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Nonaqueous Liquid Crystal Emulsions J. C. Loudet, H. Richard, G. Sigaud, and P. Poulin* Centre de Recherche Paul Pascal/CNRS, Avenue du Dr. Schweitzer, 33600 Pessac, France Received January 4, 2000. In Final Form: March 31, 2000 We study nonaqueous emulsions that are composed of liquid crystal and silicone oils. New surfactant molecules are specifically synthesized to stabilize these emulsions. The behavior of the present surfactants is reminiscent of the behavior of classical surfactants used for aqueous emulsions. Depending on their affinity for the liquid crystal or the silicone oils, they may either stabilize silicone in liquid crystal emulsions or stabilize liquid crystal in silicone emulsions. Stabilization of nonaqueous liquid crystal emulsions may be potentially useful as a new method for making electrooptical display instruments known as polymer dispersed liquid crystal devices. It also presents a way to impart specific optical properties to silicone materials traditionally used in various coating applications and in cosmetics.
Introduction Traditional emulsions are composed of water droplets dispersed in oil or of oil droplets dispersed in water.1 Emulsion droplets are stabilized against coalescence by surfactant molecules adsorbed at the oil-water interfaces. Although aqueous emulsions are widely used in various applications,2 nonaqueous emulsions composed of two nonmiscible oils are far more uncommon. Their formulation would be particularly useful for purposes such as the preparation of porous materials,3 the creation of electrooptical displays made of liquid crystal droplets,4 and the dispersion of organic compounds in polymeric matrixes and, more generally, for making systems in which the presence of water must be avoided (absence of ions, application of strong electrical fields for electrooptical devices, protection against chemical reactions and oxidation, etc.). Unfortunately, the main difficulties in making nonaqueous emulsions (i.e., oil in oil emulsions) arise from the lack of general knowledge about their stability and from the lack of appropriate surfactants. We aim in this work to achieve the stabilization of nonaqueous emulsions by synthesizing new surfactant molecules. The systems under study are composed of cyanobiphenyl and cyanoterphenyl liquid crystals and silicone oils. These oils are almost nonmiscible and may thus be used to make emulsions. Stabilizing a liquid crystal emulsion in a nonaqueous fluid presents a new method for making electrooptical display instruments known as polymer dispersed liquid crystal (PDLC) devices.4 Such devices are usually obtained via phase separation mechanisms or via liquid crystal emulsions in aqueous media. Phase separation mechanisms and the necessary drying of aqueous emulsions make difficult the control of the morphology of traditional PDLC devices. Direct emulsification in nonaqueous systems would allow these difficulties to be circumvented. An additional motivation for this work lies in the innovative features that are expected by doping with optically active liquid crystal droplets traditional silicone materials widely used in coating (1) Modern Aspects of Emulsion Science; Binks, P., Ed.; The Royal Society of Chemistry: Cambridge, U.K., 1998. (2) Encyclopedia of Emulsion Technology; Becher, P., Ed.; Dekker: New York, 1996; Vol. 4. (3) Elmes, A. R.; Hammond, K.; Sherrington, D. C. British Patent 0289238, 1988. Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (4) Drzaic, P. Liquid Crystal Dispersions; Series on Liquid Crystals, Vol. 1; World Scientific: Singapore, 1995.
applications and in cosmetics. Our method presents an alternative to the more traditional approach, which consists of chemically coupling liquid crystal molecules to silicone backbones, thereby forming polymer/liquid crystal systems.5 The present paper is focused on the description of liquid crystal/silicone emulsions and on the surfactants efficient at stabilizing such emulsions. Like common amphiphilic molecules used for aqueous systems, the surfactants that are synthesized in this work have different parts with distinct chemical affinities for the liquid crystal or for the silicone oils. Depending on the relative influence of these affinities, silicone in liquid crystal emulsions or liquid crystal in silicone emulsions can be stabilized. Emulsion systems are prepared by shearing samples that each contain a mixture of the liquid crystal, a silicone oil, and a surfactant. Optical microscopy is used to directly test the stabilities of the systems. In this paper, we first recall some basic concepts regarding emulsion stability. These concepts have long been established for aqueous emulsions, and as shown here, they can serve as a basis for understanding the behavior of nonaqueous systems. Next, we describe the experimental systems and the details of the procedure used to test emulsion stabilities. Finally, we present and discuss the obtained results, followed by some brief conclusions regarding our work. Basic Emulsion Stability Concepts Emulsions are liquid dispersions obtained with two nonmiscible fluids.1,2 Adsorbed surfactant molecules ensure the stability against coalescence. It has been empirically established that the preferential solubility of the surfactant determines the type of emulsion that can be stabilized.1 For aqueous emulsions, when it is soluble in water, the surfactant tends to stabilize oil droplets in water (direct emulsions) and, reciprocally, when it is rather soluble in oil, it tends to stabilize water droplets in oil (inverse emulsions). This empirical principle is known as Bancroft’s rule.6 In addition to coarsening through coalescence, an emulsion may coarsen through another mechanism known as Ostwald ripening.7 This mechanism is driven by the (5) Bunning, T. J.; Kreuzer, F. H. TRIP; Elsevier Ltd.: London, 1995; Vol. 3, p 318. Hikmet, R. A. M.; Lub, J. Prog. Polym. Sci. 1996, 21, 1165. (6) Bancroft, W. D. J. Phys. Chem. 1913, 17, 501. (7) Ostwald, W. Z. Phys. Chem. (Leipzig) 1900, 34, 295. Lifshitz, I. M.; Slyozov, V. V. Sov. Phys. JETP 1959, 35, 331.
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molecular diffusion of the dispersed phase through the continuous phase. Ostwald ripening is also observed in other important phenomena such as classical phase separations in binary mixtures of liquids or solids.8 The average size has been shown, theoretically and experimentally, to increase in proportion to t1/3, where t is the time.1,8 Even when the dispersed and continuous phases are slightly soluble, Ostwald ripening can be avoided by adding to the dispersed phase a component that is not soluble in the continuous phase.1,9 Shrinking of the smaller droplets is arrested by the osmotic compression of the added component that does not escape from the droplets. Though this brief summary is based on numerous reports devoted to aqueous emulsions, these concepts are likely to serve as an important basis for the design of suitable surfactants for the stabilization of nonaqueous emulsions. Experimental Section Liquid Crystals and Silicone Oils. We used liquid crystal molecules and silicone oils for making the present nonaqueous emulsions. The liquid crystal is a mixture of cyanobiphenyl and cyanoterphenyl molecules obtained from E. Merck (trade name E7). This liquid crystal is nematic at room temperature. The nematic/isotropic transition temperature is ∼62 °C. Like any classical oil, E7 can be used to make direct or inverse emulsions with water.4,10,11 Two different silicone oils were tested. The first one, purchased from Fluka, is a classical poly(dimethylsiloxane) (PDMS-DC200) oil of viscosity 1000 cP. It is referred to in the following as V1000. The second one is a PDMS cross-linkable oil obtained from Rhodia (trade name CAF3-Rhodorsil). Crosslinking spontaneously takes place when the sample is open in air. It becomes solid in a few hours. According to Rhodia, the kinetics of cross-linking depend strongly on air humidity. Even in the liquid state, the high viscosity of CAF3 makes difficult the fragmentation of this compound into small droplets emulsified in a less viscous fluid. However, CAF3 is likely to be a suitable continuous phase for an emulsion. Its high viscosity becomes an advantage in emulsifying a less viscous fluid because the mechanical stress, which is induced by a certain shear and which leads to droplet fragmentation, is proportional to the viscosity of the continuous phase.1 The solubilities of these oils in E7 were qualitatively determined by comparisons of the nematic to the isotropic endotherms of three systems: the E7 reference, a 50:50 wt % mixture of V1000 silicone oil and E7, and a 50:50 wt % mixture of CAF3 and E7. The shape of the peak recorded on a differential scanning calorimeter upon clearing (Perkin-Elmer Pyris) could then be used to estimate the purity of the liquid crystalline phase (one must note that the material E7 is not a pure compound by itself). The samples were prepared by vigorously mixing the liquid crystal and the silicone oils without surfactants, and in each case, the DSC sample pan was immediately filled with the mixture. Figure 1a shows the compared clearing endotherms for E7 and the mixture with V1000. The shape of the peak for the emulsion is not modified with respect to the E7 peak; the temperature of the end of clearing is not shifted. Moreover the integrated peak (0.79 J g-1) is close to half the value measured for E7 alone (1.37 J g-1). Considering the experimental uncertainties in preparing the cell from a heterogeneous mixture, the solubility of V1000 in E7 is too low to be estimated. The system phase-separates quickly in the absence of surfactant. As a result, it is difficult to be sure that the small amount of material transferred to the DSC cell has exactly the same composition as that of the initial mixture. (8) Gunton, J. M.; San Miguel, M.; Sahni, P. S. In Phase Transitions and Critical Phenomena; Lebowitz, J. L., Ed.; Academic Press: London 1983; Vol. 8, p 267. (9) Higushi, W. I.; Misra, J. J. Pharm. Sci. 1962, 51, 459. Kabalnov, A. S.; Pertsov, A. V.; Shchukin, E. D. Colloids Surf. 1987, 24, 19. (10) Poulin, P.; Stark, H.; Lubensky, T. C.; Weitz, D. A. Science 1997, 275, 1770. (11) Poulin, P.; Weitz, D. A. Phys. Rev. E 1998, 57, 626.
Figure 1. Thermograms recorded upon heating (2 °C min-1), showing heat flow/g of sample. (a) E7 compared to a mixture with V1000. (b) E7 compared to a mixture with CAF3. The four curves range from that for a freshly prepared emulsion (bottom curve) to that for a cured material (top curve). The curves recorded with the mixture in CAF3 are shown in Figure 1b. In this case, they are strongly damped compared to the curve for E7. This is classically expected for a nematic material spoiled with some impurity. However, if the amount of impurity is large, the transition temperature should also be greatly depressed, which is not the case, since the end of clearing is not soundly modified. In addition, a more intense endothermic signal is partly recovered as curing of CAF3 proceeds (integration yields 0.30 J g-1 measured on the upper curve of the E7/CAF3 mixture in the same temperature range as that in Figure 1a). This suggests that the solvent used in the CAF3 formulation is responsible for the modification of the liquid crystalline behavior of E7. As in the previous case, since the cell is filled from an unstable mixture, it is difficult to obtain more accurate information from the integration. A slight hysteresis upon cooling was observed for freshly prepared emulsions with CAF3. This behavior became less pronounced as curing proceeded, indicating that E7 is slightly more soluble with CAF3 and its initial solvent than with V1000 or with dry and cured CAF3. At long times, when equilibrium was reached (for both CAF3 and V1000), the recordings obtained upon cooling, not shown here, confirmed in all cases that the transition temperature was not significantly shifted. In summary, the DSC data reflected the poor compatibility of the silicone materials with the E7 liquid crystal. Thus we expected that V1000 and CAF3 could be used in combination with E7 to make nonaqueous emulsions of the liquid crystal. Although our data did not directly provide information about the solubility of liquid crystal molecules in silicone oils, they somehow reflected the chemical affinities between the silicone oils and the liquid crystal. This is why we expected the liquid crystal molecules to be poorly miscible in V1000 (since V1000
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Scheme 1
molecules are almost immiscible in the liquid crystal). Similarly, we expected a slight (but still low) solubility of liquid crystal molecules in CAF3. Finally, we note that a few additional DSC experiments performed at different compositions (with mixtures ranging from 20 to 60 wt % of E7) confirmed the main features of the previous observations. Surfactants. Surfactants suitable for stabilizing silicone oil/ liquid crystal emulsions must be composed of two different parts, each having a preferential affinity for the liquid crystal or the silicone oil. We have thus synthesized new molecules by substituting cyanobiphenyl groups through aliphatic spacers characterized by their methylene number n onto PDMS moieties of p monomer units. We describe in the following the results obtained with two molecules referred to as Si400CBP and Si12CBP, which were found to be efficient at stabilizing liquid crystal/silicone emulsions. These molecules were selected from combinations (described further) obtained by using a PDMS moiety of intermediate length and different aliphatic spacers. The poly(dimethylsiloxane) cyanobiphenyl-terminated tensioactive compound Si400CBP and the similar oligo compound Si12CBP were synthesized by using the hydrosilylation reaction shown in Scheme 1. For our conditions, we statistically expected 60% monosubstituted, 20% disubstituted, and 20% unsubstituted molecules. The last do not have amphiphilic character and are directly soluble in silicone oils. This is why unsubstituted molecules were not expected to strongly affect the interfacial behavior and the stability of the emulsion systems. The vinyl derivatives (I) were prepared from commercial products, used without further purification: 6-bromohexene or allyl bromide from Aldrich and 4-hydroxy-4′-cyanobiphenyl from Interchim. The reactions were performed in ethanol, adding KOH in equimolecular proportions with respect to the phenolic compound. Purifications were achieved by silica gel chromatography using a toluol/ethyl acetate (80:20) mixture as the eluent. The poly(dimethylsiloxane) hydride-terminated compound DMS-H31 (II) was obtained from ABCR (MW ) 28000, DPn = 400), and the corresponding oligomer with 12 siloxane elements was from Wacker. Si400CBP corresponds to n ) 11 and p = 400 (see Scheme 1) (Systems with shorter aliphatic spacers (n < 11) were also tested but were found to be less efficient at stabilizing emulsions.) Si12CBP corresponds to n ) 3 and p ) 12. The mixtures of di-, mono-, and unsubstituted polymers were extracted from the reaction bulk by GPC using toluol as the eluent. To avoid the extraction of short oligomers, we used only the first fractions obtained by GPC.
Figure 2. (a) Optical microscopy picture of silicone oil droplets stabilized in the nematic liquid crystal by the Si12CBP poly(dimethylsiloxane) surfactant. Elastic interactions mediated by the nematic medium induce the chaining of the droplets. Composition of the system: E7, 90 wt %; V1000, 8 wt %; Si12CBP, 2%. Black bar: 75 µm. (b) Schematic representation of the liquid crystal molecules around a silicone droplet. Molecules made from PDMS with p = 80 (DPn = 80, MW = 6000; from ABCR; trade name DMS-H21) were not suitable for stabilizing liquid crystal/silicone emulsions, regardless the lengths of the aliphatic spacers used. Si12CBP, in contrast to Si400CBP, has a small silicone group, and it is not soluble in silicone oils. Conversely, Si400CBP, in contrast to Si12CBP, has a large silicone group, and it is not soluble in the liquid crystal. According to the behavior of aqueous emulsions, we may expect different abilities of these molecules to stabilize either liquid crystal in silicone oil emulsions or silicone oil in liquid crystal emulsions. Finally, we note that this new class of amphiphilic molecules can presumably form micelles in silicone oils or even in liquid crystals. This has not yet been studied, and the specific features of their micellization still raise interesting and open questions. Emulsions. Each emulsion was prepared by mixing a liquid crystal sample (E7), a silicone oil sample (V1000 or CAF3), and a given amount of surfactant (Si12CBP or Si400CBP). We used a simple procedure proposed in the literature to directly obtain uniformly sized droplets in small samples.12,13 A premix was first made by gently shaking the initial mixture. This premix was composed of large droplets of about a few tens of micrometers. The system was sheared between microscope glass slides separated by about 20 µm or in the thin gap of a Couette cell.14 (12) Perrin, P. Langmuir 1998, 14, 5977. (13) Mason, T. G.; Bibette, J. Langmuir 1997, 13, 4600. (14) Bibette, J.; Mason, T. G. French Patent WO9738787, 1997.
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Figure 3. (a) Optical microscopy picture taken between crossed polarizers for liquid crystal droplets stabilized in V1000 silicone oil by the Si400CBP poly(dimethylsiloxane) surfactant. The cross pattern of the droplets reflects the radial configuration of the liquid crystal. This configuration results from the normal boundary conditions of the nematic medium at the droplet interface. White bar: 70 µm. (b) Schematic representation of the liquid crystal molecules within a droplet. With an efficient surfactant, micrometric droplets were formed by the strong uniform shear in the thin gap between the slides or in the Couette cell. If the surfactant was poor at emulsifying the droplets, the liquid crystal and silicone oil tended to phaseseparate macroscopically. We tested the stabilities of the emulsified systems by using optical microscopy images recorded on a computer via a CCD camera connected to a frame grabber. This allowed the possible growth of droplets to be studied as a function of time and the involved coarsening mechanisms, coalescence or Ostwald ripening, to be identified. The systems were considered to be stable when no significant coarsening was observed over a few hours.
Results Results. Si12CBP was found to be inefficient at emulsifying liquid crystal droplets in V1000 or CAF3 silicone oils regardless the relative proportions of the materials used. In contrast, it was found particularly efficient at emulsifying V1000 silicone oil droplets in the liquid crystal. Because of its high viscosity, CAF3 was not emulsified in the liquid crystal. As shown in Figure 2, the V1000 droplets tend to form long chains in the nematic liquid crystal. Such structures have already been observed with water droplets or solid particles in various nematic liquid crystals.10,11,15 The chaining results from elastic (15) Poulin, P.; France`s, N.; Mondain-Monval, O. Phys. Rev. E 1999, 59, 4384.
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Figure 4. Optical microscopy pictures of liquid crystal droplets emulsified in V1000 silicone oil in the presence of Si400CBP at low concentration: (a) droplets immediately after emulsification; (b) droplets after a few minutes, showing coalescence. Black bar: 20 µm.
interactions between the particles mediated by the distortions of the liquid crystal. According to recent theoretical and experimental results,10,11,16 chain formation indicates that the nematic director is normal to the droplet interface (see Figure 2b). Different structures would have been observed under distinct anchoring conditions.11,17 No coalescence or Ostwald ripening was observed for such systems over periods of ∼1 h. We may thus conclude that Si12CBP stabilizes silicone oil droplets in the liquid crystal and provides a normal anchoring to the liquid crystal molecules at the droplet interfaces. The system shown in Figure 2 contains 8 wt % V1000, 2 wt % Si12CBP, and 90 wt % E7. Other proportions were also tested. Emulsions were found to be stable for the following typical compositions: up to a few tens percent of silicone oil and from a few tenths to a few percent of Si12CBP. In contrast to Si12CBP, Si400CBP was found to be efficient at stabilizing E7 droplets in V1000 or CAF3 silicone oils. When observed between crossed polarizers, the droplets exhibit the crosslike pattern shown in Figure 3. This pattern reflects a radial structure of the nematic liquid crystal within the droplets (see Figure 3b). Radial configurations, such as chaining of silicone in liquid crystal (16) Lubensky, T. C.; Pedey, D.; Currier, N.; Stark, H. Phys. Rev. E 1998, 57, 610. (17) Mondain-Monval, O.; Dedieu, J. C.; Gulik-Krzywicki, T.; Poulin, P. Eur. Phys. J., B 1999, 12, 167.
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Figure 5. Optical microscopy pictures of E7 liquid crystal droplets stabilized by Si400CBP in a CAF3 silicone matrix: (a) immediately after emulsification; (b) after 20 min; (c) after 1 h; (d) after 5 h. Black bar: 60 µm. (Note that the apparent chainlike ordering is just arising from the initial shear between the glass slides.)
Figure 6. Average size of E7 droplets in CAF3 as a function of time. During the first hours, the droplet size increases approximately with t0.25 instead of t1/3 as in classical fluids. The droplet size is saturating at longer times.
emulsions, result from the normal anchoring of the liquid crystal molecules at the droplet interface.4 Stable liquid crystal in silicone oil emulsions typically contain up to a few tens of weight percent liquid crystal and more than 10 wt % Si400CBP. Between 1 and 10 wt % Si400CBP, the systems can be easily emulsified into micrometer-sized droplets but a rapid coalescence is observed after emulsification. As shown in Figure 4, large droplets several micrometers in diameter are observed after a few minutes. When the Si400CBP fractions exceed 10%, the systems are significantly more stable and do not coalesce over several tens of minutes. Below 1%, the systems cannot be emulsified. V1000 and CAF3 exhibit similar behavior with regard to coalescence and emulsification. However, as shown in Figure 5, coarsening occurs when the continuous phase is composed of CAF3. Such coarsening is not observed with V1000. By analyzing a series of images, we have measured the average droplet size as a function of time. The droplets grow and saturate
after a few hours. Similar trends have been observed for all the samples. However, it was difficult to obtain reproducible quantitative results. We believe that this is due to variations in the air humidity, which, according to the supplier, strongly affect the kinetics of cross-linking for CAF3. As shown in Figure 6, the growth rate of the average particle diameter is slower than the classical t1/3 scaling.1,7,8 Despite the difficulties in quantitatively reproducing our results, this important feature is always observed. We note that, assuming a power law tx to describe the behavior at early times for the system in Figure 6, we find x ∼ 0.25. Similar behavior with such a scaling has been regularly observed. Although the present coarsening is slower than t1/3, it can be ascribed to Ostwald ripening because, as shown in Figure 7, small droplets tend to shrink. Such a shrinking is not observed in systems that exhibit coalescence. As already known for aqueous systems, Ostwald ripening can be avoided by adding to the dispersed phase a small fraction of a component insoluble in the continuous phase.1,9 To avoid Ostwald ripening in a nonaqueous emulsion using this procedure and to stabilize micrometer-sized droplets in CAF3, we synthesized a polymer that is soluble in the liquid crystal. The polymer has the following formula:
It was obtained by polymerization of the corresponding
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Figure 7. A series of optical microscopy pictures of liquid crystal droplets in a CAF3 matrix as a function of time (from right to left and from top to bottom). The first picture was taken 10 min after emulsification, and there were 4 min intervals between the pictures. The droplets do not diffuse in the highly viscous matrix. However, smaller droplets are shrinking (see droplets in black circles). This particular behavior is a characteristic feature of Ostwald ripening. Black bar: 50 µm.
acrylate monomers by adding azobis(isobutyronitrile). The reaction was performed in dry dioxane at 70 °C over 8 h.18 A long macromolecule is preferentially used as the additional component because it is likely to be totally insoluble in the silicone oil. Upon the addition of 1 wt % of this polymer to E7, we observed that E7 droplets in CAF 3 did not grow at all. We note that, at this concentration, the liquid crystal is still nematic. These results show that micrometer-sized liquid crystal droplets can be stabilized in liquid or solid cross-linked silicone matrixes. Discussion The present results can be rationalized in the framework of basic knowledge about aqueous emulsions. Si12CBP stabilizes silicone in liquid crystal emulsions and Si400CBP stabilizes the liquid crystal in silicone emulsions. The surfactants are preferentially soluble in the continuous phases of the emulsions they stabilize. This behavior agrees with the empirical Bancroft rule.1,6 As shown with CAF3 matrixes, Ostwald ripening occurs when the liquid crystal and silicone materials are slightly soluble. The growth of the droplets is slower than that classically observed in phase-separating binary mixtures or coarsening emulsions.1,7,8 This difference presumably arises from the simultaneous cross-linking of the polymeric continuous phase. The solidification of the matrix is indeed expected to have two main consequences that can make coarsening slower. First, the diffusion coefficient of the molecules in the matrix is expected to decrease with increasing polymer cross-linking. Second, the solidification of the matrix exerts elastic constraints, which act against (18) Leroux, N.; Keller, P.; Achard, M. F.; Noirez, L.; Hardouin, F. J. Phys. II 1993, 3, 1289.
droplet growth or shrinkage. This is also why the coarsening saturates after a few hours. Nevertheless, the coarsening can be totally avoided by adding another component to the dispersed phase. As in the case of aqueous emulsions, if this component is insoluble in the continuous phase, the droplets do not grow. Conclusions We have studied, in this work, nonaqueous emulsions composed of liquid crystal and silicone oils. Such emulsions have been stabilized against coalescence by using amphiphilic molecules that are designed as classical surfactants used for aqueous systems. These molecules are composed of poly(dimethylsiloxane) and cyanobiphenyl groups. Depending on the size of the poly(dimethylsiloxane) group, either silicone in liquid crystal emulsions or liquid crystal in silicone emulsions can be stabilized. We have also shown that liquid crystal/silicone emulsions can be stabilized against Ostwald ripening by using an additional component in the dispersed phase of the formulation. Liquid crystal droplets dispersed in polymer films are of great interest because of their electrooptical applications.4 According to our results, such materials could be directly obtained using nonaqueous emulsions instead of using phase separation mechanisms or aqueous emulsions, which have to be dried. The absence of phase separations and drying in our systems makes it easier to control the morphologies of the obtained liquid crystal dispersions. This may provide better control of the resultant electrooptical properties. However, the materials used in this work are not optimal for electrooptical applications because of the poor mismatch between the optical indexes of the liquid crystal and silicones employed.4 Further work using different chemicals (by chemically modifying the
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silicone compounds for instance) is required for the development of high-performance electrooptical devices. Finally, we point out that the inclusion of liquid crystal droplets in silicone materials is potentially useful for imparting specific optical properties to materials traditionally employed for coatings or cosmetics. For instance, we recently stabilized silicone films doped with cholesteric liquid crystal droplets (E7 doped with chiral components) that diffract visible light. These systems can be formed using the same chemicals and procedure as those described
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in this work, presenting an alternative, with no or little modification of the liquid crystalline properties of the molecules, to an earlier approach that consists of chemically attaching chiral or nonchiral liquid crystals to silicone backbones to make polymer liquid crystals.5 Films of liquid crystal emulsions also present an alternative to traditional films that are made of encapsulated liquid crystals and that are widely used as temperature indicators. LA0000116
