Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/35—Non-linear optics
  • G02F1/355—Non-linear optics characterised by the materials used
  • G02F1/361—Organic materials
  • G02F1/3615—Organic materials containing polymers
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
  • G02B1/08—Optical elements characterised by the material of which they are made; Optical coatings for optical elements made of polarising materials

Definitions

• the following invention concerns a process of obtaining materials having optical properties, in particular a process which makes it possible to obtain films comprising a support and at least one layer comprising organic molecules oriented in an organic and/or inorganic matrix.
• the layers obtained have improved optical properties and better thermal and mechanical stability compared with the layers obtained by the methods of the prior art.
• Orientable organic molecules have optical properties related to the simultaneous presence in these molecules of deiocalised ⁇ electrons, that is to say ethylenic or aromatic bonds, and substituents which are electron donor groups or electron acceptor groups placed so as to act on these T electrons.
• These organic molecules have optical properties which are much more pronounced than those of inorganic compounds.
• the greater the permanent orientation of the molecules with respect to each other the more pronounced are the optical properties of these organic molecules.
• the orientation of the molecules can be obtained either by applying an electric field in which the molecules behave like dipoles, or by effecting a monocrystalline growth which makes it possible to obtain an ordered structure in which the molecules have a tendency to position themselves parallel to each other. In both these cases, films are obtained having, for example, non-linear optical properties or birefringent properties.
• films or solid materials comprising oriented molecules can be obtained by trapping organic molecules capable of orienting themselves in an electric field.
• organic molecules capable of orienting themselves in an electric field are introduced during the formation of an organic polymer.
• This composition is then heated to above the glass transition temperature of the polymer. In this way a viscous mixture is obtained which is placed in a strong electric field so as to allow the orientation of the molecules.
• European Patent 232 138 describes a composite consisting of an organic polymer matrix in which an in situ monocrystalline growth of organic or inorganic compounds with non-linear optical properties has been effected. This composite is obtained by extruding the polymer matrix in which the organic or inorganic compound has been dissolved. The extrusion temperature is preferably slightly higher than the highest fusion temperature of the two constituents. The crystallisation of the organic compound then takes place in situ by evaporation of the solvent.
• This technigue makes it possible to effect a growth of organic or inorganic monocrystals with an ordered structure in an organic matrix, this orientation being effected at a temperature at least equal to the glass transition temperature of the organic polymer used.
• the implementation of these methods entails many drawbacks.
• the polymers used must have a low glass transition temperature. In fact, raising the temperature too high may cause damage to the organic molecules used.
• the organic polymers which have a low glass transition temperature generally have low thermal and mechanical stability.
• the electric field used to orient the molecules is generally strong since the orientation of the molecules takes place in a highly viscous medium, of the order of 10 10 to 10 12 Pa.s.
• the percentage of molecules oriented remains low (of the order of 5%) and, in particular, the organic molecules have a tendency to return rapidly to the disordered state, which causes a very rapid deterioration over time of the optical properties obtained.
• the materials obtained keep their optical or electro- optical properties for only a few minutes.
• the patent WO 88/02131 describes a method for obtaining solid materials or fibres with optical or electro-optical properties, which consists of orienting the organic molecules in a gel or in a cross-linkable medium. This orientation is effected by applying a strong electric field, of the order of 1000 V/mm, for one hour.
• European patent application EP 406,888 describes films comprising organic molecules oriented in a thermocured resin. In all the examples of this European application, the electric field which allows the molecule orientation is applied on a partially cured layer. This process is applicable only with thermocurable resins and it does not make it possible to obtain multilayer films.
• the present invention relates to a process for obtaining, without the drawbacks or limitations mentioned above, films consisting of a support and one or more stable layers comprising organic molecules oriented in an organic and/or inorganic matrix. This process consists of orienting the organic molecules by applying an electric field to them in the course of the forming of the layer which is still in a liquid form and maintaining this orientation until a rigid polymer matrix is obtained so as to trap the oriented molecules.
• the viscosity in the liquid state is in general between 10 -1 and 4.10" 1 Pa.s.
• A preparation of a liquid solution (A) comprising either (1) organic molecules which can be oriented in an electric field and at least one organic and/or inorganic matrix precursor selected from polymer precursors, prepoly ers or polymers, or (2) at least one organic and/or inorganic matrix precursor, substituted with a radical comprising an organic part which can be oriented in an electric field,
• step(A) application of the solution obtained during step(A) to a support so as to form a layer
• step (B) of the forming of the layer by means of electrodess so as to allow the orientation of the orientable organic molecules or of the orientable organic part.
• a solvent can be added to solution (A) in order to obtain a homogeneous solution.
• a volatile one will be chosen so as to be eliminated as fast as possible during the drying stage.
• the choice of the matrix precursors is very wide.
• the matrix precursor will be chosen according to the required final properties and the ability of the precursors to be in a liquid form.
• the invention makes it possible to obtain, for example, a multilayer film with a particular orientation of the organic molecules in each layer.
• the thickness of each layer after the drying stage can be between 20 nm and several micrometers.
• the electric field is applied at the time the layer is obtained, that is to say at least during step (B) . It is maintained until the matrix is formed.
• the electric field can be applied either directly to the support by means of electrodes or by placing the layer obtained between two electrodes. These electrodes can be partially immersed in solution (A) in order to orient the organic molecules at the very moment at which the layer forms, that is to say on the surface of the solution.
• the electrodes can be covered with an insulating layer to avoid current passing through the two electrodes.
• the electric field which enables the molecules to be oriented can be static or alternating. It must be weaker than the breakdown electric field of the organic molecule to be oriented.
• "Breakdown Electric Field” means an electric field which causes damage to the organic molecules used.
• the values of the breakdown electric field of orientable organic molecules are set out in the literature. According to the invention, the value of the electric field is generally low since orientation of the organic molecules in a liquid medium does not require as much energy as orientation of the molecules in a viscous medium.
• the value of the electric field also depends on the matrix precursors used.
• the values of the electric field which is applied will preferably be between 1 V/cm and the value of the breakdown electric field of the orientable organic molecules or parts used.
• the value of the applied electric field is a macroscopic value which is not the value of the local field which actually orients the organic molecules.
• An approximate value of the local electric field will be obtained by dividing the value of the macroscopic electric field by the dielectric constant of solution (A) .
• the value of the local electric field is generally of the order of a few millivolts per cm.
• the support can be partially conductive, which makes it possible to obtain a layer comprising molecules oriented only on the conductive areas. They can be covered wholly or partially with a layer of (Sn0 2 :F) or indium tin oxide (ITO) .
• the support can, for example be a sheet of glass or plastic.
• the deposition of the oriented molecules takes place only on the conductive parts, which enables waveguides with very complex structures to be obtained easily.
• the electrodes can be punctual.
• the electrodes can be movable. The movement, size and shape of the electrodes will be determined according to the characteristics of the layer which it is desired to obtain. It is thus possible to obtain a deposition of oriented molecules determined by the size and path of the electrodes.
• a succession of areas can be obtained on the same layer with, in each area, a particular orientation of the organic molecules.
• This particular orientation can be obtained by causing the polarisation of the electrodes to be varied during the layering. It is also possible to obtain areas containing molecules oriented in a particular fashion by varying the relative angle between the direction of the electric field and the movement of the support during step B.
• the matrix precursors can be silicon alkoxides or metal alkoxides such as titanium or zirconium alkoxides, these alkoxides comprising at least one hydrolysable radical. These alkoxides can be substituted with a least one organic radical in order to improve the mechanical properties of the layers obtained.
• These precursors can be used alone or in admixture. Hydrolysis of these precursors is brought about by adding a water/alcohol mixture to solution (A) . It is possible, by using a mixture of organic and/or inorganic matrix precursors, to obtain matrices with highly varied characteristics.
• the use of a mixture of silicon and titanium alkoxides makes it possible to obtain a matrix with a high refractive index.
• the precursor is wholly or partially organic, matrices with improved mechanical and thermal properties are obtained.
• solution (A) When solution (A) is obtained from polymer precursors or prepolymer the electric field is maintained until the polymer has completely polymerised.
• This polymerisation is effected by conventional processes such as photochemical polymerisation or polymerisation by condensation on raising the temperature.
• a photochemical catalyst is added to solution (A) .
• matrix precursors a composite mixture containing at least one organic polymer precursor or prepolymer or polymer and at least one inorganic and/or organic-inorganic polymer precursor or prepolymer or polymer. In this way an organic-inorganic composite matrix is obtained.
• completely organic matrix precursors can also be used.
• organic precursors or prepolymers or polymers which may be used in the present invention is wider than in the prior art.
• the organic molecules are oriented by heating the polymer containing the organic molecules to the glass transition temperature of the polymer used.
• the choice of polymers is therefore limited to polymers which have a low glass transition temperature so as not to damage the orientable organic molecules. These polymers generally have poor mechanical properties.
• the polymer constituting the matrix can have a high glass transition temperature.
• the solution (A) is obtained from polymers and, if necessary a solvent
• the rigid matrix is obtained during the step (C) by solvent elimination. Even if the solvent elimination requires an increase of the temperature, this temperature is still lower than the glass transition temperature of the polymer used.
• the solution (A) is obtained from precursors or prepolymers and, if necessary a solvent, even when polymerisation of the organic precursor is obtained by heating, the polymerisation temperature is still lower than the glass transition temperature of the polymer used.
• an organic matrix according to the invention can be made from polymers having mechanical and thermal properties which are improved in comparison with the organic polymer matrices with a low glass transition temperature of the prior art.
• matrix precursors which can be used within the scope of the present invention are described in Applied Optics, Vol 11, February 1972, pp 428-433.
• the organic molecules or parts which can be oriented in an electric field comprise organic molecules or parts which have a permanent dipole moment or organic molecules which have a dipole moment induced by an electric field, known as polarisable molecules.
• These organic molecules can be chosen from amongst aromatic derivatives, for example benzene derivatives carrying, in a suitable position, substituents which are strong electron donors or acceptors such as 4-nitroaniline, 2-nitroaniline, 2,4-dinitroaniline, 2-chloro-4-nitroaniline, 2-chloro-5-nitroaniline, 2-methyl- 4-nitroaniline, 4-nitroanisole, 4-dimethylamimo-4'-nitro- trans-stilbene.
• the organic and/or inorganic matrix precursors and the orientable organic molecules can be replaced by one or more organic and/or inorganic matrix precursors substituted with a radical consisting of a long linear saturated alkylene chain with, at the end, an organic part capable of being oriented in an electric field.
• the saturated alkylene chain comprises preferably at least four carbon atoms so as not to interfere with the orientation of the orientable part.
• the oriented molecule is 2-methyl-4- nitroaniline
• layers are obtained which have nonlinear optical properties enabling frequency doubling elements to be manufactured.
• the orientation of the molecules can make it possible to change the refractive index of the layer obtained so as to obtain birefringent materials.
• the process of the invention makes it possible to juxtapose areas with different refractive indices so as to obtain plane or channel-shaped waveguides.
• the coating of solution (A) can be obtained by any known coating process of the prior art.
• layers can be obtained by dip-coating the support in solution (A) , or by the conventional coating methods such as kiss roll coating, spin coating, doctor blade coating or spray coating.
• Figure 1 shows the different orientations of the organic molecules (la) and the different layers comprising these oriented molecules (lb, lc, Id) which can be obtained according to the invention by varying the polarisation of the electrodes or the relative angle between the electric field and the movement of the support during step (B) .
• Figure 2 is an X-ray diffraction pattern of a layer containing MNA molecules oriented in an Si0 2 matrix.
• Figure 3 shows the preferential orientation of the MNA molecules in an amorphous Si ⁇ 2 matrix by sweeping the X-ray diffraction angle in azimuth.
• Figures 4a and 4b are X-ray diffraction pattern characteristic of a layer supersaturated with MNA molecules obtained according to the invention.
• Figures 5a and 5b are X-ray diffraction pattern characteristic of a layer comprising molecules of 2,4- dinitroaniline obtained according to the invention.
• Figures 6a and 6b are X-ray diffraction pattern characteristic of a layer comprising molecules of 3- nitroaniline obtained according to the invention.
• Figure 7 is an Absorption spectrum of dispersed red 1 (DRl)-PMMA (polymethyl ethacrylate) films on ITO/glass substrates with application of a DC-electric field
• Figures 8 to 10 are schematic representations showing how the present process can be carried out.
• Figures 8 represents the dip coating of films (3) onto a conductive substrates (2) constituted of a support (5) covered with a conductive layer (4) .
• the conductive substrates (4-5) are immersed in the solution (A) (1) containing the matrix precusors and the orientable organic molecules.
• the DC-electric field is applied before, during and after the formation of films (3) until they are completely dried.
• the obtained film (6) is constituted of non-oriented organic molecules because of the absence of a conductive layer on this side of the support.
• Figure 9 represents the dip coating of film (3) onto a support (5) , the orientation of the orientable organic molecules being obtained by placing the support (5) between the two electrodes (7) .
• Oriented films (3) are obtained on both sides of the substrate.
• the orientable molecules are oriented perpendicular to the support.
• the orientable molecules are oriented parallel to the support. In both processes, the DC-electric field is applied before, during and after the formation of the films (3) until they are completely dried.
• Figure 10 represents the dip coating of films (3, 6) onto substrates (2) constituted of a support (5) partially covered with a conductive layer according to a given design.
• the obtained film (3, 6) is constituted of oriented molecules onto the conductive design (3) and non oriented molecules onto the part of the support which is not covered with conductive layer (6) .
• EXAMPLE 1 Use of an inorganic polymer precursor.
• a solution is prepared containing 15.2 g of Si(OCH 3 ) 4 , methanol and water with an H2O/CH 3 OH molar ratio of 4 to hydrolyse the silicon alkoxide.
• MNA MNA
• a homogeneous solution is obtained.
• a glass support is then immersed in the homogeneous solution.
• the electric field is applied by means of electrodes partially immersed in the solution obtained.
• the support is withdrawn.
• a fine layer is formed and dried in the electric field (drying period 2 min) .
• the electric field applied is of the order or 30 V/cm and the thickness of the layer is of the order of 0.1 ⁇ m.
• oriented MNA molecules are obtained which are permanently oriented in an amorphous matrix of Si0 2 .
• Figure 2 shows the X-ray diffraction pattern of the layer obtained.
• the peaks observed are the peaks characteristic of an ordered phase which does not correspond to a known crystalline form.
• the pattern resembles the crystalline MNA spectrum with a displacement of the peaks towards the small angles. This displacement towards the small angles shows that the MNA molecules present in the layer are not associated in a crystal structure and that they are isolated from each other.
• Figure 3 shows the preferential orientation of the molecules in the layer by varying the X-ray diffraction sweep angle in azimuth. The obtaining of different spectra according to the sweep angle shows that the molecules in the layer obtained have a preferential orientation. If the molecules had not been oriented, identical spectra would have been obtained whatever the sweep angle.
• EXAMPLE 2 Use of a mixture of an inorganic precursor and an inorganic precursor substituted with an organic radical.
• a solution is prepared containing 7.69 g of Si(OCH 3 )4 and 33 g of (CH 3 0) 3 Si(CH 2 )3NHCH 2 CH 2 NH 2 .
• To this solution are added 30 g of methanol and 0.5 g of MNA, and then a mixture containing 12.8 g of H20 and 6 g of methanol under agitation so as to obtain a homogeneous solution. Then 1.75 g of MNA is added.
• a glass plate covered with a conductive film (Sn0 2 :F) is immersed in the homogeneous solution; a 100 V electric field is applied to the support by means of two electrodes 4mm apart.
• the electric field measured after the immersion of the support in the solution is 4 V. This value of the electric field makes possible the orientation of the organic molecules.
• the plate is then withdrawn from the solution and dried in the electric field for five minutes.
• the thickness of the layer is of the order of 3.5 ⁇ m.
• Example 2 The same solution is prepared as in Example 2 but with the addition to the solution of a quantity of MNA which makes it possible to obtain a layer supersaturated with MNA.
• Fig 4a The X-ray diffraction spectrum of this layer (Fig 4a) corresponds to that of crystalline MNA, which means that the molecules are no longer isolated from each other but on the contrary are associated in the form of crystallites.
• EXAMPLE 4 Use of a mixture of precursors comprising an inorganic precursor and an organic precursor.
• a solution is prepared containing 15.2 g of Si(OCH 3 ) 4 , 7.21 g of water and 15 g of methanol. The mixture is maintained at ambient temperature under agitation. Then a mixture of 2 g of MNA and 10 g of methyl methacrylate is added to this solution.
• ITO indium tin oxide
• An electric field (50 V/cm) is then applied and is maintained during the drying of the film. After a few minutes, the film, still placed in an electric field, is subjected to ultraviolet radiation, which polymerises the organic precursor.
• the X-ray diffraction spectrum (Fig 5a) shows the presence of 2,4-dinitroaniline crystallites in the layer obtained.
• the sweeping in azimuth of the angle (Fig 5b) of the strongest peak shows, as in Fig 3, that the 2,4- dinitroaniline molecules are well oriented.
• a solution containing Si(CH 3 )4 and an H 2 0/CH 3 OH mixture is prepared in accordance with the process of Example 5.
• a quantity of 3-nitroaniline is then added to this solution, which enables a supersaturated layer to be obtained.
• the field applied to the layer is of the order of 20 V/cm.
• the grazing angle X-ray diffraction spectrum (Fig 6a) shows the presence of a crystalline phase.
• the sweeping in azimuth of the angle (Fig 6b) of the strongest peak shows, as in Figure 3, that the 3-nitroaniline molecules are well oriented.
• EXAMPLE 7 Obtaining a multilayer film.
• a solution (1) is prepared, containing Si(OCH 3 )4, an H2OCH3OH mixture and MNA according to the process of Example 1.
• a solution (2) is then prepared, containing Si(OCH 3 ) 4 , (CH 3 0) 3 Si(CH 2 ) 3 NHCH 2 CH 2 NH 2 , an H 2 0/CH 3 OH mixture and MNA according to the process of Example 2.
• a glass support is then immersed in solution (1) .
• the electric field is applied by means of electrodes partially immersed in solution (1) .
• the support is withdrawn. In this case, the electric field is parallel to the support.
• a fine layer is formed and dried in the electric field.
• This sequence (I) is repeated three times so as to obtain a three-layer film with, in each layer, molecules with the same orientation. II.
• the film thus obtained is immersed in solution
• Sequences (I) and (II) are repeated four times.
• this layer is illuminated with a YAG:Nd laser (1.06 ⁇ m) a doubling of the frequency is observed, related to the orientation of the MNA molecules in the layer.
• a solution is prepared containing 15 grams of polymethacrylate PMMA (molecular weight 38,000, purchased from Janssen) , 40 ml of 2-ethoxyethyl acetate, 0.3 gram of disperse red 1 (DR1) and 20 ml of chlorobenzene at 60°C under stirring. Once a clear solution is obtained, the mixture is cooled to room temperature. Then 20 ml acetone is added.
• PMMA molethacrylate
• DR1 disperse red 1
• ITO-coated supports which are also served meanwhile as electrodes are immersed in it. Films are dip-coated. A 3000 V DC-electric field is applied to the substrates separated by 2 mm. Films obtained are about 2.75 ⁇ m thick. The electric field is turned on when the substrates are still in solution before being pulled out, and kept until the films are completely dried. A hair dryer is used to help the removal of solvents. The film on the nonconductive side of the substrate is mechanically removed before the absorption measurement.
• Figure 7 shows the absorbance of the obtained film with application of a DC-electric field (Fig.7a) versus a film obtained without application of the electric field (Fig.7b) .
• the difference in absorbance arised from dichroism indicates the presence of orientation of DR1 molecules in the film obtained with application of the electric field.
• the same absorption spectrum is recorded on the same film immediately after the film is dried and three months later, which indicates the absence of relaxation of chromophore orientations.
• the orientational relaxation was reported to be important in the literature when using traditional poling techniques (Corona and electrode poling) after the formation of films at above Tg of the polymer, for example in the article of J. opt. Soc. Am. B,6 (1989) 733, M.A. Morazavi, A. Knoesen, S.T. Kowel; B.G. Hihhins and A. Dienes.

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• 1992
  • 1992-02-17 FR FR9201967A patent/FR2687483A1/fr active Granted
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  • 1993-02-08 WO PCT/EP1993/000290 patent/WO1993016401A1/en active Application Filing

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