3906
Langmuir 2006, 22, 3906-3909
Polymer Micelles as Building Blocks for the Incorporation of Azobenzene: Enhancing the Photochromic Properties in Layer-by-Layer Films Ning Ma, Yapei Wang, Zhiqiang Wang, and Xi Zhang* Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing, 100084, China, and Key Lab for Supramolecular Structure and Materials, College of Chemistry, Jilin UniVersity, Changchun, 130012, China ReceiVed December 20, 2005. In Final Form: February 4, 2006 We described the use of block copolymer micelles as building blocks for the incorporation of water-insoluble photochromic species of azobenzene and the fabrication of multilayer films by alternating the deposition of the block copolymer micelles of poly(styrene-b-acrylic acid), incorporating azobenzene and poly(diallyl-dimethylammonium chloride). The azobenzene incorporated into the block copolymer micelles can undergo a reversible photoisomerization under the irradiation of UV and visible light sources. An interesting finding is that the photoisomerization of the azobenzene in the multilayer film is faster than it is in its normal solid film, but very similar to that in its diluted solution. Furthermore, the amount of azobenzene incorporated into the micelles can influence the photoisomerization rates in the films. Therefore, we expect that the block copolymer micelles may provide a proper microenvironment for the photoisomerization of azobenzene and the as-prepared polyelectrolyte/block copolymer micelle thin films will be useful for photoswitching materials.
Introduction Layer-by-layer (LbL) assembly has been a versatile method for fabricating multilayer thin films with tailored structure and composition.1 Until now, lots of LbL films with various compositions and properties have been developed by changing the building blocks2-9 or exploiting different driving forces10-13 for the film buildup. Recently, a combination of precursor assembly and LbL deposition was developed for assembling * Corresponding author. Fax: mail.tsinghua.edu.cn.
0086-10-62771149. E-mail:
xi@
(1) (a) Decher, G. Science 1997, 277, 1232. (b) Zhang, X.; Shen, J. C. AdV. Mater. 1999, 11, 1139. (2) (a) Zhang, X.; Gao, M. L.; Kong, X. X.; Sun, Y. P.; Shen, J. C. J. Chem. Soc., Chem. Commun. 1994, 1055. (b) Saremi, F.; Tieke, B. AdV. Mater. 1998, 10, 388. (c) Tedeschi, C.; Caruso, F.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841. (d) Advincula, R. C.; Fells, E.; Park, M. K. Chem. Mater. 2001, 13, 2870. (3) (a) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370. (b) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (4) Arys, X.; Fischer, P.; Joans, A.; Koetse, M.; Laschewsky, A.; Legras, R.; Wischerhoff, E. J. Am. Chem. Soc. 2003, 125, 1859. (5) Sun, J. Q.; Wu, T.; Sun, Y. P.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Cao, W. X. Chem. Commun. 1998, 1853. (6) (a) Quinn, J. F.; Caruso, F. Langmuir 2004, 20, 20. (b) Quinn, J. F.; Caruso, F. Macromolecules 2005, 38, 3414. (7) (a) Gao, M. Y.; Gao, M. L.; Zhang, X.; Yang, Y.; Yang, B.; Shen, J. C. J. Chem. Soc., Chem. Commun. 1994, 2777. (b) Schmitt, J.; Decher, G. AdV. Mater. 1997, 9, 61. (8) (a) Kong, W.; Zhang, X.; Gao, M. L.; Zhou, H.; Li, W.; Shen, J. C. Macromol. Rapid Commun. 1994, 15, 405. (b) Lvov, Y.; Lu, Z.; Schenkman, J. B.; Zu, X.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 4073. (c) Picart, C.; Lavalle, Ph.; Hubert, P.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J.-C. Langmuir 2001, 17, 7414. (d) Serizawa, T.; Yamaguchi, M.; Akashi, M. Macromolecules 2002, 35, 8656. (e) Yu, A.; Liang, Z.; Caruso, F. Chem. Mater. 2005, 17, 171. (9) (a) Emoto, K.; Nagasaki, Y.; Kataoka, K. Langmuir 2000, 16, 5738. (b) Katagiri, K.; Hamasaki, R.; Ariga, K.; Kikuchi, J. Langmuir 2002, 18, 6709. (c) Michel, M.; Vautier, D.; Voegel, J.-C.; Schaaf, P.; Ball, V. Langmuir 2004, 20, 4835. (d) Ma, N.; Zhang, H. Y.; Song, B.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2005, 17, 5065. (10) (a) Wang, L. Y.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Chi, L. F.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509. (b) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (11) (a) Xiong, H. M.; Cheng, M. H.; Zhou, Z.; Zhang, X.; Shen, J. C. AdV. Mater. 1998, 10, 529. (b) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 8518. (12) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385. (13) Zhang, F.; Jia, Z.; Srinivasan, M. P. Langmuir 2005, 21, 3389.
water-insoluble organic species or single charged functional building blocks that cannot be assembled by the conventional LbL method. For example, we have proposed the use of block copolymer micelles as matrixes for the incorporation of organic species and the fabrication of the LbL films by the alternative deposition of the block copolymer micelles and polyelectrolytes.9d With the expansion of research in LbL films, more and more attention has been paid to the multilayer films, which are endowed with special functions such as controlled release,9d,14 optical properties,15 surface wettabilities,16 and so on. Photoswitching molecules have been widely used as optical devices for photoactive systems.17 Because photochromic molecules usually exhibit much lower rates and efficiencies of photoisomerization in solid films compared to those in bulk solutions, many efforts have been made to improve the photochromic properties of these photoswitching molecules in their solid films by either physical or chemical methods. The physical method refers to distributing the photoswitching molecules into a polymer matrix for fabricating a composite film, whereas the chemical method refers to linking the photoswitching molecules covalently to the main chain or side chain of the polymers. In this paper, we attempted to use the block copolymer micelles to incorporate azobenzene, a simple photoswitching molecule, and then further fabricated the multilayer films by alternating the deposition of the azobenzeneincorporated micelles with polyelectrolytes. Moreover, we wondered if the photochromic properties of photoswitching molecules could be improved in their solid films within the environments provided by the block copolymer micelles. (14) (a) Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2004, 20, 9677. (b) Burke, S. E.; Barrett, C. J. Macromolecules 2004, 37, 5375. (15) (a) Lee, S.-H.; Balasubramanian, S.; Kim, D. Y.; Viswanathan, N. K.; Bian, S.; Kumar, J.; Tripathy, S. K. Macromolecules 2000, 33, 6534. (b) Ziegler, A.; Stumpe, J.; Toutianoush, A.; Tieke, B. Colloids Surf., A 2002, 198-200, 777. (16) (a) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. J. Am. Chem. Soc. 2004, 126, 3064. (b) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (17) (a) Irie, M. Chem. ReV. 2000, 100, 1685. (b) Yokoyama, Y. Chem. ReV. 2000, 100, 1717. (c) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. ReV. 2000, 100, 1741.
10.1021/la053441a CCC: $33.50 © 2006 American Chemical Society Published on Web 03/18/2006
Using Polymer Micelles in Azobenzene Incorporation
Langmuir, Vol. 22, No. 8, 2006 3907
Experimental Section Materials. The amphiphilic diblock copolymer used in this paper, poly(styrene-b-acrylic acid) (Mn: PS(12000)-PAA(1100), Mw/Mn ) 1.10; PS115-PAA15 for short) was purchased from Polymer Source Inc. and used as received. Poly(diallyl-dimethylammonium chloride), (PDDA; Mw ) 400 000) was obtained from Aldrich and used without further purification. Azobenzene was purchased from Fluka and recrystallized twice in ethanol before use. N,N-dimethylformamide (DMF), ethanol, and toluene were all analytical-grade products from Beijing Chemical Reagent Company. Quartz slides were purchased from the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences. Preparation of the Azobenzene-Incorporated Micelle Solution. The azobenzene-incorporated block copolymer micelles were prepared by Eisenberg’s method.18 The block copolymer PS115PAA15 and azobenzene were co-dissolved in DMF, and then deionic water was added dropwise. After a 20% volume (relative to DMF) of water was added, the solution was allowed to dialyze against deionic water for 72 h. The resulting micelle solution was filtered by a filter of 200 nm and diluted to suitable concentrations for LbL deposition. The amount of azobenzene incorporated into the micelles was determined by comparing the UV absorbance of the micelle solution at 320 nm with those of a series of concentration-known solutions in toluene. In this work, Micelle 1 denotes the PS-PAA micelle incorporating 7.5 wt % of azobenzene, and, for Micelle 2, the amount of azobenzene is 2.5 wt %. LbL Deposition. The LbL film was assembled on a quartz slide, which was first cleaned by treatment in a hot piranha solution (mixture of 98% H2SO4 and 30% H2O2 (V/V ) 7:3)) for 40 min (CAUTION: piranha solution is extremely corrosiVe) and then thoroughly washed with pure water. A hydroxy-tailored quartz slide was first immersed into PDDA aqueous solution. In this way, the substrate was covered with a PDDA layer. After rinsing with pure water and drying under a nitrogen stream, the resulting substrate was transferred into a solution of azobenzene-incorporated PS-b-PAA micelles to add a micelle layer. The immersion time was 10 min in each step. By repeating the above two steps in a cyclic fashion, the LbL multilayer film was fabricated. Both PS115-PAA15 and PDDA were of the same concentration of 0.2 mg/mL in aqueous solution. Instruments. UV-visible (UV-vis) spectra were obtained on a Hitachi U-3010 spectrophotometer. The irradiation light source for photoisomerization was a high-pressure mercury lamp with an optical fiber (RW-UVA æ05-100, purchased from Shenzhen Runwing Mechanical & Electrical Co., Ltd., China), and the intensity was 100 mW/cm2. The two band-pass filters were of the wavelengths 365 ( 10 and 420 ( 10 nm. The irradiation was carried out in an in situ mode by irradiating the quartz-supported LbL films or the solutions in the UV-vis spectrophotometer with the optical fiber so as to minimize the errors in the experiments. Transmission electron microscopy (TEM) was performed on a model H-800 electron microscope operating at an acceleration voltage of 100 kV. To prepare the TEM samples, a drop of the dilute aqueous solution was deposited onto a copper grid, which had been precoated with a thin film of polyvinyl formal and then coated with carbon. Two minutes after the deposition, the excess aqueous solution was blotted away with a strip of filter paper.
Results and Discussion The incorporation of azobenzene into the polymer thin films was carried out by a two-step assembling method that involved precursor assembly followed by LbL deposition. As shown in Scheme 1, the water-insoluble azobenzene was incorporated into the hydrophobic pockets of the block copolymer micelles on the basis of the well-known like-to-like principle. Since the block copolymer used for micelle formation is PS-b-PAA, it can selforganize into micelles spontaneously with negatively charged shells. So the azobenzene-incorporated micelles can function as (18) Moffitt, M.; Khougaz, K.; Eisenberg, A. Acc. Chem. Res. 1996, 29, 95.
Figure 1. UV-vis spectra of an eight-bilayer PDDA/Micelle 1 film. The insets are absorptions at 218 and 320 nm versus the number of bilayers, respectively. Scheme 1. The Schematic Representation for the PDDA/ Micelle-Azo Multilayer Film in Which Azobenzene Can Maintain Its Photochromic Properties Well, Similar to Its Diluted Solutions
negatively charged polyelectrolytes to assemble with positively charged polyelectrolytes for LbL film buildup. As usual, the fabrication of the PDDA/micelle-azo multilayer films was followed by UV-vis spectra. Figure 1 shows the UV spectra of an eight-bilayer PDDA/Micelle 1 film. The absorption bands at around 218 and 320 nm are identified as the π-π* transition of the benzene ring in the PS block and azobenzene, respectively, suggesting that azobenzene has been incorporated into the LbL films. From the insets of Figure 1 we can see that the absorbance at characteristic wavelengths increases in a nonlinear fashion versus the number of deposited bilayers. The nonlinear fashion is very similar to the so-called exponential growth that has been reported elsewhere.19 We wondered if the azobenzene could keep its reversible photoisomerization behavior after being incorporated into the micelles and then into the LbL films. To answer this question, we used UV light to irradiate the eight-bilayer PDDA/Micelle 1 film. As shown in Figure 2, the absorption at 320 nm decreased, and that at 250 nm increased simultaneously upon UV irradiation of 365 nm, which indicated that the azobenzene in the multilayer film underwent a trans-to-cis isomerization. There is only a slight change in the absorption at around 440 nm because of the smaller molar extinction coefficient of the cis isomer of azobenzene compared to that of the trans isomers. Moreover, the cis isomer of the azobenzene can be converted into its trans form upon irradiation of 420 nm. That is to say, the azobenzene incorporated into the LbL films can still retain its property of reversible photoisomerization. An interesting finding is that the photoisomerization of the azobenzene in an eight-bilayer PDDA/Micelle 1 film is faster (19) (a) Donald, L.; Elbert, C.; Herbert, B.; Hubbell, J. A. Langmuir 1999, 15, 5355. (b) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J.-C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531.
3908 Langmuir, Vol. 22, No. 8, 2006
Ma et al.
Figure 2. UV spectra of an eight-bilayer PDDA/Micelle 1 film before (solid line) and after (dashed line) excess UV irradiation.
Figure 4. The first-order plots of the cis-to-trans photoisomerization of azobenzene in toluene (a), in an eight-bilayer PDDA/Micelle 1 film (b), and in an eight-bilayer PDDA/Micelle 2 film (c).
Figure 3. The first-order plots of the trans-to-cis photoisomerization of azobenzene in toluene (a), in an eight-bilayer PDDA/Micelle 1 film (b), and in an eight-bilayer PDDA/Micelle 2 film (c).
than that in the normal solid film of the azobenzene, whereas it is very similar to that in its diluted solution. To confirm that there is a kinetic advantage of azobenzene photoisomerization in block copolymer micelles under other conditions, we used a first-order reaction to describe its photoisomerization by the following equation:21
(A0 - Aeq)
ln
(At - Aeq)
) ktt
(1)
where A0, At, and Aeq are the initial absorbance, the absorbance at time t, and the absorbance at the photostationary state of azobenzene at 320 nm, respectively, and kt is the rate constant of the trans-to-cis isomerization. For the azobenzene that was incorporated into the LbL films in this work, we used the absorbance of the films at 320 nm to calculate the rate constants of the isomerization. Figure 3a,b shows the first-order plots of azobenzene in a diluted solution of toluene and in the PDDA/ Micelle 1 films. The concentration of azobenzene in toluene is 0.01 mg/mL, which has a UV absorbance similar to that of the multilayer film in the UV-vis analysis. From the figure, we can clearly see that azobenzene incorporated into the multilayer films exhibits a linear fashion of plots, which is often found in the bulk solutions of azobenzene. Furthermore, the trans-to-cis conversion of azobenzene in the films is faster than that in toluene, suggesting that the block copolymer micelles can provide proper microen(20) (a) Rau, H. Photoisomerization of Azobenzenes. In Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1990; Vol. 2, p 119. (b) Mita, I.; Horie, K.; Hirao, K. Macromolecules 1989, 22, 558. (21) (a) Sung, C. S. P.; Gould, I. R.; Turro, N. J. Macromolecules 1984, 17, 1447. (b) Ueda, M.; Kim, H.-B.; Ikeda, T.; Ichimura, K. Chem. Mater. 1992, 4, 1229. (c) Se, K.; Kijima, M.; Fujimoto, T. Polymer 1997, 38, 5755.
vironments to maintain their photochromic property well. Much research has focused on the photoisomerization of azobenzene groups in polymer solid films, but, in most of the cases, the azobenzene groups underwent first-order kinetics only at the beginning of the UV irradiation and then began to deviate at a certain fraction of the trans-to-cis conversion.20b,21 A possible reason is that, in a given polymer matrix, there are no adequate sites where the local free volumes are greater than the critical size necessary for the photoisomerization of the azo groups around them. In addition, the azo groups were mostly tagged onto the polymer chains by covalent bonds in previous reports, so their molecular motions were greatly confined. In our work, the azobenzene groups are incorporated into the micelles without covalent bonding to the polymers, thus exhibiting photochromic behavior similar to that in diluted solutions. For comparison, we also prepared a solid film of azobenzene by dropping the solution of azobenzene in ethanol onto a quartz slide, and carried out a series of irradiation experiments as mentioned above. Not surprisingly, the photoisomerization occurred to only a small extent in the solid film of azobenzene, and the conversion rate was also much lower than that in the polyelectrolyte/micelle LbL film. We performed a similar kinetic study on the cis-to-trans photoisomerization of azobenzene in the LbL films upon irradiation using visible light at 420 nm. Before the experiments, the LbL films were irradiated by UV irradiation at 365 nm for 10 min to ensure that the cis-rich LbL films were in a photostationary state. For the cis-to-trans conversion of azobenzene, the kinetic equation needs to be changed to
(Aeq - A0)
ln
(Aeq - At)
) k ct
(2)
in which kc is the rate constant of the cis-to-trans photoisomerization.20 The relation between ln[(Aeq - A0)/(Aeq - At)] and t fits a direct proportion function, as shown in Figure 4, indicating that the micelles provide a similar environment for the photoisomerization of azobenzene in an organic solution. Moreover, on the basis of the slopes of curves a and b in Figure 4, the cis-to-trans conversion of azobenzene in the LbL films is also faster than that in diluted solutions of toluene. We also carried out experiments to examine the photoisomerization of azobenzene in micelle solutions as well as in multilayer films. It was interestingly found that there was no obvious difference between the conversion rates in the micelle solutions and those in the
Using Polymer Micelles in Azobenzene Incorporation
Langmuir, Vol. 22, No. 8, 2006 3909
Table 1. Photoisomerization Rate Constants of Azobenzene in Different Media kt (s-1) kc (s-1)
toluene
(PDDA/Mic. 1)8
(PDDA/Mic. 2)8
4.07 × 10-3 1.32 × 10-2
9.03 × 10-3 2.59 × 10-2
1.14 × 10-2 3.20 × 10-2
multilayer films, suggesting that the hydrophobic environment of the azobenzene molecules was well maintained upon the fabrication of the multilayer films. To investigate how the extent of crowding of azobenzene in the micellar cores can influence the photochromic behavior of the films, we decreased the amount of azobenzene incorporated into the micelles (Micelle 2) and examined its photoisomerization behavior under conditions similar to those used for Micelle 1. For an eight-bilayer film fabricated by the alternative deposition of Micelle 2 and PDDA, the kinetic behavior of the trans-to-cis isomerization is shown in Figure 3c; the cis-to-trans isomerization is shown in Figure 4c. The rate constants of azobenzene photoisomerization in different matrixes are summarized in Table 1. It is clearly seen that the LbL film with less azobenzene exhibited a larger conversion rate than that shown by the film with more azobenzene, no matter if it is in trans-to-cis or cisto-trans photoisomerizations. There have been some reports that in a compressed Langmuir-Blodgett film composed of surfactants containing azobenzene moieties, the photoisomerization can be weakened or inhibited due to the lack of necessary free volume for azobenzene moieties to complete a trans-to-cis or cis-totrans conversion.22 Similarly, it is easy to understand that, if the density of azobenzene is relatively increased in the block copolymer micelles, the photoisomerization will be slower. Similar results were obtained in the diluted solution of azobenzene in toluene. If we raise the concentration of azobenzene in its diluted solution, the conversion rate of the photoisomerization will be decreased because the increase in the crowding in the solutions can weaken the photoisomerizations to some extent. For example, when we increase the concentration of azobenzene from 0.001 mg/mL to 0.01 mg/mL, kt decreases from 4.07 × 10-3 s-1 to 2.77 × 10-3 s-1, and, at the same time, kc falls from 1.32 × 10-2 s-1 to 1.06 × 10-2 s-1. How does the block copolymer micelle look? Is there any morphological change in the micelles before and after they are loaded with azobenzene? To answer the questions, we employed TEM to observe the morphologies of the micelles. The image of micelles without any azobenzene incorporated is shown in Figure 5a; the micelles are spherical and around 30 nm in diameter. At the same time, as shown in Figure 5b, the azobenzeneincorporated micelles exhibit morphology and size very similar to that of the “empty” ones. This may reflect a similar association behavior in the block copolymers in their process of micellization, whether azobenzene is present or not, and the small amount of azobenzene incorporated cannot cause obvious change in the block copolymer micelles. The PDDA/micelle-azo multilayer films can undergo a reversible photoisomerization in a cyclic fashion. The cycles were reflected by monitoring the absorbance of the LbL films at 320 nm in the UV spectra. As shown in Figure 6, the absorbance at 320 nm performed a regular fluctuation and showed little (22) (a) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989, 5, 1378. (b) Menzel, H. Macromolecules 1993, 26, 6226.
Figure 5. TEM images of “empty” PS115-PAA15 micelles (a) and azobenzene-incorporated PS115-PAA15 micelles (b). The concentration of the micelle solution is 0.1 mg/mL.
Figure 6. Reversible photoisomerization of an eight-bilayer PDDA/ Micelle 1 film under alternating light source irradiations of 365 and 420 nm. In each irradiation, the irradiating time was 10 min to ensure that the azobenzene in the LbL films was in a photostationary state.
changes in both the trans-rich and the cis-rich multilayer films after several irradiating circulations. This result indicated that azobenzene in the LbL films maintained its photochromic behavior very well and had a fatigue resistance to some extent.
Conclusion In summary, we employed the block copolymer micelle as a matrix for the incorporation of azobenzene and then fabricated the multilayer film with PDDA. By this method, azobenzene was assembled into the LbL film, and its photochromic behavior in the film was examined by UV-vis irradiation. We found that the photoisomerization of azobenzene in the LbL film was a first-order reaction in both trans-to-cis and cis-to-trans conversions. This maintenance of photochromic properties may be ascribed to the proper microenvironment provided by the block copolymer micelles in the film. Furthermore, the PDDA/micelleazo multilayer film also exhibited a certain fatigue resistance. The application of this method to enhance the optical properties of photochromic species is greatly anticipated. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20334010, 20473045, 50573042, and 20574040) and the Major State Basic Research Development Program (Grant No. G2000078102). LA053441A
