Langmuir 2002, 18, 3875-3879
3875
Nanostructure of a Photochromic Polymer/Liquid Crystal Hybrid Monolayer on a Water Surface Observed by in Situ X-ray Reflectometry Keitaro Kago,†,§ Takahiro Seki,*,‡ Randolf R. Schu¨cke,†,⊥ Emiko Mouri,† Hideki Matsuoka,*,† and Hitoshi Yamaoka†,| Department of Polymer Chemistry, Kyoto University, Kyoto 606-8501, Japan, and Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Received September 4, 2001. In Final Form: November 16, 2001 The monolayer of a poly(vinyl alcohol) derivative having an azobenzene (Az)-containing side chain (6Az10-PVA) can induce an alignment transition of liquid crystal molecules in the monolayer by the trans-cis conformational change of the azobenzene unit (command effect). As a model system of this dynamic interface, the structural characterization of the mixed monolayer of 6Az10-PVA and a nematic liquid crystal molecule, 4′-penthyl-4-cyanobiphenyl (5CB), on a water surface was analyzed by in situ X-ray reflectometry. The depth density profile indicated that this hybrid monolayer in the trans-Az state consists of two distinct layers: a dense lower region and a less dense upper region having the same thickness of ca. 1.5 nm. This suggests that the rod molecules are stretched out in a direction perpendicular to the water surface where 5CB molecules are inserted on the lower side of the Az side chains. Light-induced structural modifications were clearly detected in the hybrid monolayer.
Introduction Photoisomerizable monolayers can induce an alignment change of liquid crystal (LC) molecules, which is referred to as “command surface”.1,2 The technique and mechanism themselves are interesting and important from the viewpoint of scientific interest, especially surface chemistry, as well as the industrial application. However, to gain deeper insights into this two-dimensional molecular system, it is necessary to obtain detailed knowledge on the mechanism and structure at a molecular level. The polymer shown in Figure 1 (left) (6Az10-PVA) (Az, azobenzene; PVA, poly(vinyl alcohol)) forms a highly photosensitive monolayer on water and solid surfaces.2-5 6Az10-PVA contains the azobenzene unit, which shows an efficient photoisomeric conformational change between trans- and cis-forms by the irradiation of visible (vis, 436 nm) and ultraviolet (UV, 365 nm) light, respectively. When LC molecules are incorporated into this monolayer, they are expected to change their orientation following the photochromic transition of azobenzene units. In fact, the hybrid monolayers composed of 6Az10-PVA and a typical nematic LC molecule, 4′-penthyl-4-cyanobiphenyl (5CB) * To whom correspondence should be addressed. † Kyoto University. ‡ Tokyo Institute of Technology. § Present address: Venture Business Laboratory, Kyoto University, Kyoto 606-8501, Japan. | Present address: Department of Materials Science, School of Engineering, The University of Shiga Prefecture, 2500 Hassakacho, Hikone City, Shiga 522-8533, Japan. ⊥ On leave from Johannes Gutenberg Universita ¨ t Mainz, Germany. (1) Ichimura, K. Chem. Rev. 2000, 100, 1847. (2) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Takami, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 211. (3) Seki, T.; Kojima, J.; Ichimura, K. J. Phys. Chem. B 1999, 103, 10338. (4) Seki, T.; Fukuda, R.; Yokoi, M.; Tamaki, T.; Ichimura, K. Bull. Chem. Soc. Jpn. 1996, 69, 2375. (5) Seki, T.; Sekizawa, H.; Morino, S.; Ichimura, K. J. Phys. Chem. B 1998, 102, 5313.
Figure 1. Chemical structures of photochromic polymer 6Az10PVA (left) and liquid crystal molecule 5CB (right).
(see Figure 1 (right)), offer an appropriate and useful interface model of the command surface.6,7 With this model system, the dynamic photoinduced molecular processes and cooperativity at the interface are precisely evaluated by spectroscopic methods.7 The above-mentioned interface model has been prepared by the Langmuir-Blodgett method, and thus accurate structural information about the Langmuir monolayer (floating monolayer on water) is of particular importance. Although its macrostructure and macrobehavior on water have been adequately confirmed by π-A (surface pressure-area/molecule) measurements3,4 and Brewster angle microscopy (BAM),5 direct information on the in situ (6) Ubukata, T.; Seki, T.; Ichimura, K. J. Phys. Chem. B 2000, 104, 4141. (7) Ubukata, T.; Seki, T.; Morino, S.; Ichimura, K. J. Phys. Chem. B 2000, 104, 4148.
10.1021/la011384p CCC: $22.00 © 2002 American Chemical Society Published on Web 04/20/2002
3876
Langmuir, Vol. 18, No. 10, 2002
Kago et al.
Figure 2. XR profiles for the 6Az10-PVA/5CB mixed monolayer on a water surface at different surface pressures for (a) transand (b) cis-forms. The circles represent data points, and the solid lines are the best fit curves by model calculation. The curves are shifted downward by one decade to avoid superposition.
nanostructure of the monolayer has not been elucidated. Atomic force microscopy (AFM) is a powerful tool for morphological observations on a molecular scale,3,8-10 but it requires the deposition process onto a solid surface, which may not reflect the “living state” of the floating monolayer. For structural evaluations on water, in situ X-ray reflectometry (XR)11,12 is a powerful tool. The potentiality of the XR technique for the study of water surface monolayer systems has already been shown by many examples.9-16 With respect to the photosensitive monolayer, we already applied the XR technique to the pure 6Az10-PVA monolayer on a water surface.13 By analyzing XR data, photoinduced changes in film thickness, interface roughness, and density profiles were successfully determined in the order of angstroms. A change in the monolayer thickness was actually detected corresponding to the conformational change of azobenzene units from the cis- to trans-forms. In the above contexts, we conducted in this work the in situ XR analysis for the hybrid monolayer composed of 6Az10-PVA and 5CB to explore the nanostructure of the hybrid film and their photoinduced structural changes. The morphology of the hybrid monolayer has been shown to be molecularly flat by AFM.6 The scanning probe microscopic method, however, only provides information on the film surface. By (8) Wang, J.; Jiang, L.; Iyoda, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. Langmuir 1996, 12, 2052. (9) Ve´lez, M.; Mukhopadhyay, S.; Muzikante, I.; Matisova, G.; Vieira, S. Langmuir 1997, 13, 870. (10) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479. (11) (a) X-ray and Neutron Reflectivity: Principles and Applications; Daillant, J., Gibaud, A. Eds.; Springer: New York, 1999. (b) X-ray Scattering from Soft-Matter Thin Films; Tolan, M., Ed.; Springer: New York, 1999. (12) Matsuoka, H.; Mouri, E.; Matsumoto, K. Rigaku J. 2001, 18, 54. (13) Kago, K.; Furst, M.; Matsuoka, H.; Yamaoka, H.; Seki, T. Langmuir 1999, 15, 2237. (14) (a) Matsumoto, K.; Mizuno, U.; Matsuoka, H.; Yamaoka, H. Macromolecules 2002, 35, 555. (b) Mouri, E.; Wahnes, C.; Matsumoto, K.; Matsuoka, H.; Yamaoka, H. Langmuir, in press. (15) Parratt, L. G. Phys. Rev. 1954, 95, 359. (16) Xue, J.; Jung, C. S.; Kim, M. W. Phys. Rev. Lett. 1992, 69, 474.
contrast, the XR technique has the advantageous feature of providing information on the interior nanostructure in the film, which may offer new understanding of this photomechanical device. Materials The synthesis and characterization of the 6Az10-PVA molecule were described elsewhere.2 For vis and UV irradiation, a 100 W Mercury lamp equipped with a wavelength selective optical filter was used. The mixed solution was prepared so that the molar ratio of 6Az10 side chains and 5CB molecules was 1:1. The solvent was chloroform. The time of irradiation required for complete photoisomerization in the system (in which the cis content was ca. 90%4) was checked by UV-vis absorption spectroscopy. For the trans-form measurement, after confirming that all molecules are in the trans-form by spectroscopy, the solution was spread on the water surface in the trough and the surface pressure was controlled. For the cis-form measurement, the solution was spread in a dark situation after the irradiation of UV light for the required time.
XR Measurement By analyzing the XR profile by model fitting, the thickness of each layer, surface and interface roughness (in root mean square (rms)), and the refractive index can be estimated.11,12 The complex refractive index n is a unique parameter representing the character of the layer.
n ) 1 - δ - iβ
(1)
The measurements were performed with an air-water interface X-ray reflectometer for laboratory use. A full description of the XR apparatus has been given elsewhere.12 The scattering vector q (q ) 4π sin θ/λ, where λ is the wavelength of the X-ray (1.5406 Å)) is changed by the incident and reflection angles. The measurements were performed under a specular condition, that is, θ ) θincident ) θreflection. The density profile normal to the surface can be evaluated from the XR profiles obtained. The time required for one measurement was about 30 min.
Results and Discussion Figure 2 shows the XR profiles of mixed monolayers for trans- and cis-forms at different surface pressures. Kiessig fringe was observed in every profile. The profiles for the
Photochromic Polymer/Liquid Crystal Monolayer
trans-form are not so largely different from each other with the change in the surface pressure. The structure of the monolayer was not largely changed by compression. This observation is in agreement with the fact that the area of one 6Az10 unit was not markedly decreased with increasing surface pressure.4 In the cis-form, on the other hand, the Kiessig fringe becomes clearer and shifted toward a lower scattering vector q as the surface pressure increased. This observation indicates that the monolayer had become denser and thicker. Comparison of the profiles for the trans- and cis-forms at the same surface pressure revealed that the Kiessig fringe was positioned at a lower scattering vector in the trans-form. Thus, the monolayer was thicker for the trans-form than the cis-form. The previous work6 showed that the molecular mixing is attained for the cospread monolayer of 6Az10-PVA and 5CB at the equimolar mixing ratio. Morphological observations by Brewster angle microscopy and atomic force microscopy and further UV-visible absorption spectroscopy on water all provided strong evidence for the ideal molecular mixing of 6Az10 side chains and 5CB molecules. Therefore, it is reasonable to proceed with analyses of the XR data without considerations of lateral (x-y direction) phase separation or structuring on the micro- or mesoscales. By using the Parratt equations,15 we fitted the data to the curves by model calculation. The solid lines in Figure 2 fit the experimental data the best. For all the profiles shown here, the data were most properly reproduced by a two-layer model. Here, the hydrophobic layer on water consisted of two layers, the less dense (upper, directing to the air) and more dense (lower, contacting with water) layers. The hydrophilic layer of PVA under the water could not be detected. This is reasonable since the electron densities of the hydrophilic region and the water subphase are comparable. Numerical data for the nanostructure of the monolayer evaluated by XR are summarized in Table 1. The two-layer model can be compared with our previous conclusion for a pure 6Az10-PVA monolayer without 5CB, that the hydrophobic layer can be analyzed by a singlelayer model.13 Figure 3 shows the depth profile of the δ value, which is nearly proportional to the electron density. The monolayer consists of a highly dense, well-defined (i.e., with sharp interface) lower layer and less dense, rather diffused upper layer. The layer thickness and the density of the trans-form increased monotonically, although slightly, as the surface pressure increased. This trend is quite reasonable. In the cis-form, the increase in thickness and density was more significant at lower surface pressures (2, 4, and 6 mN/m). In addition, the interface became sharper as the surface pressure increased. However, at higher surface pressures, the density decreased apparently as the surface pressure increased. This may be related with the collapse of the monolayer due to high compression. The fact that an additional third layer was necessary to reproduce the XR profile at the highest surface pressure for the cis-form supports this interpretation, which will be discussed later in detail. The chain lengths of both the C10 spacer (-O-CO(CH2)10-O-) of 6Az10-PVA and the 5CB molecule in an ideally stretched state are calculated to be 16 Å. In the trans-form, the estimated thickness of the lower (high density) hydrophobic part by XR was 15 Å, which exactly identical with the length of C10 and 5CB within the experimental accuracy. The thickness of the upper (less dense) layer increased with the surface pressure. Hence, the change in the total monolayer thickness is mainly due to the contribution of the upper layer. The length of the
Langmuir, Vol. 18, No. 10, 2002 3877 Table 1. Fitting Parameters of XR Curves for the 6Az10-PVA/5CB Mixed Monolayer on a Water Surface at Different Surface Pressures δ β thickness roughness (×10-6) (×10-6) (Å) (Å) trans 2 mN/m
cis
a
upper lower subphase 4 mN/m upper lower subphase 6 mN/m upper lower subphase 9 mN/m upper lower subphase 10 mN/m upper lower subphase 2 mN/m upper lower subphase 4 mN/m upper lower subphase 6 mN/m upper lower subphase 9 mN/m upper lower subphase 10 mN/m upper (1) upper (2) lower subphase
3.01 5.33 3.57 3.02 5.42 3.57 3.40 5.68 3.57 3.40 5.68 3.57 3.34 5.68 3.57 2.41 4.66 3.57 2.41 4.77 3.57 2.89 5.09 3.57 2.20 4.74 3.57 1.55 2.08 4.48 3.57
0.0046 0.0083 0.0114 0.0046 0.0083 0.0114 0.0052 0.0087 0.0114 0.0052 0.0087 0.0114 0.0052 0.0087 0.0114 0.0037 0.0073 0.0117 0.0037 0.0073 0.0114 0.0044 0.0078 0.0114 0.0044 0.0078 0.0114 0.0023 0.0031 0.0068 0.0114
14.9 14.6 1 000 000a 15.3 15.3 1 000 000 15.2 15.0 1 000 000 16.0 14.9 1 000 000 16.2 15.0 1 000 000 10.1 11.4 1 000 000 10.3 14.1 1 000 000 12.8 14.1 1 000 000 15.4 15.5 1 000 000 5.0 15.9 15.8 1 000 000
10.6 2.4 0.6 10.9 2.1 0.3 10.9 2.0 0.9 11.0 1.6 1.3 10.9 2.7 1.2 6.7 2.4 4.2 5.3 1.3 1.0 8.3 1.2 1.4 8.9 2.2 0.5 13.0 1.6 2.6 0.5
Value used for calculation, which means physically infinite.
end region of the side chain (-Az-C6H13) assuming a stretched state is calculated to be 18 Å in the trans-form. The observed thickness of the upper hydrophobic region by XR was about 15-16 Å, slightly smaller than the calculated value. Thus, the upper region should be inclined and/or disordered. This interpretation is rationalized by the fact that the electron density of the upper layer is low. The UV-vis absorption spectra show that the 6Az10 side chain and 5CB molecules are highly oriented perpendicular to the water surface in the trans-form by a cooperative effect of the 6Az10 side chain and 5CB molecule.6,7 The present results of XR are in good agreement with this observation. Compared with the monolayer of pure 6Az10-PVA,13 the present hybrid monolayer had a significantly larger total layer thickness. The observed thickness for the pure 6Az10-PVA monolayer and the hybrid one at 4 mN m-1 was 2813 and 31 Å (see Table 1), respectively. This can be again attributed to the highly perpendicular orientation. The thickness of the lower layer of the cis-form was almost the same as that of the trans-form. The upper layer, however, was significantly thinner than that of the transform. This trend is especially noticeable at lower surface pressures: it is ca. 5 Å thinner. This can be the result of the conformational change of the azobenzene group. The end region of the side chain seems to be tilted more in the cis-form, resulting in the reduction of thickness of the upper layer. Figure 4 shows the schematic representation of the nanostructure of the hybrid monolayers on the water surface deduced from the present study. 5CB molecules are located near the water surface. In the trans-form, the whole span of the 6Az10 side chain stretched nearly perpendicularly on the water surface. An interesting aspect concerning the combination of these two materials is that the length of the C10 alkylene spacer of the Az side
3878
Langmuir, Vol. 18, No. 10, 2002
Kago et al.
Figure 3. Depth profiles of the d value for the 6Az10-PVA/5CB mixed monolayer on a water surface at different surface pressures evaluated by XR profiles: (a) trans-form and (b) cis-form. The curves are shifted downward by two. The vertical short solid lines indicate the interface positions.
Figure 4. Schematic representation of the nanostructure of the 6Az10-PVA/5CB mixed monolayer on the water surface determined by the in situ XR technique.
chain well coincides with the molecular length of 5CB. For this reason, the two-dimensional array of 6Az10 side chains has spatial cavities that can accommodate 5CB molecules. This is not the case for the homologous Az polymers having a shorter spacer of C1 (6Az1-PVA) or C5 (6Az5-PVA).6 Langmuir monolayer experiments varying the molar mixing ratio of 5CB to Az unit (R ) [5CB]/[Az]) have shown that the microphase separation excluding 5CB
molecules from the hybrid layer occurs at R > 1 for the shorter spacer (C1 and C5) systems. On the other hand, the 6Az10-PVA monolayer can accommodate one additional 5CB molecule per 6Az10 side chain in the monolayer, resulting in a stable hybrid formation up to R ) 2 without generation of phase separation. In the cisform, the end region of the side chain is probably laid on the lower hydrophobic layer, which led to the thinning of the total layer. This is consistent with the expansion of the monolayer in the cis-form as revealed by the π-A isotherm. 5CB has a polar cyano group, and it is reasonable to assume that this molecule is in direct contact with the water surface16,17 also in the hybrid monolayer. However, this model has not been substantiated experimentally. The present study revealed that the density of the lower layer is larger than that of the upper layer, and there would be no doubt that the 5CB molecule is located in the lower layer with perpendicular orientation as displayed in Figure 4. The third layer observation at higher surface pressure conditions might be interesting. This layer can be explained by the new layer formation by some 5CB molecules which were squeezed out from the monolayer by too much compression. This kind of phenomenon has been expected, and in fact it was observed by the BAM technique for polysilane/fatty acid systems.18 However, the present work may be the first example to show directly the vertical nanostructure of the “squeezed-out layer”. This experimental verification indicates the particular advantage of the XR measurement as the characterization tool of the ultrathin films. The structure of the inner region of the layer can be analyzed only by XR. On the other hand, scanning probe microscopic observations provide a great deal of information on the molecular films but they are limited to surface features (geometry, friction, surface potential, etc.) placed on solid surfaces. Therefore, XR analysis is a powerful complementary tool for precise characterization of the monolayer structure. The present (17) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251. (18) Seki, T.; Ichimura, K. Langmuir 1997, 13, 1361. (19) Ubukata, T.; Seki, T.; Ichimura, K. Adv. Mater. 2000, 12, 1675.
Photochromic Polymer/Liquid Crystal Monolayer
hybrid monolayer offers a very suited example for the demonstration of this aspect. Conclusions We succeeded in observing and quantifying the nanostructure and the change of the 6Az10-PVA/5CB hybrid monolayers on a water surface caused by photoisomerization by in situ XR. The hybrid monolayer was composed of a dense lower layer and a less dense upper layer. Most likely, the liquid crystal molecules (5CB) are located in contact with the water subphase adjoining the C10 spacer of the 6Az10 side chain. Substantial structural changes are observed with UV photoirradiation. New layer forma-
Langmuir, Vol. 18, No. 10, 2002 3879
tion by squeezed-out molecules from the monolayer induced by compression was also clearly and quantitatively detected by XR. Successful characterization of the inner nanostructure should be of great help to gain understanding and newly design photochromic molecular layers for command layers and hybrid-component photomechanical devices.18 Acknowledgment. This work was supported by the Nissan Science Foundation, to which our sincere gratitude is due. LA011384P
