FR2687483A1 - PROCESS FOR OBTAINING FILMS COMPRISING ORIENTED ORGANIC MOLECULES. - Google Patents

Abstract

The present invention relates to a process of obtaining materials having organic properties. This process makes it possible in particular to obtain films with at least one layer comprising organic molecules oriented in an organic and/or inorganic matrix. The layers obtained have good thermal and mechanical stability.

Description

PROCESS FOR OBTAINING FILMS COMPRISING MOLECULES
ORIENTED ORGANICS
The present invention relates to a process for obtaining materials having optical properties, in particular a process for obtaining films with at least one layer comprising organic molecules oriented in an organic and / or inorganic matrix. The layers obtained have improved optical properties as well as better thermal and mechanical stability, compared to the layers obtained by the processes of the prior art.

 The orientable organic molecules have optical properties linked to the simultaneous presence in these molecules of delocalized 7T electrons, that is to say of ethylenic or aromatic bonds, and of substituents which are electron donor groups or attractor groups of electrons placed so as to act on these n electrons. These organic molecules have much more marked optical properties than those of inorganic compounds. In addition, the optical properties of these organic molecules are all the more marked when the permanent orientation of the molecules relative to each other is important. The orientation of the molecules can be obtained either by applying an electric field in which the molecules behave like a dipole, ie by carrying out a monocrystalline growth which makes it possible to obtain an ordered structure in which the molecules tend to be positioned parallel with respect to each other. In these two cases, films are obtained having, for example, non-linear optical properties or birefringent properties.

 Various methods are known for obtaining layers having optical properties.

 For example, such layers can be obtained by performing a monocrystalline deposition of the organic molecules defined above on a support which promotes the orientation of the molecules.

 In Japanese Journal of Applied Physics, vol 28, N 11, November 1989, pp. 2259-2263, vacuum deposition of 2-methyl-4-nitroaniline (MNA) molecules is carried out onto which alkyl chains comprising a large number of carbons. These alkyl chains promote the growth of MNA monocrystals with an ordered structure.

This ordered structure of MNA single crystals makes it possible to obtain films having non-linear optical properties. In addition, the support is covered with KCl in a crystalline form which promotes the orderly deposition of MNA molecules.

 Vacuum deposition is carried out at pressures of the order of 10 6 Torr and a temperature between 25 and 650C, the deposition rate of the MNA is estimated at 10 nm / min. This process, although difficult to implement, makes it possible to obtain thin monocrystalline films of MNA-having nonlinear optical properties.

 It is known to obtain films or massive materials comprising oriented molecules, by trapping organic molecules capable of orienting themselves in an electric field. In such processes, organic molecules capable of orienting themselves in an electric field are introduced during the formation of an organic polymer. A polymer is thus obtained in which the organic molecules have been trapped, without any particular orientation. This composition is then heated above the glass transition temperature of the polymer. A viscous mixture is thus obtained which is placed in a high electric field to allow the orientation of the molecules. The composition is then cooled in order to trap the oriented molecules under the effect of the electric field.

 European patent 232 138 describes a composite consisting of an organic polymer matrix in which monocrystalline growth has been carried out in situ of organic or inorganic compounds having non-linear optical properties. This composite is obtained by extrusion of the polymer matrix in which the organic or inorganic compound has been dissolved. The extrusion temperature is preferably slightly higher than the higher melting temperature of the two components.

The crystallization of the organic compound is then carried out in situ by evaporation of the solvent.

 This technique makes it possible to carry out growth of organic or inorganic single crystals with ordered structure in an organic matrix, this orientation being carried out at a temperature at least equal to the glass transition temperature of the organic polymer used.

 The implementation of these methods involves numerous drawbacks. The polymers used must have a low glass transition temperature. In fact, an excessive rise in temperature can lead to a deterioration of the organic molecules used. In addition, 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 high because the molecules are oriented in a very viscous medium, of the order of 1010 to 1ol2 Pua s
Despite this, the percentage of oriented molecules remains low (of the order of 5 t) and, especially the organic molecules tend to return quickly to the disordered state, which leads to a very rapid degradation in time of the optical properties obtained. . In some cases, the materials obtained retain their optical or electrooptical properties for only a few minutes.

 Other drawbacks are linked to the use of these materials. For example, when these materials are used with a laser, the impact of the laser causes a specific heating of the material used, which disorients the molecules at the point of impact of the laser and leads to a rapid deterioration of the optical properties of the material.

 Patent WO 88/02131 describes a process for obtaining solid materials or fibers having optical or electrooptic properties, which consists in orienting the organic molecules in a gel or in a crosslinkable medium. This orientation is carried out by applying a high electric field, of the order of 1000 V / mm, for one hour.

 The present invention relates to a process for obtaining, without the drawbacks mentioned above, films consisting of one or more stable layers comprising organic molecules oriented in an organic and / or inorganic matrix. This process consists in orienting the organic molecules by applying an electric field to them in a liquid medium and in maintaining this orientation until a rigid polymer structure is obtained to trap the oriented molecules. The viscosity in the liquid state is generally between 10-1 and 4.10-1 Pa.s.

The method comprises at least once the sequence of the following steps
A. preparation of a liquid solution (A) comprising either (1) organic molecules orientable in an electric field and one or more precursors of organic and / or inorganic polymers, (2) either one or more precursors of organic polymers and / or inorganic substituted by a radical comprising an organic part which can be oriented in an electric field,
B. application of the solution obtained during step (A) on a support so as to form a layer,
C. drying of the layer obtained, characterized in that, for each sequence, an electric field is applied to the layer by means of electrodes to allow the orientation of the orientable organic molecules or of the orientable organic part before the polymer was formed from the polymer precursor (s).

 The invention makes it possible, for example, to obtain a multilayer film with a particular orientation of the organic molecules in each layer. The thickness of each layer after the drying step can be between 20 nm and several micrometers.

 According to the invention, the electric field is applied when the layer is obtained, that is to say during step B or steps B and C. It is maintained until the polymer 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 the solution (A) in order to orient the organic molecules at the very moment when the layer is formed, that is to say on the surface of the solution.

The electric field which directs molecules can be static or alternating. It must be less than the breaking electric field of the organic molecule to be oriented. We call "Electric Field of
Rupture "an electric field which causes the deterioration of the organic molecules used. The values of the electric field of rupture of orientable organic molecules are listed in the literature.

 According to the invention, the value of the electric field is generally low since the orientation of organic molecules in a liquid medium does not require as much energy as the orientation of the molecules in a viscous medium. The value of the electric field which is applied will preferably be between 1 and 200 V / cm. This value is a macroscopic value which is not that of the local field which really directs the organic molecules. An approximate value of the local electric field is obtained by dividing the value of the macroscopic electric field by the dielectric constant of the solution (A). The value of the local electric field is generally of the order of a millivolt per cm.

 The support can be partially conductive, which makes it possible to obtain a layer comprising molecules oriented only on the conductive areas. It can be completely or partially covered with a layer of (SnO2: F) or indium tin oxide. The support can be, for example, a glass or plastic plate.

When the support is made conductive according to a determined drawing, the deposition of the oriented molecules is carried out only on the conductive parts, which makes it possible to easily obtain waveguides having very complex structures.

 In addition, the electrodes can be mobile. The movement, the size and the shape of the electrodes will be determined according to the characteristics of the layer that one wishes to obtain. It is thus possible to obtain a deposit of oriented molecules determined by the size and the trajectory of the electrodes.

 According to the invention, a succession of zones can be obtained on the same layer, with in each zone, a particular orientation of the organic molecules. This particular orientation can be obtained by varying the polarization of the electrodes during coating.

One can also obtain zones containing molecules oriented in a particular way by varying the relative angle between the direction of the electric field and the movement of the support during step B.

 According to the invention, the polymer precursors can be silicon alkoxides, metal alkoxides such as titanium or zirconium alkoxides, these alkoxides comprising at least one hydrolyzable radical. These alkoxides can be substituted by at least one organic radical in order to improve the mechanical properties of the layers obtained. These precursors can be used alone or as a mixture. The hydrolysis of these precursors is obtained by adding a water / alcohol mixture to the solution (A). It is possible, using a mixture of organic and / or inorganic polymer precursors, to obtain matrices having very varied characteristics.

 For example, the use of a mixture of silicon and titanium alkoxides makes it possible to obtain a matrix having a high refractive index. When the precursor is totally or partially organic, matrices are obtained having improved mechanical and thermal properties.

 According to the invention, it is possible to use as polymer precursors, a composite mixture containing at least one precursor of organic polymer and at least one precursor of inorganic and / or organic-inorganic polymer. An organic-inorganic composite matrix is thus obtained.

 In this case, the orientation of the organic molecules is always carried out in the liquid state by application of an electric field during the drying of the layer. The electric field is maintained until the organic polymer has completely polymerized. This polymerization is carried out by conventional methods such as photochemical polymerization or condensation polymerization by raising the temperature. In the case of photochemical polymerization, a photochemical catalyst is added to solution (A). The polymerization takes place during the drying step.

 In some cases, a solvent can be added to solution (A) to obtain a homogeneous solution. It will be chosen volatile in order to be eliminated quickly during the drying step.

 According to the invention, it is also possible to use totally organic precursors. The range of organic precursors which can be used in the process of the present invention is wider than in the prior art.

In fact, in the prior art, the orientation of the organic molecules is carried out by heating the polymer containing the organic molecules to the glass transition temperature of the polymer used. The choice of precursor is therefore limited to polymers having a low glass transition temperature so as not to damage the orientable organic molecules. These polymers generally have poor mechanical properties.

 In the present invention, the polymer which constitutes the matrix can have a high glass transition temperature since the orientation of the molecules takes place at ambient temperature. An organic matrix is thus obtained having improved mechanical and thermal properties compared to the matrices of organic polymers at low glass transition temperature of the prior art. Even when the polymerization of the organic precursor is obtained by heating, the polymerization temperature is always lower than the glass transition temperature of the polymer used.

 Polymer precursors which can be used in the context of the present invention are described in Applied Pics, Vol.ll, February 1972, p.428-433.

 Organic molecules orientable in an electric field include organic molecules having a permanent dipole moment or organic molecules having a dipole moment induced by an electric field, called polarizable molecules. These organic molecules can be chosen from aromatic derivatives, for example benzene, carrying in the appropriate position highly electron-donor substituents or attractors such as 4-nitroaniline, 2-nitroaniline, 2, 4-dinitroaniline, 2-chloro- 4-nitroaniline, 2-chloro-5-nitroaniline, 2-methyl-4-nitroaniline, 4-nitroanisole, 4-dimethylamino-4'-nitro-trans-stilbene.

Other molecules orientable in an electric field are described in Nonlinear Optimal Properties of Orsanic Molecules and Cristals, Vol.1, p.220-223 and in
Nonlinear Optical Properties of Orqanic and Polvmeric
Materials, ACS Symposium series 233, p.60-61.

 According to the invention, the precursor or precursors of organic and / or inorganic polymers and the orientable organic molecules can be replaced by one or more precursors of organic and / or inorganic polymer substituted by a radical consisting of a long linear alkylene chain saturated with at its end an organic part capable of orienting itself in an electric field.

The saturated alkylene chain preferably comprises at least 4 carbon atoms so as not to hinder the orientation of the orientable part.

 For example, when the oriented molecule is 2-methyl-4-nitroaniline, layers with nonlinear optical properties are obtained which make it possible to manufacture frequency doubling elements.

 The orientation of the molecules can allow the refractive index of the layer obtained to be changed to obtain birefringent materials. The method of the invention makes it possible to juxtapose zones having different refractive indices in order to obtain plane or channel-shaped waveguides.

 The coating of solution (A) can be obtained by any coating method known from the prior art. Layers can for example be obtained by dipping the support in solution (A), or by conventional coating methods such as the transfer cylinder, the spinner, the squeegee, spraying.

 FIG. 1 shows the different orientations of the organic molecules (la) and the different layers comprising these oriented molecules (lb, lc, ld) which can be obtained according to the invention by varying the polarization 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 spectrum of a layer containing MNA molecules oriented in a Silo2 matrix.

 Figure 3 shows the preferential orientation of the molecules of MNA in an amorphous matrix of SiO2 by scanning the diffraction angle X in azimuth.

 FIGS. 4a and 4b are X-ray diffraction spectra characteristic of a layer supersaturated with MNA molecules obtained according to the invention.

 FIGS. 5a and 5b are X-ray diffraction spectra characteristic of a layer comprising 2,4-dinitroaniline molecules obtained according to the invention.

 FIGS. 6a and 6b are X-ray diffraction spectra characteristic of a layer comprising 3-nitroaniline molecules obtained according to the invention.

EXAMPLES
The following examples were made from organic molecules which can be oriented in an electric field having nonlinear optical properties. The use of these molecules makes it possible to obtain excellent frequency doublers. These molecules were chosen for their ease of use, but any orientable molecule can be used in the process of the invention.

Example 1
Use of an inorganic polymer precursor.

A solution containing 15.2 g of
If (OCH3) 4, methanol and water with a H2O / CH3OH molar ratio equal to 4 to hydrolyze the silicon alkoxide.

A few grams of MNA are then added to this solution. After stirring, 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. We remove the support. A thin layer is formed and dried in the electric field (drying time 2 min). The applied electric field is to twist 30
V / cm, the thickness of the layer is of the order of 0.1 μm.

 MNA molecules are thus definitively oriented in an amorphous matrix of Six2.

 When we illuminate this layer with a YAG: Nd laser (1.06 tm), we observe a doubling of the frequency linked to the orientation of the MNA molecules in the layer.

 FIG. 2 shows the X-ray diffraction spectrum of the layer obtained. The peaks observed are the characteristic peaks of an ordered phase which does not correspond to a known crystal form. The spectrum resembles the spectrum of crystalline MNA with a displacement of the peaks towards the weak angles. This movement towards small angles demonstrates that the molecules of MNA present in the layer are not associated in crystal structure and that they are isolated with respect to each other.

 FIG. 3 highlights the preferential orientation of the molecules in the layer by varying the X-ray diffraction scanning angle in azimuth.

Obtaining different spectra according to the scanning 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 scanning angle.

 An X-ray diffraction spectrum was performed on the same layer after 8 months of storage at room temperature. A spectrum identical to FIG. 2 was obtained, which shows the stability over time of the orientation of the molecules in the layer obtained by the method of the invention. This stability is mainly due to the high glass transition temperature of the sio2 matrix.

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 (OCH3) 4 and 33 g of (CH30) 3Si (CH2) 3NHCH2CH2NH2. 30 g of methanol and 0.5 g of MNA are added to this solution, then a mixture containing 12.8 g H2O and 6 g of methanol with stirring to obtain a homogeneous solution. Then added 1.75 g of MNA
A glass plate covered with a conductive film (SnO2: F) is immersed in the homogeneous solution; an electric field of 100 V is applied to the support by means of 2 electrodes separated by 4 mm.

 Because of the conductivity of the solution, the electric field measured after immersion of the support in the solution is 4 V. This value of the electric field allows the orientation of organic molecules.

 The plate is then removed from the solution and dried in the presence of the electric field for 5 minutes.

the thickness of the layer is of the order of 3.5 μm.

 When we illuminate this layer with a YAG: Nd laser (1.06 im), we observe a doubling of the frequency linked to the orientation of the MNA molecules in the layer.

 The X-ray diffraction spectrum, when the support response is eliminated, includes 2 intense peaks at d = 0.912 nm and d = 0.309 nm. The azimuth angle scan showed as in Figure 3 that the MNA molecules are well oriented.

Example 3
The same solution is prepared as in Example 2 but by adding to this solution an amount of MNA which makes it possible to obtain a layer supersaturated with MNA.

 The X-ray diffraction spectrum of this layer (fig. 4a) corresponds to that of crystalline MNA, which implies that the molecules are no longer isolated from one another but, on the contrary, associated in the form of crystallites.

 The scanning of the azimuth angle (fig. 4b) showed, as in FIG. 3, that the crystallites of MNA are well oriented.

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 (OCH3) 4, 7.21 g of water and 15 g of methanol. The mixture is kept at room temperature and with stirring. A mixture of 2 g of MNA and 10 g of methyl methacrylate is then added to this solution.

 When the solution is homogeneous, a glass plate partially covered with tin and indium oxide (ITO) is immersed, which acts as an electrode.

 Another strip of ITO or another metal can be used as the second electrode. These electrodes must be separated by a few millimeters.

 An electric field (50 V / cm) is then applied which 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 polymerizes the organic precursor.

A film is thus obtained comprising molecules of
MNA oriented in a composite matrix sio2- polymethylmethacrylate.

 When we illuminate this layer with a YAG: Nd laser (1.06 ym), we observe a doubling of the frequency linked to the orientation of the MNA molecules in the layer.

Example 5
Obtaining a layer comprising oriented molecules of 2,4-dinitroaniline in an inorganic matrix.

 15.2 g of Si (OCH3) 4 are mixed, 7.21 g of H2O and 15 ml of methanol. Then an amount of 2,4-dinitroaniline is added which makes it possible to obtain a supersaturated layer of 2,4-dinitroaniline. A layer is obtained by dipping a glass plate in the solution obtained above. The glass plate covered with the layer is then placed between 2 electrodes by applying an electric field of 4 V / cm.

 The X-ray diffraction spectrum (fig.5a) shows the presence of 2,4-dinitroaniline crystallites in the layer obtained. The scanning of the azimuth angle (fig.5b) of the most intense peak showed as in Figure 3 that the 2,4-dinitroaniline molecules are well oriented.

Example 6
A solution containing Si (CH3) 4 and a mixture
H2O / CH3OH is prepared according to the method of Example 5. Then an amount of 3-nitroaniline is added to this solution which makes it possible to obtain a supersaturated layer.

 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 scanning of the azimuth angle (fig. 6b) of the most intense peak showed, as in FIG. 3, that the 3-nitroaniline molecules are well oriented.

Example 7
Obtaining a multilayer film
A solution (1) containing Si (OCH3) 4, a mixture of H2O / CH3OH and the MNA is prepared according to the method of example 1. A solution (2) containing Si (OCH3) 4, (CH30) 3Si is then prepared (CH2) 3NHCH2CH2NH2, a mixture of H2O / CH3OH and MNA according to the method of Example 2.

 I. A glass support is then immersed in the solution (1). The electric field is applied by means of electrodes partially immersed in the solution (1).

We remove the support. In this case, the electric field is parallel to the support. A thin layer is formed and dried in the electric field.

 This sequence (I) is repeated 3 times to obtain a 3-layer film with, in each layer, molecules having the same orientation.

 II. The film thus obtained is immersed in the solution (2). The electric field is applied by means of electrodes partially immersed in the solution (2). In this case, the polarization of the electric field is reversed with respect to the sequence (I). A thin layer is formed by removing the support from the solution (2) and dried in the electric field (sequence II).

 The sequences (I) and (II) are repeated 4 times.

 When we illuminate this layer with a YAG: Nd laser (1.06 ym), we observe a doubling of the frequency linked to the orientation of the MNA molecules in the layer.

Claims (23)

1 - Process for obtaining a film having  optical properties with at least one layer having  oriented molecules, comprising at least once the  sequence of the following steps:  A. preparation of a liquid solution comprising either  (1) organic molecules orientable in a field  electric and one or more polymer precursors  organic and / or inorganic, (2) either one or  several precursors of organic polymers and / or  inorganic substituted by a radical comprising a  organic part orientable in an electric field,  B. application of the solution obtained during  step (A) on a support so as to form a  layer,  C. drying of the layer obtained,  characterized in that, for each sequence,  applies by means of electrodes an electric field to  the layer to allow the orientation of the molecules  orientable organic or organic part  orientable of the polymer precursor (s) before  that this polymer n1 was formed from the  polymer precursors. 2 - Method according to claim 1 wherein the field  electric is applied during step B. 3 - Method according to claim 1 wherein the field  electric is applied during steps B and C. 4 - Method according to claim 1, 2 or 3 wherein  organic molecules are molecules at the moment  permanent electric dipole. 5 - Method according to claim 1,2 or 3 wherein  organic molecules are molecules at the moment  dipole induced by an electric field. 6 - Method according to any one of claims 1 to  5 wherein the polymer precursor (s) are  chosen from silicon alkoxides, alkoxides  metallic, organometallic alkoxides  comprising at least one hydrolyzable radical, or a  mixture of these various alkoxides.  7 - Method according to any one of claims  in which provision is made for  vary the relative angle between the direction of  electric field and the movement of the support in the  solution during step B.  8 - Method according to any one of claims  previous in which the orientation of the molecules  is carried out by applying the electric field  directly on a conductive support or partially  driver.  9 - Method according to any one of claims 1 to  8 in which the orientation of the molecules takes place  by placing a support covered with the layer in a  electric field. 10 - Process according to claim 9 wherein the  dimension of the electrodes is at least equal to the  support dimension. 11 - The method of claim 9 wherein the  dimension of the electrodes is less than the dimension  support. 12 - Method according to claim 11 wherein the  electrodes are pointwise. 13 - Method according to any one of claims 9 to  12 in which the electrodes are movable. 14 - Method according to any one of claims  previous ones in which we vary the polarization  electrodes during the coating step (B). 15 - Method according to any one of claims  previous in which the precursor is (CH30) 4Si  and / or (CH30) 3Si (CH3) 3NH (CH2) 2NH2 and / or the  methylmethacrylate. 16 - Method according to any one of claims  previous in which the organic molecule  orientable is chosen from 4-nitroaniline,  2-nitroaniline, 2, 4-dinitroaniline,  2-chloro-4-nitroaniline, 2-chloro-5-nitroaniline,  2-methyl-4-nitroaniline, 4-nitroanisole, 4-dimethylamino-4 '-nitro-trans-stilbene. 17 - Method according to any one of claims 8 to  16 in which the support is a glass plate  covered with a conductive layer of (SnO2: F) or  of tin and indium oxide. 18 - Method according to any one of claims  previous in which the polymer precursor  organic and / or inorganic substituted by a radical  comprising an orientable organic part is such that  the orientable organic part is connected to the  precursor by a saturated hydrocarbon chain  linear with at least 4 carbon atoms. 19 - Films likely to be obtained according to one  any of the preceding claims wherein  the precursor (s) of polymers are inorganic  and / or inorganic-inorganic. 20 - Optical articles comprising at least one film according to the  claim 19.