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Director Configuration Transitions of Polyelectrolyte Coated Liquid-Crystal Droplets Jianhua Zou, Tanmay Bera, Alicia A. Davis, Wenlang Liang, and Jiyu Fang* Advanced Materials Processing and Analysis Center, Department of Mechanical, Materials, and Aerospace Engineering, University of Central Florida, Orlando, Florida 32816, United States ABSTRACT: Liquid-crystal droplets are of great interest because of their large surface areas, rich phases, and tunable optical properties. The director configuration of liquid-crystal droplets provides a unique optical sign to detect the events occurring at the droplet surface. In this article, we report the alternating bipolar/radial configuration transitions of 4-n-pentyl-40 -cyanobiphenyl (5CB) droplets triggered by the layer-bylayer coating of negatively charged poly(styrenesulfonate sodium (PSS) and positively charged poly(diallyldimethylammonium chloride) (PDADMAC) on the droplet surface. The alternating configuration transitions are due to the interactions of the 5CB with polar versus nonpolar PDADMAC/PSS multilayer coatings. Furthermore, we find that the coating of PDADMAC/PSS multilayers makes the director configuration of the 5CB in the droplets sensitive to environmental salt concentrations.
’ INTRODUCTION Liquid crystals are a highly sensitive anisotropic material because small changes in external conditions can often trigger observable responses.1 Recently, liquid-crystal films have emerged as an optical amplification medium for sensing27 and imaging813 applications. Liquid-crystal droplets represent a new class of functional materials due to their large surface areas, rich phases, and tunable optical properties.14 The equilibrium director configuration of liquid-crystal droplets, which represents the balance between the elasticity and the surface anchoring of the liquid crystals inside the droplets, provides a unique optical sign to detect the events occurring at the droplet surface. Progress has been made in stabilizing the liquid-crystal droplets in aqueous solution and controlling their director configurations by the assembly of surfactants at the liquid crystal/water interface.1522 Polyelectrolytes are polymers whose repeating units bear electrolyte groups. These electrolyte groups dissociate in aqueous solution, making the polymers charged. It has been shown that the assembly of negatively charged poly(styrenesulfonate sodium) (PSS) at the liquid crystal/water interface can stabilize the liquid-crystal droplets in aqueous solution.2325 The PSS stabilized liquid-crystal droplets can be further coated by the layer-by-layer adsorption of poly(allylamine hydrochlorid) (PAH) and PSS through electrostatic interactions. Interestingly, the PAH/PSS multilayer coatings have a significant effect on the interaction of surfactants with the liquid crystal in the droplets.24 Poly(diallyldimethyl ammonium chloride) (PDADMAC) and PSS (Figure 1a and b) are another well-known pair for the formation of polyelectrolyte multilayers. Because of the weak coupling of PDADMAC and PSS, PDADMAC/PSS multilayers are sensitive to local environments.26 In this Article, we report r 2011 American Chemical Society
Figure 1. Chemical structures of PSS (a) and PDADMAC (b). (c) A schematic illustration of the alternating bipolar/radial transitions of the 5CB in the droplets triggered by the layer-by-layer coating of PSS and PDADMAC on the droplet surface.
the alternating bipolar/radial transitions of 4-n-pentyl-40 -cyanobiphenyl (5CB) droplets triggered by the layer-by-layer coating of negatively charged PSS and positively charged PDADMAC on the droplet surface. The alternating configuration transitions can be explained by the interactions of the 5CB with polar versus nonpolar PDADMAC/PSS multilayer coatings. We also study the effect of salt concentrations, which are a well-known stimulus in tuning the structure of polyelectrolyte multilayers, on the director configuration of the 5CB in PDADMAC/PSS-coated droplets. Received: February 27, 2011 Revised: June 10, 2011 Published: June 13, 2011 8970
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Figure 2. Polarizing optical microscopy images of PSS-coated 5CB droplets (a), PDADMAC/PSS-coated 5CB droplets (b), PSS/PDADMAC/ PSS-coated 5CB droplets (c), and (PDADMAC/PSS)2-coated 5CB droplets (d). The images were taken in water. The direction of the polarizer and analyzer is indicated by white arrows.
’ MATERIALS AND METHODS Materials. Liquid crystals used in our experiment were 4-npentyl-40 -cyanobiphenyl (5CB) from Sigma-Aldrich. 5CB has a nematic phase at room temperature with a clearing point at 35.3 °C. Poly(styrene sulfonate sodium) (PSS, Mw ≈ 70 kDa), poly(diallyldimethylammonium chloride) (PDADMAC, Mw ≈ 30 kDa), and poly(allylamine hydrochlorid) (PAH, Mw ≈ 70 kDa) from Sigma-Aldrich were used as received. Water used in our experiments was purified with Easypure II system (18 MΩ cm, pH 5.7). Experimental Methods. The assembly of PSS at the 5CB/ water interface was carried out by mixing 100 μL of 5CB, 100 mL of water, and 100 mg of PSS with a sonicator for 10 min. The PSS-stabilized 5CB droplets were purified by washing them with deionized water through centrifugation. The washing was done twice to remove excess PSS in the solution. The PSS-stabilized 5CB droplets were then coated by the layer-by-layer adsorption of positively charged PDADMAC and negatively charged PSS. The PDADMAC/PSS multilayer-coated 5CB droplets were purified by washing them with deionized water through centrifugation. Characterizations. The director configuration of the 5CB inside the PDADMAC/PSS multilayer-coated droplets in aqueous solution was characterized by a polarizing optical microscope (Olympus BX40) in transmission mode at room temperature. Transmission electron microscopy (TEM) measurements of PDADMAC/PSS multilayer capsules dried on carbon-coated copper grids were performed on a JEOL1011 microscope with an accelerating voltage of 100 kV. ζ-Potential measurements of
PDADMAC/PSS multilayer-coated 5CB droplets were carried with a Zetasizer Nano ZS90 (Malvern Instruments Inc.) at room temperature under a cell-driven voltage of 30 V.
’ RESULTS AND DISCUSSION The assembly of PSS at the 5CB/water interface was found to effectively prevent the 5CB droplets from coalescence in aqueous solution. The PSS-stabilized 5CB droplets were further coated by the layer-by-layer adsorption of positively charged PDADMAC and negatively charged PSS in 0.1 M NaCl solution (Figure 1c). During the layer-by-layer adsorption, 1 mL of PDADMAC and PSS solutions with a concentration of 1 mg/mL and 0.1 M NaCl were used. The adsorption time for each layer was 20 min. After the coating of each layer, the director configuration of the 5CB inside the droplets was observed with a polarizing optical microscope in aqueous solution at room temperature. As can be seen in Figure 2a, the PSS-coated 5CB droplets show a bipolar configuration. The bipolar configuration of the PSS-coated 5CB droplets is a result of parallel surface anchoring, while the PDADMAC/PSS-coated 5CB droplets show a radial configuration (Figure 2b), corresponding to homeotropic surface anchoring. The coating of a PSS layer on the PDADMAC/PSS-coated 5CB droplets switches the director configuration of the 5CB in the droplets back to the bipolar configuration (Figure 2c). The bipolar configuration of the 5CB in PSS/PDADMAC/PSScoated droplets turns into the radial configuration after the coating of a PDADMAC layer (Figure 2d). The alternating bipolar/radial configuration transitions of the 5CB in the droplets are observed for the 16 coating cycles that we studied here. 8971
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Figure 4. (a) Zeta potential during the layer-by-layer coating of PSS and PDADMAC on 5CB droplets. (b) TEM image of (PDADMAC/PSS)8 capsule shells dried on a carbon-coated copper grid. No staining was used. The image was taken with an accelerating voltage of 100 kV. Figure 3. (a) The director configuration transition of the 5CB in the PDADMAC/PSS-coated droplets after being exposed to PSS solutions with difference concentrations for 20 min. (b) The director configuration transition of the 5CB in the PDADMAC/PSS multilayer-coated droplets after being exposed to a PSS solution with a concentration of 0.25 mg/mL for 20 min. The data points were obtained from the statistical result of 100 droplets from each sample.
We also find that there is a minimal PSS concentration required to trigger the alternating bipolar/radial configuration transition of the 5CB in the (PDADMAC/PSS)n-coated droplets, where n represents the number of PDADMAC/PSS bilayers. Figure 3a shows the experimental results of the (PDADMAC/ PSS)1-coated 5CB droplets exposed to PSS solutions with different concentrations for a constant time (20 min). By the statistical analysis of the coated 5CB droplets, we find that all of them switch to the bipolar configuration when the concentration of PSS is larger than 0.75 mg/mL and maintain the radial configuration when the concentration is lower than 0.25 mg/mL. While in the range of 0.250.75 mg/mL, both the bipolar and the radial configurations are observed. The detailed examinations show that the radial-to-bipolar transition of small (PDADMAC/ PSS)1-coated 5CB droplets is faster than that of large (PDADMAC/PSS)1-coated 5CB droplets in this concentration range. For a constant adsorption time, we expect that the density of PSS adsorbed on the (PDADMAC/PSS)1-coated 5CB droplets increases with the increase of PSS concentrations and the decrease of droplet sizes. This result suggests that the coating of a dense PSS layer on (PDADMAC/PSS)1-coated 5CB droplets is necessary to trigger the radial-to-bipolar configuration. The minimal
PSS concentration required to trigger the alternating bipolar/ radial configuration transition of the 5CB in the droplets is found to depend on the number of PDADMAC/PSS bilayers coated on the 5CB droplets (Figure 3b). For the (PDADMAC/PSS)2coated 5CB droplets exposed to PSS solution with a 0.25 mg/mL for 20 min, approximately 30% of them transit into the bipolar configuration. In the case of (PDADMAC/PSS)3-coated 5CB droplets, approximately 50% of them become bipolar. All of the (PDADMAC/PSS)4- and (PDADMAC/PSS)5-coated 5CB droplets become the bipolar configuration after being exposed in PSS solution with a 0.25 mg/mL for 20 min. The dependence may be associated with the layer ordering of PDADMAC/PSS multilayers, which is known to increase with the number of layers.27 The zeta-potential measurements during the layer-by-layer coating of negatively charged PSS and positively charged PDADMAC on 5CB droplets show alternating changes in the zetapotential (Figure 4a), where the layer-by-layer adsorption on the PSS-stabilized 5CB droplets was carried out in PDADMAC and PSS solutions with a concentration of 1 mg/mL and 0.1 M NaCl. The adsorption time for each layer was 20 min to ensure the coating of a dense layer of PDADMAC and PSS on 5CB droplets. The zeta-potential is negative when PSS is the outer layer and positive when PDADMAC is the outer layer, respectively. The alternating changes in the zeta-potential suggest that the layerby-layer coating of PSS and PDADMAC occurs on the 5CB droplets through electrostatic interactions. To further prove that the PDADMAC/PSS multilayers are indeed coated on the 5CB droplets, we imaged the (PDADMAC/PSS)8 capsules with a transmission electron microscope (TEM) after removing the 5CB 8972
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Figure 6. Polarizing optical microscopy image of collapsed PDADMAC/PSS-coated 5CB droplets after being exposed to NaCl solution with a concentration of 3 M for 20 min. The direction of the polarizer and analyzer is indicated by white arrows.
Figure 5. (a) The director configuration transition of PDADMAC/ PSS-coated 5CB droplets as a function of NaCl concentrations. The data points were obtained from the statistical result of 100 droplets from each sample. (b) Zeta potential of PDADMAC/PSS-coated 5CB droplets as a function of NaCl concentrations.
core, according to the method reported in the literature.23 Briefly, 1 mL of ethanol was added to 1 mL of (PDADMAC/PSS)8coated 5CB droplet solution. The mixture was agitated with a vortex mixer for 15 min. The (PDADMAC/PSS)8 capsules were purified by washing them with ethanol through centrifugation. This washing process was repeated three times. Figure 4b shows a TEM image of the (PDADMAC/PSS)8 capsules dried on a carbon-coated copper grid. The wall thickness of the capsules is ∼72 nm, which is fairly consistent with the thickness of (PSS/ PDADMAC)8 layers reported in the literature.28 The layer-by-layer coating of negatively charged PSS and positively charged PDADMAC represents a charge arrangement on 5CB droplets. The entire capsule shell made by an even number of polyelectrolyte layers should be nonpolar. Taking into account the chemical structure of 5CB, the bipolar configuration of the 5CB in the droplets is likely due to the hydrophobic and/or ππ interactions between the phenyl groups of 5CB and PSS at the interface.23 In contract, the entire capsule shell made by an even number of polyelectrolyte layers is polar. The high polarity of (PDADMAC/PSS)4 multilayers was reported in the literature.29 In this case, the balance between the polar interaction of the cyano groups of the 5CB with the shell and the hydrophobic and/ or ππ interactions of the phenyl groups of the 5CB and PSS determines surface anchoring. The observed radial configuration of the 5CB in the polar shells suggests that the polar interaction is a dominant force because it is expected to induce homeotropic surface anchoring. Therefore, we conclude that the alternating
bipolar/radial configuration transition of the 5CB in the droplets triggered by the layer-by-layer coating of PSS and PDADMAC is a result of the interactions of the 5CB with polar versus nonpolar capsule shells. It is known that salt is an important stimulus to tune the structure of polyelectrolyte multilayers by attenuating the intermolecular interaction of polyelectrolyte pairs.3033 We find that the weakening of the PDADMAC/PSS bilayers coated on 5CB droplets by salt can trigger the radial-to-bipolar transition of the 5CB in the droplets (Figure 5a). In our experiments, the (PDADMAC/PSS)1-coated 5CB droplets with the radial configuration were incubated with NaCl solution with varied concentrations from 0.01 to 2.5 M for 2 h. As can be seen in Figure 5a, the radial-to-bipolar transition of (PDADMAC/PSS)1-coated 5CB droplets starts at a concentration of ∼0.5 M NaCl. When the concentration of NaCl is higher than 1.0 M, 6080% of (PDADMAC/PSS)1-coated 5CB droplets transit into the bipolar configuration. Figure 5b shows the zeta potential of (PDADMAC/PSS)1-coated 5CB droplets as a function of NaCl concentrations. The zeta potential decreases with increasing salt concentrations from an initial value of 28 mV for 0.1 M NaCl to ∼10 mV for 2.5 M NaCl. It is likely that the weakening of the PDADMAC/PSS bilayer structure on the 5CB droplets by high salt concentration reduces the polarity of the entire capsule shells. As we discussed above, the surface anchoring of the 5CB in polar shells is determined by the interplay between the hydrophobic and/or ππ interactions between the phenyl groups of 5CB and PSS and the polar interaction of the cyane group of the 5CB with the shell. The reduced polar interaction due to the incorporation of salt ions within the PDADMAC/PSS shell leads to the radial-to-bipolar configuration transition. The salt-triggerred configuration transition of the 5CB in the droplets is found to be independent of the number of PDADMA/PSS bilayers coated on the droplet surface and the droplet size. If the NaCl concentration exceeds 2.5 M, we find that the PDADMA/PSScoated 5CB droplets collapse (Figure 6). Finally, we would like to point out that the coating of PAH on the PSS-coated 5CB droplets does not trigger the bipolar-to-radial configuration transition. This may reflect the difference in the 8973
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The Journal of Physical Chemistry B layer structures between the PDADMAC/PSS pair and the PAH/PSS pair. It has been shown that the coupling of PAH and PSS is stronger than that of PDADMAC and PSS. The closest distance between the cationic and anionic charge in the PSS/PAH pair is estimated to be ∼0.25 nm,34 while the closest distance between the cationic and anionic charge in the PSS/ PDADMAC pair is ∼0.5 nm due to the presence of the methyl groups of ammonium cations. It has been shown that the polarity of (PAH/PSS)4 multilayers is much smaller than that of the (PDADMAC/PSS)4 multilayers.28 It is likely that the interaction between the polar 5CB and the polar PAH/PSS shell is not sufficient to switch the bipolar configuration of the 5CB in the droplets, which is fixed by the hydrophobic and/or ππ interactions between the phenyl groups of 5CB and PSS at the interface. In conclusion, we find that the layer-by-layer coating of PSS and PDADMAC on 5CB droplets is able to trigger the alternating bipolar/radial configuration transition of the 5CB in the droplets. The alternating configuration transition is a result of the interactions of the 5CB with polar versus nonpolar PDADMAC/ PSS multilayer coatings. Our results also show that the PDADMAC/PSS multilayer coatings make the director configuration of the 5CB in the droplets sensitive to salt concentrations.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is supported by the National Science Foundation (CBET 0931778). We thank Dr. Qun Huo for her help in the ζ-potential measurements.
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