Langmuir 2000, 16, 2975-2977
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Notes In Situ AFM Study on the Morphological Change of the Langmuir-Blodgett Film of Cadmium 10,12-Pentacosadiynoate during Polymerization Hiroaki Tachibana,*,† Yasushi Yamanaka,‡ Hideki Sakai,‡ Masahiko Abe,‡ and Mutsuyoshi Matsumoto† National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba 305-8565, Japan, and Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda 278-8510, Japan Received July 6, 1999. In Final Form: October 13, 1999
Polydiacetylenes are π-conjugated polymers obtained by solid-state polymerization on irradiation of UV light and γ-rays.1,2 This class of polymers has attracted much attention in terms of their application to nonlinear optical materials. Two spectroscopically distinct forms, blue and red, have been observed. A phase transition from the blue to the red phase of polydiacetylenes has been induced by heat treatment3,4 and irradiation by a laser pulse5,6 in single crystals. The difference in the structures of the two phases is that a significant portion of alkyl chains attached to the polymer backbone is disordered in the red phase whereas the alkyl chains are ordered in the blue phase.3,7 The Langmuir-Blodgett (LB) technique has been used for the fabrication of ultrathin films with the structures defined at the molecular level. Much work has been done on the fabrication and polymerization of LB films of amphiphilic diacetylene derivatives such as 10,12-pentacosadiynoic acid (DA(12-8)).8 It has been reported that a drastic color change occurs during the polymerization of the LB films of the cadmium salt of DA(12-8) by the irradiation of UV light.8 This structural change should be related to the above-mentioned phase transition reported for polydiacetylene crystals, but has a different feature in that the color change occurs during the polymerization in the LB films as well as after the polymerization. Little is known on the structural change accompanied by this color change during the polymerization, though the structural characterizations of the LB films during the thermochromic change9-11 and the color change after polymerization12 have been reported. † ‡
National Institute of Materials and Chemical Research. Science University of Tokyo.
(1) Polydiacetylenes of NATO ASI Series E; Bloor, D., Chance, R. R., Eds.; Nijhoff: Dordrecht, The Netherlands, 1985; Vol. 102. (2) Ogawa, T. Prog. Polym. Sci. 1995, 20, 943 and the references therein. (3) Koshihara, S.; Tokura Y.; Takeda, K.; Koda, T.; Kobayashi, A. J. Chem. Phys. 1990, 92, 7581. (4) Hankin, S. H. W.; Downey, M. J.; Sandman, D. J. Polymer 1992, 33, 5098. (5) Koshihara, S.; Tokura, Y.; Takeda, K.; Koda, T. Phys. Rev. Lett. 1992, 68, 1148. (6) Koshihara, S.; Tokura, Y.; Takeda, K.; Koda, T. Phys. Rev. B 1995, 52, 6265. (7) Tanaka, H.; Gomez, M. A.; Tonell, A. E.; Thakur, M. Macromolecules 1989, 22, 1208. (8) Tieke, B. Adv. Polym. Sci. 1985, 71, 79 and the references therein. (9) Mino, N.; Tamura, H.; Ogawa, K. Langmuir 1991, 7, 2336. (10) Deckert, A. A.; Horne, J. C.; Valentine, B.; Kiernan, L.; Fallon, L. Langmuir 1995, 11, 643.
Figure 1. Change in absorption spectrum of a single-layer LB film of DA(12-8) on irradiation of UV light.
In this paper, we report on the structural change of a single-layer DA(12-8) LB film during polymerization by the irradiation of UV light. The morphological change of the LB film is investigated by in situ atomic force microscopy (AFM). We have found that the color change is accompanied by the development of three-dimensional structures from the film surface. DA(12-8) was synthesized as reported previously.13 The measurements of surface pressure-area isotherms and the deposition of DA(12-8) monolayer were carried out on a Lauda Film balance at 17 °C. A chloroform solution of DA(12-8) was spread onto the subphase containing a Cd2+ buffer at pH ) 6.0. DA(12-8) was converted to its cadmium salt on this subphase. The monolayers were transferred at 25 mN m-1 onto solid substrates using the vertical dipping method. A low-pressure mercury lamp was used as the source of UV light. A Seiko SPA300 atomic force microscope, operating in noncontact mode, was employed to image the morphology of a single-layer DA(12-8) LB film on mica. The AFM image was obtained at line scan rates of 1 Hz using silicon cantilevers with a resonance frequency of 28 kHz and a spring constant of 1.9 Nm-1. For in situ AFM measurements, the sample was not moved from the scanner in order to follow the morphological change on photoirradiation in the same zone of the sample. During UV irradiation, the mercury lamp was placed over the sample at a distance of 19 cm. From time to time, irradiation was stopped and the in situ images of the sample were realized. All the measurements for photoirradiation were carried out at room temperature. Figure 1 shows the change in absorption spectrum of a single-layer LB film of DA(12-8). Before irradiation, no appreciable band is seen in the visible region. The polymerization process can be divided into three regimes. In the initial regime of the polymerization (irradiation time of 0 to 5 min), a band at around 650 nm develops. The film is in the blue phase. In the second regime (irradiation time of 5 to 50 min), the shoulder band situated at shorter wavelengths grows with a concomitant decrease (11) Kuriyama, K.; Kikuchi, H.; Kajiyama, T. Langmuir 1998, 14, 1130. (12) Sheth, S. R.; Leckband, D. E. Langmuir 1997, 13, 5652. (13) Tieke, B.; Lieser, G.; Wegner, G. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1631.
10.1021/la990883b CCC: $19.00 © 2000 American Chemical Society Published on Web 01/20/2000
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Figure 2. AFM images (5 × 5 µm) of a single-layer LB film of DA(12-8) with different irradiation times of UV light: (a) before irradiation, (b) irradiation time of 5 min, (c) 15 min, (d) 30 min, (e) 50 min, and (f) 90 min.
of the 650-nm band. The blue-to-red color change occurs in this regime. In the final regime (irradiation time of 50 to 90 min), the band at around 535 nm develops until the system comes to a saturated state. Further UV irradiation gave rise to a decrease in the intensity of the absorption band due to the decomposition of the polydiacetylene. Figure 2 shows in situ AFM images of a single-layer DA(12-8) LB film during the polymerization. Before the irradiation of UV light, several three-dimensional structures with a height of 5 nm above the planar surface are evident. The morphology of the DA(12-8) LB film remains unchanged in the initial regime where the film is in the blue phase (Figure 2(b)). In the second regime, a number of three-dimensional domains begin to appear from the
planar surface (Figure 2(c)). Ex situ AFM gave similar results. With increasing irradiation time, the number of three-dimensional domains increases slightly and each domain grows (Figure 2(d) and 2(e)). In the final regime, the number of three-dimensional domains does not change significantly and each domain increases its size slightly (Figure 2 (f)). The structural change of the LB films during polymerization was further investigated using FTIR spectroscopy. Figure 3 shows the transmission FTIR spectrum of a 20-layer LB film in the CH stretching region as a function of the irradiation time. Before irradiation of UV light, two strong bands are seen at ca. 2920 and 2850 cm-1, which are assigned to CH2 antisymmetric (νa(CH2)) and sym-
Notes
Figure 3. Change in transmission FTIR absorption spectrum of a 20-layer LB film of DA(12-8) as a function of the irradiation time of UV light.
metric (νs(CH2)) stretching band, respectively. In the first regime, these bands do not change significantly. In the second and the final regimes, however, both absorption bands become broader with an increase in the irradiation time of UV light though the band position does not change appreciably. Similar results were obtained by using IR reflection-absorption spectroscopy. This indicates that the packing of the alkyl chains attached to the polymer backbone becomes disordered at the point of color change,14,15 which is consistent with the phenomena observed on going from the blue to red phase in polydiacetylene crystals and LB films. From the above results, we propose a mechanism of the morphological change of the LB film during polymerization. In the initial regime of the polymerization, the absorption band at around 650 nm increases. The morphology of the film does not change significantly. In this regime, the polymerization is assumed to be a topochemical reaction without causing any significant change in the LB film structure. In the second regime where the blueto-red color change occurs, a number of three-dimensional structures appear from the film surface. The blue-to-red spectral change of the polymers is considered to be due to a decrease in the delocalization of π-electrons along the polymer backbone, which is accompanied by an orderdisorder transition of alkyl chains. This is supported by the results of IR measurements. Similar structural (14) Murakami, H.; Watanabe, Y.; Nakashima, N. J. Am. Chem. Soc. 1996, 118, 4484. (15) Saito, A.; Urai, Y.; Itoh, K. Langmuir 1996, 12, 3938.
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changes have been observed in the color change of blue to red forms upon heating the LB films of DA(12-8) polymers.9,10 We consider that the morphological change should be related to the change in the absorption spectrum. We assume the following processes: when the polymerization proceeds, the propagation of the polymer backbone will impose a significant stress on the structure of the LB films. When this stress exceeds a certain threshold, the color change will occur to release part of the stress accumulated in the film. This color change shows a decrease in the effective conjugation length of the polydiacetylene backbone, which could be caused by the disruption of the conjugation due to the displacement of some of the atoms in the polymer backbone. The color change caused by the stress in the film will lead to the modification of the structure of the LB film. This should be responsible both for the order-disorder transition of the alkyl chains and for the morphological change of the film. In the final regime, the 535-nm band develops and the morphology of the film changes only slightly until a saturated state is reached. This study reveals that the morphology of the DA(12-8) LB films changes with the blue-to-red color change during polymerization. The results suggest that the stress accumulated in the film by the polymerization, which is usually considered as a topochemical reaction, is released by the order-disorder transition of alkyl chains and also by the modification of the two-dimensional LB film structures. This morphological change is reminiscent of those of the LB films induced by the photoisomerization of azobenzene in that two-dimensional LB film structures are modified into three-dimensional structures by the photoreaction.16-20 The results further indicate that care should be taken in the study of chemical reactions in LB films. LA990883B (16) Matsumoto, M.; Tachibana, H.; Sato, F.; Terrettaz, S. J. Phys. Chem. B 1997, 101, 702. (17) Seki, T.; Tanaka, K.; Ichimura, K. Macromolecules 1997, 30, 6401. (18) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479. (19) Terrettaz, S.; Tachibana, H.; Matsumoto, M. Langmuir 1998, 14, 7511. (20) Tachibana, H.; Sato, F.; Terrettaz, S.; Azumi, R.; Nakamura, T.; Sakai, H.; Abe, M.; Matsumoto, M. Thin Solid Films 1998, 327-329, 813.
