J. Phys. Chem. B 2006, 110, 19319-19322
19319
Customized Morphologies of Self-Condensed Multisegment Polymer Nanowires Satoshi Tsukuda,† Shu Seki,*,† Masaki Sugimoto,‡ and Seiichi Tagawa† The Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan, and Quantum Beam Science Directorate, Japan Atomic Energy Agency, 1233 Watanuki-machi, Takasaki, Gunmma 370-1292, Japan ReceiVed: June 30, 2006; In Final Form: August 2, 2006
The direct formation of multisegment nanowires consisting of polymer domains by ion beam irradiation is investigated. Cross-linking reactions in the ion tracks result in localized gelation, giving isolated nanowires on substrates. It is demonstrated that the morphology of the final nanostructure is customized by appropriate selection of the ion fluence, combination of polymers, and the solvent employed for development. Octopuslike nanostructures consisting of a tangled hydrophilic polymer core and splayed hydrophobic polymer segments are successfully produced as an example of the process. The present technique provides universal feasibility for the formation of nanostructures based on “any” polymer materials in which radiation induces crosslinking reactions.
Introduction The application of high-energy ion beams has extended to many fields in recent years. In medicine, nonhomogeneous energy deposition along an ion trajectory (ion track) plays a crucial role in cancer radiotherapy,1 allowing for high spatial selectivity in the distribution of the radiation dose. The ionization and excitation processes that occur in ion tracks result in the generation of active intermediates at high density, leading to chemical reactions. The energy distribution along the ion tracks in polymers is particularly important in the case of reactions induced by ion beam exposure, and models of the energy distribution have been proposed by several groups on both experimental and theoretical bases.2-8 The direct observation and application of ion tracks in media have also attracted interest in materials science, where it is known as nuclear track fabrication.9-13 The technique has also been applied to the production of micro- and nanosized pores in materials through chemical etching of the tracks.9-11 The clear correlation between the etched pore and the characteristics of the incident charged particle has been utilized as the detectors for galactic cosmic rays in space.14 In contrast, the pored membranes produced by this method, as well as thermo-responsive etched track membranes covered with Hydro-gel, have also been examined as membrane filters for drug delivery.15-16 Our group has been examining the use of cross-linking reactions in the latent tracks for the direct formation of nanostructures.7,8,17-23 Heavy-ion irradiation of a cross-linking polymer thin film has been shown to cause cross-linking reactions along ion tracks, yielding a nanogel with reduced solubility in organic solvents. Developing irradiated samples using an organic solvent to remove the un-cross-linked polymer affords isolated nanowires with precisely controllable size and number density. Nanowires have been prepared in this way from several kinds of polymers, including polysilane, polycarbosilane, polystyrene, polythiophene, and even protein. However, these * Corresponding author. Tel: +81-6-6879-8502. Fax: +81-6-6876-3287. E-mail:
[email protected]. † The Institute of Scientific and Industrial Research, Osaka University. ‡ Quantum Beam Science Directorate, Japan Atomic Energy Agency.
nanowires have to date been formed only in films consisting of a single type of polymer. In the present study, the formation of nanowires from films consisting of two or three layers of different polymers is examined as a means of controlling the self-assembling behavior of the resultant nanowires. Experimental Section Reagents and chemicals were purchased from Wako Chemical Co. and Aldrich Chemical Co. Ltd. unless otherwise noted. Polymers of poly(methylphenylsilane) (PMPS) and poly(dimethylsilane) (PDMS) were synthesized by the Kipping reaction with sodium in refluxing toluene, n-undecane, and tetrahydrofuran (THF) from doubly distilled monomers of methylphenyldichlorosilane and dimethyldichlorosilane (Shin-Etsu Chemical) under an atmosphere of predried argon.24 Dried PDMS was pyrolyzed in an autoclave at 450 °C for 6 h.20 The pyrolyzed product, including polycarbosilane (PCS), was dissolved in toluene, and the insoluble portion was separated by filtration using a 1-µm pore-size polytetrafluorethylene (PTFE) membrane filter. PMPS and PCS were fractionally precipitated from the solution by stepwise addition of methanol, and the precipitates were collected by centrifugation. The molecular weight of PCS was measured by gel permeation chromatography (VP-10, Shimadzu) using THF as an eluent in a chromatograph equipped with four columns (Shodex KF-805L, Showa Denko). The molecular weights of PMPS and PCS used in the present study were Mn ) 1.3 × 105 and Mn ) 2.0 × 103 with dispersion of less than 1.5. Multilayer thin films were prepared by spin-coating of poly(hydroxystyrene) (PHS) dissolved in isopropyl alcohol (IPA), and PCS or PMPS dissolved in toluene, onto an Si substrate. Two-layer films were prepared by spin-coating PCS then PMPS, and three-layer films were prepared by spin-coating PMPS, PHS, then PCS. The Si substrates were cleaned in advance by exposure to O2 plasma. The samples were subsequently placed in a vacuum chamber and exposed to beams of 322-MeV 102Ru18+ (two-layer film) and 454-MeV 129Xe25+ (three-layer film) at the Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) cyclotron accelerator facility of the Japan
10.1021/jp0640981 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/14/2006
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Figure 1. AFM micrographs of PMPS nanowires of various lengths prepared by exposing PMPS films of (a) 100-, (b) 250-, and (c) 2000-nm thickness to a 454-MeV 129Xe25+ beam at an ion fluence of 3.0 × 109 ions/cm2.
Figure 2. AFM micrographs of two-segment PHS/PMPS nanowires prepared by exposing a PHS/PMPS two-layer film to a 322-MeV 102Ru18+ ion beam at an ion fluence of 1.0 × 109 ions/cm2. Images show nanowires after development with (a) benzene followed by (b) IPA. (c,d) Micrographs for the nanowires after one-step development using toluene/IPA solvents at ratios of (c) 1:2, (d) 1:1, and (e) 2:1.
Atomic Energy Agency. After irradiation, the samples were developed using organic solvents. Direct observation of the nanowires was performed by atomic force microscopy (AFM; SPI-4000, Seiko Instruments). Results and Discussion Ion irradiation at low fluence without overlapping ion tracks produces single-ion events in the target materials, yielding a cylinder-like nanostructure (nanowire) in the thin film. In crosslinking polymers, ion damage induces the cross-linking reaction gelation, and the undamaged area can be removed by development with an appropriate solvent. This technique can be used to prepare isolated nanowires on the substrate. As the incident ions penetrate through the polymer film, the length of the nanowires can be precisely controlled by changing the film thickness within the range of penetration length of the incident ions (Figure 1). The ranges of incident ions in the polymeric materials are calculated to be 45, 46, and 56 µm in PCS, PHS, and PMPS for 322-MeV 102Ru18+ ion beams, and 50, 51, and 62 µm for 454-MeV 129Xe25+ ion beams, respectively.25 It should be noted that the ranges were far longer than the thickness of the target polymer films used in the present study, and the loss of kinetic energy of incident particles penetrating the polymer films was estimated to be less than 2% of the initial energy in all cases. The use of two- and three-layer films was examined in the present study as a potential means to control the assembly
behavior of the resultant nanowires. Each of the layers of the present multilayer polymer films has a different solubility in organic solvent, allowing for multistage development. Figure 2 shows AFM images of the two-layer (PHS/PMPS) film, which was developed sequentially in benzene (to remove undamaged PMPS) and then IPA (to remove PHS) after exposure to the ion beam. The resultant nanowires were composed of a 270nm segment of PMPS and 380-nm segment of PHS, reflecting the thicknesses of each of the original films. The total length of the nanowires also matches the total thickness of the twolayer film (650 nm). We have developed the following expression of form representing the radius (r) of nanowires prepared by the present technique on the basis of the radial dose distribution in an ion track:7,8
r2 )
[ ( )]
e1/2rp LET[G(x)]N 3R ln 400πβ rc
-1
(1)
where LET, G(x), N, R, and β are deposited energy density along ion trajectories (Linear Energy Transfer, eV nm-1), efficiency of cross-linking reactions ((100 eV)-1), degree of polymerization, intrinsic viscosity index of the polymer materials, and the effective density parameter of the monomer unit (kg m-3), respectively. Equation 1 gives the estimates of r as 12.5 nm for PMPS and 10 nm for PHS, respectively, which show a good agreement with the values of r observed as 11.9 nm (PMPS) and 10.5 nm (PHS). This is apparently reflected in Figure 2b
Customized Morphologies of Polymer Nanowires
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19321
Figure 3. AFM micrographs of three-segment PMPS/PHS/PCS nanowires prepared by exposing the corresponding three-layer films to a 454-MeV 129 Xe25+ ion beam at ion fluences of (a) 1.1 × 109, (b) 5.3 × 109, and (c) 1.0 × 1010 ions/cm2 followed by development in toluene/IPA solution (2:1).
Figure 4. Selective self-entanglement of three-segment PMPS/PHS/PCS nanowires prepared by exposure of the corresponding film to a 454-MeV 129 Xe25+ ion beam at ion fluences of 1.1 × 109 (upper) and 5.3 × 109 ions/cm2 (lower).
Figure 5. AFM micrographs of octopus-like nanostructures after (a) initial preparation, (b) rewashing with IPA, and then (c) ultrasonication in IPA for 10 min.
which shows the nanowire features of a “thicker” PMPS segment linked with a “thinner” PHS segment. As the nanostructure included both hydrophilic (PHS) and hydrophobic (PMPS) parts, the final shape of the nanowire was influenced by the solvent employed for second-stage development. Figure 2 also shows the morphology of nanowires developed in three different solvents (toluene/IPA ) 2:1, 1:1, 1:2). In the 2:1 solvent, a number of nanowires became
entwined, whereas straight nanowires were obtained by development using the 1:1 mixed solvent. In the 1:2 solvent, a large tangled aggregation of nanowires was obtained on the Si substrate. It is considered that the poor adhesion between the PHS end of the nanowire and the Si substrate allowed the nanowires to become detached and to become tangled due to interaction with the solvent. PMPS nanowires, on the other hand, have been reported to adhere tightly to Si substrates.18,21,22
19322 J. Phys. Chem. B, Vol. 110, No. 39, 2006 This selective aggregation by interaction with the solvent was more prominent in the case of the three-layer films (PMPS, PHS, and PCS). Figure 3 shows AFM images of the three-segment nanowires after irradiation and single-stage development in a 2:1 mixed solvent of toluene and IPA. At low ion fluence (1.1 × 109 ions/cm2), nanowires with PMPS, PHS, and PCS segments were successfully isolated on the Si substrate. However, the central hydrophilic segment (PHS part) was entangled by the solvent (Figure 3a), resulting in a tangled central core and splayed end parts (hydrophobic PMPS and PCS). This clearly demonstrates the effect of selective selfassembly. As the ion fluence is increased, with a corresponding increase in the number density of nanowires, the tangling of the PHS segments causes aggregation of 3-4 adjacent nanowires (Figure 3b), resulting in an “octopus-like” nanostructure. The driving force for this aggregation is likely to be the strong interchain interaction in the PHS segments surrounded by the nonpolar solvent molecules. With a further increase in ion fluence, more nanowires were aggregated into each tangle of PHS segments (Figure 3c). Schematic mechanisms of the selective-assembly observed in the present case are displayed in Figure 4. It should be noted that each “octopus-like” nanostructure has an even number of “legs” in any cases of observation for the nanostructures prepared from three-layer films in the present study. The bindings of the different polymer segments at their interfaces also play an important role in the tangling of hydrophilic polymer segments with stretched hydrophobic polymer legs. In the case of the substitution of the hydrophilic PHS layer to the hydrophilic poly(vinyl alcohol) which had already been demonstrated to give nanowires upon ion beam irradiation,20 the octopus-like nanostructure was not observed even after a variety of development conditions. This may suggest that the heteromolecular cross-linking reactions at the interfaces of PHS/PCS or PHS/PMPS are important for the bindings of the multisegment nanowires. Figure 5 shows AFM images of the sample from Figure 3b after rewashing in IPA and ultrasonication in IPA for 10 min. The octopus-like nanostructure was little changed by the rewashing process, but after ultrasonication it appears to have lost the central tangle of PHS segments. It is considered that the tangled PHS core was unraveled by ultrasonication in the polar solution and separated from the hydrophobic parts. In summary, heavy-ion irradiation of two- and three-layer polymer films was shown to produce two- and three-segment nanowires directly. The morphology of the nanowires thus produced can be customized by appropriate selection of the ion fluence, combination of polymer segments, and type of organic solvent employed for development. Three-segment nanowires consisting of one hydrophobic segment and two hydrophilic segments were selectively assembled using appropriate irradiation conditions and solvents to produce octopus-like nanostructures. A diverse range of polymer combinations could be employed to achieve highly customized nanostructure morphologies. This technique is therefore potentially applicable in a wide range of fields. The nanowires of multisegments with corresponding solvent affinities and/or environmental (thermo, pH,
Tsukuda et al. etc.) responsibilities could be used as the carriers of drugs (drug delivery systems) in biological systems through their controlled aggregation properties. Polymeric systems have been often used as the suspension matrixes of metal nanoparticles, thus the present system could be used for the highly condensed supporting systems for catalysts with the huge active surface areas. Conclusion The present study provides a novel preparation technique of 1-D multi-functional nanowires consisting of “any” cross-linked polymer domains with customized morphologies via the selective entanglement of a part of the nanowires. According to the universal applicability of the present technique for polymer nanostructure formation, there is no longer need for “seeking” a specific polymer material giving self-organized nanostructures. Acknowledgment. The work was supported by a Grant-inAid from the Japan Society for the Promotion of Science, Japan. References and Notes (1) Wilson, R. R. Radiology 1946, 47, 487. (2) Magee, J. L.; Chattarjee, A. In Kinetics of Nonhomogeneous Processes; Freeman, G. R., Ed.; John Wiley & Sons: New York, 1987; Chapter 4, p 171. (3) Varma, M. N.; Baum, J. W.; Kuehner, A. J. Radiat. Res. 1975, 62, 1. (4) Kobetich, E. J.; Katz, R. Phys. ReV. 1968, 170, 391. (5) Wilson, W. E. Radiat. Res. 1994, 140, 375. (6) Pimblott, S. M.; LaVerne, J. A. J. Phys. Chem. A 2002, 106, 9420. (7) Seki, S.; Tsukuda, S.; Maeda, K.; Matsui, Y.; Saeki, A.; Tagawa, S. Phys. ReV. B 2004, 70, 144203. (8) Seki, S.; Tsukuda, S.; Maeda, K.; Tagawa, S.; Shibata, H.; Sugimoto, M.; Jimbo, K.; Hashitomi, I.; Kohyama, A. Macromolecules 2005, 38, 10164. (9) Fleischer, R. L.; Price, P. B. Science 1963, 140, 1221. (10) Price, P. B.; Walker, R. M. Nature 1962, 196, 732. (11) Fleischer, R. L.; Price, P. B.; Walker, R. M. Science 1965, 149, 383. (12) Spohr, R. In Ion Tracks and Microtechnology; Vieweg: Braunschweig, 1990. (13) Fink, D. In Fundamentals of Ion-Irradiated Polymers; Springer: Berlin, 2004. (14) Cartwright, B. G.; Shirk, E. K.; Price, P. B. Nucl Instrum. Methods 1978, 153, 457. (15) Yoshida, M.; Asano, M.; Safranj, A.; Omichi, H.; Spohr, R.; Vetter, J.; Katakai, R. Macromolecules 1996, 29, 8987. (16) Reber, N.; Ku¨chel, A.; Spohr, R.; Wolf, A.; Yoshida, M. J. Membr. Sci. 2001, 193, 49. (17) Tsukuda, S.; Seki, S.; Tagawa, S.; Sugimoto, M. Appl. Phys. Lett. 2005, 87, 233119. (18) Seki, S.; Maeda, K.; Tagawa, S.; Kudoh, H.; Sugimoto, M.; Morita, Y.; Shibata, H. AdV. Mater. 2001, 13, 1663. (19) Seki, S.; Maeda, K.; Kunimi, Y.; Tagawa, S.; Yoshida, Y.; Kudoh, H.; Sugimoto, M.; Morita, Y.; Seguchi, T.; Iwai, T.; Shibata, H.; Asai, K.; Ishigure, K. J. Phys. Chem. B 1999, 103, 3043. (20) Tsukuda, S.; Seki, S.; Tagawa, S.; Sugimoto, M.; Idesaki, A.; Tanaka, S.; Ohshima, A. J. Phys. Chem. B 2004, 108, 3407. (21) Seki, S.; Tsukuda, S.; Yoshida, Y.; Kozawa, T.; Tagawa, S.; Sugimoto, M.; Tanaka, S. Jpn. J. Appl. Phys. 2003, 42, 4159. (22) Tsukuda, S.; Seki, S.; Saeki, A.; Kozawa, T.; Tagawa, S.; Sugimoto, M.; Idesaki, A.; Tanaka, S. Jpn. J. Appl. Phys. 2004, 43, 3810. (23) Tsukuda, S.; Seki, S.; Sugimoto, M.; Tagawa, S. Jpn. J. Appl. Phys. 2005, 44, 5839. (24) Seki, S.; Koizumi, Y.; Kawaguchi, T.; Habara, H.; Tagawa, S. J. Am. Chem. Soc. 2004, 126, 3521; Seki, S.; Yoshida, Y.; Tagawa, S.; Asai, K. Macromolecules 1999, 32, 1080. (25) Ziegler, J. F.; Biersack, J. P.; Littmark, U. In The Stopping and Range of Ions in Solids; Pergamon Press: New York, 2003.
