Article pubs.acs.org/Langmuir
Reversible Thermochromic Polymer Film Embedded with Fluorescent Organogel Nanofibers Hyungwoo Kim and Ji Young Chang* Department of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul 151-744, Korea S Supporting Information *
ABSTRACT: We report a reversible thermochromic nanocomposite polymer film composed of fluorescent organogel fibers and a highly cross-linked polymer matrix. A series of cyano-substituted oligo(p-phenylenevinylene) (CN-OPV) derivatives were synthesized by the reaction of dialdehydes with phenyl or naphthyl acetonitrile under basic conditions. Among the CN-OPV derivatives, NA-DBA having naphtyl moieties and dodecyloxy chains formed a stable organogel in a cross-linkable monomeric solvent (ethylene glycol dimethacrylate). The organogel showed a thermoreversible sol−gel transition, accompanying the emission color change. A nanocomposite polymer film obtained by photopolymerization of the organogel between two quartz plates also exhibited reversible thermochromism. Under 365 nm irradiation, the orange color of the film at 25 °C became yellowish green at 120 °C. The fluorescence spectroscopy, DSC, and microscopy results determined that the thermally reversible self-assembly of NA-DBA occurred in the polymer matrix, resulting in reversible thermochromism. The melted gelator molecules at 120 °C did not diffuse into the polymer matrix probably because of poor interactions of the gelator molecules with the polymer matrix. The NA-DBA molecules dispersed in poly(methyl methacrylate), without forming a supramolecular structure, did not show thermochromism.
■
the polymer matrix.15 The self-assembled structure of the gelator was changed above the glass transition temperature of the polymer, resulting in a fluorescence color change. The initial structure was not restored by cooling, but it was recovered upon exposure to chloroform vapors. In this work, we report reversible thermochromism of the polymer composite film, prepared from a fluorescent organogel formed in a cross-linkable monomeric solvent. Cyanosubstituted oligo(p-phenylenevinylene) (CN-OPV) derivatives having two alkoxy chains were prepared as a gelator. CN-OPVs have been widely investigated as an organic luminophore because they can be modified easily by introducing various functional groups and also their optical properties are tunable by the chemical modification. For instance, Weder’s group reported thermochromism or piezochromism of CN-OPV doped polymer blends3,16,29−35 and Park’s group intensively studied self-assembly and photophysical properties of various cyanostilbene and CN-OPV derivatives.5,9,36−44 We introduced two cyano groups at β-positions from the central benzene unit of the gelators for strong fluorescence. The gelators showed a thermoreversible sol−gel transition in a nonpolar solvent, accompanying the emission color change. The gel fiberembedded cross-linked polymer film obtained by the polymerization of the organogel also displayed a reversible temperaturedependent color change. We investigated the thermochromism
INTRODUCTION Reversible thermochromic materials are of great interest because of fundamental questions over their color change mechanisms as well as their possible applications as a temperature sensor.1,2 Two approaches are widely used to achieve thermochromism, which are based on the phase transformation3−6 and chemical structural change7,8 of a chromic compound. An example of the former is a thermotropic liquid crystal, which has different morphologies depending on temperature.9−11 An example of the latter is a thermochromic leuco dye, which exists in two forms with different absorption properties.12−14 These materials are usually dispersed into a polymer matrix for practical application. The even distribution of the chromic moieties in the polymer matrix is essential to get a sharp response to temperature.15,16 An organogel is a thermoreversible viscoelastic material, consisting of a large amount of solvent molecules and an entangled nanofiber network formed by self-assembly of organogelator molecules. The organogel has various potential uses in external stimuli sensing, optoelectronics, energy harvesting, biomedical devices, and so on,17−23 but its poor structural stability often frustrates the applications. Polymerization in the gel state can be used to provide the organogel with physicochemical stability. For example, polymerization of an organogel formed in a monomeric solvent produces a polymer nanocomposite, where the supramolecular nanofibers are uniformly distributed in the polymer matrix.24−28 Ajayagosh and coworkers polymerized an organogel of a p-phenylenevinylene-based gelator in styrene and investigated the reversible self-assembly behavior of the gelator molecules in © 2014 American Chemical Society
Received: May 6, 2014 Revised: October 23, 2014 Published: October 23, 2014 13673
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
Article
mechanism of the polymer film, along with its morphological change.
■
1.87 (m, OCH2CH2, 4H), 1.40−1.36 (m, alkyl chain proton, 12H), 0.91 (t, J = 7.05 Hz, CH3, 6H). 13C NMR (125 MHz, CDCl3, ppm): δ 151.64, 136.05, 133.46, 133.33, 132.00, 128.93, 128.55, 127.72, 127.09, 126.97, 126.42, 125.95, 122.60, 118.44, 111.85, 111.45, 69.54, 31.57, 29.20, 25.90, 22.62, 13.99. IR (KBr, cm−1): 3058, 2935, 2857, 2210, 1700, 1522, 1337, 1267, 1211, 1021, 886, 851, 802, 742, 668. Anal. Calcd for C44H44N2O2: C, 83.51; H, 7.01; N, 4.43. Found: C, 83.37; H, 7.01; N, 4.43. Synthesis of NA-DBA. This compound was prepared from 2naphthylacetonitrile (219.50 mg, 1.31 mmol) and 2,5-bis(dodecyloxy)benzene-1,4-dialdehyde (300 mg, 0.60 mmol) as described for PAHBA. Yield: 75%. 1H NMR (300 MHz, CDCl3, ppm): δ 8.19 (s, Ar− H, 2H), 7.97 (s, Ar−H, 2H), 7.95−7.78 (overlap, Ar−H, 6H), 7.56− 7.53 (overlap, Ar−H, 4H), 4.17 (t, J = 6.45 Hz, OCH2, 4H), 1.91− 1.86 (m, OCH2CH2, 4H), 1.53−1.23 (m, alkyl chain proton, 36H), 0.89 (t, J = 6.75 Hz, CH3, 6H). 13C NMR (125 MHz, CDCl3, ppm): δ 151.64, 136.01, 133.45, 133.32, 131.97, 128.93, 128.55, 127.70, 127.08, 126.95, 126.41, 125.94, 122.57, 118.43, 111.81, 111.43, 69.54, 31.90, 29.68, 29.62, 29.43, 29.35, 29.25, 26.29, 22.67, 14.09. IR (KBr, cm−1): 3064, 2921, 2851, 2212, 1699, 1507, 1338, 1295, 1220, 1036, 888, 852, 804, 747, 470. Anal. Calcd for C56H68N2O2: C, 83.95; H, 8.56; N, 3.50. Found: C, 83.92; H, 8.60; N, 3.48. Fabrication of Gel Fiber-Embedded Film. The organogel of NA-DBA in EGDMA (1 wt %) was placed between two quartz plates and polymerized under UV irradiation (a high-pressure mercury arc lamp, 3 mW cm−2) in the presence of a photoinitiator (2,2-dimethoxy2-phenylacetophenone, 1 wt %) for 12 h at room temperature. The film was washed with THF and dried in vacuo. Fabrication of PMMA Film Doped with NA-DBA. To a solution of poly(methyl methacrylate) (PMMA, 138.6 mg, Mw = 120 000 g mol−1) in THF (1.6 mL) was added 1.4 mg of NA-DBA. The THF solution (50 μL, 10 wt %) was spun-cast on a quartz plate at 2500 rpm for 60 s. The resulting film was dried in vacuo.
EXPERIMENTAL SECTION
Materials. 2,5-Bis(dodecyloxy)benzene-1,4-dialdehyde and 2,5bis(hexyloxy)benzene-1,4-dialdehyde were synthesized from 1,4hydroquinone by following the synthetic procedures for 2,5bis(decyloxy)benzene-1,4-dialdehyde45 except using 1-bromododecane and 1-bromohexane, respectively, instead of 1-bromodecane. Tetrahydrofuran (THF) was dried over sodium metal and distilled. Ethylene glycol dimethacrylate (EGDMA) was purified by an aluminum oxide column for removal of an inhibitor. All other chemicals and reagent grade solvents were purchased from Aldrich and used without any further purification. Measurement. 1H and 13C NMR spectra were recorded on a Bruker Avance 300 (300 MHz) and Avance 500 (125 MHz) spectrometer, respectively. Elemental analyses were performed using a Flash EA 1112 elemental analyzer. FT-IR measurements were made on a Perkin Elmer Spectrum GX I using KBr pellets. TGA measurements were performed on a TA modulated TGA2050 with a heating rate of 10 °C min−1 under nitrogen. DSC measurements were made on a TA modulated DSC Q10 with a scanning rate of 10 °C min−1 under nitrogen. X-ray diffraction patterns were obtained using a Bruker Xps GADDS (Cu Kα radiation, λ = 1.54 Å). UV−vis spectra were obtained using a Sinco S-3150 spectrometer. Fluorescence measurements were performed on a Shimadzu RF5301PC spectrofluorometer. Fluorescence images were obtained by using a Carl Zeiss-LSM510 confocal laser scanning microscope. SEM images were obtained by using a JEOL JSM-6330F microscope. TEM images were taken by using a JEM1010 microscope operating at 80 kV. Optical textures were observed by a Leica DM LP equipped with a Mettler Toledo FP 82HT heating stage and a Mettler Toledo FP 90 central process controller. Synthesis of PA-HBA. To a solution of phenylacetonitrile (0.15 mL, 1.31 mmol) and 2,5-bis(hexyloxy)benzene-1,4-dialdehyde (200 mg, 0.60 mmol) in 100 mL of THF/ethanol (1:1, v/v) was added a solution of NaOH (95.47 mg, 2.39 mmol) in ethanol (5 mL). After stirring for 2 h at room temperature, orange precipitates were filtered and dried in vacuo. Yield: 78%. 1H NMR (300 MHz, CDCl3, ppm): δ 8.04 (s, Ar−H, 2H), 7.90 (s, Ar−H, 2H), 7.69−7.72 (overlap, Ar−H, 4H), 7.49−7.40 (overlap, Ar−H, 6H), 4.13 (t, J = 6.45 Hz, OCH2, 4H), 1.88−1.83 (m, OCH2CH2, 4H), 1.49−1.34 (m, alkyl chain proton, 12H), 0.90 (t, J = 7.05 Hz, CH3, 6H). 13C NMR (125 MHz, CDCl3, ppm): δ 151.57, 136.11, 134.73, 129.21, 129.08, 126.09, 125.82, 118.35, 111.80, 111.42, 69.49, 31.54, 29.17, 25.86, 22.58, 13.95. IR (KBr, cm−1): 3077, 2931, 2856, 2210, 1700, 1522, 1363, 1249, 1215, 1043, 945, 903, 856, 759, 669. Anal. Calcd for C36H40N2O2: C, 81.17; H, 7.57; N, 5.26. Found: C, 81.16; H, 7.65; N, 5.17. Synthesis of PA-DBA. This compound was prepared from phenylacetonitrile (0.15 mL, 1.31 mmol) and 2,5-bis(dodecyloxy)benzene-1,4-dialdehyde (300 mg, 0.60 mmol) as described for PAHBA. Yield: 80%. 1H NMR (300 MHz, CDCl3, ppm): δ 8.04 (s, Ar− H, 2H), 7.90 (s, Ar−H, 2H), 7.72−7.69 (overlap, Ar−H, 4H), 7.49− 7.40 (overlap, Ar−H, 6H), 4.13 (t, J = 6.45 Hz, OCH2, 4H), 1.87− 1.83 (m, OCH2CH2, 4H), 1.48−1.25 (m, alkyl chain proton, 36H), 0.88 (t, J = 6.75 Hz, CH3, 6H). 13C NMR (125 MHz, CDCl3, ppm): δ 151.57, 136.11, 134.72, 129.20, 129.08, 126.09, 125.83, 118.35, 111.80, 111.43, 69.50, 31.91, 29.67, 29.62, 29.56, 29.38, 29.34, 29.21, 26.22, 22.67, 14.09. IR (KBr, cm−1): 3065, 2921, 2851, 2213, 1699, 1508, 1363, 1294, 1222, 1031, 905, 761, 721, 687. Anal. Calcd for C48H64N2O2: C, 82.24; H, 9.20; N, 4.00. Found: C, 81.95; H, 9.30; N, 3.94. Synthesis of NA-HBA. This compound was prepared from 2naphthylacetonitrile (219.98 mg, 1.32 mmol) and 2,5-bis(hexyloxy)benzene-1,4-dialdehyde (200 mg, 0.60 mmol) as described for PAHBA. Yield: 81%. 1H NMR (300 MHz, CDCl3, ppm): δ 8.19 (s, Ar− H, 2H), 7.97 (s, Ar−H, 2H), 7.95−7.78 (overlap, Ar−H, 6H), 7.56− 7.53 (overlap, Ar−H, 4H), 4.18 (t, J = 6.45 Hz, OCH2, 4H), 1.91−
■
RESULTS AND DISCUSSION Scheme 1 shows the synthetic procedures of a series of cyanosubstituted oligo(p-phenylenevinylene) (CN-OPV) derivatives. Scheme 1. Synthesis of CN-OPV Derivatives
2,5-Bis(dodecyloxy)benzene-1,4-dialdehyde and 2,5-bis(hexyloxy)benzene-1,4-dialdehyde were prepared from 1,4hydroquinone and reacted with phenyl or naphthyl acetonitrile under basic conditions to give four CN-OPV derivatives as orange solids. The compounds having phenyl and hexyl or dodecyl groups were named PA-HBA or PA-DBA, respectively, and the naphthyl and hexyl or dodecyl groups NA-HBA or NADBA, respectively. Among four CN-OPV derivatives, NA-DBA having naphthyl moieties and dodecyloxy chains showed the best gelation ability, followed by PA-DBA with phenyl groups and 13674
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
Article
dodecyloxy chains. NA-DBA formed stable organogels in nhexane, n-decane, 1,2-dichloroethane, and ethylene glycol dimethacrylate (EGDMA). PA-DBA gelated n-decane and EGDMA, but was precipitated in n-hexane and 1,2-dichloroethane. PA-HBA and NA-HBA with hexyloxy chains gelated only EGDMA. Critical gelation concentrations (CGCs) were determined by the vial inversion method and are summarized in Supporting Information Table S1. We focused on NA-DBA in our further research, which formed the most stable organogel. Figure 1a,b shows TEM and
by the naked eye. Figure 2 shows the photographs of NA-DBA in EGDMA taken under 365 nm irradiation at the temperature ranging from 30 to 85 °C. As the temperature increased, the color gradually changed from orange to green. A gel fiber-embedded polymer film was prepared by the polymerization of an organogel. The organogel of NA-DBA in EGDMA (1 wt %) was placed between two quartz plates and photopolymerized in the presence of a photoinitiator to give a nanocomposite polymer film. The fluorescent gel fibers embedded in the film could be observed by confocal laser scanning microscopy (CLSM) (Figure 3), suggesting that the supramolecular structure of NA-DBA was maintained even after the polymerization.
Figure 1. (a) TEM and (b) SEM images of the dry gels obtained from NA-DBA in EGDMA (1 wt %), and (c) optical image of the crystals obtained by slow evaporation of the THF solution of NA-DBA.
SEM images of the dry gels obtained from NA-DBA in EGDMA (1 wt %), respectively. Fibrous network structures with fiber diameters ranging from 95 to 380 nm were observed in both images. The X-ray diffractograms of NA-DBA measured in the dry gel state are shown in Supporting Information Figure S1. In the midangle region, the dry gel showed three peaks with dspacings of 20.53, 10.27, and 6.92 Å, respectively. The length of the rigid part in NA-DBA was calculated to be 20.57 Å by geometry optimization using the Forcite module of Material Studio, indicating that three peaks corresponded to (100), (200), and (300) reflections of the layered structure, respectively. Fibrous crystals of NA-DBA obtained by slow evaporation of a THF solution of NA-DBA (Figure 1c), also showed peaks with d-spacings of 20.63, 10.06, and 6.92 Å. These results suggested that NA-DBA molecules were similarly stacked to form layers in the dry gel and crystalline states. The organogel of NA-DBA in EGDMA (1 wt %) was thermally stable up to around 60 °C. Above 60 °C, the gel gradually became a sol. The absorption and emission spectra of the sol and gel of NA-DBA in EGDMA are shown in Supporting Information Figure S2. The absorption and emission spectra of the sol were similar to those of a THF solution of NA-DBA (10−5 M) (Supporting Information Figure S3). In the sol state, the maximum absorption peak appeared at 439 nm, which was red-shifted to 507 nm after gelation, indicating the elongation of the effective conjugation resulted from the charge transfer interaction and conformational change of the gelator molecule.35,36,46 In the emission spectra obtained by excitation at 400 nm, the maximum peak was shown at 523 nm in the sol state and bathochromically shifted to 580 nm after gelation. The emission color change was also observable
Figure 3. CLSM image of the gel fiber-embedded EGDMA film excited at 488 nm.
The gel fiber-embedded polymer film exhibited a reversible color change depending on temperature, showing an opaque orange color at 25 °C and a transparent yellow color at 120 °C (Figure 4a). A temperature-dependent fluorescence change was
Figure 4. (a) Photographs of the gel fiber-embedded EGDMA film (1 wt % of NA-DBA) taken at 25 and 120 °C. (b) Photographs of the gel fiber-embedded EGDMA film taken under 365 nm irradiation at the temperature ranging from 30 to 120 °C.
Figure 2. Photographs of NA-DBA in EGDMA (1 wt %) taken under 365 nm irradiation at the temperature ranging from 30 to 85 °C. 13675
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
Article
Figure 5. (a) Emission spectra of the gel fiber-embedded EGDMA film (1 wt % of NA-DBA) obtained at 25 and 120 °C (λex = 400 nm). (b) CIE color coordinates of NA-DBA in EGDMA (1 wt %) in the sol, gel, and film states. Red and blue arrows indicate heating and cooling, respectively.
Figure 6. Emission spectra of the gel fiber-embedded EGDMA film (1 wt % of NA-DBA) obtained at 25 °C (a) after five heating−cooling cycles (λex = 400 nm), (b) after thermal treatment for 12 h at 150 °C, and (c) after immersion in THF. The insets show the change of emission intensity at 584 nm compared to the initial emission.
also observed. As the film temperature increased under 365 nm irradiation, the orange color changed rapidly to yellowish green above 110 °C without any deformation of the film (Figure 4b). The emission change of the film was further investigated by fluorescence spectroscopy. The film showed an emission peak at 584 nm at 25 °C. As the film temperature increased to 120 °C, the emission peak was hypsochromically shifted to 531 nm (Figure 5a). The CIE color coordinates of the film were calculated to be x = 0.30, y = 0.64 at 120 °C and x = 0.55, y = 0.45 at 25 °C (Figure 5b), which were close to those of the sol and gel of NA-DBA in EGDMA (1 wt %), respectively. The CIE color coordinates in the sol and gel states were x = 0.27, y = 0.66 and x = 0.51, y = 0.48, respectively, suggesting that the emission color turned from yellowish green to yellowish orange by the sol−gel transition. The spectral change was reversible, so the original emission pattern was recovered when the film was cooled to room temperature. The thermochromic behavior of the film was comparable to that of the gel, suggesting that the fibrous supramolecular structure of NA-DBA molecules in the film was reversibly responsive to the temperature change. The film showed almost the same emission spectrum even after five heating−cooling cycles (Figure 6a). After heating at 150 °C for 12 h, the film also displayed the same emission as the original one (Figure 6b). These results corroborated that the gelator molecules in the melt state remained stable in the preformed space within the polymer matrix and reassembled to form the fibers on cooling. However, when the film was immersed in THF, where NA-DBA showed good solubility, the emission intensity decreased due to the dissolving out of the gelator molecules from the film. After 48 h in THF, the emission maximum was shifted to 536 nm, indicating that the
assembled structure of the gelator molecules was destroyed (Figure 6c). Supporting Information Figure S4 shows the DSC thermograms of NA-DBA, the organogel in EGDMA, and the gel fiberembedded EGDMA film. The organogel of NA-DBA in EGDMA (1 wt %) showed an endothermic peak at 72 °C on heating for the gel to sol transition and an exothermic peak at 46 °C for the sol to gel transition. In the crystalline state, NADBA showed an endothermic peak at 124 °C for the melt transition on heating and an exothermic peak at 101 °C for crystallization on cooling. The gel fiber-embedded film exhibited a similar thermal behavior to that of crystalline NADBA, showing an endothermic peak at 120 °C on heating and an exothermic peak at 104 °C on cooling. The thermal reversibility of the fibrous supramolecular structure of NA-DBA in the polymer film was also confirmed by SEM and TEM. The organogel of NA-DBA in EGDMA (1 wt %) was photopolymerized on a carbon-coated copper grid. The resulting polymer sample was heated to 120 °C and cooled down to room temperature. Its morphological change was then investigated. Supporting Information Figure S5 shows SEM and TEM images of the polymer film. The images obtained before and after thermal treatment were almost the same, showing the gel fibers. It was noteworthy that the dry gel fibers did not show the same thermal reversibility as the gel fibers embedded in the polymer matrix. Figure 7 shows optical textures of the gel fiberembedded polymer film observed at 25 and 120 °C. In the asprepared film, orange-colored, micrometer-sized fiber bundles were observed. When heated up to 120 °C, the film turned yellow, without significant change of the structure. However, in 13676
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
■
Article
CONCLUSION We demonstrated a reversible thermochromism of an organogel fiber-embedded polymer film. NA-DBA consisting of a CNOPV unit, naphthyl moieties, and dodecyloxy chains formed a stable organogel in EGDMA. The emission of the organogel noticeably changed from yellowish green to yellowish orange during the thermoreversible sol−gel transition. The organogel fiber-embedded polymer film was prepared by photopolymerization of the organogel, in which the gel fibers were uniformly dispersed. The film showed the reversible thermochromism, originating from the supramolecular structural change of the gel fibers embedded in the polymer matrix. The emission of the film was also reversibly changed by heating and cooling. At 120 °C, the gelator molecules in the fibers temporarily disassembled and emitted yellowish green light. After being cooled to 25 °C, the gelator molecules reassembled into gel fibers, and the emission of the film returned to the initial state. These results suggested that the interactions of the gelator molecules with the polymer matrix were similar to those with EGDMA in the gel state.
Figure 7. Optical micrographs of EGDMA film (1 wt % of NA-DBA) observed at 25 (a) and 120 °C (b). Optical micrographs of the asprepared dry gel observed at 25 (c) and 120 °C (d).
the case of the dry gel, the fibrous structure was completely destroyed at 120 °C and not restored when cooled to 25 °C. Given the fluorescence spectroscopy, DSC, and microscopy results, it was concluded that the thermally reversible selfassembly of NA-DBA occurred in the polymer matrix, resulting in reversible thermochromism. Although there was a possibility of the melted gelator molecules diffusing into the polymer matrix, we presumed that poor interactions of the gelator molecules with the polymer matrix were favorable for the reversible formation of the fibrous structure. Accordingly, the NA-DBA molecules dispersed in the polymer film without forming a supramolecular structure did not show thermochromism. We prepared a poly(methyl methacrylate) (PMMA) film containing 1 wt % of NA-DBA by spin-casting a solution of PMMA and NA-DBA in THF on a quartz plate. The PMMA film showed an emission at 513 nm similar to that of the monomeric gelator. The polymer film prepared from EGDMA containing 0.1 wt % of NA-DBA also did not display thermochromism, as the gelator molecules could not assemble to form gel fibers because of the low gelator concentration below CGC. The emissions of both the films were independent of the temperature, which was in definite contrast to that of the gel fiber-embedded film as shown in Figure 8.
■
ASSOCIATED CONTENT
S Supporting Information *
Critical gelation concentrations of the gelators, X-ray diffraction patterns of NA-DBA, absorption and emission spectra of NADBA in EGDMA, DSC thermograms, and SEM and TEM images of the EGDMA film. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Tel: +82-2-880-7190. Fax: +82-2-885-1748. E-mail: jichang@ snu.ac.kr. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was supported by grants from the Midcareer Researcher Program through NRF grant (No. 2010-0017552) funded by National Research Foundation of Korea.
■
REFERENCES
(1) Wang, X. D.; Wolfbeis, O. S.; Meier, R. J. Luminescent Probes and Sensors for Temperature. Chem. Soc. Rev. 2013, 42, 7834−7869. (2) Pucci, A.; Bizzarri, R.; Ruggeri, G. Polymer Composites with Smart Optical Properties. Soft Matter 2011, 7, 3689−3700. (3) Crenshaw, B. R.; Weder, C. Deformation-Induced Color Changes in Melt-Processed Photoluminescent Polymer Blends. Chem. Mater. 2003, 15, 4717−4724. (4) Zhao, Z.; Lamb, J. W. Y.; Tang, B. Z. Self-Assembly of Organic Luminophores with Gelation-Enhanced Emission Characteristics. Soft Matter 2013, 9, 4564−4579. (5) An, B.-K.; Gierschner, J.; Park, S. Y. π-Conjugated Cyanostilbene Derivatives: A Unique Self-Assembly Motif for Molecular Nanostructures with Enhanced Emission and Transport. Acc. Chem. Res. 2012, 45, 544−554. (6) Liu, C.; Lu, Y.; He, S.; Wang, Q.; Zhao, L.; Zeng, X. The Nature of the Styrylindolium Dye: Transformations Among its Monomer, Aggregates and Water Adducts. J. Mater. Chem. C 2013, 1, 4770−4778. (7) Bao, S.; Fei, B.; Li, J.; Yao, X.; Lu, X.; Xin, J. H. Reversible Thermochromic Switching of Fluorescent Poly(vinylidenefluoride) Composite Containing Bis(benzoxazolyl)stilbene Dye. Dyes Pigm. 2013, 99, 99−104.
Figure 8. Emission spectra of the PMMA film (1 wt % of NA-DBA) and the EGDMA films (0.1 and 1 wt % of NA-DBA) (λex = 400 nm). Photographs of the PMMA film and the EGDMA film (1 wt % of NADBA) taken under 365 nm are shown in the insets. 13677
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
Article
(8) Seeboth, A.; Lötzsch, D.; Ruhmann, R. First Example of a NonToxic Thermochromic Polymer Material-Based on a Novel Mechanism. J. Mater. Chem. C 2013, 1, 2811−2816. (9) Yoon, S.-J.; Kim, J. H.; Kim, K. S.; Chung, J. W.; Heinrich, B.; Mathevet, F.; Kim, P.; Donnio, B.; Attias, A.-J.; Kim, D.; Park, S. Y. Mesomorphic Organization and Thermochromic Luminescence of Dicyanodistyrylbenzene-Based Phasmidic Molecular Disks: Uniaxially Aligned Hexagonal Columnar Liquid Crystals at Room Temperature with Enhanced Fluorescence Emission and Semiconductivity. Adv. Funct. Mater. 2012, 22, 61−69. (10) Seredyuk, M.; Gaspar, A. B.; Ksenofontov, V.; Reiman, S.; Galyametdinov, Y.; Haase, W.; Rentschler, E.; Gütlich, P. Room Temperature Operational Thermochromic Liquid Crystals. Chem. Mater. 2006, 18, 2513−2519. (11) Sage, I. Thermochromic Liquid Crystals. Liq. Cryst. 2011, 38, 1551−1561. (12) Nakamura, K.; Kobayashi, Y.; Kanazawa, K.; Kobayashi, N. Thermoswitchable Emission and Coloration of a Composite Material Containing a Europium(III) Complex and a Fluoran Dye. J. Mater. Chem. C 2013, 1, 617−620. (13) Schäfer, C. G.; Gallei, M.; Zahn, J. T.; Engelhardt, J.; Hellmann, G. P.; Rehahn, M. Reversible Light-, Thermo-, and MechanoResponsive Elastomeric Polymer Opal Films. Chem. Mater. 2013, 25, 2309−2318. (14) Azizian, F.; Field, A. J.; Heron, B. M. Reductive Alkylation of Aminofluorans: A Simple Route to Intrinsically Thermochromic Fluorans. Dyes Pigm. 2013, 99, 432−439. (15) Srinivasan, S.; Babu, P. A.; Mahesh, S.; Ajayaghosh, A. Reversible Self-Assembly of Entrapped Fluorescent Gelators in Polymerized Styrene Gel Matrix: Erasable Thermal Imaging via Recreation of Supramolecular Architectures. J. Am. Chem. Soc. 2009, 131, 15122−15123. (16) Kunzelman, J.; Chung, C.; Mather, P. T.; Weder, C. Shape Memory Polymers with Built-in Threshold Temperature Sensors. J. Mater. Chem. 2008, 18, 1082−1086. (17) Terech, P.; Weiss, R. G. Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels. Chem. Rev. 1997, 97, 3133−3159. (18) Sangeetha, N. M.; Maitra, U. Supramolecular Gels: Functions and Uses. Chem. Soc. Rev. 2005, 34, 821−836. (19) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Organogels as Scaffolds for Excitation Energy Transfer and Light Harvesting. Chem. Soc. Rev. 2008, 37, 109−122. (20) Babu, S. S.; Prasanthkumar, S.; Ajayaghosh, A. Self-Assembled Gelators for Organic Electronics. Angew. Chem., Int. Ed. 2012, 51, 1766−1776. (21) Samanta, S. K.; Bhattacharya, S. Aggregation Induced Emission Switching and Electrical Properties of Chain Length Dependent πGels Derived from Phenylenedivinylene Bis-pyridinium Salts in Alcohol−Water Mixtures. J. Mater. Chem. 2012, 22, 25277−25287. (22) Basak, S.; Nanda, J.; Banerjee, A. Assembly of Naphthalenediimide Conjugated Peptides: Aggregation Induced Changes in Fluorescence. Chem. Commun. 2013, 49, 6891−6893. (23) Basak, S.; Nanda, J.; Banerjee, A. A New Aromatic Amino Acid Based Organogel for Oil Spill Recovery. J. Mater. Chem. 2012, 22, 11658−11664. (24) Gu, W.; Lu, L.; Chapman, G. B.; Weiss, R. G. Polymerized Gels and ‘Reverse Aerogels’ from Methyl Methacrylate or Styrene and Tetraoctadecylammonium Bromide as Gelator. Chem. Commun. 1997, 543−544. (25) Beginn, U.; Sheiko, S.; Möller, M. Self-Organization of 3,4,5Tris(octyloxy)benzamide in Solution and Embedding of the Aggregates into Methacrylate Resins. Macromol. Chem. Phys. 2000, 201, 1008−1015. (26) Zubarev, E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. Scaffolding of Polymers by Supramolecular Nanoribbons. Adv. Mater. 2002, 14, 198−203. (27) Kang, S. H.; Jung, B. M.; Chang, J. Y. Polymerization of an Organogel Formed by a Hetero-bifunctional Gelator in a Monomeric
Solvent: Preparation of Nanofibers Embedded in a Polymer Matrix. Adv. Mater. 2007, 19, 2780−2784. (28) Kang, S. H.; Jung, B. M.; Kim, W. J.; Chang, J. Y. Embedding Nanofibers in a Polymer Matrix by Polymerization of Organogels Comprising Heterobifunctional Organogelators and Monomeric Solvents. Chem. Mater. 2008, 20, 5532−5540. (29) Crenshaw, B. R.; Weder, C. Phase Separation of ExcimerForming Fluorescent Dyes and Amorphous Polymers: A Versatile Mechanism for Sensor Applications. Adv. Mater. 2005, 17, 1471−1476. (30) Kinami, M.; Crenshaw, B. R.; Weder, C. Polyesters with Built-in Threshold Temperature and Deformation Sensors. Chem. Mater. 2006, 18, 946−955. (31) Crenshaw, B. R.; Burnworth, M.; Khariwala, D.; Hiltner, A.; Mather, P. T.; Simha, R.; Weder, C. Deformation-Induced Color Changes in Mechanochromic Polyethylene Blends. Macromolecules 2007, 40, 2400−2408. (32) Kunzelman, J.; Crenshaw, B. R.; Weder, C. Self-Assembly of Chromogenic DyesA New Mechanism for Humidity Sensors. J. Mater. Chem. 2007, 17, 2989−2991. (33) Crenshaw, B. R.; Kunzelman, J.; Sing, C. E.; Ander, C.; Weder, C. Threshold Temperature Sensors with Tunable Properties. Macromol. Chem. Phys. 2007, 208, 572−580. (34) Tang, L.; Whalen, J.; Schutte, G.; Weder, C. Stimuli-Responsive Epoxy Coatings. ACS Appl. Mater. Interfaces 2009, 1, 688−696. (35) Kunzelman, J.; Crenshaw, B. R.; Kinami, M.; Weder, C. SelfAssembly and Dispersion of Chromogenic Molecules: A Versatile and General Approach for Self-Assessing Polymers. Macromol. Rapid Commun. 2006, 27, 1981−1987. (36) An, B.-K.; Gihm, S. H.; Chung, J. W.; Park, C. R.; Kwon, S.-K.; Park, S. Y. Color-Tuned Highly Fluorescent Organic Nanowires/ Nanofabrics: Easy Massive Fabrication and Molecular Structural Origin. J. Am. Chem. Soc. 2009, 131, 3950−3957. (37) Yoon, S.-J.; Kim, J. H.; Chung, J. W.; Park, S. Y. Exploring the Minimal Structure of a Wholly Aromatic Organogelator: Simply Adding Two β-Cyano Groups to Distyrylbenzene. J. Mater. Chem. 2011, 21, 18971−18973. (38) Seo, J.; Chung, J. W.; Cho, I.; Park, S. Y. Concurrent Supramolecular Gelation and Fluorescence Turn-on Triggered by Coordination of Silver Ion. Soft Matter. 2012, 8, 7617−7622. (39) Kim, S.; Yoon, S.-J.; Park, S. Y. Highly Fluorescent Chameleon Nanoparticles and Polymer Films: Multicomponent Organic Systems that Combine FRET and Photochromic Switching. J. Am. Chem. Soc. 2012, 134, 12091−12097. (40) Chung, J. W.; An, B.-K.; Park, S. Y. A Thermoreversible and Proton-Induced Gel-Sol Phase Transition with Remarkable Fluorescence Variation. Chem. Mater. 2008, 20, 6750−6755. (41) Shin, S.; Gihm, S. H.; Park, C. R.; Kim, S.; Park, S. Y. WaterSoluble Fluorinated and PEGylated Cyanostilbene Derivative: An Amphiphilic Building Block Forming Self-Assembled Organic Nanorods with Enhanced Fluorescence Emission. Chem. Mater. 2013, 25, 3288−3295. (42) Park, J. W.; Nagano, S.; Yoon, S.-J.; Dohi, T.; Seo, J.; Seki, T.; Park, S. Y. High Contrast Fluorescence Patterning in CyanostilbeneBased Crystalline Thin Films: Crystallization-Induced MassFlow Via a Photo-Triggered Phase Transition. Adv. Mater. 2014, 26, 1354−1359. (43) Kim, J. H.; An, B.-K.; Yoon, S.-J.; Park, S. K.; Kwon, J. E.; Lim, C.-K.; Park, S. Y. Highly Fluorescent and Color-Tunable Exciplex Emission from Poly(N-vinylcarbazole) Film Containing Nanostructured Supramolecular Acceptors. Adv. Funct. Mater. 2014, 24, 2746− 2753. (44) Chung, J. W.; Yoon, S.-J.; Lim, S.-J.; An, B.-K.; Park, S. Y. DualMode Switching in Highly Fluorescent Organogels: Binary Logic Gates with Optical/Thermal Inputs. Angew. Chem., Int. Ed. 2009, 48, 7030−7034. (45) Wang, B.; Wasielewski, M. R. Design and Synthesis of Metal Ion-Recognition-Induced Conjugated Polymers: An Approach to Metal Ion Sensory Materials. J. Am. Chem. Soc. 1997, 119, 12−21. (46) Wang, B.; Wang, Y.; Hua, J.; Jiang, Y.; Huang, J.; Qian, S.; Tian, H. Starburst Triarylamine Donor−Acceptor−Donor Quadrupolar 13678
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
Article
Derivatives Based on Cyano-Substituted Diphenylaminestyrylbenzene: Tunable Aggregation-Induced Emission Colors and Large TwoPhoton Absorption Cross Sections. Chem.Eur. J. 2011, 17, 2647− 2655.
13679
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Reversible Thermochromic Polymer Film Embedded with Fluorescent Organogel Nanofibers Hyungwoo Kim and Ji Young Chang* Department of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul 151-744, Korea S Supporting Information *
ABSTRACT: We report a reversible thermochromic nanocomposite polymer film composed of fluorescent organogel fibers and a highly cross-linked polymer matrix. A series of cyano-substituted oligo(p-phenylenevinylene) (CN-OPV) derivatives were synthesized by the reaction of dialdehydes with phenyl or naphthyl acetonitrile under basic conditions. Among the CN-OPV derivatives, NA-DBA having naphtyl moieties and dodecyloxy chains formed a stable organogel in a cross-linkable monomeric solvent (ethylene glycol dimethacrylate). The organogel showed a thermoreversible sol−gel transition, accompanying the emission color change. A nanocomposite polymer film obtained by photopolymerization of the organogel between two quartz plates also exhibited reversible thermochromism. Under 365 nm irradiation, the orange color of the film at 25 °C became yellowish green at 120 °C. The fluorescence spectroscopy, DSC, and microscopy results determined that the thermally reversible self-assembly of NA-DBA occurred in the polymer matrix, resulting in reversible thermochromism. The melted gelator molecules at 120 °C did not diffuse into the polymer matrix probably because of poor interactions of the gelator molecules with the polymer matrix. The NA-DBA molecules dispersed in poly(methyl methacrylate), without forming a supramolecular structure, did not show thermochromism.
■
the polymer matrix.15 The self-assembled structure of the gelator was changed above the glass transition temperature of the polymer, resulting in a fluorescence color change. The initial structure was not restored by cooling, but it was recovered upon exposure to chloroform vapors. In this work, we report reversible thermochromism of the polymer composite film, prepared from a fluorescent organogel formed in a cross-linkable monomeric solvent. Cyanosubstituted oligo(p-phenylenevinylene) (CN-OPV) derivatives having two alkoxy chains were prepared as a gelator. CN-OPVs have been widely investigated as an organic luminophore because they can be modified easily by introducing various functional groups and also their optical properties are tunable by the chemical modification. For instance, Weder’s group reported thermochromism or piezochromism of CN-OPV doped polymer blends3,16,29−35 and Park’s group intensively studied self-assembly and photophysical properties of various cyanostilbene and CN-OPV derivatives.5,9,36−44 We introduced two cyano groups at β-positions from the central benzene unit of the gelators for strong fluorescence. The gelators showed a thermoreversible sol−gel transition in a nonpolar solvent, accompanying the emission color change. The gel fiberembedded cross-linked polymer film obtained by the polymerization of the organogel also displayed a reversible temperaturedependent color change. We investigated the thermochromism
INTRODUCTION Reversible thermochromic materials are of great interest because of fundamental questions over their color change mechanisms as well as their possible applications as a temperature sensor.1,2 Two approaches are widely used to achieve thermochromism, which are based on the phase transformation3−6 and chemical structural change7,8 of a chromic compound. An example of the former is a thermotropic liquid crystal, which has different morphologies depending on temperature.9−11 An example of the latter is a thermochromic leuco dye, which exists in two forms with different absorption properties.12−14 These materials are usually dispersed into a polymer matrix for practical application. The even distribution of the chromic moieties in the polymer matrix is essential to get a sharp response to temperature.15,16 An organogel is a thermoreversible viscoelastic material, consisting of a large amount of solvent molecules and an entangled nanofiber network formed by self-assembly of organogelator molecules. The organogel has various potential uses in external stimuli sensing, optoelectronics, energy harvesting, biomedical devices, and so on,17−23 but its poor structural stability often frustrates the applications. Polymerization in the gel state can be used to provide the organogel with physicochemical stability. For example, polymerization of an organogel formed in a monomeric solvent produces a polymer nanocomposite, where the supramolecular nanofibers are uniformly distributed in the polymer matrix.24−28 Ajayagosh and coworkers polymerized an organogel of a p-phenylenevinylene-based gelator in styrene and investigated the reversible self-assembly behavior of the gelator molecules in © 2014 American Chemical Society
Received: May 6, 2014 Revised: October 23, 2014 Published: October 23, 2014 13673
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
Article
mechanism of the polymer film, along with its morphological change.
■
1.87 (m, OCH2CH2, 4H), 1.40−1.36 (m, alkyl chain proton, 12H), 0.91 (t, J = 7.05 Hz, CH3, 6H). 13C NMR (125 MHz, CDCl3, ppm): δ 151.64, 136.05, 133.46, 133.33, 132.00, 128.93, 128.55, 127.72, 127.09, 126.97, 126.42, 125.95, 122.60, 118.44, 111.85, 111.45, 69.54, 31.57, 29.20, 25.90, 22.62, 13.99. IR (KBr, cm−1): 3058, 2935, 2857, 2210, 1700, 1522, 1337, 1267, 1211, 1021, 886, 851, 802, 742, 668. Anal. Calcd for C44H44N2O2: C, 83.51; H, 7.01; N, 4.43. Found: C, 83.37; H, 7.01; N, 4.43. Synthesis of NA-DBA. This compound was prepared from 2naphthylacetonitrile (219.50 mg, 1.31 mmol) and 2,5-bis(dodecyloxy)benzene-1,4-dialdehyde (300 mg, 0.60 mmol) as described for PAHBA. Yield: 75%. 1H NMR (300 MHz, CDCl3, ppm): δ 8.19 (s, Ar− H, 2H), 7.97 (s, Ar−H, 2H), 7.95−7.78 (overlap, Ar−H, 6H), 7.56− 7.53 (overlap, Ar−H, 4H), 4.17 (t, J = 6.45 Hz, OCH2, 4H), 1.91− 1.86 (m, OCH2CH2, 4H), 1.53−1.23 (m, alkyl chain proton, 36H), 0.89 (t, J = 6.75 Hz, CH3, 6H). 13C NMR (125 MHz, CDCl3, ppm): δ 151.64, 136.01, 133.45, 133.32, 131.97, 128.93, 128.55, 127.70, 127.08, 126.95, 126.41, 125.94, 122.57, 118.43, 111.81, 111.43, 69.54, 31.90, 29.68, 29.62, 29.43, 29.35, 29.25, 26.29, 22.67, 14.09. IR (KBr, cm−1): 3064, 2921, 2851, 2212, 1699, 1507, 1338, 1295, 1220, 1036, 888, 852, 804, 747, 470. Anal. Calcd for C56H68N2O2: C, 83.95; H, 8.56; N, 3.50. Found: C, 83.92; H, 8.60; N, 3.48. Fabrication of Gel Fiber-Embedded Film. The organogel of NA-DBA in EGDMA (1 wt %) was placed between two quartz plates and polymerized under UV irradiation (a high-pressure mercury arc lamp, 3 mW cm−2) in the presence of a photoinitiator (2,2-dimethoxy2-phenylacetophenone, 1 wt %) for 12 h at room temperature. The film was washed with THF and dried in vacuo. Fabrication of PMMA Film Doped with NA-DBA. To a solution of poly(methyl methacrylate) (PMMA, 138.6 mg, Mw = 120 000 g mol−1) in THF (1.6 mL) was added 1.4 mg of NA-DBA. The THF solution (50 μL, 10 wt %) was spun-cast on a quartz plate at 2500 rpm for 60 s. The resulting film was dried in vacuo.
EXPERIMENTAL SECTION
Materials. 2,5-Bis(dodecyloxy)benzene-1,4-dialdehyde and 2,5bis(hexyloxy)benzene-1,4-dialdehyde were synthesized from 1,4hydroquinone by following the synthetic procedures for 2,5bis(decyloxy)benzene-1,4-dialdehyde45 except using 1-bromododecane and 1-bromohexane, respectively, instead of 1-bromodecane. Tetrahydrofuran (THF) was dried over sodium metal and distilled. Ethylene glycol dimethacrylate (EGDMA) was purified by an aluminum oxide column for removal of an inhibitor. All other chemicals and reagent grade solvents were purchased from Aldrich and used without any further purification. Measurement. 1H and 13C NMR spectra were recorded on a Bruker Avance 300 (300 MHz) and Avance 500 (125 MHz) spectrometer, respectively. Elemental analyses were performed using a Flash EA 1112 elemental analyzer. FT-IR measurements were made on a Perkin Elmer Spectrum GX I using KBr pellets. TGA measurements were performed on a TA modulated TGA2050 with a heating rate of 10 °C min−1 under nitrogen. DSC measurements were made on a TA modulated DSC Q10 with a scanning rate of 10 °C min−1 under nitrogen. X-ray diffraction patterns were obtained using a Bruker Xps GADDS (Cu Kα radiation, λ = 1.54 Å). UV−vis spectra were obtained using a Sinco S-3150 spectrometer. Fluorescence measurements were performed on a Shimadzu RF5301PC spectrofluorometer. Fluorescence images were obtained by using a Carl Zeiss-LSM510 confocal laser scanning microscope. SEM images were obtained by using a JEOL JSM-6330F microscope. TEM images were taken by using a JEM1010 microscope operating at 80 kV. Optical textures were observed by a Leica DM LP equipped with a Mettler Toledo FP 82HT heating stage and a Mettler Toledo FP 90 central process controller. Synthesis of PA-HBA. To a solution of phenylacetonitrile (0.15 mL, 1.31 mmol) and 2,5-bis(hexyloxy)benzene-1,4-dialdehyde (200 mg, 0.60 mmol) in 100 mL of THF/ethanol (1:1, v/v) was added a solution of NaOH (95.47 mg, 2.39 mmol) in ethanol (5 mL). After stirring for 2 h at room temperature, orange precipitates were filtered and dried in vacuo. Yield: 78%. 1H NMR (300 MHz, CDCl3, ppm): δ 8.04 (s, Ar−H, 2H), 7.90 (s, Ar−H, 2H), 7.69−7.72 (overlap, Ar−H, 4H), 7.49−7.40 (overlap, Ar−H, 6H), 4.13 (t, J = 6.45 Hz, OCH2, 4H), 1.88−1.83 (m, OCH2CH2, 4H), 1.49−1.34 (m, alkyl chain proton, 12H), 0.90 (t, J = 7.05 Hz, CH3, 6H). 13C NMR (125 MHz, CDCl3, ppm): δ 151.57, 136.11, 134.73, 129.21, 129.08, 126.09, 125.82, 118.35, 111.80, 111.42, 69.49, 31.54, 29.17, 25.86, 22.58, 13.95. IR (KBr, cm−1): 3077, 2931, 2856, 2210, 1700, 1522, 1363, 1249, 1215, 1043, 945, 903, 856, 759, 669. Anal. Calcd for C36H40N2O2: C, 81.17; H, 7.57; N, 5.26. Found: C, 81.16; H, 7.65; N, 5.17. Synthesis of PA-DBA. This compound was prepared from phenylacetonitrile (0.15 mL, 1.31 mmol) and 2,5-bis(dodecyloxy)benzene-1,4-dialdehyde (300 mg, 0.60 mmol) as described for PAHBA. Yield: 80%. 1H NMR (300 MHz, CDCl3, ppm): δ 8.04 (s, Ar− H, 2H), 7.90 (s, Ar−H, 2H), 7.72−7.69 (overlap, Ar−H, 4H), 7.49− 7.40 (overlap, Ar−H, 6H), 4.13 (t, J = 6.45 Hz, OCH2, 4H), 1.87− 1.83 (m, OCH2CH2, 4H), 1.48−1.25 (m, alkyl chain proton, 36H), 0.88 (t, J = 6.75 Hz, CH3, 6H). 13C NMR (125 MHz, CDCl3, ppm): δ 151.57, 136.11, 134.72, 129.20, 129.08, 126.09, 125.83, 118.35, 111.80, 111.43, 69.50, 31.91, 29.67, 29.62, 29.56, 29.38, 29.34, 29.21, 26.22, 22.67, 14.09. IR (KBr, cm−1): 3065, 2921, 2851, 2213, 1699, 1508, 1363, 1294, 1222, 1031, 905, 761, 721, 687. Anal. Calcd for C48H64N2O2: C, 82.24; H, 9.20; N, 4.00. Found: C, 81.95; H, 9.30; N, 3.94. Synthesis of NA-HBA. This compound was prepared from 2naphthylacetonitrile (219.98 mg, 1.32 mmol) and 2,5-bis(hexyloxy)benzene-1,4-dialdehyde (200 mg, 0.60 mmol) as described for PAHBA. Yield: 81%. 1H NMR (300 MHz, CDCl3, ppm): δ 8.19 (s, Ar− H, 2H), 7.97 (s, Ar−H, 2H), 7.95−7.78 (overlap, Ar−H, 6H), 7.56− 7.53 (overlap, Ar−H, 4H), 4.18 (t, J = 6.45 Hz, OCH2, 4H), 1.91−
■
RESULTS AND DISCUSSION Scheme 1 shows the synthetic procedures of a series of cyanosubstituted oligo(p-phenylenevinylene) (CN-OPV) derivatives. Scheme 1. Synthesis of CN-OPV Derivatives
2,5-Bis(dodecyloxy)benzene-1,4-dialdehyde and 2,5-bis(hexyloxy)benzene-1,4-dialdehyde were prepared from 1,4hydroquinone and reacted with phenyl or naphthyl acetonitrile under basic conditions to give four CN-OPV derivatives as orange solids. The compounds having phenyl and hexyl or dodecyl groups were named PA-HBA or PA-DBA, respectively, and the naphthyl and hexyl or dodecyl groups NA-HBA or NADBA, respectively. Among four CN-OPV derivatives, NA-DBA having naphthyl moieties and dodecyloxy chains showed the best gelation ability, followed by PA-DBA with phenyl groups and 13674
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
Article
dodecyloxy chains. NA-DBA formed stable organogels in nhexane, n-decane, 1,2-dichloroethane, and ethylene glycol dimethacrylate (EGDMA). PA-DBA gelated n-decane and EGDMA, but was precipitated in n-hexane and 1,2-dichloroethane. PA-HBA and NA-HBA with hexyloxy chains gelated only EGDMA. Critical gelation concentrations (CGCs) were determined by the vial inversion method and are summarized in Supporting Information Table S1. We focused on NA-DBA in our further research, which formed the most stable organogel. Figure 1a,b shows TEM and
by the naked eye. Figure 2 shows the photographs of NA-DBA in EGDMA taken under 365 nm irradiation at the temperature ranging from 30 to 85 °C. As the temperature increased, the color gradually changed from orange to green. A gel fiber-embedded polymer film was prepared by the polymerization of an organogel. The organogel of NA-DBA in EGDMA (1 wt %) was placed between two quartz plates and photopolymerized in the presence of a photoinitiator to give a nanocomposite polymer film. The fluorescent gel fibers embedded in the film could be observed by confocal laser scanning microscopy (CLSM) (Figure 3), suggesting that the supramolecular structure of NA-DBA was maintained even after the polymerization.
Figure 1. (a) TEM and (b) SEM images of the dry gels obtained from NA-DBA in EGDMA (1 wt %), and (c) optical image of the crystals obtained by slow evaporation of the THF solution of NA-DBA.
SEM images of the dry gels obtained from NA-DBA in EGDMA (1 wt %), respectively. Fibrous network structures with fiber diameters ranging from 95 to 380 nm were observed in both images. The X-ray diffractograms of NA-DBA measured in the dry gel state are shown in Supporting Information Figure S1. In the midangle region, the dry gel showed three peaks with dspacings of 20.53, 10.27, and 6.92 Å, respectively. The length of the rigid part in NA-DBA was calculated to be 20.57 Å by geometry optimization using the Forcite module of Material Studio, indicating that three peaks corresponded to (100), (200), and (300) reflections of the layered structure, respectively. Fibrous crystals of NA-DBA obtained by slow evaporation of a THF solution of NA-DBA (Figure 1c), also showed peaks with d-spacings of 20.63, 10.06, and 6.92 Å. These results suggested that NA-DBA molecules were similarly stacked to form layers in the dry gel and crystalline states. The organogel of NA-DBA in EGDMA (1 wt %) was thermally stable up to around 60 °C. Above 60 °C, the gel gradually became a sol. The absorption and emission spectra of the sol and gel of NA-DBA in EGDMA are shown in Supporting Information Figure S2. The absorption and emission spectra of the sol were similar to those of a THF solution of NA-DBA (10−5 M) (Supporting Information Figure S3). In the sol state, the maximum absorption peak appeared at 439 nm, which was red-shifted to 507 nm after gelation, indicating the elongation of the effective conjugation resulted from the charge transfer interaction and conformational change of the gelator molecule.35,36,46 In the emission spectra obtained by excitation at 400 nm, the maximum peak was shown at 523 nm in the sol state and bathochromically shifted to 580 nm after gelation. The emission color change was also observable
Figure 3. CLSM image of the gel fiber-embedded EGDMA film excited at 488 nm.
The gel fiber-embedded polymer film exhibited a reversible color change depending on temperature, showing an opaque orange color at 25 °C and a transparent yellow color at 120 °C (Figure 4a). A temperature-dependent fluorescence change was
Figure 4. (a) Photographs of the gel fiber-embedded EGDMA film (1 wt % of NA-DBA) taken at 25 and 120 °C. (b) Photographs of the gel fiber-embedded EGDMA film taken under 365 nm irradiation at the temperature ranging from 30 to 120 °C.
Figure 2. Photographs of NA-DBA in EGDMA (1 wt %) taken under 365 nm irradiation at the temperature ranging from 30 to 85 °C. 13675
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
Article
Figure 5. (a) Emission spectra of the gel fiber-embedded EGDMA film (1 wt % of NA-DBA) obtained at 25 and 120 °C (λex = 400 nm). (b) CIE color coordinates of NA-DBA in EGDMA (1 wt %) in the sol, gel, and film states. Red and blue arrows indicate heating and cooling, respectively.
Figure 6. Emission spectra of the gel fiber-embedded EGDMA film (1 wt % of NA-DBA) obtained at 25 °C (a) after five heating−cooling cycles (λex = 400 nm), (b) after thermal treatment for 12 h at 150 °C, and (c) after immersion in THF. The insets show the change of emission intensity at 584 nm compared to the initial emission.
also observed. As the film temperature increased under 365 nm irradiation, the orange color changed rapidly to yellowish green above 110 °C without any deformation of the film (Figure 4b). The emission change of the film was further investigated by fluorescence spectroscopy. The film showed an emission peak at 584 nm at 25 °C. As the film temperature increased to 120 °C, the emission peak was hypsochromically shifted to 531 nm (Figure 5a). The CIE color coordinates of the film were calculated to be x = 0.30, y = 0.64 at 120 °C and x = 0.55, y = 0.45 at 25 °C (Figure 5b), which were close to those of the sol and gel of NA-DBA in EGDMA (1 wt %), respectively. The CIE color coordinates in the sol and gel states were x = 0.27, y = 0.66 and x = 0.51, y = 0.48, respectively, suggesting that the emission color turned from yellowish green to yellowish orange by the sol−gel transition. The spectral change was reversible, so the original emission pattern was recovered when the film was cooled to room temperature. The thermochromic behavior of the film was comparable to that of the gel, suggesting that the fibrous supramolecular structure of NA-DBA molecules in the film was reversibly responsive to the temperature change. The film showed almost the same emission spectrum even after five heating−cooling cycles (Figure 6a). After heating at 150 °C for 12 h, the film also displayed the same emission as the original one (Figure 6b). These results corroborated that the gelator molecules in the melt state remained stable in the preformed space within the polymer matrix and reassembled to form the fibers on cooling. However, when the film was immersed in THF, where NA-DBA showed good solubility, the emission intensity decreased due to the dissolving out of the gelator molecules from the film. After 48 h in THF, the emission maximum was shifted to 536 nm, indicating that the
assembled structure of the gelator molecules was destroyed (Figure 6c). Supporting Information Figure S4 shows the DSC thermograms of NA-DBA, the organogel in EGDMA, and the gel fiberembedded EGDMA film. The organogel of NA-DBA in EGDMA (1 wt %) showed an endothermic peak at 72 °C on heating for the gel to sol transition and an exothermic peak at 46 °C for the sol to gel transition. In the crystalline state, NADBA showed an endothermic peak at 124 °C for the melt transition on heating and an exothermic peak at 101 °C for crystallization on cooling. The gel fiber-embedded film exhibited a similar thermal behavior to that of crystalline NADBA, showing an endothermic peak at 120 °C on heating and an exothermic peak at 104 °C on cooling. The thermal reversibility of the fibrous supramolecular structure of NA-DBA in the polymer film was also confirmed by SEM and TEM. The organogel of NA-DBA in EGDMA (1 wt %) was photopolymerized on a carbon-coated copper grid. The resulting polymer sample was heated to 120 °C and cooled down to room temperature. Its morphological change was then investigated. Supporting Information Figure S5 shows SEM and TEM images of the polymer film. The images obtained before and after thermal treatment were almost the same, showing the gel fibers. It was noteworthy that the dry gel fibers did not show the same thermal reversibility as the gel fibers embedded in the polymer matrix. Figure 7 shows optical textures of the gel fiberembedded polymer film observed at 25 and 120 °C. In the asprepared film, orange-colored, micrometer-sized fiber bundles were observed. When heated up to 120 °C, the film turned yellow, without significant change of the structure. However, in 13676
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
■
Article
CONCLUSION We demonstrated a reversible thermochromism of an organogel fiber-embedded polymer film. NA-DBA consisting of a CNOPV unit, naphthyl moieties, and dodecyloxy chains formed a stable organogel in EGDMA. The emission of the organogel noticeably changed from yellowish green to yellowish orange during the thermoreversible sol−gel transition. The organogel fiber-embedded polymer film was prepared by photopolymerization of the organogel, in which the gel fibers were uniformly dispersed. The film showed the reversible thermochromism, originating from the supramolecular structural change of the gel fibers embedded in the polymer matrix. The emission of the film was also reversibly changed by heating and cooling. At 120 °C, the gelator molecules in the fibers temporarily disassembled and emitted yellowish green light. After being cooled to 25 °C, the gelator molecules reassembled into gel fibers, and the emission of the film returned to the initial state. These results suggested that the interactions of the gelator molecules with the polymer matrix were similar to those with EGDMA in the gel state.
Figure 7. Optical micrographs of EGDMA film (1 wt % of NA-DBA) observed at 25 (a) and 120 °C (b). Optical micrographs of the asprepared dry gel observed at 25 (c) and 120 °C (d).
the case of the dry gel, the fibrous structure was completely destroyed at 120 °C and not restored when cooled to 25 °C. Given the fluorescence spectroscopy, DSC, and microscopy results, it was concluded that the thermally reversible selfassembly of NA-DBA occurred in the polymer matrix, resulting in reversible thermochromism. Although there was a possibility of the melted gelator molecules diffusing into the polymer matrix, we presumed that poor interactions of the gelator molecules with the polymer matrix were favorable for the reversible formation of the fibrous structure. Accordingly, the NA-DBA molecules dispersed in the polymer film without forming a supramolecular structure did not show thermochromism. We prepared a poly(methyl methacrylate) (PMMA) film containing 1 wt % of NA-DBA by spin-casting a solution of PMMA and NA-DBA in THF on a quartz plate. The PMMA film showed an emission at 513 nm similar to that of the monomeric gelator. The polymer film prepared from EGDMA containing 0.1 wt % of NA-DBA also did not display thermochromism, as the gelator molecules could not assemble to form gel fibers because of the low gelator concentration below CGC. The emissions of both the films were independent of the temperature, which was in definite contrast to that of the gel fiber-embedded film as shown in Figure 8.
■
ASSOCIATED CONTENT
S Supporting Information *
Critical gelation concentrations of the gelators, X-ray diffraction patterns of NA-DBA, absorption and emission spectra of NADBA in EGDMA, DSC thermograms, and SEM and TEM images of the EGDMA film. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Tel: +82-2-880-7190. Fax: +82-2-885-1748. E-mail: jichang@ snu.ac.kr. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was supported by grants from the Midcareer Researcher Program through NRF grant (No. 2010-0017552) funded by National Research Foundation of Korea.
■
REFERENCES
(1) Wang, X. D.; Wolfbeis, O. S.; Meier, R. J. Luminescent Probes and Sensors for Temperature. Chem. Soc. Rev. 2013, 42, 7834−7869. (2) Pucci, A.; Bizzarri, R.; Ruggeri, G. Polymer Composites with Smart Optical Properties. Soft Matter 2011, 7, 3689−3700. (3) Crenshaw, B. R.; Weder, C. Deformation-Induced Color Changes in Melt-Processed Photoluminescent Polymer Blends. Chem. Mater. 2003, 15, 4717−4724. (4) Zhao, Z.; Lamb, J. W. Y.; Tang, B. Z. Self-Assembly of Organic Luminophores with Gelation-Enhanced Emission Characteristics. Soft Matter 2013, 9, 4564−4579. (5) An, B.-K.; Gierschner, J.; Park, S. Y. π-Conjugated Cyanostilbene Derivatives: A Unique Self-Assembly Motif for Molecular Nanostructures with Enhanced Emission and Transport. Acc. Chem. Res. 2012, 45, 544−554. (6) Liu, C.; Lu, Y.; He, S.; Wang, Q.; Zhao, L.; Zeng, X. The Nature of the Styrylindolium Dye: Transformations Among its Monomer, Aggregates and Water Adducts. J. Mater. Chem. C 2013, 1, 4770−4778. (7) Bao, S.; Fei, B.; Li, J.; Yao, X.; Lu, X.; Xin, J. H. Reversible Thermochromic Switching of Fluorescent Poly(vinylidenefluoride) Composite Containing Bis(benzoxazolyl)stilbene Dye. Dyes Pigm. 2013, 99, 99−104.
Figure 8. Emission spectra of the PMMA film (1 wt % of NA-DBA) and the EGDMA films (0.1 and 1 wt % of NA-DBA) (λex = 400 nm). Photographs of the PMMA film and the EGDMA film (1 wt % of NADBA) taken under 365 nm are shown in the insets. 13677
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
Article
(8) Seeboth, A.; Lötzsch, D.; Ruhmann, R. First Example of a NonToxic Thermochromic Polymer Material-Based on a Novel Mechanism. J. Mater. Chem. C 2013, 1, 2811−2816. (9) Yoon, S.-J.; Kim, J. H.; Kim, K. S.; Chung, J. W.; Heinrich, B.; Mathevet, F.; Kim, P.; Donnio, B.; Attias, A.-J.; Kim, D.; Park, S. Y. Mesomorphic Organization and Thermochromic Luminescence of Dicyanodistyrylbenzene-Based Phasmidic Molecular Disks: Uniaxially Aligned Hexagonal Columnar Liquid Crystals at Room Temperature with Enhanced Fluorescence Emission and Semiconductivity. Adv. Funct. Mater. 2012, 22, 61−69. (10) Seredyuk, M.; Gaspar, A. B.; Ksenofontov, V.; Reiman, S.; Galyametdinov, Y.; Haase, W.; Rentschler, E.; Gütlich, P. Room Temperature Operational Thermochromic Liquid Crystals. Chem. Mater. 2006, 18, 2513−2519. (11) Sage, I. Thermochromic Liquid Crystals. Liq. Cryst. 2011, 38, 1551−1561. (12) Nakamura, K.; Kobayashi, Y.; Kanazawa, K.; Kobayashi, N. Thermoswitchable Emission and Coloration of a Composite Material Containing a Europium(III) Complex and a Fluoran Dye. J. Mater. Chem. C 2013, 1, 617−620. (13) Schäfer, C. G.; Gallei, M.; Zahn, J. T.; Engelhardt, J.; Hellmann, G. P.; Rehahn, M. Reversible Light-, Thermo-, and MechanoResponsive Elastomeric Polymer Opal Films. Chem. Mater. 2013, 25, 2309−2318. (14) Azizian, F.; Field, A. J.; Heron, B. M. Reductive Alkylation of Aminofluorans: A Simple Route to Intrinsically Thermochromic Fluorans. Dyes Pigm. 2013, 99, 432−439. (15) Srinivasan, S.; Babu, P. A.; Mahesh, S.; Ajayaghosh, A. Reversible Self-Assembly of Entrapped Fluorescent Gelators in Polymerized Styrene Gel Matrix: Erasable Thermal Imaging via Recreation of Supramolecular Architectures. J. Am. Chem. Soc. 2009, 131, 15122−15123. (16) Kunzelman, J.; Chung, C.; Mather, P. T.; Weder, C. Shape Memory Polymers with Built-in Threshold Temperature Sensors. J. Mater. Chem. 2008, 18, 1082−1086. (17) Terech, P.; Weiss, R. G. Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels. Chem. Rev. 1997, 97, 3133−3159. (18) Sangeetha, N. M.; Maitra, U. Supramolecular Gels: Functions and Uses. Chem. Soc. Rev. 2005, 34, 821−836. (19) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Organogels as Scaffolds for Excitation Energy Transfer and Light Harvesting. Chem. Soc. Rev. 2008, 37, 109−122. (20) Babu, S. S.; Prasanthkumar, S.; Ajayaghosh, A. Self-Assembled Gelators for Organic Electronics. Angew. Chem., Int. Ed. 2012, 51, 1766−1776. (21) Samanta, S. K.; Bhattacharya, S. Aggregation Induced Emission Switching and Electrical Properties of Chain Length Dependent πGels Derived from Phenylenedivinylene Bis-pyridinium Salts in Alcohol−Water Mixtures. J. Mater. Chem. 2012, 22, 25277−25287. (22) Basak, S.; Nanda, J.; Banerjee, A. Assembly of Naphthalenediimide Conjugated Peptides: Aggregation Induced Changes in Fluorescence. Chem. Commun. 2013, 49, 6891−6893. (23) Basak, S.; Nanda, J.; Banerjee, A. A New Aromatic Amino Acid Based Organogel for Oil Spill Recovery. J. Mater. Chem. 2012, 22, 11658−11664. (24) Gu, W.; Lu, L.; Chapman, G. B.; Weiss, R. G. Polymerized Gels and ‘Reverse Aerogels’ from Methyl Methacrylate or Styrene and Tetraoctadecylammonium Bromide as Gelator. Chem. Commun. 1997, 543−544. (25) Beginn, U.; Sheiko, S.; Möller, M. Self-Organization of 3,4,5Tris(octyloxy)benzamide in Solution and Embedding of the Aggregates into Methacrylate Resins. Macromol. Chem. Phys. 2000, 201, 1008−1015. (26) Zubarev, E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. Scaffolding of Polymers by Supramolecular Nanoribbons. Adv. Mater. 2002, 14, 198−203. (27) Kang, S. H.; Jung, B. M.; Chang, J. Y. Polymerization of an Organogel Formed by a Hetero-bifunctional Gelator in a Monomeric
Solvent: Preparation of Nanofibers Embedded in a Polymer Matrix. Adv. Mater. 2007, 19, 2780−2784. (28) Kang, S. H.; Jung, B. M.; Kim, W. J.; Chang, J. Y. Embedding Nanofibers in a Polymer Matrix by Polymerization of Organogels Comprising Heterobifunctional Organogelators and Monomeric Solvents. Chem. Mater. 2008, 20, 5532−5540. (29) Crenshaw, B. R.; Weder, C. Phase Separation of ExcimerForming Fluorescent Dyes and Amorphous Polymers: A Versatile Mechanism for Sensor Applications. Adv. Mater. 2005, 17, 1471−1476. (30) Kinami, M.; Crenshaw, B. R.; Weder, C. Polyesters with Built-in Threshold Temperature and Deformation Sensors. Chem. Mater. 2006, 18, 946−955. (31) Crenshaw, B. R.; Burnworth, M.; Khariwala, D.; Hiltner, A.; Mather, P. T.; Simha, R.; Weder, C. Deformation-Induced Color Changes in Mechanochromic Polyethylene Blends. Macromolecules 2007, 40, 2400−2408. (32) Kunzelman, J.; Crenshaw, B. R.; Weder, C. Self-Assembly of Chromogenic DyesA New Mechanism for Humidity Sensors. J. Mater. Chem. 2007, 17, 2989−2991. (33) Crenshaw, B. R.; Kunzelman, J.; Sing, C. E.; Ander, C.; Weder, C. Threshold Temperature Sensors with Tunable Properties. Macromol. Chem. Phys. 2007, 208, 572−580. (34) Tang, L.; Whalen, J.; Schutte, G.; Weder, C. Stimuli-Responsive Epoxy Coatings. ACS Appl. Mater. Interfaces 2009, 1, 688−696. (35) Kunzelman, J.; Crenshaw, B. R.; Kinami, M.; Weder, C. SelfAssembly and Dispersion of Chromogenic Molecules: A Versatile and General Approach for Self-Assessing Polymers. Macromol. Rapid Commun. 2006, 27, 1981−1987. (36) An, B.-K.; Gihm, S. H.; Chung, J. W.; Park, C. R.; Kwon, S.-K.; Park, S. Y. Color-Tuned Highly Fluorescent Organic Nanowires/ Nanofabrics: Easy Massive Fabrication and Molecular Structural Origin. J. Am. Chem. Soc. 2009, 131, 3950−3957. (37) Yoon, S.-J.; Kim, J. H.; Chung, J. W.; Park, S. Y. Exploring the Minimal Structure of a Wholly Aromatic Organogelator: Simply Adding Two β-Cyano Groups to Distyrylbenzene. J. Mater. Chem. 2011, 21, 18971−18973. (38) Seo, J.; Chung, J. W.; Cho, I.; Park, S. Y. Concurrent Supramolecular Gelation and Fluorescence Turn-on Triggered by Coordination of Silver Ion. Soft Matter. 2012, 8, 7617−7622. (39) Kim, S.; Yoon, S.-J.; Park, S. Y. Highly Fluorescent Chameleon Nanoparticles and Polymer Films: Multicomponent Organic Systems that Combine FRET and Photochromic Switching. J. Am. Chem. Soc. 2012, 134, 12091−12097. (40) Chung, J. W.; An, B.-K.; Park, S. Y. A Thermoreversible and Proton-Induced Gel-Sol Phase Transition with Remarkable Fluorescence Variation. Chem. Mater. 2008, 20, 6750−6755. (41) Shin, S.; Gihm, S. H.; Park, C. R.; Kim, S.; Park, S. Y. WaterSoluble Fluorinated and PEGylated Cyanostilbene Derivative: An Amphiphilic Building Block Forming Self-Assembled Organic Nanorods with Enhanced Fluorescence Emission. Chem. Mater. 2013, 25, 3288−3295. (42) Park, J. W.; Nagano, S.; Yoon, S.-J.; Dohi, T.; Seo, J.; Seki, T.; Park, S. Y. High Contrast Fluorescence Patterning in CyanostilbeneBased Crystalline Thin Films: Crystallization-Induced MassFlow Via a Photo-Triggered Phase Transition. Adv. Mater. 2014, 26, 1354−1359. (43) Kim, J. H.; An, B.-K.; Yoon, S.-J.; Park, S. K.; Kwon, J. E.; Lim, C.-K.; Park, S. Y. Highly Fluorescent and Color-Tunable Exciplex Emission from Poly(N-vinylcarbazole) Film Containing Nanostructured Supramolecular Acceptors. Adv. Funct. Mater. 2014, 24, 2746− 2753. (44) Chung, J. W.; Yoon, S.-J.; Lim, S.-J.; An, B.-K.; Park, S. Y. DualMode Switching in Highly Fluorescent Organogels: Binary Logic Gates with Optical/Thermal Inputs. Angew. Chem., Int. Ed. 2009, 48, 7030−7034. (45) Wang, B.; Wasielewski, M. R. Design and Synthesis of Metal Ion-Recognition-Induced Conjugated Polymers: An Approach to Metal Ion Sensory Materials. J. Am. Chem. Soc. 1997, 119, 12−21. (46) Wang, B.; Wang, Y.; Hua, J.; Jiang, Y.; Huang, J.; Qian, S.; Tian, H. Starburst Triarylamine Donor−Acceptor−Donor Quadrupolar 13678
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
Langmuir
Article
Derivatives Based on Cyano-Substituted Diphenylaminestyrylbenzene: Tunable Aggregation-Induced Emission Colors and Large TwoPhoton Absorption Cross Sections. Chem.Eur. J. 2011, 17, 2647− 2655.
13679
dx.doi.org/10.1021/la502932x | Langmuir 2014, 30, 13673−13679
