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Reverse-Mode Operation Switchable Nematic Emulsions G. Di Profio, F. P. Nicoletta,* G. De Filpo, and G. Chidichimo Dipartimento di Chimica, Universita` degli Studi della Calabria, 87036 Rende (CS), Italy Received July 31, 2001. In Final Form: December 21, 2001 Switchable nematic emulsions are dispersions of micron-sized liquid crystal droplets in a monomer matrix, which can be switched from an opaque OFF state to a transparent ON state by application of very low electric fields. In this paper, we present our preliminary results concerning the behavior of switchable nematic emulsions operating in a reverse mode, that is, droplets characterized by a transparent OFF state that can be electrically turned into an opaque ON state. Radial droplets of liquid crystal with a negative dielectric anisotropy have been prepared with a well-defined transparent state by choosing a monomer mixture that matched the average refractive index of the liquid crystal droplets. The application of an electric field determines the reorientation of liquid crystal directors through an elastic distortion and the achievement of a more scattering state. The effect of temperature on the electro-optical response is also reported.
I. Introduction Emulsions are dispersions of liquid droplets of one immiscible fluid in another one. They are of considerable technological importance with applications in widespread fields such as foods, drugs, paints, and coatings.1 Emulsions are generally metastable systems because of attractive interactions among drops. They tend to coalesce into larger ones, and a net separation between the two fluids can appear. To prevent coalescence, surface-active molecules are added in emulsions. Surfactants provide strong short-range repulsions (Coulombic, steric, or other repulsive interactions), which counterbalance the attractive forces and give a long-term stability. Nematic emulsions are formed by nematic liquid crystal droplets dispersed in a continuous liquid solvent. They have been the object of both experimental and theoretical studies.2-11 The kinetic stability of nematic emulsions is strongly affected by viscosity and surface tension of both fluids. The use of matrixes with low viscosity values generally results in metastable droplets. The presence of anisotropic properties in one fluid changes the nature of interactions between the droplets and the host matrix. In fact, liquid crystal directors can assume different alignments (for example, tangent and normal) at the droplet surface, giving rise to different structures (bipolar and radial configurations, respectively).2,3 The distortion of the nematic director field in droplets can induce interac* To whom correspondence should be addressed. E-mail:
[email protected]. (1) Becher, P. Emulsions, Theory and Practise, 2nd ed.; Reinhold: New York, 1965. (2) Meyer, R. B. Phys. Rev. Lett. 1969, 22, 918. (3) Dubois-Violette, E.; Parodi, O. J. Phys. (France) 1969, 30, C457. (4) Candau, S.; Le Roy, P.; Debeauvais, F. Mol. Cryst. Liq. Cryst. 1973, 23, 283. (5) Volovik, G. E.; Lavrentovich, O. D. Sov. Phys. JETP 1983, 58, 1159. (6) Poulin, P.; Stark, H.; Lubensky, T. C.; Weitz, D. A. Science 1997, 275, 1770. (7) Lavrentovich, O. D.; Tarent’ev, E. M. Sov. Phys. JETP 1986, 64, 1237. (8) De Filpo, G.; Lanzo, J.; Nicoletta, F. P.; Chidichimo, G. J. Appl. Phys. 1998, 84, 3581. (9) De Filpo, G.; Lanzo, J.; Nicoletta, F. P.; Chidichimo, G. J. Appl. Phys. 1999, 85, 2894. (10) Lavrentovich, O. D. Sov. Tech. Phys. Lett. 1988, 14, 73. (11) Golovataya, N. M.; Kurik, M. V.; Lavrentovich, O. D. Liq. Cryst. 1990, 7, 287.
tions among droplets,3,10,11 and several different configurations can be observed in the presence of either additives or external fields.12-15 Bipolar droplets are characterized by a director field parallel to the interfaces with two point defects at opposite ends of the droplets and a cylindrical symmetry. The application of an electric field involves the reorientation of liquid crystal directors along a direction parallel to the external field (if the dielectric anisotropy of mesogens is positive) through a simple rotation of droplet directors with no additional elastic deformation. Radial droplets are characterized by a director field perpendicular to the droplet walls with a central point defect and a spherical symmetry. Such a configuration is not easily found in nature in agreement with the FriedelCreagh-Kmetz rule, which predicts the incompatibility between long-term stability and radial configuration in emulsions. As a consequence, the simplest way to achieve a normal alignment in liquid crystal droplets is the addition of small amounts of surfactants. The application of an external electric field determines the reorientation of liquid crystal directors through an additional elastic deformation and the achievement of the axial configuration. To induce reorientation, the electric energy must overcome the elastic free energy difference between the ON and OFF states. The existence of this energy difference in radial droplets gives reorientation fields larger and relaxation times faster (as they scale inversely with the square of reorientation fields) than those in bipolar emulsions. Recently, nematic emulsions have attracted high interest since nematic droplets dispersed in a highly viscous monomer matrix can be used as cheap, fluid, switchable electro-optical devices.8,9,16 Films are prepared by cooling the initial isotropic mixture of monomers and liquid crystal at a controlled rate in order to give micron-sized bipolar droplets. They can be switched from an opaque state to (12) Kralj, S.; Zumer, S. Phys. Rev. A 1992, 45, 2461. (13) Xu, F.; Kitzerow, H. S.; Crooker, P. P. Phys. Rev. A 1992, 46, 6535. (14) Prost, D.; De Gennes, P. G. The Physics of Liquid Crystals; Clarendon: Oxford, 1993. (15) Drzaic, P. S. Liquid Crystal Dispersions; World Scientific: Singapore, 1995. (16) Lanzo, J.; Nicoletta, F. P.; De Filpo, G.; Chidichimo, G. Liq. Cryst. 2000, 27, 1029.
10.1021/la011217p CCC: $22.00 © 2002 American Chemical Society Published on Web 03/20/2002
Reverse-Mode Switchable Emulsions
a transparent state by application of very low electric fields. The relaxation times are quite long (∼15 min) but can be drastically reduced if a radial configuration is induced by the addition of small amounts of surface-active molecules.17 In addition to being used as optical shutters, nematic emulsions are also attractive for displays, smart windows, and automotive applications. In particular, a great interest is around reverse-mode operation shutters, which are transparent in the absence of the external field and opaque when the field is turned on. They are important in applications that require a transparent “default” state (e.g., safety reasons). Some reverse-mode devices have been obtained with liquid crystal dispersions in a solid polymer matrix. Such electro-optical shutters have been prepared by using dual-frequency addressable liquid crystal,18,19 by polymerization of nematic emulsions,20 by using rough surfaces21 and gel systems,22-24 and by modification of the interfacial properties of liquid crystal droplets.25 Nevertheless, no nematic emulsion has been up to now prepared and investigated as a long-term stable, reverse-mode, switchable device, due to the intrinsic metastability and inadequate droplet size of these fluid systems. The light-scattering properties of nematic emulsions at high droplet densities are affected by the interference between the waves scattered from different droplets and multiple scattering. The first effect is mainly related to the refractive index difference between neighboring droplets, while the second influences the scattering profile for long light paths. Multiple scattering can be neglected when considering the film transmittance. Radial droplets in their OFF state scatter less light than bipolar droplets as the refractive index difference between neighboring droplets is not as large as in droplets with tangential wall alignment.15 In addition, the transmittance in the OFF state can be opportunely modulated by properly matching the refractive indices of the liquid crystal droplets and the monomer matrix. In this paper, we present our preliminary results concerning the preparation of reversemode operation switchable nematic emulsions, that is, the achievement of a well-defined transparent OFF state in radial switchable emulsions, which can be turned into a more scattering state by application of an electric field. The influence of temperature on the electro-optical properties of such reverse-mode emulsions is also reported. II. Experimental Section ZLI4788-000 (Merck) was used as the nematic liquid crystal. Such a eutectic mixture is characterized by a nematic-isotropic transition temperature of 84 °C, a negative dielectric anisotropy (∆ ) -5.7), and refractive indices no ) 1.486 and ne ) 1.640. no and ne are the perpendicular and parallel refractive indices to the liquid crystal director, respectively. The average refractive index of ZLI4788-000 is consequently 1.537 (n j ) (2no + ne)/3). The monomer was a mixture of bisphenol A glycerolate diacrylate (BAGD, Aldrich) and CN945B85 (an aliphatic urethane (17) Koval’chuck, A. V.; Lavrentovich, O. D.; Sergan, V. V. Sov. Tech. Phys. Lett. 1989, 17, 529. Nicoletta, F. P.; Lanzo, J.; De Filpo, G.; Chidichimo, G. Langmuir 2001, 17, 534. (18) Nolan, P.; Coates, D. Mol. Cryst. Liq. Cryst. 1991, 8, 75. (19) Gotoh, M.; Murai, H. Appl. Phys. Lett. 1992, 60, 3102. (20) Nicoletta, F. P.; De Filpo, G.; Lanzo, J.; Chidichimo, G. Appl. Phys. Lett. 1999, 74, 3945. (21) Macchione, M.; Cupelli, D.; De Filpo, G.; Nicoletta, F. P.; Chidichimo, G. Liq. Cryst. 2000, 27, 917. (22) Hikmet, R. A. M. Appl. Phys. Lett. 1990, 68, 1. (23) Yang, D. K.; Chien, L. C.; Doane, J. W. Appl. Phys. Lett. 1992, 60, 392. (24) Nakamura, K.; Yoshida, H.; Hanaoka, K. U.S. Patent 5,189,540, 1993. (25) Nicoletta, F. P.; De Filpo, G.; Cupelli, D.; Macchione, M.; Chidichimo, G. Appl. Phys. Lett. 2001, 79, 4325.
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Figure 1. Bipolar droplets of liquid crystal with negative dielectric anisotropy show an opaque OFF state, which can be electrically turned into a more opaque state (a). On the contrary, the OFF state of radial droplets is rather transparent and can be electrically driven in an opaque state (b). Both ON states are characterized by a random distribution of liquid crystal directors in the plane perpendicular to the electric field. triacrylate, 85% in hexandiol diacrylate from Cray Valley-Total) in different weight ratios in order to match n j . The presence of polymerizable moieties in matrix components plays no part in the functioning of the emulsions, and whatever material characterized by a high viscosity value and matched refractive index can be used. Moieties can allow the conversion of nematic emulsions in polymer-dispersed liquid crystals if a polymerization initiator is added. Samples were prepared by weighing the appropriate amounts of liquid crystal and monomers in vials. They were doped with different weight percentages of surfaceactive agents such as lecithins, polyoxyethylene ethers, quaternary ammonium compounds, and linear alcohol ethoxylates. The best results are generally obtained with few percent of dodecyltrimethylammonium bromide (5-10 wt %) for which a welldefined perpendicular alignment of liquid crystal directors at the droplet interface is achieved. Mixtures were stirred at 100 °C, and then a small quantity was introduced by capillarity in homemade cells, whose thickness was set to be about 25 µm by means of glass spacers. Uniform, micron-sized droplets were obtained by cooling cells at a rate of 50 °C/min. The electrooptical properties of emulsions were measured using the optical line reported in a previous work.26 The intensity of incident light passing through cells containing pure monomer mixtures was assumed as full-scale intensity. The rise and decay times, τrise and τdecay, are defined as the time required by the cells to align the liquid crystal directors perpendicularly to the applied electric field and the time interval required by directors to lose their planar alignment once the field is turned off, respectively. They were measured by monitoring the drive signal (frequency ) 1 kHz, Vrms ) 100 V) and the response of the photodiode using a digital storage oscilloscope interfaced with a computer. The sample temperature was kept constant with a thermostatic bath, whose stability was better than 0.1 °C. Measurements were performed only 1 h after each temperature change.
III. Results and Discussion III.1. Electro-Optical Properties of Reverse-Mode Operation Switchable Emulsions at Room Temperature (T ) 20 °C). Figure 1 shows the basic idea followed in this work. If an electric field is applied to a micronsized emulsion with a negative dielectric anisotropy liquid crystal and a tangential alignment of directors, a transition from an opaque state (OFF state transmittance ∼1%) to a more opaque state (ON state transmittance ∼0.1%) is observed (Figure 1a) as the electric field forces the liquid crystal directors to orient randomly within the film plane. In fact, such random alignment of directors among neighboring droplets increases the light scattering. If nematic droplets are characterized by a normal alignment of their directors at the liquid crystal/matrix interface, the OFF state transmittance will reach the value of some (26) Chidichimo, G.; Huang, Z.; Caruso, C.; De Filpo, G.; Nicoletta, F. P. Mol. Cryst. Liq. Cryst. 1997, 299, 379.
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Figure 2. Transmittance dependence on the external electric field of reverse-mode switchable emulsions: bipolar droplets (a) and radial droplets with BAGD wt % ) 70% (b), BAGD wt % ) 75% (c), and BAGD wt % ) 80% (d).
percent as radial droplets scatter less light than bipolar droplets because of the lower refractive index difference among neighboring radial droplets (Figure 1b). The OFF state transmittance value can be increased by choosing a monomer matrix whose refractive index is matched with the average refractive index of the droplets. In fact, the scattering cross section of a nematic droplet (whose radius is R) dispersed in a matrix (whose refractive index is nmatrix) is given according to the anomalous diffraction approximation by27
(
σ ) 2πR4K2
ndroplet -1 nmatrix
)
2
(1)
where ndroplet is the average refractive index of a liquid crystal droplet and K is the wave vector of impinging light. Because the transmittance, T, is related to σ by the relation27
T ) exp[-βσd]
(2)
where β is the droplet density and d is the sample thickness, it is evident that a larger value of T can be gained if the condition ndroplet ) nmatrix is fulfilled and the other parameters are kept constant. The average refractive index of radial droplets in their OFF state is about equal to the average refractive index of liquid crystal in the absence of a spatial dependence of the nematic order j. parameter, that is, ndroplet ∼ n As shown in Figure 2, the OFF and ON transmittances are very low for bipolar droplets, while well-defined values can be obtained for radial emulsions when the monomer matrix reaches the matching condition. Such a condition is gained for 75 wt % of bisphenol A in the monomer mixture. Lower transmittance values are obtained if the bisphenol A percentage is raised to 80 wt % or lowered to 70 wt %. Small changes in the optical properties of samples can arise from slight dissimilarities in droplet density and radius. Nevertheless, it should be considered that monomer matrix composition is quite a delicate factor that affects transmittances. In fact, the solubility of liquid crystal in monomer matrix and monomer molecules in liquid crystal changes the refractive index values. As a consequence, the OFF state transmittance never reaches the theoretical value of 100%. In addition, the anomalous diffraction model does not take into account some factors that can reduce the transmittance, such as correlations between adjacent droplets and multiple scattering effects.15 (27) Zumer, S. Phys. Rev. A 1998, 37, 4006.
Figure 3. Typical electro-optical response of a reversemode operation nematic emulsion. The driving frequency is 1 kHz.
The switching electric fields, E10% and E90%, defined as the fields at which the OFF state transmittance has lost 10% and 90%, respectively, of the difference between the maximum and minimum transmittances, are 0.2 and 1.5 V µm-1. Such field strengths are not different from the values found in radial droplets with positive dielectric anisotropy.17 As the transmittance decreases immediately when the field is applied, it can be speculated that the droplet directors in our reverse-mode emulsions adopt a range of reorientation with respect to the applied field rather than a unique value of 90° needed for a threshold behavior (i.e., the transmittance does not keep a welldefined value until a particular strength of the electric field is reached). The electro-optical response at 20 °C of reverse-mode operation emulsions is reported in Figure 3. The rise times are rather faster (always
