Article pubs.acs.org/Macromolecules
Discrimination of Alcohol Molecules Using Hydrogen-Bridged Cholesteric Polymer Networks Chin-Kai Chang,*,†,‡ Cees W. M. Bastiaansen,†,§ Dirk J. Broer,† and Hui-Lung Kuo‡ †
Department of Chemical Engineering and Chemistry, Functional Materials and Devices, Eindhoven University of Technology, Eindhoven, Netherlands ‡ Materials & Chemical Research Laboratory, Industrial Technology Research Institute, Hsinchu, Taiwan § School of Engineering and Materials Science, Queen Mary University of London, London, U.K. S Supporting Information *
ABSTRACT: This paper presents a novel hydrogen-bridged cholesteric liquid crystal (CLC) polymer networks with a porosity. This kind of material is adopted to distinguish methanol from ethanol. Because of the diversity of molecular affinities in ethanol and methanol with the hydrogen-bridged cholesteric polymer networks, the reflected colors of the cholesteric polymer networks are obviously different upon the uptake of the alcohol solutions with different ratios of methanol/ethanol. Furthermore, the composition of the monomers was altered to obtain the different sensitivities of the different alcohol concentrations and ratios of methanol/ethanol. The reduction of cross-link density and increasing amount of alkyloxybenzoic acid in polymer networks can enhance the distinguishing ability between methanol and ethanol. The function of change in transmission-valley wavelength of CLC versus the alcohol concentration and the ratio of methanol/ethanol was also obtained from the experimental results.
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INTRODUCTION Some authors have recently shown the sensing ability of liquid crystal (LC) for sensor application.1−3 The molecular orientation of LC can be altered by external stimuli. The molecular structure of LCs includes a special functional group, the orientation of which is altered by the formation of particular chemical bonds. Thus, LCs can be used to sense chemical agents and are of considerable interest in terms of gas sensor.4,5 Furthermore, LCs have the advantages of fast response, reversibility, and high selectivity as used to sense chemical agents. Cholesteric liquid crystal (CLC) is characterized by its helical structure. The molecules in each layer have an average axis of orientation, which rotates by a small angle from layer to layer. A selected reflection wavelength and the reflection bandwidth of the CLC are represented as
of CLC will reveal different reflected colors of CLC. Therefore, the reflected color of a CLC can be exploited to identify chemical vapors according to the extent of the change in the helical pitch upon the absorption of chemical vapors.6−11 Methanol and ethanol are the two alcohol molecules commonly used in the chemical industry, and these two alcohol molecules can usually be found in synthetic wine. The detection of alcohol molecules is very useful in daily life, and they were used to investigate the sensing capacity of CLC polymer networks in this study. Counterfeit wine contains both ethanol and methanol, and the methanol is harmful to humans. The difference between the properties of methanol and ethanol are not obvious, and it was difficult to determine the methanol concentration in counterfeit wine. Therefore, it was clearly necessary to find a way to distinguish methanol from ethanol. Optical detection in distinguishing methanol from ethanol will be an effective method because of its direct visualization and fast response, and CLC has a capability of providing different colors response under the external stimuli. In this work, a novel CLC material is proposed to distinguish methanol from ethanol. The CLC materials used in this work are hydrogenbridged CLC polymer networks with a porosity. Hydrogenbonded LC polymer has previously been reported to be able to form the nanopore system in a smectic phase.12 The hydrogen-
λ=n×p Δλ = Δn × p
(1)
where n is the average refractive index, Δn is the birefringence, and p is the helical pitch. The helical pitch can also be altered by external stimuli (such as temperature, electricity, and chemical bonding), and the change in helical pitch is shown by a variation of the reflected color. The molecular structure of CLC can be modified to absorb the special analyte, and the uptake of the special analyte will affect the helical pitch of CLC (swelling or shrinkage). In eq 1, the change in the helical pitch © 2012 American Chemical Society
Received: April 6, 2012 Revised: May 7, 2012 Published: May 16, 2012 4550
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Figure 1. Chemical structures RM82, RM105, 6OBA, 6OBAM, LC756, 5CB, and I369.
bonded LC polymer can be actuated in humidity,13 pHcontrolled14 environments, and organic solvents.15 Hydrogen-bridged CLC polymer has carboxylic moieties, and its hydrogen bonds can be activated to absorb organic molecules as treated with alkaline solution. The ionic carboxylic groups can distinguish methanol from ethanol according to the different ionic strengths of alcohol molecules with carboxylic moiety. Furthermore, the differences in molecular affinity between methanol and ethanol with a CLC helical structure comprised of carboxylic moieties are also different. The absorption of methanol and ethanol induces different swelling states of the CLC helical structure and reveals different reflected colors of the CLC. The porosity was also incorporated into the CLC polymer networks. Generation of porosity into the CLC polymer works increases the surface-area-to-volume ratio, and the porosity generation in polymer networks can enhance the sensing ability of hydrogen-bridged CLC polymer. Although the properties of ethanol and methanol are very similar, this kind of CLC polymer can amplify the properties of ethanol and methanol, revealing variations of the reflected color of the CLC. The colorimetry of the CLC can be used to identify methanol and ethanol because the CLC helical structure will form different sizes as absorbed in methanol and ethanol. Therefore, the CLC is a versatile tool for distinguishing methanol from ethanol.
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RM105 are not alcohol-responsive reactive mesogens; thus, they are capable of preserving the integrity of the polymer. Furthermore, RM82 will provide more cross-link density and stronger polymer integrity for CLC films. The chiral diacrylate used in this study was LC756, which was obtained from BASF. Each of these exerts a twisting force in the liquid crystal, which adopts a helical structure upon the addition of the chiral dopant. The photopolymerized alkyloxybenzoic acids used in this study were 6OBA and 6OBAM, which were obtained from Philips and Synthon, respectively. They form the dimer through the hydrogen bond. The generation of porosity used in this study was 5CB, which was obtained from Merck. 5CB is a nonreactive mesogen, which is not chemically cross-linked by a photopolymerization process. Therefore, 5CB can be extracted to create porosity in the polymer network by heat treatment after photopolymerization. The photoinitiator used in this study was I369, which was obtained from Ciba. All the CLC mixtures were dissolved in tetrahydrofuran (THF) solution (1:1). Figure 1 schematically depicts the chemical compounds RM82, RM105, 6OBA, 6OBAM, 5CB, and I369. An inkjet printer (Dimatix) was used to print the CLC mixtures on triacetyl cellulose (TAC) films which were prerubbed using a flannel sheet, and the photopolymerization process was performed in the nitrogen atmosphere. The temperature of the printing substrate depends on the properties of the CLC mixture. It was necessary to decrease the temperature of the printing substrate for a CLC mixture containing fewer diacrylates in order to preserve the helicoidal molecular order. The temperature of the printing substrate was set from 38 to 53 °C in this study. The printing area was 11 × 11 mm. The CLC mixtures were made with different amounts of reactive mesogen and alkyloxybenzoic acid to obtain different CLC films, and these CLC films had different sensitivities to the alcohol solution. The intensity of UV light and exposure time used in photopolymerization were 18 mW/cm2 and 150 s, respectively. CLC films with nonreactive mesogen were heated on a hot plate at 105 °C for 1 h in a nitrogen-circulation atmosphere after
MATERIALS AND METHODS
The reactive mesogens used in this study were RM82 and RM105, which were obtained from Merck. The RM82 and RM105 were diacrylate and monoacrylate mesogens, respectively. RM82 and 4551
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Table 1. Recipes of Different CLC mixtures and Corresponding Film Thicknessesa
a
(%)
Mix1
Mix2
Mix3
Mix4
Mix5
Mix6
Mix7
Mix8
RM82 RM105 6OBA 6OBAM 5CB LC756 I369 film thickness (μm)
21.1 37.1 18 18 0 5.1 0.7 7.2
23.4 0 27 27 17.4 4.5 0.7 6.8
32.1 0 22.4 22.4 18 4.6 0.7 6.8
14.4 26.4 18 18 18 4.5 0.7 6.6
18 32.4 13.4 13.4 18 4.4 0.7 6.7
5 36 18 18 18 4.3 0.7 6.6
27.5 13.2 18 18 18 4.6 0.7 6.9
0 53.8 11.5 11.5 18 4.5 0.7 6.3
For mixtures containing 5CB, the film thicknesses are obtained after the extraction of 5CB.
Figure 2. Optical transmission spectra of CLC films and SEM. (a) Optical transmission spectra of CLC films at different steps. Insets are the schematic drawings of different molecular structures. (b) SEM is cross-sectional micrograph of the CLC film after the extraction of 5CB. This CLC film is made from Mix3, and scale bar in (b) is 2 μm. reducing in transmission-valley wavelength of the CLC film is close to the ratio of 5CB in the CLC mixture. Therefore, this heating treatment can extract 5CB from the CLC film completely. Furthermore, the change in thickness of this CLC film is only 5.6% (from 7.2 to 6.8 μm) after extraction of 5CB. It also means that this CLC film contains porosity after extraction of 5CB. The scanning electron micrograph (SEM) in Figure 2b also shows that it has porosity in the CLC film after the extraction of 5CB. The sizes of porosity are about 50−120 nm. Previous literatures16,17 had shown that the porosity size in polymer networks can be increased as increasing amount of nonreactive mesogen into the mixture. Therefore, the porosity size in the CLC polymer networks also can be controlled as altering the amount of 5CB in the mixture. After the extraction of 5CB, the process of breaking hydrogen bond is performed. The CLC film was placed in sodium hydroxide solution, and the helical structure of CLC with breaking hydrogen bond is swollen. In Figure 2a, the CLC film made from Mix3 revealed a redshift with a central transmission-valley wavelength of 595 nm caused by the swelling of the helical pitch. As for the final drying process, the CLC film made from Mix3 revealed a blue-shift and reverted to a central transmission-valley wavelength of 418 nm. The sensing ability of the CLC film was performed after the drying process for the different alcohol solutions.
photopolymerization to extract 5CB. The color reflected by the CLC films showed a blue shift because the amounts of the LC had been reduced, causing the helical pitch of the CLC to shrink. It generated porosity in the CLC polymer networks after the extraction of 5CB. The amount of 5CB was extracted from CLC film can be estimated by the change in transmission-valley wavelength of CLC film. 6OBA and 6OBAM stuck to each other physically to form a hydrogen bond by means of the carboxylic moieties. Therefore, the hydrogen bond was needed to break to enable the absorption of the alcohol molecules. The CLC films were placed in a sodium hydroxide solution (0.05 M) for 20 min after the extraction of 5CB to break the hydrogen bonds. During the process of breaking hydrogen bond, 6OBA and 6OBAM will form carboxylic salt (COO−Na+). The helical structure of CLC was swollen in sodium hydroxide solution after breaking the hydrogen bonds of the CLC networks, and the reflected color of the CLC films had a red-shift. The CLC films were dried on a hot plate at 75 °C for 1 h in a nitrogen-circulation atmosphere after the process of breaking hydrogen bond. These CLC films showed a blue shift and reverted to reflecting their original color after the drying process. Then the CLC films were placed into different alcohol solutions with different alcohol concentrations and ratios of methanol/ethanol. After reaching equilibrium, an AvaSpec spectrometer (AvaSpec-USB2, version 7.3) was used to measure the transmission spectra of the CLC films. Table 1 shows the recipes of the different CLC mixtures used in this study. The CLC film made from Mix1 is without a porosity, and its sensitivity for alcohol solutions will be compared to CLC with a porosity. Figure 2a shows the different optical transmission spectra of the CLC film and corresponding schematic molecular structures at different stages. This CLC film in Figure 2a was made from Mix3. After the extraction of 5CB, this CLC film revealed a blue-shift with a central transmission-valley wavelength of 419 nm caused by the extraction of 5CB. The reduction in transmission-valley wavelength of the CLC film is about 18.3% (from 513 to 419 nm) after the extraction of 5CB. The ratio of 5CB is 18% in the CLC mixture. The ratio of
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RESULTS AND DISCUSSION Optical Transmission Spectra of CLC Films in Alcohol Solutions. The color reflected by the CLC would reveal its pitch. From eq 1, the reflected color is red-shifted as the CLC pitch increases. The wavelength of reflected color from CLC film is corresponding with central transmission-valley wavelength of CLC film. The CLC films were placed in an alcohol solution for 10 min. Then they were removed from the alcohol 4552
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solutions to measure the transmission spectra of the CLC films. The alcohol solutions were ethanol concomitant with methanol and kept at a mixture of 60% water and 40% of methanol/ ethanol. The ratio of methanol/ethanol was changed. Figure 3
the CLC to expand more because of the higher molecular affinity with the hydrogen-bridged CLC networks. In Figure 3, the CLC films were made from Mix3, and they contained more alkyloxybenzoic acids. This means that the CLC films made from Mix3 can absorb more alcohol molecules. If the CLC has insufficient polymer integrity and absorbs more alcohol molecules, this will cause the CLC polymer structure to relax. This means that the variation of the transmission-valley wavelength will not be obvious. The polymer integrity and ability of absorbing alcohol molecule will influence the sensing ability of the CLC for alcohol solutions. For the hysteresis of the sensing ability, alcohols remained in the hydrogen-bridged CLC networks even after the alcohol solutions around the CLC films evaporated (Supporting Information). The alcohol molecules had stronger interaction with the hydrogen-bridged CLC networks. Therefore, this CLC sensor only has partially reversibility for detection of alcohol molecules because the molecular affinity of alcohol molecules is close to the molecular affinity of carboxylic moiety. Variations of Transmission-Valley Wavelength for CLC Films in Alcohol Solutions. The variation of the transmission-valley wavelength versus different ratios of methanol/ethanol for the CLC films is also discussed. The change in the transmission-valley wavelength (Δλ) is determined by comparing the initial valley transmission wavelength transmitted from the CLC with it after the alcohol solutions have been absorbed. Figure 4a shows the Δλ of the CLC films with different ratios of methanol/ethanol. In Figure
Figure 3. Optical transmission spectra of CLC in alcohol solutions with different ratios of methanol/ethanol. These CLC films are made from Mix3.
shows that the CLC films reflect different colors in the alcohol solutions with different ratios of methanol/ethanol. In Figure 3, the CLC films were made from Mix3. This reveals that the size of the helical structure of the CLC with the uptake of alcohol solutions increased and thus red-shifted the spectrum of the CLC. The alcohol solutions with a higher concentration of ethanol caused a substantially greater shift. The hydroxyl group of alcohol molecules interacted with the carboxylic moieties of the CLC networks, and the alcohol molecules were absorbed by the CLC films through ionic interaction. For the interaction between hydrogen-bridged CLC polymer networks and alcohol molecules, we consider the hydrogen bonding force, dipolar intermolecular force, and dispersion force between molecules according to Hildebrand solubility parameters.18 In solubility parameters, the values of methanol and ethanol are 29 MPa0.5 and 26 MPa0.5, respectively. The alcohol molecule was absorbed by carboxylic moieties of the hydrogen-bridged CLC polymer networks, and benzoic acid is part of such CLC polymer networks. The value of benzoic acid is 22 MPa0.5 in a solubility parameter, and it is close to ethanol. Therefore, the molecular affinity of ethanol with a hydrogenbridged CLC is higher than methanol. Hydrogen-bridged CLC polymer absorbs more ethanol molecules. Alcohol solutions containing more ethanol molecules caused the CLC films to exhibit a larger red-shift. More specifically, the color reflected by the CLC films differed according to the different swelling states of the helical structure. The absorption of the alcohol molecules with different molecular polarities in the helical structure comprised of hydrogen bonds enhanced the effect of this difference on the color reflected by the films. When comparing the change in the transmission wavelength of the CLC in a lower ethanol concentration with that in a higher ethanol concentration, it was seen that the variation of the transmission-valley wavelength was larger in the lower ethanol concentration, as shown in Figure 3. Also in Figure 3, the transmission bands and intensity of the valley become broader and larger as the ethanol concentration increases, respectively. For the alcohol solutions containing a higher ethanol concentration, this causes the helical structure of the CLC to swell more, and the order of the helical structure is reduced. The ethanol molecule causes the helical structure of
Figure 4. Change in the transmission-valley wavelength of CLC films in alcohol solutions: (a) change in the transmission-valley wavelength of CLC films with different ratios of methanol/ethanol, and alcohol concentration keep at 40%; (b) solid and dashed lines represent changes in the transmission-valley wavelength of CLC films with and without porosity, respectively in alcohol solutions, with different alcohol concentrations and ratios of methanol/ethanol. 4553
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Figure 5. Contour plots of Δλ versus alcohol concentration and ratio of methanol/ethanol: (a) CLC films are made from Mix4; (b) CLC films are made from Mix8; (c) CLC films are made from Mix3; (d) CLC films are made from Mix6.
4a, the alcohol concentration was also kept at 40%, and the ratio of methanol/ethanol was changed. It was found that the hydrogen-bridged CLC networks showed different selected properties for distinguishing methanol from ethanol. Figure 4a shows that Δλ of the CLC films increased as the ratios of methanol/ethanol decreased. An increase in the number of ethanol molecules absorbed caused the helical structure of the CLC to swell more because the molecular affinity of ethanol is larger than that of methanol with hydrogen-bridged CLC polymer networks. Therefore, the color reflected by the CLC films showed a larger red-shift as the ratios of methanol/ethanol decreased. Furthermore, Figure 4a shows that the slope and Δλ increased as the amount of 6OBA and 6OBAM increased and the cross-link density in polymer networks decreased. The slope in Figure 4a denotes the selectivity between methanol and ethanol of the CLC sensor. It can be concluded that the discrimination between methanol and ethanol is enhanced remarkably by an increase in the amount of 6OBA and 6OBAM. The increase in the slope is not quite as obvious as decreasing the cross-link density in Figure 4a. When comparing Mix4 and Mix6 with Mix7, the ratios of RM82/RM105 are different and kept at the same amount of 6OBA and 6OBAM. In Figure 4a, the slope of Mix6 is a little larger than in Mix4, and the cross-link density is the smallest in Mix6. Monoacrylate provides less polymer integrity than diacrylate, and this makes the CLC polymer more flexible. This more flexible polymer structure enhances the sensing ability, but it is not obvious. Furthermore, the Δλ are not a linear response versus higher ethanol concentration for Mix2, Mix3, and Mix6. This is because their own polymer integrity cannot sustain their ability
to absorb a higher number of ethanol molecules. Ethanol molecules will cause the helical structure of the CLC to swell more and cause the reduction of order for the helical structure substantially. Therefore, the Δλ is not obvious with a higher ethanol concentration for these CLC films. In Figure 4a, the CLC films made from Mix1 have no porous structure, and the slope is very small. This means that the porous structure can improve the ability of the CLC film to sense the alcohol solution. Figure 4b also compares the effect of porous structure on the detection of alcohol molecules at different alcohol concentrations and different ratios of methanol/ethanol. In Figure 4b, the solid and dashed lines are the Δλ versus alcohol concentration and the ratio of methanol/ethanol for Mix1 and Mix4, respectively. The alcohol concentration represents the sum of methanol and ethanol concentrations. Figure 4b also shows that the slope increases for the CLC films with a porosity for alcohol concentration. The porosity generation in polymer networks enhanced the sensitivity and selectivity of hydrogen-bridged CLC films to detect alcohol molecules. Modeling Δλ versus Alcohol Concentration and Ratio of Methanol/Ethanol. It can be seen from Figure 4b that both the different alcohol concentration and ratios of methanol/ethanol influence the Δλ of CLC film in alcohol solutions. Therefore, there are two unknowns for one Δλ. More specifically, the test of an alcohol solution is performed to obtain one Δλ by using one CLC film, but the real ethanol and methanol concentration still cannot be determined. Therefore, the formulation of Δλ as functions of alcohol concentration and ratio of methanol/ethanol needs to be determined. A linear 4554
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relationship of Δλ with alcohol concentration and ratio of methanol/ethanol is presented as
showed a slight red-shift after increasing environmental temperature (Supporting Information), as a consequence of thermal expansion of the helical structure of the CLC. The colorimetry of the CLC is an effective means of identifying the concentration of methanol to monitor the safety of a wine by analyzing the contents of the alcohol solution. Therefore, a sensor device based on CLC has the potential to be applied to alcohol sensors in the future.
(2)
Δλ = AX − BY + C
where X is the alcohol concentration, Y is the ratio of methanol/ethanol, A is the sensitivity to alcohol concentration, B is the sensitivity to ratio of methanol/ethanol, and C is the Δλ of CLC film for the response of pure water. The methanol will cause the transmission spectra of the CLC to have a smaller Δλ than the ethanol. Therefore, the BY has a negative contribution for Δλ in eq 2. It is assumed that the experimental results with Δλ versus alcohol concentrations and ratios of methanol/ethanol are linear relationships. The Δλ versus alcohol concentration and ratio of methanol/ethanol are plotted in Figure 5. Figures 5a, 5b, 5c, and 5d show the Δλ of CLC films made from Mix4, Mix8, Mix3, and Mix6 versus the alcohol concentration and the ratio of methanol/ethanol, respectively. When comparing these figures, it can be observed that the CLC films have a higher sensitivity for the ratio of methanol/ethanol as CLC contains more alkyloxybenzoic acids and less the cross-link density. Furthermore, a least-squares method was adopted to determine A and B in eq 2 from the experimental results. Table 2 shows the A and B of all the CLC mixtures, and it can be seen that the sensitivities to alcohol concentration and
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Figures SI-1 and SI-2. This material is available free of charge via the Internet at http://pubs.acs.org.
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Mix1
Mix2
Mix3
Mix4
Mix5
Mix6
Mix7
Mix8
0.74 2.5 65
7.02 22.35 80
4.79 17.35 48
2.44 8.52 40
1.1 4.4 36
3.86 9.55 80
1.88 7.24 20
2.71 5.18 34
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported from the Industrial Technology Research Institute, Taiwan (Grant A101W1D000). REFERENCES
(1) Woltman, S. J.; Jay, G. D.; Crawford, G. P. Nat. Mater. 2007, 6, 929−938. (2) Hussain, A.; Pina, A. S.; Roque, A. C. A. Biosens. Bioelectron. 2009, 25, 1−8. (3) Lin, I. H.; Miller, D. S.; Bertics, P. J.; Murphy, C. J.; de Pablo, J. J.; Abbott, N. L. Science 2011, 27, 1297−1300. (4) Adgate, J. L.; Bartekova, A.; Raynor, P. C.; Griggs, J. G.; Ryan, A. D.; Acharya, B. R.; Volkmann, C. J.; Most, D. D.; Lai, S.; Bonds, M. D. J. Environ. Monit. 2009, 11, 49−55. (5) Xu, H.; Bi, X.; Ngo, X.; Yang, K. L. Analyst 2009, 134, 911−915. (6) Han, Y.; Pacheco, K.; Bastiaansen, C. W. M.; Broer, D. J.; Sijbesma, R. P. J. Am. Chem. Soc. 2010, 132, 2961−2967. (7) Chang, C. K.; Kuo, H. L.; Tang, K. T.; Chiu, S. W. Appl. Phys. Lett. 2011, 99, 073504. (8) Chang, C. K.; Chiu, S. W.; Kuo, H. L.; Tang, K. T. Appl. Phys. Lett. 2012, 100, 043501. (9) Kirchner, N.; Zedler, L.; Mayerhofer, T. G.; Mohr, G. J. Chem. Commun. 2006, 1512−1514. (10) Mujahid, A.; Stathopulos, H.; Lieberzeit, P. A.; Dickert, F. L. Sensors 2010, 10, 4887−4897. (11) Dickert, F. L.; Haunschild, A.; Hofmann, P. Fresenius' J. Anal. Chem. 1994, 350, 577−581. (12) Gonzalez, C. L.; Bastiaansen, C. W. M.; Lub, J.; Loos, J.; Lu, K.; Wondergem, H. J.; Broer, D. J. Adv. Mater. 2008, 20, 1246−1252. (13) Harris, K. D.; Bastiaansen, C. W. M.; Lub, J.; Broer, D. J. Nano Lett. 2005, 5, 1857−1860. (14) Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J. J. Microelectromech. Syst. 2007, 16, 480−488. (15) Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J. Macromol. Rapid Commun. 2006, 27, 1323−1329. (16) Li, W.; Cao, H.; Kashima, M.; Liu, F.; Cheng, Z.; Yang, Z.; Zhu, S.; Yang, H. J. Polym. Sci., Part B: Polym. Phys. 2008, 46, 2090−2099. (17) Dolgov, L.; Yaroshchuk, O.; Qiu, L. Mol. Cryst. Liq. Cryst. 2007, 468, 335−344. (18) Solubility Parameters: Theory and Application; Burke, J., Ed.; The Oakland Museum of California: Oakland, CA, 1984.
Table 2. Determined Coefficients for Linear Function of Δλ versus Alcohol Concentration and Ratio of Methanol/ Ethanol A B C
ASSOCIATED CONTENT
S Supporting Information *
the ratio of methanol/ethanol were obviously enhanced with an increment of 6OBA and 6OBAM. In Table 2, it also can find that the sensitivities to alcohol concentration and the ratio of methanol/ethanol can only be enhanced slightly as decreasing the cross-link density. The determined A and B can provide the formulation of Δλ. It can use two CLC films to obtain two Δλs and determine the alcohol concentration and the ratio of methanol/ethanol in an alcohol solution.
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CONCLUSIONS In summary, this paper has demonstrated the discrimination between methanol and ethanol by the CLC films. The helical structure of the CLC was swollen by the uptake of the alcohol molecule by an analysis of the optical transmission spectra. The helical order of the CLC reduced as more alcohol molecules were absorbed. Based on the analysis of Δλ, the experimental results revealed a red-shift in the color reflected by the CLC as it absorbed the alcohol molecule, and the ethanol caused a larger red shift. A linear formation was also obtained for Δλ versus alcohol concentration and ratio of methanol/ethanol from the experimental results by using a least-squares method, and this is useful for an application in sensing an alcohol solution. Therefore, the hydrogen-bridged CLC film with a porosity can be used to identify methanol and ethanol molecules by their molecular polarities. Furthermore, the CLC polymer networks used in this study are fully crosslinked networks. Their properties are very stable under the variation of external temperature. The color of the CLC films 4555
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