pubs.acs.org/Langmuir © 2010 American Chemical Society
Superamphiphilic Janus Fabric Ho Sun Lim,† Song Hee Park,‡ Song Hee Koo,‡ Young-Je Kwark,‡ Edwin L. Thomas,† Youngjin Jeong,*,‡ and Jeong Ho Cho*,‡ †
Department of Materials Science and Engineering, Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States, and ‡Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul 156-743, Korea Received September 24, 2010. Revised Manuscript Received October 28, 2010
Janus fabrics with superamphiphilicity were fabricated via electrospinning of polyacrylonitrile (PAN). PAN nanofibrous mats were formed on an aluminum foil substrate and then thermally treated to cause hydrolysis. An identical PAN solution was subsequently electrospun onto the hydrolyzed PAN layer, followed by peeling off of the bicomposite film from the collector substrate to produce a free-standing Janus fabric. On one side, the electrospun PAN mat exhibited superhydrophobic properties, with a water contact angle of 151.2°, whereas the initially superhydrophobic PAN sheet on the opposite side of the fabric was converted to a superhydrophilic surface (water contact angle of 0°) through hydrolysis of the surface functional groups induced by the thermal treatment. The resulting Janus fabrics exhibited both superhydrophobicity, repelling water on the one side, and superhydrophilicity, absorbing water on the other side. The organic solvent resistance of the PAN nanofibrous sheets was remarkably improved by incorporation of a tetraethyl orthosilicate. This facile and simple technique introduces a new route for the design and development of functional smart, robust fabrics from an inexpensive, commercially available polymer.
1. Introduction Producing functional smart fabrics using commercially available polymers has attracted great interest for numerous applications, including high-performance medical implants, filter media, chemical nanoreactors, biological/chemical sensors, and electronic devices.1-8 Research efforts have led to the development of waterproof and breathable materials, which initiated a revolution in functional garment technologies. For instance, a waterproofbreathable polytetrafluoroethylene (PTFE) membrane having a microporous structure displays the fascinating ability to completely prevent the penetration of water droplets from one side, while easily evaporating water vapor molecules across the membrane.9 The watertight and respirable qualities are attributed to the size of the microscopic pores in the hydrophobic PTFE materials, which are smaller than the diameter of the average water droplet but much larger than water vapor molecules. The performance of such functionalities can be further enhanced via control over both *Corresponding author: E-mail:
[email protected] (J.H.C.), yjeong@ ssu.ac.kr (Y.J.).
(1) Dzenis, Y. Science 2008, 319, 419–420. (2) Li, D.; Xia, Y. Nano Lett. 2003, 4, 933–938. (3) Greiner, A.; Wendorff, J. H. Angew. Chem., Int. Ed. 2007, 46, 5670–5703. (4) Li, W.-J.; Mauck, R. L.; Tuan, R. S. J. Biomed. Nanotechnol. 2005, 1, 259–275. (5) Lee, S. W.; Lee, H. J.; Choi, J. H.; Koh, W. G.; Myoung, J. M.; Hur, J. H.; Park, J. J.; Cho, J. H.; Jeong, U. Nano Lett. 2010, 10, 347–351. (6) Anzenbacher, P.; Palacios, M. A. Nature Chem. 2009, 1, 80–86. (7) Subbiah, T.; Bhat, G. S.; Tock, R. W.; Pararneswaran, S.; Ramkumar, S. S. J. Appl. Polym. Sci. 2005, 96, 557–569. (8) Mosinger, J.; Lang, K.; Plistil, L.; Jesenska, S.; Hostomsky, J.; Zelinger, Z.; Kubat, P. Langmuir 2010, 26, 10050–10056. (9) Kennedy, M. E.; Hollenbaugh, D. R. U.S. Patent 7,521,010, 2009. (10) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (11) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (12) Zhai, L.; Cebeci, F. C-.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349–1353. € (13) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777–7782. (14) Lim, H. S.; Han, J. T.; Kwak, D.; Jin, M.; Cho, K. J. Am. Chem. Soc. 2006, 128, 14458–14459. (15) Lim, H. S.; Kwak, D.; Lee, D. Y.; Lee, S. G.; Cho, K. J. Am. Chem. Soc. 2007, 129, 4128–4129.
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the geometric details of the surface texture and the inherent surface free energy of the smooth surface.10-17 A combination of their easy manufacturing and high performance makes functional smart fabrics attractive for a number of practical applications. In this study, we demonstrate a facile fabrication of a biphasic Janus fabric from electrospun fibers composed of polyacrylonitrile (PAN), which is one of commercially available polymers. The Janus fabric features a superhydrophobic surface that forces water away on one side and a superhydrophilic surface that attracts water on the other side. Electrospinning is a simple, convenient, and effective method to produce continuous fibers with micro- and nanosized diameters that may be used to direct hierarchical surface structures on the top surface of any shape of large area substrate.18-22 Thus, the electrospinning technique allows nanofibrous nanostructures to versatilely apply in a wide range of scientific and industrial fields including filtration technologies, tissue engineering, optical devices, and sensors, taking advantage of their unique characteristics such as large surface area-to-volume ratio and ease of fabrication. In particular, to achieve the Janus fabric, we have paid attention to a unique nature of PAN: a hydrophobic PAN surface may be easily converted to a hydrophilic one as a result of hydrolysis induced by thermal treatment (Figure 1).23-25
(16) Lim, H. S.; Lee, S. G.; Lee, D. H.; Lee, D. Y.; Lee, S.; Cho, K. Adv. Mater. 2008, 20, 4438–4441. (17) Lafuma, A.; Quere, D. Nature Mater. 2003, 2, 457–460. (18) McCann, J. T.; Marquez, M.; Xia, Y. J. Am. Chem. Soc. 2006, 128, 1436– 1437. (19) Ma, M. L.; Titievsky, K.; Thomas, E. L.; Rutledge, G. C. Nano Lett. 2009, 9, 1678–1683. (20) Huang, Z.; Zhang, Y.; Kotaki, M.; Ramakrishna, S. Compos. Sci. Technol. 2003, 63, 2223–2253. (21) Bergshoef, M. M.; Vancso, G. J. Adv. Mater. 1999, 11, 1362–1365. (22) Ma, M.; Gupta, M.; Li, Z.; Zhai, L.; Gleason, K. K.; Cohen, R. E.; Rubner, M. F.; Rutledge, G. C. Adv. Mater. 2007, 19, 255–259. (23) Chemat, F.; Poux, M.; Berlan, J. J. Chem. Soc., Perkin Trans. 1994, 2, 2597– 2602. (24) Nakamura, S.; Otake, T.; Matsuzaki, K. J. Appl. Polym. Sci. 1972, 16, 1817–1825. (25) Hay, J. N. J. Polym. Sci., Part A: Polym. Chem. 1968, 6, 2127–2135.
Published on Web 11/12/2010
DOI: 10.1021/la103829c
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Figure 1. Schematic representation for thermal hydrolysis of polyacrylonitrile at 200 °C in air.
2. Experimental Details Fabrication of Janus Fabric. Polyacrylonitrile (PAN) was purchased from Misui Chemicals, Inc. N,N-Dimethylformamide (DMF) and tetraethyl orthosilicate (TEOS) were purchased from Sigma-Aldrich. A 15 wt % PAN (Mw = 300 000 g/mol) solution was dissolved in DMF. Subsequently, TEOS was added to the solution to yield mass ratios of 0, 30, or 50 wt % TEOS relative to that of PAN (PAN-TEOS 0%, PAN-TEOS 30%, and PANTEOS 50%, respectively), and the solutions were stirred for 6 h at room temperature. The polymer solution was electrospun into nanofibers under a 16 kV voltage applied over a collection distance of 15 cm between the needle and a grounded aluminum foil collection substrate. In order to impart superhydrophilicity, the as-electrospun fibrous mats were heated in air at 200 °C for 1 h, and then the same polymer solution was electrospun onto the thermally treated PAN fibrous mats to make Janus fabrics. The Janus fabrics were obtained by peeling off the electrospun mats from the collector substrate. Characterization. The morphologies of the electrospun PAN fibrous mats were imaged by scanning electron microscopy (JEOL JSM-6360). Water contact angle (CA) measurements were conducted with a drop shape analysis system (Kr€ uss). Fourier transform infrared (FT-IR) spectra were recorded on an FTIR6300 (JASCO) spectrometer in transmittance mode with a resolution of 4 cm-1. The superamphiphilic behavior of the free-standing electrospun mats was quantitatively evaluated by measuring the transmissivity of water using a gravimetric absorbency testing system. The porosity and pore size distribution of the fibrous mats were measured by using CFP-1200-AEL (Porous Materials Inc.).
3. Results and Discussion A 15 wt % PAN (Misui Chemicals, Inc., Mw=300 000 g/mol) solution was dissolved in N,N-dimethylformamide (DMF) and electrospun into fibers under a 16 kV voltage applied over a collection distance of 15 cm between the needle and a grounded aluminum foil collection substrate. Figure 2a shows the morphology of the continuous electrospun PAN fibers, which featured an average diameter of 280 nm and a uniform diameter distribution. The thickness of the fiber mat was 12.7 ( 0.8 μm. This fabric also showed a uniform pore size distribution of 2.84 ( 0.23 μm as shown in Figure 3. As spun, the water contact angle (CA) of the PAN nanofibrous web was 151.2 ( 2°. Such superhydrophobicity arises from a combination of the low surface energy of the PAN and the hierarchical re-entrant architectures formed by (26) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Science 2007, 318, 1618–1622. (27) Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 18200–18205. (28) Acatay, K.; Simsek, E.; Ow-Yang, C.; Menceloglu, Y. Z. Angew. Chem., Int. Ed. 2004, 39, 5322–5325.
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electrospinning.26-28 Even though the water CA for a smooth PAN surface was 71.9 ( 1°, the electrospun PAN mat displayed the superhydrophobic character due to enhanced surface roughness (Table 1). In addition, this fabric showed superhydrophobicity of “gecko” states with strong water-adhesive properties.29 Even when the fibrous mat was vertically tilted or turned upside down, the water droplet did not roll off. This high water-adhesive behavior arises from a large van der Waals force between the nanofibrous structures of the electrospun PAN mats and water droplets. To impart superhydrophilicity, the as-electrospun fibrous mats were next heated in air at 200 °C for 1 h to hydrolyze the PAN. As a result, the water CA for the smooth PAN surface was changed from 71.9 ( 1° to 37.5 ( 1° after thermal hydrolysis. It was attributed that, during this process, the nitrile groups (1) partially transformed into amides (2) or oxidized further to carboxylic acids (3) (Figure 1).23-25 As shown in Figure 2b, despite treating the material above Tg (∼140 °C), the fibrous morphology of the electrospun mats did not undergo any substantial change after heat treatment. Interestingly, however, the water CA of the hydrolyzed surface dramatically decreased to 0°, displaying a superhydrophilic state. These results demonstrate the thermally induced superhydrophobic-to-superhydrophilic transition made by the electrospun PAN fibrous mats. In order to verify the chemical changes in the PAN nanofibers before and after thermal treatment, FT-IR signals were observed (Figure 4a). The peak at 2242 cm-1, assigned to stretching vibrations of the nitrile (CtN) bond, was slightly reduced, while a new small absorption peak was observed at 1685 cm-1, assigned to the stretching vibration of the carbonyl bond (CdO) of the amide (-CONH2) or carboxylic acid (-COOH). The IR spectral changes indicated that the nitrile groups of the PAN were converted into the carboxylic acid and amide functions by thermally induced hydrolysis. Likewise, the C 1s and N 1s peaks of the PAN mats were investigated by X-ray photoemission spectroscopy (XPS) (Figure 4b). The C 1s peak of the PAN nanofibers, prior to thermal treatment, could be fit to two peak components, at 284.6 and 286.3 eV, attributed to C-C/C-H and CtN, respectively. However, two new species, an amide at 288.2 eV and a carboxylic acid at 289.9 eV, were observed after hydrolysis, consistent with the FT-IR results. The integrated areas of each C 1s peak, which were sensitive to the nitrile to amide or carboxyl group transition, were found to be 10.7% and 14.4%, respectively, whereas that of the C-C/C-H peak did not change after thermal treatment. The initial N 1s spectra could be fit only to a single peak at 401.0 eV, corresponding to the nitrogen in the CtN bond. After thermal treatment, however, a new peak at a binding energy of 399.9 eV, which was assigned to the amide bond, appeared. Nitriles may be hydrolyzed to amides, which then may undergo a secondary hydrolysis to a carboxylic acid group. Carboxylic acid groups behave autocatalytically to promote the reactivity of nitriles and induce more hydrolysis of the PAN nanofibers.23-25 As a result, the chemical composition of the outer surface regions of the PAN nanofiber mats changed via thermally induced partial hydrolysis, which produced a wetting transition, from superhydrophobic to superhydrophilic. Superamphiphilic materials were formed by subsequently electrospinning the solution onto the hydrolyzed superhydrophilic fibrous mats. However, this electrospinning process was initially limited, since the hydrolyzed PAN fibers were altered by the DMF solvent during electrospinning of the second layer. For example, when 1 mL of DMF was dropped onto the mats, the PAN film did (29) Jin, M.; Feng, X. J.; Feng, L.; Sun, T. L.; Zhai, J.; Li, T. J.; Jiang, L. Adv. Mater. 2005, 17, 1977–1981.
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Figure 2. SEM images and contact angles of the electrospun polyacrylonitrile (PAN) nanofibrous mats with and without 50 wt % TEOS (a, c) before and (b, d) after heat treatment, respectively. Table 1. Water Contact Angles (deg) of Spin-Coated and Electrospun PAN before and after the Heat Treatment PAN-TEOS0%
before anneal after anneal
PAN-TEOS30%
PAN-TEOS50%
spin-coated
electrospun
spin-coated
electrospun
spin-coated
electrospun
71.9 37.5
151.2 ∼0
70.5 35.7
150.3 ∼0
71.2 36.9
152.8 ∼0
Figure 3. Pore size distribution for the PAN nanofibrous mats.
not retain its nanofibrous structure as shown in Figure S1 (see the Supporting Information). On the other hand, as tetraethyl orthosilicate (TEOS) was added to the PAN solution to enhance the stability and solvent resistance of the PAN fibers, no significant morphological changes were observed in the PAN surface spun with 50 wt % TEOS after exposure to DMF.30 The strong solvent resistance of the TEOS-enforced PAN fibers was attributed to the formation of a silica network between the PAN chains (30) Park, S. H.; Lee, S. M.; Lim, H. S.; Han, J. T.; Lee, D. R.; Shin, H. S.; Jeong, Y.; Kim, J.; Cho, J. H. ACS Appl. Mater. Interfaces 2010, 2, 658–662.
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Figure 4. (a) FT-IR and (b) XPS spectra of the electrospun PAN nanofibrous mats before and after thermal treatment at 200 °C for 1 h under ambient conditions. DOI: 10.1021/la103829c
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Figure 6. Water adsorption behavior of the electrospun PAN nanofibrous mats before and after the thermal treatment. Figure 5. Photographs of the free-standing biphasic Janus fabric displaying superamphiphilicity.
via cross-linking of the hydrolyzed TEOS moieties. These results suggested that the increased stability would prevent solvent damage during deposition of the second layer, which permitted fabrication of Janus fabrics that encouraged water to dewet one side of the sheet and to wet the other. Parts c and d of Figure 2 show SEM images for the electrospun PAN mat with 50 wt % TEOS before and after thermal treatment, respectively, indicating uniform fibrous morphologies. As the electrospun PAN web without TEOS, the fibrous morphologies of that with 50 wt % TEOS did not change after thermal treatment. In contrast with Figure 2a,b, the average diameter of the as-spun fibers increased to 410 nm upon incorporation of TEOS due to the increase in the solution viscosity. A porosity and surface area obtained from a measurement of mercury porometry were 83.6 ( 3.2% and 230 m2/g on this fabric, respectively. Table 1 summarizes the water CAs of spin-coated and electrospun PAN-TEOS 0%, PAN-TEOS 30%, and PAN-TEOS 50% nanofibrous mats before and after thermal treatment. We could not observe a significant change in the surface wettability with increasing the content of the reinforcing TEOS in the case of the spin-coated smooth PAN surfaces. Moreover, the water CAs of the PAN mats containing TEOS were almost indistinguishable from those of the fibrous surfaces formed without TEOS, and the superhydrophobic nature of the surface was retained. Most likely, the PAN chains preferentially surface-segregated in the fibers, yielding surface properties that were relatively independent of the fiber diameter.30 Thermal hydrolysis also successfully converted the surface wettability of the TEOS-embedded PAN sheets to superhydrophilic, similar to the PAN web spun without TEOS. Moreover, their fibrous structures can highly amplify not only the hydrophobicity but also the hydrophilicity on the electrospun mats. The Janus fabrics were obtained by peeling off the electrospun mats from the collector substrate. The topmost surface of the freestanding mats exhibited superhydrophobic properties, while the underside, which had been separated from direct contact with the foil, displayed a superhydrophilic character. The water adsorption behaviors of the individual two phases of the resulting Janus mats were characterized by exposing each surface to water droplets stained with Procion red. As shown in Figure 5, the superhydrophilic face, the thermally treated fibrous mat, fully absorbed the water droplet, whereas the superhydrophobic side, the second
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electrospun layer, strongly repelled water. The areal size of the detached free-standing fabrics depended only on the size of the collector, so that these fabrics are further scalable. The superamphiphilic behavior of the free-standing electrospun mats was quantitatively evaluated by measuring the transmissivity of water using a gravimetric absorbency testing system.31 Figure 6 shows the changes in water permeability of either side of the electrospun Janus fabrics as a function of water exposure time. The quantity of water absorbed at the superhydrophobic face was almost zero after loading to water for 1 min. On the other hand, that at the superhydrophilic phase sharply increased upon initial exposure and then reached a stationary state of 0.6 g within 15 s. These results indicated that the superhydrophilic side easily attracted water into the material matrix, while the superhydrophobic surface did not absorb water at all, as expected.
4. Conclusion In conclusion, biphasic Janus fabrics were developed via a sequential three-step procedure of electrospinning, heating, and electrospinning of PAN-TEOS. Our core strategy combined the production of hierarchical geometric architectures by electrospinning and the superhydrophobic-to-superhydrophilic wetting transition induced by thermal hydrolysis of the PAN nanofibers. In particular, this simple process used the single commercial polymer to make a Janus fabric that displayed antisymmetric wetting behavior: a superhydrophilic surface on one side imbibed water, while the superhydrophobic one on the other side repelled water, without the use of complex surface treatments. This robust and tailored method opens a new route for the design and development of functional smart fabrics from inexpensive, commercially available polymers. Acknowledgment. The work was supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-2008-357-D00069), Korea. Edwin L. Thomas thanks the NSF grant (DMR-0804449), United States. Supporting Information Available: Additional data. This material is available free of charge via the Internet at http:// pubs.acs.org. (31) Hong, C. J.; Kim, J. B. Fibers Polym. 2007, 8, 218–224.
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