding claims wherein the elastomer material is ferroelectric. 11. A free-standing liquid crystal elastomer film obtainable by the method according to any of the preceding claims. 12. A free-standing liquid crystal elastomer film of area 10mm 2 -40, 000mm 2 13. A film according to claim 12 wherein the area is 100mm 2 -20, 000mm 2 14. A film according to claim 12 wherein the area is 500mm 2 -20, 000mm 2 . 15. A free-standing liquid crystal elastomer film suitable for use in a transducer device. 16. A film according to claim 15 of area 10mm 2 -40, 000mm 2 17. A film according to claim 15 wherein the area is 100mm 2 -20, 000mm 2 18. A film according to claim 15 wherein the area is 500mm 2 -20, 000mm 2 . 19. A pyroelectric device comprising two spaced electrodes and a layer of a liquid crystal elastomer film according to claim 11 enclosed between the electrodes. 20. A piezoelectric device comprising two spaced electrodes and a layer of a liquid crystal elastomer film according to claim 11 enclosed between the electrodes. 21. An optical recording medium comprising a recording layer which comprises an elastomer film according to claim 11. 22. A pyroelectric device comprising two spaced electrodes and a layer of a liquid crystal elastomer film according to any of claims12-14. 23. A piezoelectric device comprising two spaced electrodes and a layer of a liquid crystal elastomer film according to any of claims12-14. 24. An optical recording medium comprising a recording layer which comprises a layer of a liquid crystal elastomer film according to any of claims12-14. 25. A free-standing liquid crystal elastomer film.

Inventors: BIGGS ANDREW GRAHAM, BLACKWOOD KEITH MORAY, DAILEY STUART, BIGGS, ANDREW, GRAHAM, BLACKWOOD, KEITH, MORAY, DAILEY, STUART
Applicants: BIGGS ANDREW GRAHAM, BLACKWOOD KEITH MORAY, DAILEY STUART, SECR DEFENCE, BIGGS, ANDREW, GRAHAM, BLACKWOOD, KEITH, MORAY, DAILEY, STUART, THE SECRETARY OF STATE FOR DEFENCE
Application year: 2000
Main classification: G02F1
IPC codes: B29C39, B29C41, B29L7, C09K19, G02F1, H01L37, H01L41