pubs.acs.org/Langmuir © 2010 American Chemical Society
Flexible, Transparent Nanocomposite Film with a Large Clay Component and Ordered Structure Obtained by a Simple Solution-Casting Method Kazuhiro Shikinaka,† Kazuto Aizawa,† Nozomu Fujii,† Yoshihito Osada,‡ Masatoshi Tokita,§ Junji Watanabe,§ and Kiyotaka Shigehara*,† †
Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei 184-8588, Japan, ‡ Advanced Science Institute, Riken, Wako 351-0198 Japan, and §Department of Organic and Polymer Materials, Tokyo Institute of Technology, Ookayama 152-8552, Japan Received April 15, 2010. Revised Manuscript Received June 24, 2010
A flexible, transparent nanocomposite (NC) film with 57 wt % inorganic components was obtained by the simple casting of a solution of Laponite and modified organic molecules through a sol-gel reaction. The NC film has solvent resistance and a disco-nematic liquid-crystalline-like structure of Laponite that originates from the cross linking of Laponite by silanol agents and the large amount of Laponite in the film.
Introduction Polymer-clay nanocomposites (NCs) have special properties (e.g., excellent mechanical strength, excellent gas-barrier properties, and ionic conductivity).1-7 These are achieved by, for example, organic premodification to the clay nanoparticles8 and polymer-clay multilayer formation.9-13 The properties of NCs depend on the feed ratio and the dispersed uniformity of the components. In particular, it is expected that the physical properties of NCs are drastically improved by an increase in the amount of clay with anisotropic shape (e.g., fibril or disk) because of the formation of ordered structure. However, an increase in the amount of the clay component often causes structural inhomogeneities (e.g., inadequate dispersion) that have negative effects on the optical and mechanical properties. Therefore, general NCs consist of only a few weight percent of clay, and the addition of ordered structure is achieved by a limited cumbersome technique such as the multilayer formation method.9-13 Recently, an NC was obtained by the direct mixing of poly(vinyl alcohol) and montmorillonite (MTM) and doctor blading of the mixture.14 This achieved a high proportion of MTMs with unidirectional alignment. However, this NC lacks flexibility and transparency. *Author to whom correspondence should be addressed. E-mail: jun@ cc.tuat.ac.jp. Tel: þ81-42-388-7052. Fax: þ81-42-381-8175.
(1) Saegusa, T.; Chujo, Y. Makromol. Chem., Macromol. Symp. 1992, 64, 1–9. (2) Chujo, Y.; Tamaki, R. MRS Bull. 2001, 26, 389–392. (3) Mark, J. E. Macromol. Symp. 2003, 201, 77–83. (4) Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. J. Mater. Res. 1993, 8, 1179–1184. (5) Giannelis, E. P. Adv. Mater. 1996, 8, 29–35. (6) Haraguchi, K.; Takehisa, T. Adv. Mater. 2002, 14, 1120–1124. (7) Haraguchi, K.; Ebato, M.; Takehisa, T. Adv. Mater. 2006, 18, 2250–2254. (8) Tajouria, T.; Bouchrihab, H.; Hommelc, H. Polymer 2003, 44, 6825–6833. (9) Tang, Z.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2, 413– 418. (10) Dundigalla, A.; Lin-Gibson, S.; Ferreiro, V.; Malwitz, M. M.; Schmidt, G. Macromol. Rapid Commun. 2005, 26, 143–149. (11) Stefanescu, E. A.; Daly, W. H.; Negulescu, I. I. Macromol. Mater. Eng. 2008, 293, 651–656. (12) Bonderer, L. J.; Studart, A. R.; Gauckler, L. J. Science 2008, 319, 1069– 1073. (13) Podsiadlo, P.; Liu, Z.; Paterson, D.; Messersmith, P. B.; Kotov, N. A. Adv. Mater. 2007, 19, 949–955. (14) Walther, A.; Bjurhager, I.; Malho, J.-M.; Pere, J.; Ruokolainen, J.; Berglund, L. A.; Ikkala, O. Nano Lett., published online Mar 10, 2010, http:// dx.doi.org/10.1021/nl1003224.
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In this letter, we report of the use of a simple solution-casting method to create a novel flexible, transparent NC film with a large component of Laponite RD with an ordered structure. Laponite RD (hereafter referred to as Laponite) is a relatively uniform diskshaped synthetic clay that is 25 nm in diameter and 1 nm thick;15 it is widely used as a constituent of NC.6,7,16 Because the surface of Laponite has a negative charge and the edges show a positive charge at neutral pH, it can disperse randomly in water. Because of this electrostatic interaction, a solution of Laponite of a few (∼3) weight percent undergoes gelation (i.e., the random dispersion state of Laponite is fixed through electrostatic interaction). In this study, the modification of an organic polymer and Laponite in a dispersed solution was performed by a sol-gel reaction for the homogeneous mixing of an organic polymer with a large amount of Laponite. It has been reported that poly(ethylene glycol) (PEG) interacts with the surface of Laponite and prevents the aggregation of Laponite by forming a steric layer.17 Furthermore, Laponite has silanol groups on its edge, and a sol-gel reaction on these silanol groups has been performed.18 Here, PEG interacts with Laponite in a dispersed solution, and a sol-gel reaction is performed to fix a steric layer of PEG and Laponite. The use of PEG would give additional biomedical applications for the nanocomposite. We used a sol-gel reaction to synthesize an NC consisting of Laponite and PEG with a large inorganic component. We then obtained an NC film with an ordered structure of the components by casting the NC solution.
Experimental Procedure Materials. Synthetic Laponite RD, Naþ0.7[(Si8Mg5.5Li0.3)O20(OH)4]0.7-, was purchased from Rockwood Additives Ltd. Other chemicals were purchased from Kanto, Tokyo Kasei, Wako, and Aldrich. All chemicals were used as received, and deionized water was purified with a Milli-Q Plus system (Millipore, Eschborn, Germany). Synthesis of Triethoxysilane Modified with Poly(ethylene glycol) (s-PEG). The procedure is arranged as described in a (15) www.laponite.com (Rockwood Additives Ltd.). (16) Liff, S. M.; Kumar, N.; McKinley, G. H. Nat. Mater. 2007, 6, 76–83. (17) Bujdak, J.; Hackett, E.; Giannelis, E. P. Chem. Mater. 2000, 12, 2168–2174. (18) Bl€ummel, J.; Perschmann, N.; Aydin, D.; Drinjakovic, J.; Surrey, T.; Garcia, M. L.; Kessler, H.; Spatz, J. P. Biomaterials 2007, 28, 4739–4747.
Published on Web 06/29/2010
DOI: 10.1021/la101496b
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previous study.18 A 300 mL four-necked flask containing methoxy-terminated poly(ethylene glycol)750 (mPEG750; 15.0 g) was dried in vacuo at 90 °C for 1 h. At 50 °C, the dried mPEG750 was mixed with 3-isocyanateopropyl-triethoxysilane (3-ITPS; 5.45 g) in 150 mL of 1,4-dioxane (20.0 mM mPEG750) in the presence of 0.454 g (4.05 mM) of 1,4-diazabicyclo[2,2,2]octane and refluxed for 24 h. Then, the solvent was evaporated and the raw products were purified by recrystallization from toluene/hexane 1:4 to yield a white waxen solid (95%). Sol-Gel Reaction of s-PEG and Laponite. The procedure is arranged as described in a previous study.19 A 100 mL Erlenmeyer flask containing Laponite (0.3 g) in deionized water (20 mL) was stirred at 50 °C for 3 h. In another 100 mL Erlenmeyer flask, a solution of s-PEG (1.68 g, 1.68 mM) and deionized water (40 mL) was prepared and the pH was adjusted to 4.3 with acetic acid. This solution was heated to 50 °C and stirred for 20 min. Then it was mixed with the Laponite dispersion, and the mixture was stirred at 50 °C for 21 h. The product was collected via vacuum filtration and rinsed with deionized water using a 100 nm Millipore filter. The products were freeze dried and stored at r.t.
Figure 1. (a) Freeze-dried powder of modified Laponite. (b) NC film that consists of 57 wt % (w/w) inorganic components. The thickness of the NC film is 20 μm.
Cast Film Preparation of Organically Modified Laponite. Modified Laponite solution (3.0 or 5.0 wt %, w/w, defined as the amount of Laponite) was obtained by the dispersion of freezedried powder of modified Laponite in deionized water. This was cast onto a Teflon surface with various thicknesses at r.t. by simply dropping the solution or using a Baker film applicator. It was then dried in air for ∼3 days. Tensile Stress-Strain Measurement. The tensile stressstrain measurements were performed on cast films using a Tensilon universal tester (Tensilon RTC-1250, Orientec Co.) at 25 °C. The sample strips were 2 mm wide and 20 mm long. Five millimeters from each end of the sample was clamped with polyimide tape to prevent the sample from slipping out of the machine. The data were measured using a load cell (UR-10 L-A, Orientec Co.) at a 100 mm/min head speed. The initial modulus (E) and tensile fracture strength (δ) were found by averaging the values for three samples. E was determined by the average slope over the range of 0-0.01 of the strain ratio from the stress-strain curve. Scanning Electron Microscopy. Scanning electron microscopy (SEM) observation was performed using a JSM-6510 (JEOL, Tokyo, Japan) operating at a 10 kV acceleration voltage. The fracture surface of the film after the quench by liquid nitrogen was observed with no staining. Small-Angle X-ray Scattering. The small-angle X-ray scattering (SAXS) experiments were performed using Ni-filtered Cu KR radiation (NanoSTAR-U, Bruker AXS, Germany) at 50 kV and 100 mA, providing a KR1 wavelength of 1.54 A˚. An image of the scattering pattern was obtained at a frame size of 1024 pixels 1024 pixels using the 2D photon-counting HI-STAR detector system. The q range covered in the SAXS measurement was 0.0085 to 0.20 A˚-1, where q = 4π sin(θ/2)/λ and θ is the scattering angle. The exposure time and the specimen-to-detector distance were 3600 s and 1.056 m, respectively.
Results and Discussion To modify PEG and Laponite through a sol-gel reaction, triethoxysilane modified with PEG (Scheme S1, hereafter referred to as s-PEG) was synthesized and was reacted with Laponite. The reaction was confirmed by FT-IR (Figure S1), 29Si solidstate NMR (Figure S2), and XRD (Figure S3) as shown in the Supporting Information. TGA analysis (Figure S4) revealed that the s-PEG-modified Laponite consisted of 57 wt % inorganic components. (19) Wheeler, P. A.; Wang, J.; Baker, J.; Mathias, L. J. Chem. Mater. 2005, 17, 3012–3018.
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Figure 2. Typical stress-strain curve for the NC film.
The modified Laponite can be preserved as a freeze-dried powder (Figure 1a) and can be dispersed in deionized water. The cast film of the modified Laponite solution (hereafter referred to as the NC film) was obtained by the simple method described in Figure 1. Despite containing 57 wt % inorganic components, the NC film has a high light transmittance of >90% at 400-800 nm (Figure S5). The NC film did not dissolve and swell in various organic solvents (e.g., methanol and chloroform) for at least 1 week. In particular, the film never melted in deionized water (a good solvent for PEG and Laponite) despite being heated to 80 °C. A cast film of Laponite solution simply mixed with PEG in the same ratio (hereafter referred to as the simple mixed film) easily dissolves in deionized water. Therefore, the modification of Laponite by the silane agent induces solvent resistance. TGA analysis and liquid-state NMR spectra (Figure S6) confirmed the modification of the excess amount of s-PEG with respect to Laponite and the polymerization of the triethoxysilanes of s-PEG. Therefore, excess triethoxysilanes cross-link Laponite originating from the gradual condensation of the modified Laponite solution until being air dried. It is expected that the cross linking makes a 3D network of Laponite and triethoxysilane (Figure S7) and induces the solvent resistance of the NC film. The NC film has flexibility as shown in Figure 1b, although the simple mixed film is brittle. A tensile stress measurement was performed as shown in Figure 2. The initial modulus (E), fracture strain (ε), and tensile fracture strength (δ) of the film are 11.0 MPa, 0.088, and 28.7 MPa, respectively. The NC film exhibits a J-shaped stress-strain curve. This is fundamentally different from that of other polymer-clay composite films12,13 and similar to that of ordinary living tissues (e.g., skin and veins).20,21 These results encourage us to use the NC film as a bioalternate material. Because the simple mixed film fractured at low strain (ε = 0.015, Figure S8), it is believed that the modification caused (20) Vincent, J. F. V. Structural Biomaterials; Princeton University Press: Princeton, NJ, 1990. (21) Vogel, S. Comparative Biomechanics: Life’s Physical World; Princeton University Press: Princeton, NJ, 2003.
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Figure 3. SEM image of the NC film.
by the silane agent induces the flexibility of the NC film. Because the cast film of a Laponite solution is too brittle for a tensile stress measurement to be performed, PEG also provides flexibility to the NC film. The DSC curve of the NC film has no peak because of the crystalline structure of the PEG. A peak is observed at 21 °C on the DSC curve of the simple mixed film (Figure S9). This result indicates that the PEG in the NC film is completely amorphous (i.e., the interaction between PEG and Laponite through the sol-gel reaction prevents the crystallization of the PEG chains). SEM shows the overall aligned structure parallel to the surface of the film (Figure 3). Figure 4 shows the SAXS results for the NC film. The isotropic (in the x-z plane) and the anisotropic (in the x-y plane) scattering patterns indicate that the film has a disconematic liquid-crystalline-like structure. Because the PEG is amorphous in the NC film, it is expected that the Laponite platelets are predominantly aligned parallel to the surface of the film (i.e., in the x-z plane) as illustrated in Figure 4a. Although the NC film was obtained by simple casting and air drying of the PEG-modified Laponite solution, the ordered structure of Laponite emerged. As mentioned above, the large amount of Laponite platelets in the modified Laponite solution (57 wt %) induces its ordered alignment in the NC film according to the excludedvolume (entropic) effect on the molecules described by Onsager.22 A gradual condensation of the modified Laponite solution during air drying occurs, and this induces the alignment of Laponite in the overall area of the film. Because of the ordered structure of the Laponite, the NC film exhibits birefringence under the polarized optical microscope (POM, Figure S10). In conclusion, a flexible, transparent NC film with 57 wt % inorganic components was obtained by organic modification through a sol-gel reaction in a dispersed solution of the components. The NC film was prepared by simple casting methods without the assistance of cumbersome techniques such as organicinorganic multilayer formation.9-13 Surprisingly, the NC film has (22) Onsager, L. Ann. N.Y. Acad. Sci. 1949, 51, 627–659.
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Figure 4. (a) Schematic image of the SAXS experiment system. (b, Left) 2D SAXS patterns from the x-y plane (i.e., perpendicular to the surface of the film) and (right) x-z plane (i.e., parallel to the surface of the film).
solvent resistance even for water although it consists of Laponite and PEG, which are water-soluble molecules. Furthermore, the disco-nematic liquid-crystalline-like structure of Laponite emerged in the NC film although PEG is completely amorphous. Because the NC film presented here has special properties, it can be used for various advanced research and technological applications. In particular, the NC film is expected to be suitable for use as a biomaterial (e.g., tissue adhesive material and artificial skin) because of the biocompatibility of the components. PEG has also been considered to be a solid polymer electrolyte.23,24 Because the Laponite platelets are aligned in one direction and PEG is completely amorphous in the NC film, the film has high ionic conductivity, making it suitable for use as a solid polymer electrolyte. Thus, the NC film has the potential to induce material innovation in various fields. Supporting Information Available: An experimental section discussing the characterization of modified Laponite, Fourier transform infrared (FT-IR) spectra, nuclear magnetic resonance (NMR) spectra, wide-angle X-ray diffraction (XRD), thermogravimetric analysis (TGA), UV-vis spectra, differential scanning calorimetry (DSC), and polarized optical microscopy (POM). This material is available free of charge via the Internet at http://pubs.acs.org. (23) Ratner, M. A.; Shriver, D. F. Chem. Rev. 1988, 88, 109–124. (24) Kitajima, S.; Tominaga, Y. Macromolecules 2009, 42, 5422–5424.
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