pubs.acs.org/Langmuir © 2009 American Chemical Society
Facile Reversible UV-Controlled and Fast Transition from Emulsion to Gel by Using a Photoresponsive Polymer with a Malachite Green Group Yugui Jiang, Pengbo Wan, Huaping Xu, Zhiqiang Wang, and Xi Zhang* Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China
Mario Smet Department of Chemistry, University of Leuven, Celestijnenlaan 200F box 2404, B-3001, Leuven, Belgium Received March 16, 2009. Revised Manuscript Received April 12, 2009 In this paper we describe the facile reversible UV-controlled and fast transition from emulsion to gel by using a photoresponsive polymer with a malachite green group. The photoresponsive polymer with the hydrophobic malachite green group can be used for the formation of an oil-in-water emulsion. However, upon UV irradiation of 5 min, the photochromic malachite green group could be ionized to its corresponding cation, leading to the transformation from emulsion to gel. Upon shaking, such gel can recover the emulsion state, and further UV irradiation can turn the emulsion into gel again. Such transition from emulsion to gel by photochemical reaction and reverse shaking treatment can be repeated several times. It is anticipated greatly that this line of research may provide new insight into the mechanism behind stimuli-responsive systems, facilitating the design and synthesis of new responsive molecules for the fabrication of stimuli-responsive materials with designed functions.
1. Introduction The physicochemical properties of stimuli-responsive materials, such as surface wetting, solution properties, and others, can be changed depending on different external stimuli from the environment.1 Gels represent a different class of soft materials, which has diverse applications in fields such as regenerative medicine, drug delivery, biosensing, environmental remediation and others. They could be ideally suited as responsive materials, because they combine the elastic behavior of a solid with the microviscous *Corresponding author. E-mail:
[email protected]. (1) (a) Jiang, Y.; Wan, P.; Smet, M.; Wang, Z.; Zhang, X. Adv. Mater. 2008, 20, 1972. (b) Jiang, Y.; Wang, Y.; Ma, N.; Wang, Z.; Smet, M.; Zhang, X. Langmuir 2007, 23, 4029. (c) Jiang, Y.; Wang, Z.; Xu, H.; Chen, H.; Zhang, X.; Smet, M.; Dehaen, W.; Hirano, Y.; Ozaki, Y. Langmuir 2006, 22, 3715. (d) Jiang, Y.; Wang, Z.; Yu, X.; Shi, F.; Xu, H.; Zhang, X.; Smet, M.; Dehaen, W. Langmuir 2005, 21, 1986. (e) Wan, P.; Jiang, Y.; Wang, Y.; Wang, Z.; Zhang, X. Chem. Commun. 2008, 5710. (2) (a) Isobe, Y.; Sudo, A.; Endo, T. Macromolecules 2006, 39, 7783. (b) Fu, Q.; Rao, G. V. R.; Ward, T. L.; Lu, Y.; Lopez, G. P. Langmuir 2007, 23, 170. (c) Wang, C.; Zhang, D.; Zhu, D. Langmuir 2007, 23, 1478. (3) Choi, H. S.; Ooya, T.; Huh, K. M.; Yui, N. Biomacromolecules 2005, 6, 1200. (4) (a) Kawano, S.-i.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2004, 126, 8592. (b) Tsuchiya, K.; Orihara, Y.; Kondo, Y.; Yoshino, N.; Ohkubo, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2004, 126, 12282. (c) Wang, C.; Zhang, D.; Zhu, D. J. Am. Chem. Soc. 2005, 127, 16372. (5) (a) Naota, T.; Koori, H. J. Am. Chem. Soc. 2005, 127, 9324. (b) Paulusse, J. M. J.; van Beek, D. J. M.; Sijbesma, R. P. J. Am. Chem. Soc. 2007, 129, 2392. (6) (a) Ogoshi, T.; Takashima, Y.; Yamaguchi, H.; Harada, A. J. Am. Chem. Soc. 2007, 129, 4878. (b) Ghoussoub, A.; Lehn, J.-M. Chem. Commun. 2005, 5763. (c) Chen, J.; McNeil, A. J. J. Am. Chem. Soc. 2008, 130, 16496. (7) (a) Suzuki, M.; Nakajima, Y.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Langmuir 2003, 19, 8622. (b) Ahmed, S. A.; Sallenave, X.; Fages, :: :: :: F.; Mieden-Gundert, G.; Muller, W. M.; Muller, U.; Vogtle, F.; Pozzo, J.-L. Langmuir 2002, 18, 7096. (c) Lucas, L. N.; van Esch, J.; Kellogg, R. M.; Feringa, B. L. J. Chem. Soc., Chem. Commun. 2001, 759. (d) de Jong, J. J. D.; Lucas, L. N.; Kellogg, R. M.; van Esch, J. H.; Feringa, B. L. Science 2004, 304, 278. (e) Geiger, C.; Stanescu, M.; Chen, L.; Whitten, D. G. Langmuir 1999, 15, 2241. (f) Murata, K.; Aoki, M.; Nishi, T.; Ikeda, A.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1991, 1715. (g) de Loos, M.; van Esch, J.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 2001, 40, 613. (h) Moriyama, M.; Mizoshita, N.; Yokota, T.; Kishimoto, K.; Kato, T. Adv. Mater. 2003, 15, 1335. (i) Frkanec, L.; Jokic, M.; Makarevic, J.; c, M. J. Am. Chem. Soc. 2002, 124, 9716. Wolsperger, K.; Zini
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properties of fluids. Stimuli-responsive gels can be tuned by different external stimuli from the environment such as heat,2 light, pH,3 redox,4 ultrasound,5 etc.6 It is known that gels are commonly prepared by heating the gelators in an appropriate solvent until the substance dissolves, followed by cooling of the clear solution. There are very few examples of gelations that do not require a heating process;7 even stimuli-responsive gels always require a heating process. Especially, in the fields of scientific research and industrial application, there is a growing interest in the design of new targeted delivery systems containing entrapped bioactive molecules. As such molecules are often thermally sensitive, methods for the preparation of gels that avoid heating seem to be highly desirable. To overcome such limitation, a possible solution is the development of a novel way to fabricate in situ and fast stimuli-induced reversible gelation without heating treatments, instead of the normal “thermal-set” gel. Photocontrol as an attractive alternative provides a very broad range of tunable parameters, e.g., wavelength, duration, and intensity, which can be modulated in the potential applications. Without need of additional substances, light is one of the most desirable stimuli for clean and rapid control of solution properties and others with specific direction and position. Without the heating treatments, the reversible transition from gel to solution can be easily realized under UV irradiation,7b,8 while the reversible transition of UV-responsive gels from solution to gel is very important but limitedly reported.9 Irie et al. have reported that the gel-sol transition of a polystyrene-carbon disulfide gel having 10.5 mol % azobenzene groups could be induced isothermally (8) (a) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komri, T.; Olrseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664. (b) Koumura, N.; Kudo, M.; Tamaoki, N. Langmuir 2004, 20, 9897. (c) Tomatsu, I.; Hashidzume, A.; Harada, A. Macromolecules 2005, 38, 5223. (d) Kume, S.; Kuroiwa, K.; Kimizuka, N. Chem. Commun. 2006, 2442. (e) Yagai, S.; Iwashima, T.; Kishikawa, K.; Nakahara, S.; Karatsu, T.; Kitamura, A. Chem.;Eur. J. 2006, 12, 3984. (9) Irie, M.; Iga, R. Macromolecules 1986, 19, 2480.
Published on Web 05/01/2009
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Figure 1. The synthetic route to the malachite green monomer and the polymerization of malachite green monomer and acrylamide (poly-MG-AAm).
at -52 °C at high polymer concentration in CS2, i.e., UV irradiation converted the sol to the gel state, whereas visible irradiation induced the transition from the gel to the sol state.9 The realization and combination of these two kinds of transition behavior, i.e., UV-induced transition from gel to solution, and transition from solution to gel, may be important in complex controlled delivery systems, which usually require integrating more complex functionalities in many potential applications. In addition, for most UV-responsive gels, long irradiation times, the requirement of additional species, and irreversible response10 are the shortcomings for many research and industrial applications. In this paper we intend to realize the facile reversible UVcontrolled transition from emulsion to gel using a water-soluble copolymer with a malachite green group (poly-MG-AAm) at low concentration, through combination of a simple emulsion process and short-time UV irradiation.
2. Experimental Section Materials. 4-Vinylbenzoic acid and 4-bromo-N,N-dimethylaniline were purchased from Alfa Aesar Co. Acrylamide was purchased from Beijing Chemical Reagent Co. (China) and recrystallized in chloroform before use. n-Butyl lithium solution in hexane was purchased from Chemetall Chemicals Co. All Other chemicals were analytical-grade reagents and used as received. All aqueous solutions were prepared with deionized water. The synthesis and copolymerization of the vinyl monomer of malachite green leucohydroxide are shown in Figure 1. The content of malachite green group in the polymer is estimated to be about 5.1 per molecule (0.7 mol %), which is calculated from the absorption spectrum using the absorption coefficient of 6.7 104 dm 3 mol-1 3 cm-1 for a malachite green functionality in aqueous solution at pH 4.00 at a maximum absorption wavelength of around 620 nm.11 Synthesis of the Vinyl Monomer of Malachite Green Leucohydroxide (4). 4-Vinylbenzoic acid (1) (1.5 g, 10.1 mmol) was dissolved in 40 mL methanol in a 100 mL round-ground flask, then 3 mL H2SO4 was added under stirring. The mixture was stirred for 24 h in an oil bath at about 80 °C and kept in an inert Ar atmosphere. After the solvent was evaporated under vacuum, a NaHCO3 solution was added, and then dichloromethane was added to extract the residue (3 50 mL). The organic layer was collected and dried by anhydrous sodium sulfate, and evaporated in vacuum. The compound methyl 4-vinylbenzoate (2) was obtained after column chromatography (SiO2, CH2Cl2/petroleum ether v:v 4:1) as white solid (1.3 g, 79%). 1H NMR (CDCl3): 7.99 (d, 2H, m-H of PhCdC), 7.45 (d, 2H, o-H of PhCdC), c, M. Langmuir 2005, 21, 2754. (10) Miljanic, S.; Frkanec, L.; Meic, Z.; Zini (11) Nakayama, Y.; Matsuda, T. J. Controlled Release 2003, 89, 213.
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6.74 (q, 1H, PhCHdC), 5.85 (d, 1H, cis-H of PhCHdCH2), 5.37 (d, 1H, trans-H of PhCHdCH2), 3.90 (s, 3H, COOCH3). 4-Bromo-N,N-dimethylaniline (5.4 g, 27.0 mmol) was dissolved in 50 mL anhydrous tetrahydrofuran (THF) and the solution was kept at -78 °C in a mixture bath of liquid nitrogen and ethanol under an argon atmosphere. A hexane solution of n-butyl lithium (n-BuLi) (13.0 mL, 32.5 mmol) was injected gradually into the THF solution with stirring. To the mixture was added dropwise THF solution of methyl 4-vinylbenzoate (2) (2.2 g, 13.6 mmol). The reaction mixture was allowed to warm slowly to room temperature and then stirred overnight. After the reaction, THF was evaporated off under vacuum and water was added to the residue. The aqueous phase was then neutralized by the addition of 0.1 mol 3 dm-3 hydrochloric acid. Extraction with dichloromethane three times, dried by anhydrous sodium sulfate, followed by vacuum evaporation of the solvent, afforded a dark-green oily product. Precipitation of the crude product was carried out sufficiently from methanol to 150 mL petroleum ether to yield a pale-green solid of vinyl monomer of malachite green leucohydroxide (4) (3.0 g, 60%). The precipitate was dried under vacuum and stored in a dark desiccator. 1H NMR (DMSO-d6): 7.35 (d, 2H, m-H of PhCdC), 7.17 (d, 2H, o-H of PhCdC), 6.97 (d, 4H, o-H of NPh), 6.70 (dd, 1H, PhCHdC), 6.62 (d, 4H, m-H of NPh), 5.77 (d, 1H, cis-H of PhCHdCH2), 5.21 (d, 1H, trans-H of PhCHdCH2), 2.85 (s, 12H, -NCH3). MS ESI m/z = 355.37 found for C25H27N2+, calcd 355.22.
Copolymerization of the Vinyl Monomer of Malachite Green Leucohydroxide and Acrylamide (poly-MG-AAm). The radical copolymerization of the vinyl monomer of malachite green leucohydroxide (4) (36.1 mg, 0.097 mmol) with acrylamide (510.5 mg, 7.18 mmol) was carried out in DMSO at 60 °C for 3 days, 2,2’-azobis(isobutyronitrile) (AIBN) was used as the initiator (7.4 mg).11 After the polymerization, the crude polymer was precipitated by addition of a large amount of methanol, and was separated from the solution by filtration. Reprecipitation was carried out from aqueous solution to methanol three times to remove nonreacted monomers and initiators completely. The last precipitate was dried under vacuum and stored in the dark. 1H NMR (DMSO-d6): 7.23 (Ph-H), 6.88 (Ph-H), 2.11 (-CH-), 1.51 (-CH2-). Gel permeation chromatography (GPC) data (water): Mn = 51 000, Mw = 110 000. Preparation of the Emulsion and UV Irradiation. We first used poly-MG-AAm aqueous solution to form an oil-in-water emulsion of water and different solvents by shaking for 3 min. The emulsion type was determined by observing the dilution of the emulsion in both oil and water. A drop of as-prepared oilin-water emulsion is immediately dispersed in water, while not dispersed in oil (e.g., cyclohexane, etc.). Then after 5 min UV irradiation, the as-prepared emulsion turned into a gel.
Preparation of the Scanning Electron Microscopy (SEM) Sample. The sample was prepared on clean Si substrates. The Si DOI: 10.1021/la900916m
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substrates were cleaned by immersing them in piranha solution (98% sulfuric acid/30 wt % H2O2, v:v = 70:30) for 30 min. Caution: piranha solution is highly corrosive and reacts violently with organic materials. It should be handled with great care. Upon removal, these cleaned Si slides were rinsed with large amounts of deionized water and sonicated in water many times, and then dried in a stream of high-purity nitrogen before the preparations of the SEM samples. The sample in tube was placed in an ice water bath during the UV irradiation. The ice water bath is used to minimize the heat effect during the UV irradiation, while the distance between the sample and the UV irradiator is about 3-4 cm. To prepare the SEM samples, the emulsion before or after UV irradiation, was deposited onto a Si substrate, placed in a refrigerator for 30 min and freeze-dried in the freeze-drying machine at -50 °C. General Techniques. NMR spectra were recorded at room temperature on a JEOL JNM-ECA300 spectrometer with tetramethylsilane as internal standard. Mass spectra spectra were recorded on a PE Sciex API 3000 apparatus. The molecular weight was determined by a Wyatt DAWN EOS multiangle light scattering spectrometer. UV-visible (UV-vis) spectra were performed on a Hitachi U-3010 spectrophotometer. The UV irradiation light source was a high-pressure mercury lamp with an optical fiber (purchased from Shenzhen Runwing Mechanical & Electrical Co., Ltd., China), and the intensity was 1500 mW/cm2. The SEM images are recorded on a JEOL JSM-7401F field emission scanning electron microscope at 1.0 kV. All the pictures are taken by a Nikon D80 digital camera.
3. Results and Discussion The UV-responsive water-soluble copolymer of the malachite green derivative and acrylamide (poly-MG-AAm), as shown in Figure 2, was successfully synthesized for facile reversible UV-controlled transition from emulsion to gel. It is well-known that the malachite green group as an electrically neutral form behaves as a hydrophobic group when no stimulus of UV is employed; however, upon a stimulus of UV irradiation, the neutral malachite green can be ionized into its corresponding delocalized triphenylmethyl cation and a hydroxide anion that is released to the aqueous phase.1a,1b Therefore, poly-MG-AAm is a suitable candidate for fine-tuning of the solution properties. The synthetical routes to the malachite green monomer and the copolymerization of the malachite green monomer are shown in the experimental section. We have done the copolymerization with different malachite green monomer ratios; finally we chose the UV-responsive water-soluble poly-MG-AAm with a malachite green content of about 5.1 per molecule (0.7 mol %) in order to realize the reversible UV-controlled transition from emulsion to gel. To exclude the influence of pH on malachite green, the
sample solutions were prepared in a pH 9.18 buffer solution of sodium tetraborate if not specifically indicated. The photoinduced transformation of the aqueous solution of poly-MG-AAm was evidenced by UV-vis spectroscopy. The typical absorption-spectral change of the poly-MG-AAm in a buffer solution at pH 9.18 before and after UV irradiation is shown in Figure 3a,b, respectively. The UV irradiation time for the different curves in Figure 3a was 0, 1, 10, 40, and 100 s, respectively. Before UV irradiation, the absorption peak at around 259 nm, which is assigned to the electrically neutral form of the malachite green group, can be clearly seen. Upon UV irradiation, this absorption band decreased rapidly; concomitantly, a new absorption band appeared at around 620 nm, indicating the formation of the expected triphenylmethyl cation. With increased UV irradiation time, the color of the poly-MG-AAm solution changed from near-colorless to green. As triphenylmethyl cations of poly-MG-AAm can thermally recover its electrically neutral form, we also employed UV-vis spectroscopy to monitor the reverse reaction of the 100 s UV irradiated poly-MG-AAm buffer solution. Figure 3b shows a typical absorption-spectra change of the poly-MG-AAm solution after 100 s UV irradiation, as a function of time after removal of the UV irradiation. The time elapsed between the different recordings in Figure 3b was 6, 51, 103, 166, 219, and 268 min after removal of the UV irradiation, respectively. We found that, after removal of the UV irradiation, the absorption band at around 620 nm, which is assigned to the positively charged form of poly-MG-AAm, decreased gradually, as shown in Figure 3b, indicating that the cationic form returns to
Figure 2. Schematic illustration of reversible photochemical reaction.
Figure 3. (a) Absorption-spectral changes of poly-MG-AAm buffer solution at pH 9.18 before UV irradiation and after stepwise total 100 s UV irradiation at room temperature of 22 °C. The concentration of poly-MG-AAm aqueous solution is 0.13 mg/mL. (b) Absorption-spectral changes of such 100 s UV irradiated poly-MG-AAm buffer solution at pH 9.18 at room temperature of 22 °C, with different time after removal of the UV irradiation (from top to bottom: 0, 6, 51, 103, 166, 219, 268 min). The concentration of poly-MG-AAm aqueous solution is 0.13 mg/mL. 10136 DOI: 10.1021/la900916m
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the electrically neutral form. The recovery of such poly-MG-AAm aqueous solution takes several hours at ambient temperature, as shown in Figure S1 (Supporting Information). In order to achieve the successful transformation from emulsion to gel, we should first find out the suitable concentration of poly-MG-AAm used in the emulsion preparation. In this paper, the apolar solvent selected is cyclohexane, if not specifically indicated. The emulsion we used is an oil-in-water type emulsion, which is determined by observing the dilution of the emulsion in both cyclohexane and water. We investigated the concentration effect of the poly-MG-AAm aqueous solution on the transition from emulsion to gel at poly-MG-AAm concentrations of 0.08, 0.16, 0.33, 0.65, 1.30, 2.58, and 7.75 mg/mL. We used 0.5 mL of poly-MG-AAm aqueous solution and 2 mL of cyclohexane to prepare the emulsion at poly-MG-AAm concentrations of 0.65, 1.30, 2.58, and 7.75 mg/mL; further UV irradiation can turn the emulsion into gel; at the poly-MG-AAm concentration of 0.16 and 0.33 mg/mL, 2 mL cyclohexane cannot be emulsified completely under the same condition, but the transformation from emulsion to gel can still be obtained; at poly-MG-AAm concentration of 0.08 mg/mL, no successful transformation from emulsion to gel can be obtained. Therefore, we chose the poly-MGAAm concentration of 1.3 mg/mL for further experiments. To get the clear transformation from emulsion to gel, we should determine the suitable ratio of aqueous solution to cyclohexane to prepare the emulsion of poly-MG-AAm. We tried different volume ratios of aqueous poly-MG-AAm solution to cyclohexane, e.g., v:v = 3:20, 5:20, 10:20, and 20:20 at the poly-MG-AAm concentration of 1.3 mg/mL. It is found that, at the ratio of 3:20, the mixture can form a partial oil-in-water emulsion with excess cyclohexane, while, at ratios of 10:20 and 20:20, the oil-in-water emulsions are formed with excess water. Therefore, the ratio of aqueous solution to cyclohexane, v:v = 5:20, was further used to prepare the oil-in-water emulsion. Transformation from emulsion to gel can be successfully realized under UV irradiation. As mentioned above, we first used the poly-MG-AAm aqueous solution to form an oil-in-water emulsion of water and cyclohexane (v:v = 5:20) by shaking for 3 min; then, after 5 min UV irradiation, the as-prepared emulsion turned into a gel, which is shown by reversing the tube as depicted in Figure 4. For the ratio of 10:20, and 20:20, the emulsion can also be transformed into gel by the 5 min UV irradiation. The solvents shown in Table 1 can be used for the UV-controlled transition from emulsion to gel in a similar way. In order to exclude the heat
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effect during the UV irradiation, we have done a control experiment by placing the emulsion sample of cyclohexane in an ice water bath during the UV irradiation, and found that the transition from emulsion to gel is similar to that obtained by placing the emulsion sample in air. We also placed the emulsion sample in a 60 °C water bath, and found that this treatment cannot induce the transition from emulsion to gel itself. We believe the transition from emulsion to gel should be due to the UV-responsive behavior of the copolymer of malachite green, poly-MG-AAm. It is anticipated that the reversible switch between emulsion and gel is due to the conformation variation of the copolymer upon UV irradiation. Before UV irradiation, the electrically neutral malachite green group behaves only as a conventional nonionic hydrophobic group. After UV irradiation, the photochromic moiety of malachite green is ionized to its corresponding intensely colored delocalized triphenylmethyl cation, and a hydroxide anion Table 1. The Solvent Used for UV-Controlled Emulsion-Gel Transition no.
solvent
W:Oa
no.
solvent
W:Oa
1 2 3 4 5
cyclohexane 5:20 6 n-octane 5:20 n-dodecane 5:20 7 petroleum ether 5:20 n-hexane 5:20 8 benzene 6:20 n-heptane 5:20 9 toluene 5:20 5:20 10 xylene 6:20 n-pentaneb a W:O is defined as the volume ratio of aqueous poly-MG-AAm solution to different solvent in our experiments (v: v). b Because of the low boiling point of n-pentane at about 36 °C, the emulsion was placed in ice water bath to minimize the effect of UV irradiation on the evaporation of n-pentane.
Figure 5. Schematic illustration of a possible mechanism behind the UV-responsive transition from emulsion to gel.
Figure 4. The photograph of reversible solution property switches of emulsion of cyclohexane before UV irradiation and after UV irradiation. The concentration of poly-MG-AAm buffer solution at pH 9.18 is 1.3 mg/mL. Langmuir 2009, 25(17), 10134–10138
Figure 6. Reversible solution property switches of emulsion of cyclohexane before UV irradiation and after UV irradiation, as a function of treatment times in several cycles of UV irradiation and shaking. The UV irradiation period is 300 s every time, and the concentration of poly-MG-AAm buffer solution at pH 9.18 is 1.3 mg/mL. DOI: 10.1021/la900916m
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Figure 7. SEM images of sample of emulsion before UV irradiation (a) and after UV irradiation (b) on Si substrate. The UV irradiation period is 300 s, and the concentration of poly-MG-AAm buffer solution at pH 9.18 is 1.3 mg/mL.
is released to the aqueous phase, which leads to the change of the conformation of the copolymer. The very rapid photoionization with a high quantum efficiency, which produces the photoinduced positive charge of the poly-MG-AAm by UV irradiation and brings about remarkable environmental changes from an electrically neutral to an ionic state, can be used as the driving force for photocontrol of physical properties (Figure 5). From the above-mentioned UV-vis data, it was anticipated that, after removal of UV stimulus, the positively charged form can recover to the electrically neutral form gradually. Indeed, the reversibility of the UV-responsive behavior of poly-MG-AAm is clearly demonstrated, e.g., the color of the emulsion can be changed from green to colorless after removal of UV irradiation and keeping in the dark for several hours, but the formed gel can remain in most cases. Without any treatment, such gel can remain in the reverse tube for weeks. To realize the quick switching of gel to emulsion, we shaked the tube. Upon shaking, the gel can recover the fluid state, and further UV irradiation can turn the emulsion into gel, again. The shaking can destroy the network, and change the gel into emulsion. Such transition from emulsion to gel by photochemical reaction and reverse shake treatment of poly-MG-AAm can be repeated several times, as shown in Figure 6, which can increase the usefulness of such UV-responsive system in potential applications. In order to confirm the reversible UV-controlled transition from emulsion to gel, we employed SEM to observe the morphology of the emulsion before and after UV irradiation. To prepare the SEM samples, the oil-in-water emulsion before or after UV irradiation was deposited onto a Si substrate and then freezedried. As indicated by the SEM images in Figure 7, the morphology of the emulsion after UV irradiation is different from the one before UV irradiation. Some interconnected contorted structure can be seen clearly in the SEM images for the emulsion after the UV irradiation, which could be responsible to the UV-induced network in such oil-in-water emulsions. Since the malachite green is also a pH-responsive molecule, we did the above-mentioned experiments employing a pH 9.18 buffer solution, to exclude the pH responsive effect. Furthermore, we investigated the pH effect of the poly-MG-AAm aqueous solution on the transition from emulsion to gel, in pH 0, 4.00, 6.86, 9.18,
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and 14 solutions. Because of the pH responsive properties, the asprepared emulsion with pH 6.86 buffered poly-MG-AAm solutions shows a light green color before UV irradiation, in contrast to the near-colorless one with pH 9.18 buffered poly-MG-AAm solution, but still can be changed into gel upon UV irradiation. For the pH 4.00, in most cases, we cannot obtain similar results, and in our experiments no stable emulsion can be formed at pH 0, as it is reported that malachite green group can be turned into a colorless species at low pH values of 0-2 as a kind of traditional pH indicator. When the pH value of solution is as high as 14, we found that the malachite green is colorless, and cannot show the UV-responsiveness because the UV-induced malachite green cation readily reacts with the OH- anion immediately to recover the neutral form of malachite green.
4. Conclusions In conclusion, we have demonstrated the preparation and characterization of a malachite green derived copolymer (polyMG-AAm). By using short-time UV irradiation of its emulsion at low poly-MG-AAm concentration, we can realize the quick transition of solution properties from emulsion to gel, without heating process. This could be especially interesting and fundamentally important for many material and biomaterial applications. The great attention to such responsive materials may be helpful for acquiring better knowledge to enable new insights into such systems, leading to broad potential applications in controlled on-off switching of solution properties. It is anticipated that this research may open a route to the design of new stimuliresponsive materials. Acknowledgment. The authors thank the National Basic Research Program of China (2007CB808000), the National Natural Science Foundation of China (20473045, and 20334010), and a bilateral grant of the Flemish government (BIL07/04) for financial support. Supporting Information Available: Further details of the characterization of poly-MG-AAm. This material is available free of charge via the Internet at http://pubs.acs.org.
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