© Copyright 1999 American Chemical Society
MARCH 30, 1999 VOLUME 15, NUMBER 7
Letters Direct Observation of Photoisomerization of a Polymer Monolayer on a Water Surface by X-ray Reflectometry Keitaro Kago,† Maren Fu¨rst,†,§ Hideki Matsuoka,† Hitoshi Yamaoka,*,† and Takahiro Seki‡ Department of Polymer Chemistry, Kyoto University, Kyoto 606-8501, Japan, and Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Received August 24, 1998. In Final Form: January 25, 1999 X-ray reflectivity (XR) measurements were carried out for monolayers of poly(vinyl alcohol) containing a photochromic azobenzene side chain (6Az10-PVA) on a water surface. Kiessig fringes were observed for specular measurement for 6Az10-PVA monolayers on a water surface. By the irradiation of visible and ultraviolet light, azobenzene shows a photoisomeric conformational change between trans and cis forms, respectively. The structural change of a 6Az10-PVA monolayer on a water surface was detected by in-situ XR measurement. By analyzing the XR data, it was indicated that the thickness of the monolayer became thicker for the trans form than for the cis form. This reflects that the side chain should stretch to the direction perpendicular to the water surface with the conformational change from cis to trans form, in addition to the longer chain length for the trans form.
Introduction The polymer shown in Figure 1 (6Az10-PVA) is an interesting amphiphilic molecule, which forms a photosensitive monolayer on a water surface. 6Az10-PVA consists of a hydrophilic polymer backbone of poly(vinyl alcohol) and a hydrophobic side chain which contains photosensitive azobenzene. The azobenzene shows a photoisomeric conformational change between trans and cis forms by irradiation of vis (436 nm) and UV (365 nm) light, respectively. By the conformational change from trans to cis form, the area per side chain increases and the structure of the monolayer changes accordingly. Thus, this polymer forms a photosensitive monolayer. Since this photoisomerization is reversible, this monolayer should * To whom correspondence should be addressed. † Kyoto University. ‡ Tokyo Institute of Technology. § On leave from Johannes Gutenberg Universita ¨ t Mainz, Germany.
Figure 1. Chemical structure of 6Az10-PVA. The sample of n ) 10 and x ) 0.18 was used in this study.
repeat contraction and expansion by periodical irradiation of vis and UV light, which can be applied with a photomechanical device. Seki et al. have extensively studied 6Az10-PVA as a photosensitive polymer mono-
10.1021/la981084g CCC: $18.00 © 1999 American Chemical Society Published on Web 03/06/1999
2238 Langmuir, Vol. 15, No. 7, 1999
Letters
layer.1-3 Although its functionality has been clearly confirmed by π-A measurements and its surface structure observed with a Brewster angle microscope,4 direct information on the in-situ conformation of this polymer could not be obtained. The structure of the monolayer after deposition on a substrate could be observed by atomic force microscopy,5 but it should be different from the real structure of a monolayer on a water surface. Not only this polymer monolayer but also, in most cases, the structure of a monolayer on a water surface has been just an assumption from the π-A isotherm and surface potential as a function of the area per molecule. X-ray and neutron reflectivity techniques (XR, NR) are a relatively new but quite powerful tool to study surface and interface structures.6,7 Their large advantage is that the nanostructure of the surface and the interface can be directly determined. Hence, recently, extensive XR and NR works on solid films,8-12 liquid-liquid interfaces,13 and monolayers of lipids and polymers, and two-dimensional micelles on water14-20 have been done. Recently, Yamaoka et al. constructed an X-ray reflectivity (XR) apparatus which can be applied to a liquid surface for laboratory use.21-25 Since this XR apparatus has a Langmuir-Blodgett (LB) trough at the sample position, a direct analysis of the in-situ structure of the monolayer on a water surface has become possible. Although the XR apparatus is made by using a conventional X-ray generator, not using a synchrotron source, it can measure the reflectivity down to 10-8 with a very high resolution due to a high power generator and high-performance collimation system. (1) Seki, T. Supramol. Sci. 1996, 3, 25. (2) Seki, T.; Sekizawa, H.; Fukuda, R.; Tamaki, T.; Yokoi, M.; Ichimura, K. Polym. J. 1996, 28, 613. (3) Seki, T.; Fukuda, R.; Yokoi, M.; Tamaki, T.; Ichimura, K. Bull. Chem. Soc. Jpn. 1996, 69, 2375. (4) Seki, T.; Sekizawa, H.; Morino, S.; Ichimura, K. J. Phys. Chem. B 1998, 102, 5313. (5) Seki, T.; Tanaka, K.; Ichimura, K. Macromolecules 1997, 30, 6401. (6) Russell, T. P. Mater. Sci. Rep. 1990, 5, 171. (7) Zhou, X. L.; Chen, S. H. Phys. Rep. 1995, 257, 223. (8) Orts, W. J.; van Zaten, J. H.; Wu, W.; Satija, S. K. Phys. Rev. Lett. 1993, 71, 867. (9) Wallace, W. E.; van Zaten, H. H.; Wu, W. Phys. Rev. E 1995, 52, R3329. (10) Anastasiadis, S. H.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F. J. Chem. Phys. 1990, 92, 5677. (11) Karim, A.; Satija, S. K.; Douglas, J. F.; Ankner, J. F.; Fetters, L. J. Phys. Rev. Lett. 1994, 73, 3407. (12) Torikai, N.; Matsushita, Y.; Noda, I.; Karim, A.; Satija, S. K.; Han, C. C. Physica B 1995, 213/214, 694. (13) McClain, B. R.; Lee, D. D.; Carvalho, B. L.; Mochrie, S. G. J.; Chen, S. H.; Litster, J. D. Phys. Rev. Lett. 1994, 72, 246. (14) Lee, E. M.; Simister, E. A.; Thomas, R. K. Mol. Cryst. Liq. Cryst. 1990, 179, 151. (15) Vaknin, D.; Kjær, K.; Ringsdorf, H.; Blankenburg, R.; Piepenstock, M.; Diederich, A.; Loesche, M. Langmuir 1993, 9, 1171. (16) Li, Z.; Zhao, W.; Quinn, J.; Rafailovich, M. H.; Slkolov, J.; Lennox, R. B.; Eisenberg, A.; Wu, X. Z.; Kim, M. W.; Sinha, S. K.; Tolan, M. Langmuir 1995, 11, 4785. (17) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381. (18) Bosio, L.; Benattar, J. J.; Rieutord, F. Rev. Phys. Appl. 1987, 22, 775. (19) Als-Nielsin, J.; Pershan, P. S. Nucl. Instrum. Methods 1983, 208, 545. (20) Weiss, A. H.; Deutsch, M.; Braslau, A.; Ocko, B. M.; Pershan, P. S. Rev. Sci. Instrum. 1986, 57, 2554. (21) Matsuoka, H.; Yamaoka, H. Proc. Risø Int. Symp. Mater. Sci. 1997, 18, 437. (22) Kago, K.; Matsuoka, H.; Endo, H.; Eckelt, J.; Yamaoka, H. Supramol. Sci. 1998, 5, 349. (23) Yamaoka, H.; Matsuoka, H.; Kago, K.; Eckelt, J.; Yoshitome, R. Chem. Phys. Lett. 1998, 295, 24. (24) Yamaoka, H.; Matsuoka, H.; Kago, K.; Endo, H.; Eckelt, J. Physica B 1998, 248, 280. (25) Kago, K.; Matsuoka, H.; Yamaoka, H.; Ijiro, K.; Shimomura, M. Submitted.
Figure 2. (a) XR curves of a 6Az10-PVA monolayer on the water surface at 4 mN/m for trans and cis forms. The curve for the cis form is shifted downward by one decade to avoid superposition. (b) Rq4 versus q plot of a 6Az10-PVA monolayer on the water surface at 4 mN/m for trans and cis forms. Table 1. Structural Parameters for a 6Az10-PVA Monolayer on a Water Surface in trans and cis Forms Estimated by X-ray Reflectivity Measurementsa surface pressure (mN/m) 4
11
δ β thickness roughness (×10-6) (×10-6) (Å) (Å) trans 6Az10 PVA subphase cis 6Az10 PVA subphase trans 6Az10 PVA subphase cis 6Az10 PVA subphase
3.70 4.10 3.57 3.49 3.80 3.57 3.80 4.15 3.57 3.75 4.15 3.57
0.0057 0.0073 0.0114 0.0051 0.0068 0.0114 0.0066 0.0074 0.0114 0.0066 0.0074 0.0114
27.5 4.0 24.0 2.1 29.3 3.6 27.6 2.8
3.0 0.7 1.7 2.8 0.1 3.5 2.8 2.9 1.0 2.4 3.0 1.8
a δ and β are the difference from unity of the real part and the coefficient of the imaginary part of the refractive index n, that is, n ) 1- δ -iβ. The δ value is almost proportional to electron density.
In this study, we performed a direct and highly quantitative analysis of the structural changes induced
Letters
Langmuir, Vol. 15, No. 7, 1999 2239 The XR measurements were performed in angular steps of 0.01, 0.02, and 0.02° and at accumulation times of 2, 15, and 20 s for a reflection angle (θ) of 0-0.5, 0.5-2.0, and 2.0-3.0°, degrees, respectively.
Results and Discussions
Figure 3. (a) XR curves of a 6Az10-PVA monolayer on the water surface at 11 mN/m for trans and cis forms. The curve for the cis form is shifted downward by one decade to avoid superposition. (b) Rq4 versus q plot of a 6Az10-PVA monolayer on the water surface at 11 mN/m for trans and cis forms.
by photoisomerization of a polymer monolayer of 6Az10PVA on a water surface by utilizing our XR instrument. Experimental Section Samples. The synthesis and characterization of the 6Az10PVA molecule have been described elsewhere.1-3 We used a sample of n ) 10 and x ) 0.18 (see Figure 1). For vis and UV irradiation, a 100 W Mercury lamp equipped with a wavelength selective optical filter was used. The time for irradiation required for complete photoisomerization in the system (in which the cis content was ca. 90%3) was checked by UV-vis absorption spectroscopy. Absorption peaks at 350 and 440 nm are characteristic for trans and cis forms, respectively. By utilizing the peaks, the cis form was confirmed to be stable for 1-2 h. For the trans form measurement, after checking the trans form of all molecules by spectroscopy, the chloroform solution dissolving the polymer was spread on the water surface in the trough and the surface pressure was controlled. For the cis form measurement, the chloroform solution was spread on the water surface in the trough after the required irradiation of UV light. XR Apparatus. The details of the XR apparatus have been fully described elsewhere.21-24 With this apparatus, we succeeded in detecting a change in thickness of a lipid monolayer on water as small as 2 Å.21-24
Figure 2 a shows the XR profile (reflectivity (R) as a function of θ or the scattering vector q ()(4π/λ) sin θ)) for trans and cis form monolayers of 6Az10-PVA at a surface pressure of 4mN/m (a rather low pressure). Although broad, the Kiessig fringe was clearly observed at the q of about 0.25 Å-1. The monolayer was certainly detected by our XR instrument. A small but significant difference between the trans and cis forms was observed in the XR profiles for the q range from 0.3 to 0.4 Å-1. (In Figure 2b, Rq4 is plotted as a function of q.26 The slight difference of the Kiessig fringe is clearly shown.) The solid lines in Figure 2a are the best fit curves by model fitting, which gives us the thickness of each layer, the roughness of each interface, and the optical parameters for each layer as shown in Table 1. Since the trans form has a smaller area per molecule than the cis form, the same surface pressure means denser packing of molecules for the trans form. Thus, the thickness of the hydrophobic part (6Az10) is larger for the trans form (27.5 Å) than for the cis form (24.0 Å). For the hydrophilic part (PVA) under water, a similar trend is observed (4.0 and 2.1 Å). This change in thickness was only about 2-3 Å, but the difference was obvious in the XR profile in Figure 2a and b, as was the case for phospholipid monolayers.21-24 Figure 3a shows a similar comparison at 11 mN/m. The difference in XR profiles between trans and cis forms is more obvious at this higher pressure. By plotting Rq4 as a function of q, the characteristic of the Kiessig fringe is pronounced, as shown in Figure 3b. The structural parameters obtained by model fitting are also shown in Table 1. The trend in structural change for trans-cis isomerization is the same as that observed at 4 mN/m. However, by comparison of cis form data for 4 and 11 mN/m, the thicknesses of the hydrophobic and hydrophilic parts are larger for higher pressure: 24.0 and 27.6 Å for the 6Az10 side chain layer, and 2.1 and 2.8 Å for the PVA layer for 4 and 11 mN/m, respectively. Since the packing should be denser at a higher pressure, the molecules should stretch in a direction perpendicular to the water surface. The thickness change in the 6Az10 side chain layer is about 3.5 and 1.7 Å at surface pressures of 4 and 11 mN/ m, respectively. These values are consistent with the change in length of the azobenzene group. Hence, the change in this part is mainly caused by a trans-cis conformational change of the azobenzene group. The change at a higher surface pressure is smaller because the denser packing of the molecule in the layer restricts the conformational change. Figure 4 shows a schematic representation of the conformational change of the 6Az10PVA monolayer on the water surface detected in the present XR study. Although the cis form of azobenzene has been believed to be in contact with the water surface at lower surface pressure, but not for higher surface pressure, XR did not show a clear transition between these two conformations. Such a transition might be a continuous rather than a drastic change. However, the larger (26) Lvov, Y.; Decher, G.; Mo¨hwald, H. Langmuir 1993, 9, 481.
2240 Langmuir, Vol. 15, No. 7, 1999
Letters
Figure 4. Schematic representation of photoisomerization of a 6Az10-PVA monolayer on the water surface at surface pressures of 4 and 11 mN/m directly estimated by in-situ XR measurements.
thickness change (3.5 Å) for the 6Az10 side chain layer for 4 mN/m than that for 11 mN/m (1.7 Å) is consistent with the model in Figure 4, taking the effect of the hydrophilicity increase for the azobenzene group into account. Conclusions The structural change of polymer monolayers on the water surface caused by photoisomerization of the photosensitive group on a polymer side chain has been observed directly and quantitated by in-situ XR measurement. The XR profile clearly showed the change in thickness of 2-3 Å due to the trans-cis conformational change in the hydrophobic side chain. Extended XR
experiments, such as those at higher surface pressure conditions, in which a large change is expected, are now underway. Acknowledgment. This work was financially supported by the New Energy Development Organization (NEDO) project of the Ministry of International Trade and Industry of Japan, and also by a Grant-in-Aid for Scientific Research on Priority Areas (Molecular Superstructure-Design and Creation) by the Ministry of Education, Science, Sports and Culture of Japan (08231237, 09217230), to whom our sincere gratitude is due. LA981084G
