FR2883883A1 - COMPLEX MOLECULES HAVING A HIGH DISPERSING POWER BASED ON LIQUID CRYSTAL FUNCTION AND METAL OXIDE PARTICLES - Google Patents

Abstract

The subject of the invention is a new type of complex chemical molecules comprising liquid crystal chains capable of constituting a dispersive element for liquid crystal molecules within a polymer matrix, characterized in that it corresponds to the following chemical formula: FN1M (OR) N2AN3BN4With M: metal of Si, Ti, ZrF type: group comprising a liquid crystal functionR, A, B: aliphatic groupsAndN1 + N2 + N3 + N4> = 1N1 between 1 and 3N2 between 1 and 3N3 between 0 and 2N4 between 0 and 2Applications: Electro-optical materials based on liquid crystals for optical shutters and in particular for display screens

Description

COMPLEX MOLECULES HAVING A HIGH DISPERSING POWER BASED ON

  LIQUID CRYSTAL FUNCTION AND OXIDE PARTICLE

METALLIC

  The field of the invention is that of electrooptical materials and more specifically that of electrooptical materials based on liquid crystal dispersed in a polymer. This type of material is particularly interesting in the field of optical shutters and in particular in the field of display screens.

  Polymer dispersed liquid crystal (PDLC) materials represent a fairly large class of organic optical components. Presenting historically droplets of liquid crystals with micrometer sizes, the PDLCs now include materials for which the mesomorphic domains have dimensions of a few tens of nanometers.

  The presence of liquid crystals allows electrical addressing of these components and opens broad perspectives in terms of switchable optical functions. The separation of photoinduced phases that leads to the formation of mesomorphic domains allows a very rich structuring of these new materials but is currently a poorly understood step and sometimes limiting manufacturing processes.

  In general, the liquid crystal is soluble in the monomer and not in the polymer derived from the monomer by photocrosslinking.

  This results in the formation of liquid crystal droplets leading to phase separation between liquid crystal and polymer molecules. The creation of these droplets makes it possible to create diffusing or diffracting elements according to the sizes of the domains that can be oriented under the effect of an electric field.

  These optical changes are all the more marked as the phase separation between the liquid crystal and the polymer is effective.

  Bunning and Sutherland Switchable orthorhombic F photonic crystals formed by holographic polymerization-induced phase separation of liquid crystal R. L. Sutherland, V.P. Tondiglia, L.V. Natarajan, S. Chandra, D. Tomlin, T. J. Bunning, Optics Express Oct. 7, 2002 Vol. No. 20 p1074)) underline during the recording of H-PDLC films the difficulties of effectively controlling phase separation (H-PDLCs are composite materials in which polymer and crystal stratum domains are organized liquid).

  The present invention is placed in this general problem of highly conditioning phase separation for the optical properties of materials, and proposes to introduce nucleation sites in the complex: monomer, liquid crystal initially mixed homogeneously so as to obtain a separation greatly improved phase during the photoinduced polymerization of the monomer.

  More specifically, the subject of the invention is a complex chemical molecule comprising a liquid crystal chain grafted onto a nanoparticle capable of constituting a dispersive element for liquid crystal molecules within a polymer matrix. More precisely, this complex chemical molecule is characterized in that it corresponds to the following chemical formula: FNIM (OR) N2AN3BN4 With M: Si, Ti, Zr F type metal: group comprising a liquid crystal function R, A, B: aliphatic groups and N 1 + N 2 + N 3 + N 4 I N between 1 and 3 N 2 between 1 and 3 N 3 between 0 and 2 N 4 between 0 and 2 Advantageously the liquid crystal function can be cyano-biphenyl type or cyanoterphényl.

  Advantageously, the group comprising the liquid crystal function comprises a long aliphatic chain typically comprising from 5 to 12 carbons so as to separate the metal from said liquid crystal function.

  The subject of the invention is also a method for synthesizing a complex chemical molecule according to the invention of the sol-gel process type, making it possible to use the grafting of liquid crystal molecules on the surface of oxide nanoparticles.

  More precisely, such a synthesis process is characterized in that it comprises the following steps: the reaction of an alkoyl silane in the presence of a group comprising a liquid crystal function and an alcohol or vinyl functional group leading to the formation of a silicon alkoxide - the grafting of silicon alkoxide molecules to the surface of silica-type oxide particles.

  Advantageously, the grafting can be carried out in the presence of a colloidal solution of silica.

  Advantageously, the grafting step may be carried out in a solvent of the DMF (dimethylformamide) type.

  According to a variant of the invention, the grafting is followed by a purification step allowing a better dispersion of the subsequent liquid crystal molecules, sought in the case of the preparation of dispersed particles of liquid crystal in a polymer matrix.

  This purification step may advantageously be carried out in the presence of a good solvent for the liquid crystal functions, typically it may be a dichloromethane type solvent.

  The invention finally relates to a composite material particularly suitable for organic optical component applications, comprising liquid crystal molecules, highly dispersible molecules according to the invention compatible with said liquid crystal molecules and a polymer matrix.

  The present invention will be better understood and other advantages will become apparent on reading the following description and with the appended figures, in which: FIG. 1 illustrates molecules according to the invention comprising liquid crystal functions grafted onto a particle of silica; Figure 2 illustrates the formation of dimers during the grafting step.

  In general, the complex molecules according to the invention comprise long aliphatic chains grafted onto the surface of a particle which can be of silica type via siloxane bridges, as illustrated in FIG.

  The free end of the chains is terminated by a cyanobiphenyl group which can bind to commercially available conventional liquid crystal molecules (BL24, BL093 Merck).

  In general, the sol-gel process used according to the present invention makes it possible to use the grafting of liquid crystal molecules on the surface of oxide nanoparticles.

  We will describe in more detail below the different steps of the process of synthesis of dispersive molecules in the case of the use of silica oxide particles: I) The liquid crystal molecule The liquid crystal part must be able to be associated to an alkoxy-silane group in order subsequently to produce a siloxane bridge with the particles of silica or more generally of oxide to be grafted. The commercial liquid crystal mixtures are optimized to ensure optimum chemical stability over time and thus do not have any chemical function capable of reacting with a silicon atom.

  Advantageously, the liquid crystal may be of the cyanobiphenyl or cyano-terphenyl type for the following reasons: The presence of a long aliphatic chain characteristic of this family of liquid crystals makes it possible to spatially separate (typically 11 carbons) the anchoring at the surface particles and dipole function (cyano-biphenyl).

  If chemically very close to the commercial mixtures used (BL24 or BL093), the effects of the silica on the nematic phase will be minimized by the interaction of the cyano-biphenyl groups of the grafted molecules with those of the free molecules.

  The cyano-biphenyl or cyano-terphenyl can be considered as generic liquid crystal molecules and allow to consider the subsequent grafting of molecules with more specific electro-optical properties.

  Examples of Liquid Crystal Molecules Used in the Invention and Synthesis Method: 1.1) Alcoholic Cyano-biphenyl In a 100 ml tricolor flask, 0143 g (25.6 mmol) of potassium hydroxide pellets are dissolved in 4 ml of ethanol while heating slightly .

  0.5 g (2.5 mmol) of 4-cyano-4'-hydroxobiphenyl (white salt) are added. A yellow precipitate appears before redissolving once the solution at reflux.

  0.64 g (2.5 mmol) of 11-bromo-1-undecanol are added dropwise. The reflux is then maintained for 16 h. 2 times 10 ml of ethanol are added to complete the evaporation of the solvent. Extraction is carried out with a water / dichloromethane (50/50) mixture using a separating funnel.

  The organic phase is then dried with magnesium sulphate and evaporated.

  The purification is carried out by chromatography. A series of eluents is prepared: (500 mL CH 2 Cl 2 + 2 mL EtOH)) (500 mL CH 2 Cl 2 + 4 mL EtOH)) (500 mL CH 2 Cl 2 + 20 mL EtOH)),

N C

  (241131 NO2 vIoI, Mit .: 365.51 1.2) Cyano-biphenyl vinyl In a 100 mL trash flask, 0.86 g (154 mmol) of potassium hydroxide pellets are dissolved in 25 mL of ethanol with gentle heating.

  3 g (15.4 mmol) of 4-cyano-4'-hydroxobiphenyl (white salt) are added. A yellow precipitate appears before redissolving once the solution at reflux.

  3.814 ml (15.4 mmol) of 11-bromo-1-undecene are added dropwise. The reflux is then maintained for 16 h. 2 times 10 ml of ethanol are added to complete the evaporation of the solvent. The potassium bromide salts are then filtered on sintered glass. Ethanol is extracted with a rotavapor.

  An oil at 70 C crystallizes at room temperature.

  The purification by chromatography is carried out in a mixture of hexane and dichlormethane (50/50) or by recrystallization in ethanol.

CN - KOI1

HO

E T011

CN

  C13H9NO Mol. Wt .: 195.22 3g (0.0154 mol) 0.86 g (0.154 mol) C14143 Mol. Wt.:2311 ') d: I.063 3.814 ml (0.1) 154 Mole)

CN

B + OK

  (-24hl2ON0 Mol. Wt .: 347.117 Il) Synthesis of precursors From the formation of these groups comprising a liquid crystal function and an alcohol or vinyl function, it is possible to synthesize precursors which will be intended to be grafted onto particles of metal oxides.

  11.1) Formation of a precursor from an alcoholic liquid crystal 20 Formation of an isocyanate: In a 10 mL flask, 0.58 g (1.58 mmol) of alcoholic liquid crystal (1.1) are dissolved in 1 mL of tetrahydrofuran ( THF) anhydrous.

  0.416 g (1.68 mmol) of 3-isocyanatopropyl-triethoxysilane (ICPTEOS) are then added. 18 μL (0.08 mmol) of 0.15% dibutyltin dilaurate serves as the catalyst for the reaction, stirring under a nitrogen atmosphere is maintained for 4 h. The solvent is evaporated in a rotary evaporator.

  The purification is carried out by chromatography. Tetrahydrofuran / cyclohexane eluent (30/70).

  The following compound is obtained: EtO C34H52N2O6SiMol. Wt .: 612,87 If OEt

CN

  The propyl-isocyanate group from the synthesis on ICPTEOS ensures a high mobility between the polar head Si (OEt) nR3_ and the rest of the molecule corresponding to the liquid crystal function. Cyanobiphenyl will thus retain better mobility vis-à-vis the rest of the nematic phase when this type of molecules will be dispersed in the liquid crystal, thus avoiding possible phenomena of anchoring and alignments on the surface of the particle. .

  11.2.1) Formation of a precursor from a vinyl liquid crystal by platinum hydrosilylation In the case of ethylenic bonds, a direct hydrosilylation by a silicon alkoxide in the presence of platinum as a catalyst non-quantitatively gives a product of mediocre quality. A 2-step procedure is preferred: The molecule to be grafted is placed in the presence of chlorosilane with platinum and carbon as catalyst; The reaction product is then ethoxylated in basic medium (in the presence for example of triethylamine) to give the final silicon alkoxide.

  Into a 50 ml tube is introduced 37 ml of triethoxysilane, 35 mg of vinyl liquid crystal 10 ml of anhydrous toluene and 30 l of Pt complexed in a xylene solution. The tube is closed and the mixture is brought to 100 C for 48 H. The final solution is slightly yellow. Xylene and triethoxysilane are evaporated at the vane pump always at 100 C. The traces of platinum are eliminated with animal black.

  The purification by chromatography is eluted with chloroform.

CN H - Si {OL1) 3

I

CN

  11.2.2) Hydrosilylation reaction by chlorosilanes: In a 250 ml tube 1 g (2.8 mmol) of vinyl liquid crystal is introduced with 60 mg of Pt / C (10%). The whole is passed under a nitrogen atmosphere and 30 ml of anhydrous tetrahydrofuran are added with 871.4 μl (8.6 mmol) of trichlorosilane. Stirring is maintained for 16 h. The solvent and the unreacted trichlorosilane are evaporated at the vane pump. The solid is then redispersed in 30 mL of anhydrous tetrahydrofuran.

  3.6 ml (25 mmol) of triethylamine are added, a white smoke then emerges.

  Finally 3 ml of ethanol are introduced, precipitation of amine salts which are filtered on sintered glass. Platinum traces are removed with activated charcoal.

  Purification by chromatography is eluted with dichloromethane. A white solid is obtained with a yield of 93%.

  C30114SNO If Mol. Wt ,: 511.77 11.2.3) Hydrosilylation reaction by a methyl-chlorosilane Liquid crystal functionalized with a methylchlorosilane An identical protocol is used for the synthesis of compounds which ultimately have only two hydrolysable functions.

  HSiCl 2 (CH 3): 149.68 μL, vinyl liquid crystal: 500 mg, THF: 15 mL, Pt / C: 13 mg, N (Et) 3: 2 mL, EtOH: 1.6 mL, A white solid is obtained with a yield of 68. %. 1K / 5i IG 8 1U

  C2y1143NO3Si Mol. WL: 481.74 11.2.4) Hydrosilylation reaction by dimethylchlorosilane An identical protocol is used for the synthesis of compounds which ultimately have only a hydrolysable function.

  HSiCl (CH 3) 2: 3.3 mL, vinyl liquid crystal: 3.5 g, THF: 100 mL, Pt / C: 175 mg, N (Et) 3: 12.54 mL, EtOH: 10.5 mL, A white solid is obtained.

  (2y1141NO2Si Mol. WL: 451.7 III) Synthesis of the particle by grafting the precursor onto a silica oxide particle Commercial colloidal silica is used as a typical support particle for grafting liquid crystal molecules. The grafting of organic molecules is presented as a special case of the sol-gel process. Finally, the purification step made possible by the use of a monoethoxylated monomer will be described.

  The commercial solutions have suspensions of particles with fairly mono-dispersed dimensions and whose diameters remain of the order of 10 to 20 nm, compatible with the minimum size desired for the nucleation sites.

  On the other hand, the silica is soluble for very basic solutions (pH> 10), the hydrolysis will then be catalyzed in an acid medium, which requires the solution to be stable at low pH.

  The stabilization of high concentration colloidal silica is often achieved by the addition of organic binders in the case of steric stabilization, or by adsorption on the surface of ions that provide electrostatic stabilization. In order to optimize the grafting of the liquid crystal molecules, a solution of colloidal silica without additive has been retained in order not to block the access of the -OH sites to the liquid crystal molecules.

  Nissan Chemical proposes a suspension of particles (MAST) in methanol without additive and stable at pH = 1.7. The advertised characteristic sizes are below 20 nm.

  It should be noted that the liquid crystal molecules are poorly soluble in methanol, which is a too polar solvent, which limits the solute concentration and therefore the effectiveness of the grafting. A good grafting solvent must properly solubilize the monomers and ensure the stability of the colloidal silica suspension.

  The silica is sold in methanol and transferred to DMF. The solvent transfer is carried out by adding to a volume V of methanolic solution and its DMF equivalent. The methanol is evaporated while heating (60 ° C.) under a high vacuum (0.1 mmHg). After filtration at 0. 2 m, the silica concentration remains 330 g / l.

  The surface of the silica has both adsorbed water molecules and protons, so the hydrolysis of the monomer molecules can be carried out without the addition of acid. NMR analyzes show traces of water in the DMF solution which remains a hydrophilic solvent. The catalysis of the hydrolysis reaction by molecules mainly adsorbed on the surface of the particles makes it possible to promote the subsequent grafting by condensation of the molecules rather than the condensation in the form of a dimer.

  The grafting is carried out by carrying a mixture of liquid crystal and silica in refluxing DMF with stirring for 24 hours. The solution is filtered at 0.2 m and then stored in an inert atmosphere.

  IV) Purification At the end of the grafting step, in addition to the grafted silica particles, ethoxylated or hydrolysed monomer molecules can remain in solution as well as a large quantity of dimer. The ratios of the rates of the grafting and dimerization reactions estimated show that it is impossible to obtain a significant grafting of the silica without, at the same time, forming a dimer.

  The dimeric molecules can bind to the molecules grafted onto the silica as shown in FIG. 2. As the attachment takes place primarily via the cyano-biphenyl groups, it is possible to imagine a dimer envelope around the particles. grafted as shown in the figure below. This layer prevents the adsorption of the liquid crystal molecules that will come from the nematic phase during the implementation of the nucleation sites in the final system. It is therefore important to successfully purify the solution containing the grafted particles.

  The dimer molecules have a polar part with Si-O-Si groups which is not visible in the case of the molecules grafted onto the silica. A significant difference in solubility is therefore observed in an organic solvent.

  The dimer molecules can be recrystallized at low temperature (4 C) and separated from the grafted particles by centrifugation. The supernatant (DMF + particles) is then separated. The addition of 1% water is then sufficient to precipitate the grafted particles. Once separated by purification, these particles can be perfectly redispersed in dichloromethane.

  The elimination of the dimer can therefore be done by exchange between solvents and precipitation. The hydrolysed or ethoxylated molecules, for their part, do not reprecipitate. The grafting step is continued until the disappearance of the monomer, which does not improve the grafting but allows a more efficient purification.

  V) Inclusion of nucleation sites in "nanodrop" films of the HPDLC type The objective, by manufacturing these artificial nucleation sites, is to experimentally dispose of a device for controlling the density of mesomorphic domains within nanoguide materials. ". Without the addition of artificial sites, the droplet density is, in the same way as the size of the drops, related to the light intensity leading to the crosslinking. By artificially introducing nucleation sites that may be present in a mixture, from the beginning of the nanodomain recording in the case of H-PDLCs, it is possible to control the density of these droplets and to obtain smaller drops while increasing the total volume fraction represented by the drops of liquid crystal.

  Experimentation In order to highlight the effect of nucleation site on the formation of "nanodrop" materials, particles initially in chloroform are introduced into a commercial liquid crystal mixture (BL24). The chloroform is then evaporated under a reduced atmosphere.

  Two precursor mixtures (73% by weight of monomer - NOA81- / 27% by mass of liquid crystal -BL24) are then prepared, one only with grafted nanoparticles. U.V. irradiation (200 mW / cm 2 for 4 min) allows the formation of the nanodrop film.

  The films obtained are quite different. Visible diffusion is more important for the film prepared with artificial nucleation sites.

  It is possible to calculate in the two previous cases the volume fraction of liquid crystal in the form of drops.

  É An average drop size of = 420 nm and a density d = 3.6 1018 part / m3 gives xvo1 = 14.8% É An average drop size of X170 nm and a density = 7.2 25 1019 part / m3 gives x oi = 28.6% For the same size of drop, the increase in the viscosity of the initial mixture causes a clear decrease in the overall volume fraction since only 14.8% of the volume is occupied by the drops.

  The inclusion of artificial nucleation sites, on the other hand, leads to a new system where all of the liquid crystal is in the form of droplets.

  It is interesting to note that in this case, the end of growth of the droplets is not to be related to the gel point of the polymer network but to the absence of mesogenic molecules in the matrix in formation.

Claims (8)

  A complex chemical molecule comprising a liquid crystal chain, capable of constituting a dispersive element for liquid crystal molecules within a polymer matrix, characterized in that it corresponds to the following chemical formula: FNI M (OR) N2AN3BN4 With M Si, Ti, Zr F type metal: group comprising a liquid crystal function R, A, B: aliphatic groups 1 and NI + N2 + N3 + N4 1 N between 1 and 3 N2 between 1 and 3 N3 inclusive between 0 and 2 N4 between 0 and 2 2. Molecule according to claim 1, characterized in that the liquid crystal chain is of the cyano-biphenyl or cyano-terphenyl type.   3. Molecule according to one of claims 1 or 2, characterized in that the group comprising the liquid crystal function comprises a long aliphatic chain which can typically comprise between 5 and 12 carbon atoms so as to separate the metal of said liquid crystal function .   4. A method for synthesizing a complex molecule according to one of claims 1 to 3, characterized in that it comprises the following steps: the reaction of an alkoxy-silane in the presence of a group comprising a crystal function liquid and an alcohol or vinyl function leading to the formation of a silicon alkoxide; and grafting of silicon alkoxide molecules to the surface of silica-type oxide particles. 5. Synthesis process according to claim 4, characterized in that the grafting is carried out in the presence of a colloidal solution of silica.   6. Synthesis process according to one of claims 4 or 5, characterized in that the grafting step can be carried out in a solvent of the DMF (dimethylformamide) type.   7. Synthesis process according to one of claims 4 to 6, characterized in that it comprises a purification step following the grafting step, in a good solvent liquid crystal functions, dichloromethane type.   8. A composite material suitable for organic optical component applications, comprising liquid crystal molecules, complex chemical molecules according to one of claims 1 to 3 having liquid crystal functions, compatible with said liquid crystal molecules, and a polymer matrix.