pubs.acs.org/Langmuir © 2009 American Chemical Society
Director Configuration of Liquid-Crystal Droplets Encapsulated by Polyelectrolytes Jianhua Zou and Jiyu Fang* Advanced Materials Processing and Analysis Center, Department of Mechanical, Materials, and Aerospace Engineering, University of Central Florida, Orlando, Florida 32816 Received November 10, 2009. Revised Manuscript Received December 1, 2009 Liquid-crystal 4-n-pentyl-40 -cyanobiphenyl (5CB) droplets dispersed in aqueous solution are prepared by the assembly of poly(styrenesulfonic acid) (PSSH) and poly(styrenesulfonate sodium) (PSSNa) at the 5CB/water interface. The micrometer sized PSSH-coated 5CB droplets in the space confinement formed by two parallel glass slides break up into submicrometer sized droplets under evaporation-induced flow. We find that the size reduction of the PSSH-coated droplets is accompanied by the bipolar-to-radial configuration transition of the 5CB in the droplets, while the PSSNa-coated 5CB droplets show no size-dependent configuration transition in the same size range. Our results suggest that the size-dependent director configuration of liquid-crystal droplets encapsulated by polyelectrolytes can be modulated by changing the interface conditions, which is important in designing liquid-crystal droplets for optical and biological applications.
Introduction Liquid-crystal droplets dispersed in a host matrix have raised considerable interest because of their tunable optical properties and potential applications in displays.1 In the absence of external fields, the director configuration of liquid-crystal droplets is determined by a balance between the surface anchoring energy and the bulk elasticity of the liquid crystal in the droplets.2 The surface anchoring tends to orient the director of the liquid crystal in the droplets. However, the deformation of the director costs the elastic energy of the liquid crystal. A number of director configurations including radial, axial, bipolar, and concentric can be formed in liquid-crystal droplets by the balance between the surface anchoring and the elasticity. It is known that the bipolar configuration of liquid-crystal droplets relates to the parallel anchoring, which is characterized by two point defects at the droplet surface, called “boojum”, while the radial configuration of liquid-crystal droplets corresponds to the homeotropic anchoring, which only contains a single point defect at the droplet center, called “hedgehog”. Recently, the investigations on the director configuration transition of liquid-crystal droplets have been carried out for the reason that the optical properties of liquid-crystal droplets can be tuned by changing their director configurations. The reported approaches for triggering the director configuration transition of liquid-crystal droplets include changing the boundary conditions at the droplet surface3-6 and applying external stimuli such as electric fields7 and shear flow.8 *To whom correspondence should be addressed. E-mail: jfang@ mail.ucf.edu. (1) Drzaic, P. J. Liquid crystal Dispersions; World Scientific: Singapore, 1995. (2) De Gennes, P. G.; Prost, J. The Physics of Liquid Crystals; Oxford University Press: New York, 1995. (3) Lavrentovich, O. D. Liq. Cryst. 1998, 24, 117. (4) Prishchepa, O. O.; Shabanov, A. V.; Zyryanov, V. Ya. Phys. Rev. E 2005, 72, 031712. (5) Heppenstall-Butler, M.; Williamson, A. M.; Terentjev, E. M. Liq. Cryst. 2005, 32, 77. (6) Lopez-Leon, T.; Fernandez-Nieves, A. Phys. Rev. E 2009, 79, 021707. (7) Xu, F.; Kitzerow, H. S.; Crooker, P. P. Phys. Rev. A 1992, 46, 6535. (8) Fernandez-Nieves, A.; Link, D. R.; Marquez, M.; Weitz, D. A. Phys. Rev. Lett. 2007, 98, 087801.
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Liquid-crystal droplets dispersed in water are of particular interest because they can be easily manipulated and investigated individually.9-12 The method of stabilizing liquid-crystal droplets in water is to use appropriate surface active surfactants, which can assemble at the liquid crystal/water interface. The surfactants assembled at the interface can also induce the surface order, giving rise to a stable director configuration of the liquid crystal inside the droplets. Much progress has been made in stabilizing the liquid-crystal droplets and controlling the director configuration of the liquid crystal in the droplets by the tailoring of surface active surfactants.3-6,13-20 Polyelectrolytes are polymers whose repeating units bear an electrolyte group. The electrolyte group dissociates in aqueous solution, making the polymers charged. It has been shown that poly(styrenesulfonate sodium) (PSSNa) can electrostatically stabilize liquid crystal 40 -pentyl-4-cyanobiphenyl (5CB) droplets in water and induce a bipolar configuration.21 The PSSNa-stabilized 5CB droplets with a bipolar configuration can be further coated by the layer-by-layer adsorption of polyelectrolytes to engineer functional materials for sensor applications.21,22 (9) Wood, T. A.; Gleeson, H. F.; Dickinson, M. R.; Wright, A. J. Appl. Phys. Lett. 2004, 84, 4292. (10) Gleeson, H. F.; Wood, T. A.; Dickinson, M. Philos. Trans. R. Soc. London, Ser. A 2006, 364, 2789. (11) Abedin, K. S.; Kerbage, C.; Fernandez-Nieves, A.; Weitz, D. A. Appl. Phys. Lett. 2007, 91, 091119. (12) Brasselet, E.; Murazawa, N.; Juodkazis, S.; Misawa, H. Phys. Rev. E 2008, 77, 041704. (13) Tongcher, O.; Sigel, R.; Landfester, K. Langmuir 2006, 22, 4504. (14) El-Sadek, R.; Roushdy, M.; Magda, J. Langmuir 2007, 23, 7907. (15) Spillmann, C. M.; Naciri, J.; Wahl, K. J.; Garner, Y. H., III; Chen, M. S.; Ratna, B. R. Langmuir 2009, 25, 2419. (16) Toquer, G.; Phou, T.; Monge, S.; Grimaldi, A.; Nobili, M.; Blanc, C. J. Phys. Chem. B 2008, 112, 4157. (17) Poulin, P.; Stark, H.; Lubensky, T. C.; Weitz, D. A. Science 1997, 275, 1770. (18) Loudet, J. C.; Richard, H.; Sigaud, G.; Poulin, P. Langmuir 2000, 16, 6724. (19) Zhao, Y.; Mahajan, N.; Fang, J. Y. Small 2006, 2, 364. Fang, J. Y. J. Mater. Chem. 2007, 17, 3479. (20) Simon, K. A.; Sejwal, P.; Gerecht, R. B.; Luk, Y.-Y. Langmuir 2007, 23, 1453. (21) Tjipto, E.; Cadwell, K. D.; Quinn, J. F.; Johnston, A. P. R.; Caruso, F.; Abbott, N. L. Nano Lett. 2006, 6, 2243. (22) Sivakumar, S.; Wark, K. L.; Gupta, J. K.; Abbott, N. L.; Caruso, F. Adv. Funct. Mater. 2009, 19, 2260.
Published on Web 12/11/2009
DOI: 10.1021/la904257j
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Figure 1. Polarizing optical microscopy images of PSSH-coated 5CB droplets in the space confinement formed by two parallel glass slides. These images were taken from the same sample at (a) the beginning, (b) 6 min later, and (c) 8 min later. The white arrows shown in (c) indicate the escaped radial configuration. Scheme 1. Chemical Structures of PSSH, PSSNa, and 5CB
In this article, we report the formation of 5CB droplets dispersed in water by the assembly of poly(styrenesulfonic acid) (PSSH) and poly(styrenesulfonate sodium) (PSSNa) at the 5CB/ water interface. The micrometer sized PSSH-coated 5CB droplets confined in the space formed by two parallel glass slides break into submicrometer droplets under evaporation-induced flow. We find that the size reduction of the PSSH-coated 5CB droplets is accompanied by the bipolar-to-radial configuration transition of the 5CB in the droplets. The size-dependent configuration transition, which occurs for the PSSH-coated 5CB droplets, is not observed for the PSSNa-coated droplets.
Experimental Section The liquid crystal used in our experiment was 40 -pentyl-4cyanobiphenyl (5CB) (Sigma-Aldrich), which has a nematic phase with a clearing point at 35.3 °C. Poly(styrenesulfonic acid) (PSSH) (Mw ∼ 75 000) and poly(styrenesulfonate sodium) (PSSNa) (Mw ∼ 70 000) from Sigma-Aldrich were used as received. Water used in our experiments was purified with an Easypure II system (18 MΩ cm, pH 5.7). Microscope cover glass slides were purchased from Fisher Scientific and cleaned by using piranha solution (3:1 H2SO4/H2O2). PSSH-coated 5CB droplets were formed by mixing 5 μL of 5CB, 50 mL of water, and 28 μL of PSSH water solution (18 wt.%) with a sonicator (19 w, 60 Hz) for 10 min. PSSNa-coated 5CB droplets were prepared by mixing 5 μL of 5CB, 50 mL of water, and 5 mg of PSSNa with a sonicator for 10 min. The director configuration of the PSSH- and PSSNacoated 5CB droplets dispersed in water was observed by using a polarizing optical microscope (Olympus BX40) in transmission mode at room temperature.
Results and Discussion Both PSSH- and PSSNa-coated 5CB droplets are found to be stable for months in aqueous solution, suggesting that a significant fraction of PSSH and PSSNa is assembled at the 5CB/water interface against the coalescence of 5CB droplets. The chemical structures of PSSH, PSSNa, and 5CB are shown in Scheme 1. The stability of PSS-coated 5CB droplets has been suggested to be a result of the amphiphilic nature of PSS (the aromatic group is hydrophobic, while the sulfonate group is hydrophilic) and/or the π-π interaction between the phenyl groups of 5CB and PSS.21 At 7026 DOI: 10.1021/la904257j
the 5CB/water interface, we expected that the hydrophobic phenyl groups are in contact with 5CB, while the hydrophilic sulfonate groups are in contact with water. In our experiments, 5 μL of PSSH- or PSSNa-coated 5CB droplet solution was added on a cover glass (2.2 cm 2.2 cm) and then another cover glass was placed on it. The droplet solution confined between these two cover glasses spread until it balanced the weight on top of it. The thickness of the confined solution film was estimated to be ∼10 μm at initial stages and decreased over time. The size and director configuration of PSSH- or PSSNa-coated 5CB droplets in the confined space were characterized with a polarizing optical microscope over a period of time in transmission mode. We find that the average size of PSSH-coated 5CB droplets quickly decreases from ∼2.4 to ∼0.6 μm within 8 min and appears to be little change thereafter. The average size was derived from the statistical results of 200 droplets. Interestingly, the size reduction of the PSSH-coated 5CB droplets is accompanied by the bipolar-to-radial configuration transition. As can be seen in Figure 1a, the micrometer sized PSSH-coated 5CB droplets show a bipolar configuration when viewed with crossed polarizers at the very beginning, whereas 6 min later we observe the appearance of a large number of submicrometer sized 5CB droplets, which show a radial configuration with Maltese crosses (Figure 1b). These initially observed micrometer 5CB droplets (Figure 1a) completely disappear 8 min later (Figure 1c). It is known that bare 5CB droplets in water tend to have a bipolar configuration. In addition, there is no coalescence for these submicrometer sized 5CB droplets observed over time. Therefore, we conclude that the submicrometer 5CB droplets are still coated by PSSH. The point defect of the submicrometer PSSH-coated 5CB droplets is not always at the center of the droplets. Some of them have a point defect which locates at the position between the surface and center of the droplets (Figure 1c), known as an escaped radial configuration. We find that the escaped radial droplets are unstable and eventually transit into the radial droplets by moving the point defect to the center of the droplets over time. This may suggest that the escaped radial configuration is a metastable phase during the transition from a bipolar to a radial configuration. It is known that the translational diffusion of a bipolar droplet can cause the change of its apparent optical texture. A bipolar liquid-crystal droplet can show an apparent cross texture when its optical symmetry axis is oriented parallel to the path of the light (i.e., perpendicular to the crossed polarizers). To confirm that the observed Maltese cross texture comes from the radial configuration, we followed individual submicrometer PSSH-coated 5CB droplets when they moved and rotated in the confined solution film. As can be seen in Figure 2, there is no optical texture change over time, confirming the radial configuration for the submicrometer sized 5CB droplets. Langmuir 2010, 26(10), 7025–7028
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Figure 2. Polarizing optical microscopy images of submicrometer PSSH-coated 5CB droplets in the space confinement. The white arrows indicate the droplet which we followed over time.
Figure 3. (a, b) Polarizing optical microscopy images of the breaking of a deformed large PSSH-coated 5CB droplet into a middle droplet and a small droplet. The small droplet broken from the large droplet has a radial configuration indicated by the white arrow. (c) Schematic representation of the breaking of a large droplet into small droplets under the confinement and evaporation-induced flow.
The formation of submicrometer sized PSSH-coated 5CB droplets is a result of the breaking up of micrometer sized PSSH-coated 5CB droplets in the space confinement made by two parallel glass slides. We find that the motion of large PSSHcoated 5CB droplets in the space confinement is often restricted. The restricted large droplets deform and break into small droplets over time. The small droplet of submicrometer size broken from the deformed large droplet shows a radial configuration (Figure 3a), while the middle droplet of micrometer size broken from the deformed large droplet remains bipolar and changes its optical appearance as it rotates (Figure 3b). It is recognized that Langmuir 2010, 26(10), 7025–7028
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the evaporation of solvent can induce convection in the interior of a liquid film. In the space confinement, the evaporation-induced flow is evident by the motion of PSSH-coated 5CB droplets along the same direction (e.g., flow direction). At the same time, the space distance between two parallel cover glasses decreases over time due to the evaporation of solvent. As a result, the motion of large droplets is restricted by the space confinement and deformed by flow-induced shear force. When the flow-induced shear force exceeds the surface tension of the large droplet, it breaks into small droplets (Figure 3c). Two control experiments were carried out to verify our proposed mechanism. We first introduced the micrometer sized PSSH-coated 5CB droplets into a glass channel (50 μm width and 30 μm depth) with two open ends. In such a case, the solvent evaporation was allowed, but the glass channel had no restriction for the motion of the micrometer sized 5CB droplet. We found that the micrometer sized droplets remained intact under the evaporation-induced flow, indicating that the confinement is essential for the breaking of micrometer sized PSSH-coated 5CB droplets. The second control experiment was conducted in a confined space formed by two parallel glass slides separated and sealed by a 10 μm Mylar spacer. In this setup, the evaporation was prohibited, and thus, no flow was generated. Although we found that large PSSH-coated droplets were restricted by the confinement, they remained intact over time. These control experiments suggest that the space confinement and flowinduced shear force are essential for the breaking of micrometer sized PSSH-coated 5CB droplets. This size-dependent configuration transition of liquid-crystal droplets has generated a great deal of interest.3,23-27 In a recent work reported by Gupta et al.,28 5CB was filled into the preformed polyelectrolyte multilayer capsules with diameters of 10 ( 0.22, 8 ( 0.20, 5 ( 0.19, 3 ( 0.18, 1 ( 0.04, and 0.7 ( 0.08 μm. The multilayer capsules were prepared by sequential deposition of PSSNa and poly(allylamine hydrochloride) (PAH) on charged silica templates. The 5CB in the PSS/PAH multilayer capsules shows a bipolar configuration for a diameter larger than 3 μm and a radial configuration for a diameter of ∼0.7 μm. The breaking up of PSSH-coated 5CB droplets provides a convenient way to elucidate the size-dependent configuration of liquid-crystal droplets. By the statistical analysis of PSSH-coated 5CB droplets with different sizes during the breaking process, we find that all droplets larger than 1.2 μm show a bipolar configuration and all droplets smaller than 0.6 μm show a radial configuration (Figure 4), while in the size range of 0.6-1.2 μm, both bipolar and radial configurations were observed. The ratio of radial/bipolar droplets increases rapidly when the droplet size decreases in this range (Figure 4). It is generally believed that the director configuration of liquid-crystal droplets is a result of the competition of the surface anchoring strength and the Frank elastic constant of the liquid crystal in the droplets.2 Previous theoretical calculation including the splay K11, twist K22, and bend K33 elastic constants suggests a uniform configuration for smaller liquid crystal droplets.3 After considering the effect of the saddle-splay K24 and the splay-bend K13 elastic constants of the liquid crystal in the droplets, the radial configuration is predicted to be more stable than the uniform configuration in smaller (23) Huang, W.; Tuthill, C. F. Phys. Rev. E 1994, 49, 570. (24) Erdmann, J. H.; Zumer, S.; Doane, J. W. Phys. Rev. Lett. 1990, 64, 1907. (25) Vennes, M.; Martin, S.; Gisler, T.; Zentel, R. Macromolecules 2006, 39, 8326. (26) Tixier, T.; Heppenstall-Butler, M.; Terentjev, E. M. Langmuir 2006, 22, 2365. (27) Kadivar, E. Phys. Rev. E 2009, 80, 011701. (28) Gupata, J. K.; Sivakumar, S.; Caruso, F.; Abbott, N. L. Angew. Chem., Int. Ed. 2009, 48, 1652.
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Figure 4. Dependence of director configuration of PSSH-coated 5CB droplets on droplet size. Each data point was obtained from the statistical results of 100 droplets.
Figure 5. Polarizing optical microscopy image of PSSNa-coated 5CB droplets in the space confinement formed by two parallel glass slides. All the droplets show a bipolar configuration in the size range of 0.5-2.0 μm.
droplets,28 which agrees with the observed size-dependent configuration of PSSH-coated 5CB droplets. We further study how the change of the boundary conditions affects the size-dependent configuration by using PSSNa as a surface active surfactant to stabilize the 5CB droplets in water. As can be seen from Figure 5, the PSSNa-coated 5CB droplets show a size distribution in the size range of 0.5-2.0 μm, which is broader than that of the PSSH-coated 5CB droplets. Interestingly, the size-dependent configuration transition, which occurs for the PSSH-coated 5CB droplets, is not observed for the
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PSSNa-coated droplets. All the PSSNa-coated 5CB droplets show a bipolar configuration in the size range of 0.5-2.0 μm (Figure 5). This suggests that there is a difference in the surface anchoring energy between the PSSH- and PSSNa-coated 5CB droplets. PSSNa has the same backbone as PSSH, but they have different countercations (Scheme 1). It is known that Hþ is a less hydrated cation than Naþ (hydration number of 2.7 and 3.5 for Hþ and Naþ, respectively). Therefore, PSSNa is expected to adopt a more stretched conformation in the 5CB/water interface due to the stronger electrostatic repulsion, compared with PSSH. The stretched PSSNa can form a densely packed layer on the 5CB droplet, as is evident from the stability of PSSNa-coated 5CB droplets. Under the same confined condition as PSSH-coated 5CB droplets, we found that the confined PSSNa-coated 5CB droplets were strong enough to survive the evaporation-induced flow without breaking up. The less hydrated PSSH may form a less stretched conformation at the 5CB/water interface due to the weak electrostatic repulsion. It is known that the stretched and aligned polymer chains by a rubbing process can induce parallel alignment of liquid crystals.29,30 We expect that the more stretched conformation of PSSNa can generate stronger surface anchoring for parallel alignment of the 5CB at the interface to prevent the bipolar-to-radial transition as the PSSNa-coated 5CB droplet size is reduced. In addition, we noted that the bipolar-toradial configuration transition of the 5CB in the polyelectrolyte multilayer capsules with PSS as an internal layer occurred as the size reduced.28 It is likely that the PSS at the internal surface of polyelectrolyte multilayer capsules may adopt different conformations, compared to the PSS assembled at the 5CB/water interface. In conclusion, the 5CB droplets dispersed in water have been prepared by the assembly of PSSH and PSSNa at the 5CB/water interface. The micrometer sized PSSH-coated 5CB droplets in the space confinement formed with two parallel glass slides are unstable and break into submicrometer droplets under evaporation-induced flow. The size reduction is accompanied by the bipolar-to-radial configuration transition of the 5CB in the droplets, while the bipolar configuration of PSSNa-coated 5CB droplets remains unchanged when their size is reduced from micrometer to submicrometer ranges. An understanding of the influence of surface conditions on the size-dependent director configuration of liquid-crystal droplets is an important step in designing liquid-crystal droplets for optical and biological applications. Acknowledgment. This work is supported by the National Science Foundation (CBET 0931778). (29) Geary, J. M.; Goodby, J. W.; Kmetz, A. R.; Patel, J. S. J. Appl. Phys. 1987, 62, 4100. (30) Van Aerie, N. A. J. M.; Barmento, M.; Hollering, R. W. J. Appl. Phys. 1993, 74, 3111.
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