Photoinduced Radical Generation and Self-Assembly of

Photoinduced Radical Generation and Self-Assembly of Tetrathiafulvalene into the Mixed-Valence State in the Poly(vinyl chloride) Film under UV Irradia...
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Photoinduced Radical Generation and Self-Assembly of Tetrathiafulvalene into the Mixed-Valence State in the Poly(vinyl chloride) Film under UV Irradiation Kazuo Tanaka, Fumiyasu Ishiguro, and Yoshiki Chujo* Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Received June 23, 2009. Revised Manuscript Received July 28, 2009 The photoinduced self-assembly and the formation of the mixed-valence state of tetrathiafulvalene (TTF) in the solid state are reported. The polymer composites containing TTF in poly(vinyl chloride) (PVC) were prepared, and oxidation of TTF by chlorine radical generated by UV irradiation in PVC was investigated. The formation of the mixed-valence state of TTF in the composite films by UV irradiation was observed, and the resulting TTF radical species, including the mixed-valence state after the photopatterning, exhibited extremely high stability in the composite films. Finally, we performed the fabrication of the gradient materials of the radical concentrations on the TTF/PVC composites with the photopatterning.

Introduction The use of nanofillers is a facile and valid strategy for creating optical and electrical properties in polymer materials, and the resulting nanohybrid materials are suitable as a scaffold for building multifunctional opto-electro devices with nanotechnology.1,2 Nanostructured composites containing a well-defined structural motif often lead to multifunctional characteristics exceeding those of their component parts.3 In addition, metastable conformations and electronic states such as the mixedvalence states which are often observed for metal-metal-bonded or metal-conjugated compounds can be fixed and express curious properties with the assistance of the polymer matrices in the composite materials.4-6 Therefore, the regulation and assistance in the formation of the self-assembled structure in the polymer matrices according to the preprogrammed design are greatly important in the creation of high-performance nanocomposite materials. Tetrathiafulvalene (TTF)-based charge transfer complexes have been widely studied and known to form crystals consisting *To whom correspondence should be addressed. E-mail: chujo@chujo. synchem.kyoto-u.ac.jp. Fax: þ81-75-383-2605. Phone: þ81-75-383-2604.

(1) (a) Holder, E.; Tessler, N.; Rogach, A. L. J. Mater. Chem. 2008, 18, 1064. (b) Kymakis, E.; Amaratunga, G. A. J. Rev. Adv. Mater. Sci. 2005, 10, 300. (c) Moulart, A.; Marrett, C.; Colton, J. Polym. Eng. Sci. 2004, 44, 588. (d) Park, M.; Chin, B. D.; Yu, J.-W.; Chun, M.-S.; Han, S.-H. J. Ind. Eng. Chem. 2008, 14, 382. (2) (a) Scott, J. C.; Bozano, L. D. Adv. Mater. 2007, 19, 1452. (b) Fox, R. T.; Wani, V.; Howard, K. E.; Bogle, A.; Kempel, L. J. Appl. Polym. Sci. 2008, 107, 2558. (c) Sudha, J. D.; Sivakala, S.; Prasanth, R.; Reena, V. L.; Radhakrishnan Nair, P. Compos. Sci. Technol. 2009, 69, 358. (3) (a) Kang, J.; Shin, N.; Jang, D. Y.; Prabhu, V. M.; Yoon, D. Y. J. Am. Chem. Soc. 2008, 130, 12273. (b) Andreasen, J. W.; Jrgensen, M.; Krebs, F. C. Macromolecules 2007, 40, 7758. (c) Deng, X.; Zheng, L.; Yang, C.; Li, Y.; Yu, G.; Cao, Y. J. Phys. Chem. B 2004, 108, 3451. (d) Gebeyehu, D.; Maenning, B.; Drechsel, J.; Leo, K.; Pfeiffer, M. Sol. Energy Mater. Sol. Cells 2003, 79, 81. (4) (a) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 10, 247. (b) Allen, G. C.; Hush, N. S. Prog. Inorg. Chem. 1967, 8, 357. (c) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. Chem. Rev. 2001, 101, 2655. (d) Lewis, I. C.; Singer, L. S. J. Chem. Phys. 1965, 43, 2712. (e) Rathore, R.; Kumar, A. S.; Lindeman, S. V.; Kochi, J. K. J. Org. Chem. 1998, 63, 5847. (f) Kochi, J. K.; Rathore, R.; Le Magueres, P. J. Org. Chem. 2000, 65, 6826. (g) Joergensen, T.; Hansen, T. K.; Becher, J. Chem. Soc. Rev. 1994, 23, 41. (h) Nielsen, M. B.; Lomholt, C.; Becher, J. Chem. Soc. Rev. 2000, 29, 153. (i) Nakagawa, M.; Ishii, D.; Aoki, K.; Seki, T.; Iyoda, T. Adv. Mater. 2005, 17, 200. (5) (a) Bulinski, M.; Kuncser, V.; Plapcianu, C.; Krautwald, S.; Franke, H.; Rotaru, P.; Filoti, G. J. Phys. D: Appl. Phys. 2004, 37, 2437. (b) Miller, L. L.; Duan, R. G.; Hong, Y.; Tabakovic, I. Chem. Mater. 1995, 7, 1552. (6) Iyoda, M.; Hasegawa, M.; Miyake, Y. Chem. Rev. 2004, 104, 5085.

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of segregated stacks of TTF and acceptors.6-8 The preparation of the polymer composites containing the TTF charge transfer complexes has been established, and these materials were applied in various manners such as a transparent conductive material.9 TTF radical cation can work as an acceptor, and the mixture of neutral TTF and TTF radical cation can form the mixed-valence complex.10,11 Self-assembly stacking formation is applicable as a driving force for mechanical motion of molecular switches.12 We have recently reported that the formation of the mixed-valence state of TTF can be regulated by the coexisting organic anion (7) (a) Segura, J. L.; Nartı´ n, N. Angew. Chem., Int. Ed. 2001, 40, 1372. (b) Kobayashi, N.; Naito, T.; Inabe, T. Adv. Mater. 2004, 16, 1803. (c) Wallis, J. D.; Griffiths, J. P. J. Mater. Chem. 2005, 15, 347. (d) Rovira, C. Chem. Rev. 2004, 104, 5289. (e) Kobayashi, A.; Fujiwara, E.; Kobayashi, H. Chem. Rev. 2004, 104, 5243. (f) Fabre, J. M. Chem. Rev. 2004, 104, 5133. (g) Saito, G.; Yoshida, Y. Bull. Chem. Soc. Jpn. 2007, 80, 1. (h) Horiuchi, S.; Yamochi, H.; Saito, G.; Sakaguchi, K.; Kusunoki, M. J. Am. Chem. Soc. 1996, 118, 8604. (8) (a) Kitamura, T.; Nakaso, S.; Mizoshita, N.; Tochigi, Y.; Shimomura, T.; Moriyama, M.; Ito, K.; Kato, T. J. Am. Chem. Soc. 2005, 127, 14769. (b) Bryce, M. R. J. Mater. Chem. 1995, 5, 1481. (c) de Caro, D.; Malfant, I.; Savy, J.-P.; Valade, L. J. Phys.: Condens. Matter 2008, 20, 184012. (d) Iyoda, M.; Hasegawa, M.; Enozawa,  H. Chem. Lett. 2007, 36, 1402. (e) Puigmartí-Luis, J.; Laukhin, V.; Perez del Pino, A.; Vidal-Gancedo, J.; Rovira, C.; Laukhina, E.; Amabilino, D. B. Angew. Chem., Int. Ed.  Ujaque, G.; Rovira, 2007, 46, 238. (f) Puigmartí-Luis, J.; Minoia, A.; Perez del Pino, A.; C.; Lledos, A.; Lazzaroni, R.; Amabilino, D. B. Chem.;Eur. J. 2006, 12, 9161. (g) Wu, H.; Zhang, D.; Su, L.; Ohkubo, K.; Zhang, C.; Yin, S.; Mao, L.; Shuai, Z.; Fukuzumi, S.; Zhu, D. J. Am. Chem. Soc. 2007, 129, 6839. (h) Bryce, M. R.; Moore, A. J.; Batsanov, A. S.; Howard, J. A. K.; Petty, M. C.; Williams, G.; Rotello, V.; Cuello, A. J. Mater. Chem. 1999, 9, 2973. (9) (a) Tracz, A.; Jeszka, J. K.; Sroczynska, A.; Ulanski, J.; Plocharski, J.; Yamochi, H.; Horiuchi, S.; Saito, G. Synth. Met. 1997, 86, 2173. (b) Jeszka, J. K.; Tracz, A.; Sroczynska, A.; Kryszewski, M.; Yamochi, H.; Horiuchi, S.; Saito, G.; Ulanski, J. Synth. Met. 1999, 106, 75. (c) Mas-Torrent, M.; Laukhina, E.; Rovira, C.; Veciana, J.; Tkacheva, V.; Zorina, L.; Khasanov, S. Adv. Funct. Mater. 2001, 11, 299. (d) Suzuki, S.; Emilie, P.; Urano, T.; Takahara, S.; Yamaoka, T. Polymer 2005, 46, 2238. (10) (a) Torrance, J. B.; Scott, B. A.; Welber, B.; Kaufman, F. B.; Seiden, P. E. Phys. Rev. B 1979, 19, 730. (b) Naka, K.; Ando, D.; Wang, X.; Chujo, Y. Langmuir 2007, 23, 3450. (c) Iyoda, M.; Hasegawa, M.; Kuwatani, Y.; Nishikawa, H.; Fukami, K.; Nagase, S.; Yamamoto, G. Chem. Lett. 2001, 30, 1146. (11) Tanaka, K.; Kunita, T.; Ishiguro, F.; Naka, K.; Chujo, Y. Langmuir 2009, 25, 6929. (12) (a) Yoshizawa, M.; Kumazawa, K.; Fujita, M. J. Am. Chem. Soc. 2005, 127, 13456. (b) Kitahara, T.; Shirakawa, M.; Kawano, S.; Beginn, U.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 14980. (c) Lyskawa, J.; Salle, M.; Balandier, J.-Y.; Le Derf, F.; Levillain, E.; Allain, M.; Viel, P.; Palacin, S. Chem. Commun. 2006, 2233. (d) Chiang, P.-T.; Chen, N.-C.; Lai, C.-C.; Chiu, S.-H. Chem.;Eur. J. 2008, 14, 6546. (e) Azov, V. A.; Gomez, R.; Stelten, J. Tetrahedron 2008, 64, 1909. (f) Aprahamian, I.; Olsen, J.-C.; Trabolsi, A.; Stoddart, J. F. Chem.;Eur. J. 2008, 14, 3889. (g) Nakanishi, T.; Kojima, T.; Ohkubo, K.; Hasobe, T.; Nakayama, K.; Fukuzumi, S. Chem. Mater. 2008, 20, 7492.

Published on Web 08/28/2009

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Scheme 1. Proposed Reaction Scheme for Oxidation and Generation of Radicals of TTF

species in the casting films composed of the random networks of the mixed-valence TTF nanowires.11 Next, interest was directed to the precise regulation of the amount and distribution of the radical and mixed-valence TTF in the solid films for realizing further applications of TTF-based materials such as advanced opto-electro or microfabricated devices.13 Herein, we report the photoinduced self-assembly and formation of the mixed-valence state of TTF in the solid state. The polymer composites containing TTF in poly(vinyl chloride) (PVC) were prepared, and oxidation of TTF by chlorine radical generated by UV irradiation in PVC was investigated. The formation of the mixed-valence state of TTF in the composite films by UV irradiation was observed, and TTF radical species in the composite can exist at least for 2 months with less degradation. In addition, the conversion of the neutral TTF into the mixed-valence state of TTF can be regulated by irradiation time. We also demonstrate the fabrication of the spin concentration gradient materials prepared with photopatterning. This is the first example, to the best of our knowledge, to regulate the formation of the mixed-valence TTF via solid-state reaction.

Figure 1. Time course of change in UV-vis/NIR absorption spectra of the PVC composite films containing 10 wt % TTF under UV irradiation (254 ( 20 nm) at 25 °C for 0, 10, 30, 60, 120, 180, 240, and 300 min. The films were obtained from a THF solution of PVC in the presence or absence of 10 wt % TTF after 12 h at 30 °C by casting the solution.

Results and Discussion UV irradiation to chlorinated hydrocarbons can generate chlorine radical which is a strong oxidizer (Scheme 1).14 We applied this photoinduced chlorine generation to oxidize TTF to form TTF radical species.15 The solution of PVC in THF was added to TTF (10 wt %) and cast onto the vessels. After 12 h at 30 °C, transparent yellow films were obtained. Phase separation or inhomogeneity was less frequently observed from the appearance of the films. The TTF/PVC composite films were irradiated with a transilluminator (254 ( 20 nm) at 25 °C. The films turned brown after UV irradiation, indicating the generation of TTF cation radical. Oxidation of TTF and the conversion to radical species can be monitored with UV-vis/NIR measurements. The TTF/PVC composite films were treated with UV light for 2 h, and the films were analyzed. In the UV-vis/NIR spectra of the films, two evident absorption bands at 523 nm derived from the π-π* transition of the TTF cation radical and 772 nm originated from TTF cation dimers appeared after photoreaction (Figure 1).11 In particular, the increase in the magnitude of the broad absorption band from 1200 nm assigned to the mixed-valence state of TTF was significantly observed after UV irradiation.11 These data clearly indicate that oxidation of TTF and the formation of the mixed-valence state can proceed in the film. In the FTIR spectra (Figure 2), a broad absorption band from 2000 cm-1 assigned as the mixed-valence state appeared in the composite film after UV irradiation.16 In addition, ESR signals of the eluent of the film in (13) Ko, H. C.; Stoykovich, M. P.; Song, J.; Malyarchuk, V.; Choi, W. M.; Yu, C.-J.; Geddes, J. B., III; Xiao, J.; Wang, S.; Huang, Y.; Rogers, J. A. Nature 2008, 454, 748. (14) (a) Decker, C.; Balandier, M. Polym. Photochem. 1984, 5, 267. (b) Gibb, W. H.; MacCallum, J. R. Eur. Polym. J. 1973, 10, 533. (15) Guldi, D. M.; Snchez, L.; Nartn, N. J. Phys. Chem. B 2001, 105, 7139. (16) (a) Bozio, R.; Zanon, I.; Girlando, A.; Pecile, C. J. Chem. Phys. 1979, 71, 2282. (b) Cooke, G.; Dhindsa, A. S.; Song, Y. P.; Williams, G.; Batsanov, A. S.; Bryce, M. R.; Howard, J. A. K.; Petty, M. C.; Yarwood, J. Synth. Met. 1993, 55-57, 3871.

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Figure 2. FTIR spectra of the PVC composite films containing 10 wt % TTF before and after UV irradiation for 2 h at 25 °C. The films were obtained from a THF solution of PVC in the presence or absence of 10 wt % TTF after 12 h at 30 °C by casting the solution.

Figure 3. ESR spectrum at 25 °C of the acetonitrile eluent of the PVC composite film containing 10 wt % TTF after UV irradiation for 2 h at 25 °C.

acetonitrile after photoirradiation were detected (Figure 3). The g value (2.008) of the ESR signals showed good agreement with that of the TTF cation radical (g = 2.008).17 These data strongly support the oxidation of TTF resulting in cation radical and the mixed-valence state of TTF. Interestingly, these results suggest that the self-assmebly of TTF molecules through the solid film (17) (a) Coffen, D. L.; Chambers, J. Q.; Williams, D. R.; Garrett, P. E.; Canfield, N. D. J. Am. Chem. Soc. 1971, 93, 2258. (b) Gerson, F. J. Mol. Struct. 1986, 141, 27.

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Figure 4. SEM images of the PVC films (a) before and (b) after UV irradiation (254 ( 20 nm) and the PVC composite films with 10 wt % TTF (c) before and (d) after UV irradiation (254 ( 20 nm) for 2 h. The films were obtained from a THF solution of PVC in the presence or absence of 10 wt % TTF after 12 h at 30 °C by casting the solution on a silicon wafer.

Figure 5. Time course of conversion of neutral TTF to TTF cation radical in the PVC composite films containing 10 wt % TTF under UV irradiation (254 ( 20 nm) at 25 °C within 20 min. The films were obtained from a THF solution of PVC in the presence or absence of 10 wt % TTF after 12 h at 30 °C by casting the solution on a silica glass.

should occur by UV irradiation. The π-π stacking interaction with the neutral TTF and TTF cation radical resulting in the formation of the mixed-valence state could be the driving force for formation of the self-assembly structure. To investigate the morphology of the TTF/PVC composite films before and after UV irradiation for 2 h, we observed the surface of the casting films via scanning electron microscopy (SEM) (Figure 4). Before UV irradiation, similar monotonous images were obtained from the samples with or without loading of TTF. On the other hand, white spots were generated only in the TTF/PVC composite film after UV irradiation. These data suggest that the cation radical and the mixed-valence state of TTF could generate microphase separation in the film through the PVC matrix during photoirradiation. 1154 DOI: 10.1021/la902246z

We evaluated the photoreaction rate of TTF oxidation in the TTF/PVC composite films, and to investigate the homogeneity of the oxidation reaction in the solid films, we compared the reaction rate with that of the liquid phase. The absorption bands at 523 and 772 nm were monitored versus the time of UV irradiation at 25 °C using the TTF/PVC composite films (Figure 5). Correspondingly, the increase in the magnitude of the absorption bands at both wavelengths, which represents the transformation of TTF yielding cation radical, was observed, and the reaction was almost finished within 20 min of irradiation (t50% = 4.5 min). Compared to the conversion rate of the liquid-phase photoreaction in the TTF solutions of PVC (t50% = 3.9 min), oxidation of TTF can proceed efficiently even in the solid films in high yield. These data suggest that chlorine radical generated by UV irradiation can immediately react with TTF even in the solid films, and the oxidation of TTF could concurrently occur through the composite films under photoirradiation. In the kinetic experiments, the thin films (1 μm) were used because of the intrinsic absorption of TTF in the UV region, and the formation of the mixed-valence state of TTF was slightly observed in these experiments. It was assumed that fast TTF consumption could provide less time for the TTF radical cation to meet the neutral TTF. We evaluated the stability of TTF cation radical in the composite films. From the longitudinal monitoring of the time course of the absorption spectra, we found that TTF radical cation can exist for at least 2 months in open air under ambient conditions (Figure 6). Even the absorption around 1600 nm which represents the mixed-valence state of TTF was slightly changed. From the SEM observations of the surface of the composites, less significant differences in the morphology of the white dots were observed from the sample 1 week after UV irradiation (Figure S1 of the Supporting Information). These data indicate that the aggregation of TTF species triggered by UV irradiation can be Langmuir 2010, 26(2), 1152–1156

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Figure 6. Time course of the change in the absorption spectra of the PVC composite films containing 10 wt % TTF after UV irradiation for 2 h at 25 °C. The samples were placed under ambient conditions in open air.

Figure 8. Dynamic mechanical curves (depicting E0 , E00 , and tan δ) of the PVC composites containing 10 wt % TTF before and after UV irradiation for 2 h at 25 °C. The films were obtained from a THF solution of PVC in the presence or absence of 10 wt % TTF after 12 h at 30 °C by casting the solution.

Figure 7. Photopatterned composite film (50 μm) containing 10 wt % TTF under UV irradiation (254 ( 20 nm) at 25 °C for 0, 3, 10, 30, and 60 min. (a) Photograph and (b) UV-vis/NIR absorption spectra of the PVC composite film.

slightly diffused in the composite films. The hole hopping from radical species to neutral state could hardly occur in the composites. These results suggest that TTF radical species can be preserved in the composite films. The deactivation of the radical species caused by moisture or oxygen might be prevented by wrapping in the polymer matrices. We executed the photopatterning for manufacturing the gradient concentration of the TTF radical species in the TTF/PVC composites. The UV irradiation time with the TTF/PVC composite films was modulated, and the radical concentrations in the composites were evaluated from the absorption spectra in UV-vis/ NIR measurements. Figure 7a represents the image of the film with Langmuir 2010, 26(2), 1152–1156

the gradient concentration of TTF cation radical. By increasing the irradiation time, the film became brown, and the radical concentration linearly increased in the film with the time of UV irradiation (Figure 7b). These results are summarized as follows. The radical concentration of TTF in polymer composites can be tuned by the UV irradiation time, and the resulting gradient concentration of TTF radical species can be memorized in the composites. The influence of UV irradiation on the thermomechanical properties of the PVC composites was evaluated by dynamic mechanical analysis (DMA; temperature scan at 1 Hz). DMA curves using the PVC composites containing 10 wt % TTF are shown in Figure 8. The modulus and the DMA glass transition temperatures (Tg) calculated from the peak of tan δ were determined (Tg = 44 °C, before reaction) from the DMA curves. The UV irradiation to the PVC composites had a slight effect on storage and loss modulus. In contrast, a new glass transition process was observed in the PVC composite after UV irradiation (Tg = 39 and 51 °C). In the polymer composites containing a large amount of filler, plasticizing effects have often been observed.18 The aggregation of the mixed-valence TTF might enhance the dissociation of the polymer chains, followed by a reduction in the Tg values of the composites.

Conclusion We describe the photoinduced self-assembly and formation of the mixed-valence TTF in polymer films. Our experiments suggest that TTF molecules can assemble through the PVC matrix to (18) Tanaka, K.; Adachi, S.; Chujo, Y. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, in press.

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form stacks, triggered by the oxidation. The radical concentration in the composite was tailored. In addition, the TTF radical species, including the mixed-valence state after the photopatterning, exhibited extremely high stability in the composite films. The materials which have the gradient of the radical spin concentration in the sole material have gradually received attention for their potential application as spintronic devices. We can expect that TTF polymer composites will be described as highly designable functional materials such as all organic spintronic materials as well as opto-electro devices.

Experimental Section General. UV-vis/NIR absorption spectra were recorded with a SHIMADZU UV-3600UV-vis-NIR spectrophotometer. The TTF/PVC films for the UV-vis/NIR spectra were deposited on a silica glass plate by casting the mixing solutions after they had been stirred for 1 h. Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer 1600 infrared spectrometer. After photoreaction for 2 h at 25 °C, the TTF/PVC composite films were soaked in acetonitrile, and the eluent was used for ESR measurements. The sample was sealed in a glass capillary (outside diameter of 0.5 mm) with a quartz tube (outside diameter of 5 mm) and subjected to ESR measurements taken with a JEOL JES SRE-2X spectrometer at the X-band (9.1 GHz) at 25 °C. The g values were determined using a Mn2þ-MgO solid solution as a standard. Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-5600 instrument operated at an

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accelerating voltage of 15 kV. Dynamic mechanical analysis (DMA) was performed on a SDM5600/DMS210 instrument (Seiko Instrument, Inc.) with the heating rate of 2 °C/min at 1 Hz with 1% strain under air. TTF was purchased from Tokyo Chemical Industry Co., Inc. PVC (n ∼ 1100) was purchased from Wako Pure Chemical Industries and used after reprecipitation in acetonitrile. UV irradiation was generated with an LMS-20E instrument (254 ( 20 nm, 4.6 mW/cm2) at 25 °C. Exponential approximations based on the first-order equation were used for determining the half-times of consumption of neutral TTF. Preparation of Polymer Composites. A typical preparation of TTF/PVC films is shown here. The THF solution (4 mL) containing 200 mg of PVC and 20 mg of TTF was prepared and cast onto the substrates. After drying at 30 °C for 12 h, the resulting films were used for a series of measurements. The thickness could be tuned by changing the volume of the mixture solutions (1 or 50 μm3).

Acknowledgment. This research was supported by the Casio Science Promotion Foundation. We thank Prof. Kazuyoshi Tanaka and Dr. Akihiro Ito of Kyoto University for the instruction and the kind advice about ESR measurements. Supporting Information Available: SEM images of the UVirradiated PVC composite films (Figure S1) and Dynamic mechanical curves (depicting tan δ) of the PVC composites (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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