FULL PAPER DOI: 10.1002/asia.201402230
Aqueous and Air-Compatible Fabrication of High-Performance Conductive Textiles Xiaolong Wang,[a, b] Casey Yan,[a, b] Hong Hu,[a, b] Xuechang Zhou,[a, b] Ruisheng Guo,[a] Xuqing Liu,[a, b] Zhuang Xie,[a, b] Zhifeng Huang,[c] and Zijian Zheng*[a, b]
Abstract: This paper describes a fully aqueous- and air-compatible chemical approach to preparing high-performance conductive textiles. In this method, the surfaces of textile materials are first modified with an aqueous solution of double-bond-containing silane molecules to form a surface-anchoring layer for subsequent in situ free-radical polymerization of [2(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) in the air. Thin layers of poly-METAC (PMETAC) are therefore covalently
grafted on top of the silane-modified textile surface. Cu- or Ni-coated textiles are finally fabricated by electroless deposition (ELD) onto the PMETACmodified textiles. Parameters including polymerization time, temperature, and ELD conditions are studied to optimize the whole fabrication process. Keywords: conducting materials · electroless deposition · polymerization · radicals · wearable electronics
Introduction
terference protection. At the moment, conductive textiles can be readily fabricated by depositing a layer of conductive metals on the textile surface by means of techniques such as galvanic deposition, atomic layer deposition, solution process of Al precursor composite, and electroless deposition (ELD).[6–17] Among these techniques, ELD is particularly attractive because it does not require expensive fabrication devices and can be carried out under ambient conditions on a large scale.[18–20] Previously, we have demonstrated the fabrication of conductive textiles by ELD of Cu and Ni onto various textile surfaces that have been modified with a thin layer of polyelectrolyte brushes.[21–27] The polyelectrolyte brushes that covalently tether one end on the surface served as a seeding layer for ELD as well as an adhesion layer between the metal coating and the textile surface, thereby providing metal coatings on textiles with remarkable durability against many cycles of bending, stretching, and even washing tests. However, two major drawbacks exist in our previous approach. Firstly, surface-initiated atom-transfer radical polymerization (SI-ATRP) was employed to prepare the polyelectrolyte brushes on the textile surface. Although ATRP is recognized as a green chemical approach for polymer synthesis,[28–31] it is very time-consuming, offers low yields, and requires N2 protection during the synthesis process. Therefore, it is difficult to scale up the fabrication with ATRP. Secondly, the synthesis requires the use of trichlorosilane chemicals in toluene, in which the HCl released in the reaction can dramatically degrade the textile performance.
Wearable electronics, one of the emerging areas of electronics, are in high demand, especially in on-the-go entertainment, military, sports, wearable displays, solar cells, and real-time health-monitoring devices.[1–5] With increasingly large applications that require high-performance flexible conductive interconnects, contacts and electrodes, wearable electronics rely heavily on metal-coated textiles not only just to provide desirable wearing comfort, but also functions and applications such as electrical conductivity, electrostatic discharge, electromagnetic shielding, and radio-frequency in[a] Dr. X. Wang, C. Yan, Prof. H. Hu, Dr. X. Zhou, R. Guo, X. Liu, Z. Xie, Prof. Z. Zheng Nanotechnology Centre, Institute of Textiles and Clothing The Hong Kong Polytechnic University Hung Hom, Kowloon, Hong Kong SAR (China) Fax: (+ 852) 27731432 E-mail: [email protected] [b] Dr. X. Wang, C. Yan, Prof. H. Hu, Dr. X. Zhou, X. Liu, Z. Xie, Prof. Z. Zheng Advanced Research Centre for Fashion and Textiles The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen (China) [c] Prof. Z. Huang Department of Physics Hong Kong Baptist University Kowloon, Hong Kong SAR (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402230.
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The as-made conductive textiles exhibit sheet resistance as low as 0.2 W sq 1, which makes them highly suitable for use as conductive wires and interconnects in flexible and wearable electronic devices. More importantly, the chemical method is fully compatible with the conventional “pad-dry-cure” fabrication process in the textile manufacturing industry, thus indicating that it is very promising for high-throughput and roll-to-roll fabrication of high-performance metal-coated conductive textiles in the future.
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Moreover, toluene is undesirable in textiles owing to its adverse effects on the human body. To overcome the challenges mentioned above, this paper describes a facile and versatile in situ free-radical polymerization approach to synthesizing thin layers of polyelectrolyte brushes on top of various textile materials including cotton, nylon, polyester, Kevlar, and spandex. In this proposed fabrication method, double-bond-containing silane molecules are first attached to the target textile by a condensation reaction in an aqueous solution, and subsequent in situ freeradical polymerization of monomer in water yields a layer of polymer brushes. Such a polyelectrolyte brush layer has been demonstrated to be an effective seeding and adhesion layer for subsequent ELD of Cu and Ni to yield conductive textiles in which the metal-deposited textiles show excellent mechanical and electrical properties. Polymerization and metal-deposition conditions including polymerization time, polymerization temperature, and ELD time have been studied. Importantly, the whole chemical process is carried out in aqueous and air environments, and this in situ polymerization method is also compatible with the conventional rollto-roll fabrication process in textile finishing, therefore allowing a high-throughput fabrication in the textile industry in the future.
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surfaces with double-bond-containing silane molecules, 2) synthesis of a polyelectrolyte brush by means of in situ free-radical polymerization, and 3) deposition of metal on the polyelectrolyte brush by ion exchange and ELD. Various textiles including cotton, nylon, polyester, Kevlar, and spandex were used as substrates. Apart from cotton, all textiles were exposed to an oxygen plasma before solution reactions. In a typical experiment, textile fabrics were immersed in 10 % (v/v) vinyltrimethoxysilane (VTMS) in deionized (DI) water to allow the reaction of silane molecules with hydroxyl groups on the surface of textiles through a condensation reaction. Subsequently, the VTMS-modified fabrics (VTMStextile) were immersed in a 10 % (v/v) [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) aqueous solution for 60 min at 80 8C to carry out in situ free-radical polymerization on the textile surface using potassium persulfate (KPS) as initiator (see the Supporting Information for detailed chemical reactions). As a result, PMETAC-coated fabrics (PMETAC-textile) were yielded. PMETAC textile was then immersed in a 5 mm (NH4)2PdCl4 aqueous solution for 15 min to immobilize PdCl42 by ion exchange. With thorough rinsing with deionized (DI) water, textile fabrics were finally immersed in a Cu ELD plating bath for 30 min at room temperature, which resulted in Cu-coated fabrics (Cu textile). It should be noted that thorough rinsing with DI water was carried out at the end of each step to avoid physisorption of any unattached chemicals.
Results and Discussion 1. Preparation of Conductive Textiles
2. Comparative Study on Si Wafer
Figure 1 outlines the chemical fabrication of metal-coated textiles, which typically involves 1) modification of textile
It is worthy of note that the detailed study of the in situ free-radical polymerization process directly on textiles is difficult because of their rough surfaces and the existence of physisorbed chemicals. Therefore, we carried out comparative experiments in which we synthesized PMETAC brushes on flat Si wafers by following the same procedures and conditions. Prior to any chemical modifications, Si wafers were exposed to oxygen plasma to render the surface hydrophilic. Then they were immersed in an aqueous VTMS solution (pH 4.5) for 15 min to yield VTMS-deposited Si wafer (VTMS-Si). Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used to confirm surface dep-
Figure 1. Schematic illustration of the fabrication of conductive textiles by the formation of polyelectrolyte brushes by means of in situ free-radical polymerization and subsequent ELD of the metal layer on top of the polyelectrolyte brushes. Drawing is not to scale.
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Figure 2. ATR-FTIR spectra of raw Si, VTMS-Si, and PMETAC-Si.
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osition of VTMS (Figure 2). Strong peaks at 1408 and 1599 cm 1, which are attributed to stretching vibrations of C=C of VTMS, indicate successful deposition of VTMS onto Si wafer. Peaks at 1004 and 1026 cm 1 are attributed to the Si-O-Si stretching vibrations of VTMS, which suggests cross-linking between VTMS. Another peak at 752 cm 1 is ascribed to the Si C stretching vibration in VTMS. Subsequently, in situ free-radical polymerization on VTMS-Si was carried out by immersing the substrates in an aqueous monomer solution under air for a certain amount of time at 60–80 8C. PMETAC-grafted silicon wafer (PMETAC-Si) is evidenced by the carbonyl peak at 1700–1800 cm 1. Successful grafting of PMETAC could also be confirmed by X-ray photoelectron spectroscopy (XPS; Figure 3). The unmodified Si wafer is mainly composed of Si and O signals. After silanization and in situ free-radical polymerization of PMETAC, distinctive signals at 404.1 (N 1s) and 200.5 eV (Cl 2p) indicate the successful polymerization of PMETAC on the Si wafer. Importantly, without using VTMS as an anchoring layer, no polymer coating was found after the polymerization process. The polymerization kinetics were then studied by measuring the PMETAC brush thickness at different polymerization temperatures and times. It was found that the thickness of the PMETAC brushes proportionally increases from approximately 4–20 nm when the polymerization time increases from 15 to 60 min, depending on polymerization temperature (Figure 4). After 30 min polymerization, the thickness of the PMETAC brushes does not show rapid increase when the polymerization time is prolonged further. When observed by atomic force microscopy (AFM), the obtained PMETAC brushes (40 min polymerization) are dense and uniform with a surface roughness of 6.2 nm (see the Supporting Information). PMETAC-Si was then immersed in PdCl42 solution for ion exchange, in which PdCl42 ions absorbed on the positively charged quaternary ammonium groups.[32] Through XPS analysis, prominent Pd signals at binding energy values of 532.4 (Pd 3p3/2), 343.3 (Pd 3d3/2), and 337.8 eV (Pd 3d5/2) confirm the successful immobilization of PdCl42 moieties on PMETAC during the ion-exchange process (Figure 3C). Finally, after brief rinsing with DI water, ELD of Cu was carried out by immersing the PdCl42 -loaded substrate into a Cu plating bath for 5–30 min. Loaded palladium moieties act as effective catalytic sites for ELD of Cu.[33–36] It was found that the Cu film reaches a thickness of approximately 50 nm rapidly after 5 min of ELD. After that, its thickness increased proportionally with ELD time at a lower deposition rate of approximately 3 nm per min (Figure 5). After 30 min of plating, the thickness of the Cu film on the Si wafer increased to approximately 140 nm. The AFM topography of Cu coating is shown in the Supporting Information. The size of the Cu particle aggregates is estimated to be approximately 100 nm, and the resistance of the as-deposited Cu layer measured by a conventional four probe method is approximately 0.45 W sq 1.
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Figure 3. A) XPS spectra of modified silicon wafers. Detailed scan of the PMETAC-Si sample B) before and C) after ion exchange.
3. Fabrication of Metal-Coated Cotton Fabrics After knowing the polymerization details, we then performed similar procedures on cotton fabrics to yield Cucoated cotton fabrics (Cu cotton). Surface modification of the cotton fabric was also characterized by ATR-FTIR. As shown in the Supporting Information, strengthened peaks at 1410 and 1602 cm 1 are attributed to the stretching vibration of C=C, whereas another peak at 752 cm 1 is ascribed to the
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layer, Figure 6C) look similar to that of unmodified cotton (Figure 6A). However, water contact angle measurements reveal their dramatic difference in surface wettability. After VTMS treatment, the cotton surface becomes largely hydrophobic owing to the presence of dense vinyl groups (Figure 6B, inset). After growing the water-absorbing PMETAC brushes, the cotton surface becomes hydrophilic again (Figure 6C, inset). After ELD, the surface of Cu cotton is covered with a layer of densely packed Cu particles (Figure 6D), which are further confirmed by cross-sectional SEM and energy-dispersive X-ray spectroscopy (EDS) (see the Supporting Information). The Cu film uniformly surrounds the cotton fiber with almost the same thickness all around. Notably, the Cu film coats the surface only instead of smearing into the depth direction. No cracks are found on the surface of Cu cotton. The substrate surface becomes hydrophobic again, which is a typical property of Cu thin films in the air. As cotton fabrics are very light in weight, it is possible to observe the weight gain during the ELD process. As shown in Figure 7A, the weight of Cu cotton increases as a first-order function of ELD time, and reaches approximately 112 % after 30 min of ELD plating. This is consistent with our previous finding that the thickness of the Cu coating on Si increases linearly during the ELD process. We then studied the electrical resistance of Cu-cotton samples. It should be noted that the conventional four-probe measurement used for Si wafer samples is not suitable for Cu cotton because the fabric surface is extremely rough and its resistance primarily comes from the fiber-to-fiber and yarn-to-yarn contact resistances. Therefore, we adopted a homemade four-probe method based on a recent report by Jur et al.[12] This modified method takes into account of the contact resistance of textile structures and is especially designed for measuring the resistance of fabrics (see the Supporting Information, or the Experimental Section for details). Before ELD on 20 nm-thick PMETAC cotton, the sample is electrically insulated. With 15 min of ELD, Cu cotton becomes electrically conductive, yet its resistance remains as high as approximately 50 W sq 1. The high resistance of the Cu cotton can be explained by the poor Cu coverage on the surface, as shown in SEM images in the Supporting Information. When the ELD time is prolonged to 30 min, as a consequence of the increased weight ( 12 %) imparted on the cotton fabric by Cu deposition, the resistance of Cu cotton at 30 min of ELD decreased dramatically to approximately 1.2 W sq 1. The resistance decreased slowly to 0.2 W sq 1 when the ELD time was further extended to 90 min. The thickness of the PMETAC brushes is important in determining the sheet resistance. When using thinner brushes (e.g., 5 nm), the resistance increased to approximately 80 W sq 1 under the same ELD conditions after 30 min (Figure 7C). The increase in resistance can be explained by the fact that thinner PMETAC brushes immobilize less catalytic palladium moieties, thereby leading to a slower rate of ELD and thinner Cu film. Without the PMETAC brushes, Cu cotton can also be obtained. However, the Cu coating is dis-
Figure 4. Thickness of PMETAC brushes as a function of polymerization time on Si wafers.
Figure 5. Thickness of Cu layer fabricated on Si wafers as a function of ELD time.
stretching vibration of Si C in VTMS. All these peaks confirm the deposition of VTMS on the cotton surface. PMETAC cotton shows a strong C=O peak at 1716 cm 1, which suggests successful polymerization of PMETAC on cotton fabric. The surface morphology of the as-made PMETAC cotton and Cu cotton were directly visualized by scanning electron microscopy (SEM). Typical surface morphologies of VTMS cotton (Figure 6B) and PMETAC cotton ( 20 nm polymer
Figure 6. SEM images of A) unmodified cotton fabric, B) VTMS cotton, C) PMETAC cotton obtained at 40 min polymerization, and D) Cu cotton obtained at 30 min ELD. Insets are the corresponding images of water contact-angle measurements.
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Figure 7. A) Weight increase of Cu cotton at different ELD plating times. Resistance of Cu cotton at different B) ELD plating times and C) PMETAC brush thicknesses. D) Electrical resistance aging test of the as-made Cu cotton.
continuous and only small portions of the cotton fibers are covered with Cu (see the Supporting Information). The color of the Cu-coated cotton without PMETAC brushes is dark brown, and the resistance is typically > 200 MW sq 1, even when the ELD time is 90 min. To evaluate the conductivity of the as-made Cu cotton under aging, the resistance of Cu cotton (40 min polymerization and 30 min ELD) was recorded when it was stored under air for a period of time (Figure 7D). It was found that the sheet resistance increased by 128 % after one day and then remained almost constant for the rest of the week. This significant increase in resistance is due to the partial oxidation of Cu into Cu2O, which was shown in our previous study.[24] The instability can be solved by capping a layer of air-stable metals or by directly depositing air-stable metals. For example, Ni-coated cotton fabrics (Ni cotton) were fabricated with a similar process (see the Supporting Information). The as-made Ni-coated (thickness of approximately 320 nm) cotton fabrics exhibited excellent electrical conductance with a resistance of approximately 7 W sq 1 after being freshly made, and this resistance did not show a significant increase after the sample was stored for one year in the air. Finally, we tested the electrical resistance change of Cu cotton upon bending. First of all, the sample was bent at different radii of curvature from 30 nm down to 0.5 mm. Surprisingly, the electrical resistance decreased as the bending angles decreased (Figure 8A). This can be explained by the fact that when Cu cotton is bent to a small radius, higher tensile strain is applied to the sample. In this case, the
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cotton fibers and yarns of the fabric are actually stretched to form a more compact contact between each other, thus leading to a lower contact resistance of the whole fabric. We further bent and unbent this sample at r = 5 mm for 1000 times as illustrated in the inset of Figure 8B. It was found that the resistance increased 3.5-fold after the cyclic tests. We believe that this is due to the continuous abrasion among the cotton fibers and yarns of the fabric. As discussed above, when the fabric is repeatedly stretched and relaxed during the bending tests, fibers and yarns are continuously switching between compact and loose forms of each other. As such, friction among them occurs, which might lead to partial removal of the surface Cu. 4. Applications on Other Conductive Textiles and Feasibility of Scale Production
As this chemical approach is solution processable, it can be readily extended to fabricating metal-coated conductive fibers, yarns, and fabrics on various materials. For proof-ofconcept purposes, we fabricated Cu-coated spandex monofi-
Figure 8. A) Electrical resistance change of Cu cotton at different bending angles. B) Cyclic tests of electrical resistance of Cu cotton with bending radius = 5 mm.
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Figure 9. SEM images of A) Cu-coated nylon fabric, B) polyester fabric, C) Kevlar yarns, D) and spandex monofilaments. The insets are SEM images of fibers in high magnification.
laments, cotton and Kevlar yarns, and nylon and polyester fabrics by the same procedures and under the same conditions (20 % monomer concentration, 40 min polymerization, 30 min ELD; shown in Figure 9). For nylon, polyester, Kevlar, and spandex, a treatment of oxygen plasma for 5 min prior to silanization is needed to coat their surfaces with functional hydroxyl groups for further reactions. Apart from spandex, all these Cu-coated textiles possess resistance as low as approximately 20 W sq 1, which is low enough to be used as conductors in many electronic devices. For example, Cu-coated cotton yarns were used as conducting wires in an integrated circuit to power a light-emitting diode (LED). The highly conductive cotton yarns are flexible and robust enough to be wrapped around a pencil while still functioning as conductors for the LED (Figure 10A). Very importantly, the Cu-coated cotton yarns can be sewn onto a piece of fabric to form a simple circuit pattern. Figure 10B shows an example on a Lycra knit. The patterned circuit works even under large uniaxial or biaxial stretching, which is highly desirable for wearable electronics (Figure 10C, D). Importantly, because whole chemical fabrication process is carried out in aqueous solutions and in the air, it is fully compatible with industrial roll-to-roll-type “pad-dry-cure” textile finishing process. As a proof-of-concept, VTMSmodified cotton fabrics pre-dipped in METAC aqueous solution were passed through a pair of padding mangles (Figure 10E) by using a “three-dip, three-nip” method at room temperature. Subsequently, the cotton fabrics were cured for 40 min at 80 8C to allow polymerization. Finally, the PMETAC-grafted cotton fabrics were passed to 15 min of ion exchange and then 30 min of ELD. The resulting Cucoated cotton fabrics show an average resistance of 14 W sq 1, thus indicating that the chemical approach is very promising for roll-to-roll fabrication of highly conductive metal-coated textiles.
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Figure 10. Digital images of electrical circuits containing a commercial LED, a 9 V battery, and the as-made Cu-coated cotton yarns. The conductive yarn is wrapped A) around a pencil and B–D) sewn onto a piece of Lycra fabric. C) Uniaxial or D) biaxial stretching of the Lycra fabric does not show a significant effect on the performance of the circuit. E) Digital image of a typical padding machine.
Conclusion In conclusion, we have reported an aqueous- and air-compatible approach to preparing highly conductive textiles based on the high-throughput formation of PMETAC brushes by means of in situ free-radical polymerization and subsequent ELD of metal onto the brushes. This approach possesses several features that are superior to previous studies that use SI-ATRP methods, which are summarized in Table 1. First, VTMS anchoring molecules can react with textiles effectively in aqueous medium, and do not need to use anhydrous toluene, which is undesirable in any textile processing or products. Second, the use of VTMS avoids the generation of HCl, which is a significant drawback when using trichlorosilane in SI-ATRP. Therefore, the tensile strength shows significant improvement over SI-ATRP methods. Third, in situ free-radical polymerization is much faster than SI-ATRP, in which the grafting of PMETAC brushes only takes approximately 30 min, whereas SI-ATRP
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Table 1. Comparison of properties of Cu cotton fabricated by means of an in situ free-radical polymerization method and SI-ATRP.
silane modification polymerization environment polymerization time sheet resistance tensile strength
In situ free-radical polymerization
SI-ATRP
VTMS in water air 30 min 10 1 W sq 1 130 % cotton
Br-terminating trichlorosilane in anhydrous toluene N2 24 h 10 1 W sq 1 89 % cotton
tion B was 9.5 mL L 1 formaldehyde in water. The electroless plating of Ni was performed in the following steps in the plating bath that consisted of 40 g L 1 nickel(II) sulfate hexahydrate, 20 g L 1 sodium citrate, 10 g L 1 lactic acid, and 1 g L 1 dimethylamine borane (DMAB) in water: A nickel stock solution of all components except the DMAB reductant was prepared in advance. A DMAB aqueous solution was prepared separately. The stock solutions were prepared for a 4:1 volumetric proportion of nickel-to-reductant stocks in the final electroless plating bath. All ELDs were carried out at room temperature.
takes approximately 24 h to achieve similar PMETAC brush thickness. The resulting metallic textiles show similar performance. The efficiency is hence 48-fold improved when using in situ polymerization. Fourth, the whole fabrication process can be carried out under ambient conditions, which overcomes the drawback of N2 protection and use of expensive metal catalyst that is required in SI-ATRP. Therefore, the approach is fully compatible with conventional roll-to-roll fabrication processes in the industry. The chemical approach reported herein shows remarkable potential towards largescale production in the future.
Control Experiments Control experiments were carried out on Si wafers. Si wafer was used as substrate to investigate the details of PMETAC and Cu films. The Si wafer was first treated by oxygen plasma for 5 min, silanized, then followed by in situ free-radical polymerization and Cu ELD. The surface morphology and thickness of as-made PMETAC and Cu films were investigated.
Experimental Section Materials
Characterization
All chemicals were purchased from Aldrich. The inhibitor in the monomer, METAC (80 % (w/v) in H2O), was removed by elution through a neutral alumina plug before use. Potassium persulfate (KPS) was used as received. Cotton fabric used in the experiments was pretreated by singeing, desizing, scouring, bleaching, and mercerizing, followed by washing with toluene, ethanol, and water to remove any possible impurities; it was then dried at room temperature and cured at 105 8C for 24 h. Spandex and Kevlar fiber, and nylon and polyester fabric were used as received.
ATR-FTIR were recorded with a Perkin–Elmer Spectrum 100 spectrometer. Chemical composition information about the samples was obtained by XPS. The measurement was carried out with a Sengyang SKL-12 spectrometer using MgKa radiation. The morphology of the fibers was investigated with a scanning electron microscope (TM3000, Hitachi). Atomic force microscopy was performed with a XE-100 (Park Systems) instrument in tapping mode to characterize the surface morphology of modified Si wafer.
Silanization
A homemade four-probe measurement method was used to quantify the effective conductivity of metal coatings on fibrous structures. One-sided copper tapes (3M) were used to prepare the probe electrodes, as shown in the Supporting Information. The metal-coated fabrics were cut into strips of 30 mm long and 10 mm wide. The sample was placed on the central region of the four-probe apparatus, sufficiently far from the edge of the parallel electrodes. A glass slide with dimensions of 25 mm 75 mm 6.25 mm was placed on top of the sample, and measurements were conducted by loading weights (500 g) to the top of the glass slide. In the measurement, current was supplied to the outer electrode pair and voltage was measured between the inner electrodes.
Resistance Measurement
Silanization of textiles was carried out by immersing in 10 % (v/v) vinyltrimethoxysilane (VTMS) for 15 min to allow reactions with the hydroxyl groups of fibers and fabrics through a condensation reaction. Cotton was directly immersed in the solution for silanization because there are plenty of activated hydroxyl groups on its surface, whereas other materials—for example, spandex and Kevlar fiber, and nylon and polyester fabric—were freshly treated with oxygen plasma for 5 min to form a chemically activated surface before immersion. The silanized textiles were finally rinsed with water and dried at 110 8C for 15 min. In Situ Free-Radical Polymerization The silanized textiles were immersed in a mixture of 20 % (v/v) METAC aqueous solution (100 mL) and potassium persulfate (KPS; 60 mg) to carry out the polymerization for 60 min (or other specific time) at 80 8C with stirring under ambient conditions. Finally, the textiles were washed with plenty of water and dried under vacuum at 45 8C for 2 h.
Acknowledgements Z.J.Z. acknowledges GRF of Hong Kong (project no. 5030/12P) and The Hong Kong Polytechnic University (project no. A-SA74) for financial support of this work. X.W. and C.Y. contributed to this work equally.
Electroless Deposition (ELD) ELD was carried out following the procedures in our previous reports.[19–25] PdCl42 was loaded by immersing PMETAC-coated textiles into a 5 mm (NH4)2PdCl4 aqueous solution for 15 min. After thorough washing with water, the textiles were coated with a metal film by immersion in a homemade ELD plating bath under stirring, followed by water rinsing and blow drying. The Cu ELD was performed in a plating bath that contained a 1:1 mixture of freshly prepared solutions A and B. Solution A consisted of 12 g L 1 sodium hydroxide, 13 g L 1 copper(II) sulfate pentahydrate, and 29 g L 1 potassium sodium tartrate tetrahydrate. Solu-
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[21] Y. Yu, C. Yan, Z. Zheng, Adv. Mater. 2014, DOI: 10.1002/ adma.201305558. [22] Y. Yu, J. Zeng, C. Chen, Z. Xie, R. Guo, Z. Liu, X. Zhou, Y. Yang, Z. J. Zheng, Adv. Mater. 2014, 26, 810 – 815. [23] C. Yan, Z. Zheng, J. Fiber Bioeng. Inf. 2013, 6, 117 – 128. [24] R. Guo, Y. Yu, Z. Xie, X. Liu, X. Zhou, Y. Gao, Z. Liu, F. Zhou, Y. Yang, Z. Zheng, Adv. Mater. 2013, 25, 3343 – 3350. [25] X. Liu, X. Zhou, Y. Li, Z. Zheng, Chem. Asian J. 2012, 7, 862 – 870. [26] X. Wang, H. Hu, Y. Shen, X. Zhou, Z. Zheng, Adv. Mater. 2011, 23, 3090 – 3094. [27] X. Liu, H. Chang, Y. Li, W. T. S. Huck, Z. Zheng, ACS Appl. Mater. Interfaces 2010, 2, 529 – 535. [28] V. Coessens, T. Pintauer, K. Matyjaszewski, Prog. Polym. Sci. 2001, 26, 337 – 377. [29] J. S. Wang, K. Matyjaszewski, Macromolecules 1995, 28, 7901 – 7910. [30] F. J. Xu, K. G. Neoh, E. T. Kang, Prog. Polym. Sci. 2009, 34, 719 – 761. [31] F. Zhou, Z. Zheng, B. Yu, W. Liu, W. T. S. Huck, J. Am. Chem. Soc. 2006, 128, 16253 – 16258. [32] Q. Ye, X. Wang, H. Hu, D. Wang, S. Li, F. Zhou, J. Phys. Chem. C 2009, 113, 7677 – 7683. [33] O. Azzaroni, S. E. Moya, A. A. Brown, Z. Zheng, E. Donath, W. T. S. Huck, Adv. Funct. Mater. 2006, 16, 1037 – 1042. [34] O. Azzaroni, Z. Zheng, Z. Yang, W. T. S. Huck, Langmuir 2006, 22, 6730 – 6733. [35] Z. Liu, H. Hu, B. Yu, M. Chen, Z. Zheng, F. Zhou, Electrochem. Commun. 2009, 11, 492 – 495. [36] G. O. Mallory, J. B. Hajdu, Electroless Plating, American Electroplaters and Surface Finishers Society, Orlando, 1990. Received: March 13, 2014 Revised: April 9, 2014 Published online: May 27, 2014
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Aqueous and Air-Compatible Fabrication of High-Performance Conductive Textiles Xiaolong Wang,[a, b] Casey Yan,[a, b] Hong Hu,[a, b] Xuechang Zhou,[a, b] Ruisheng Guo,[a] Xuqing Liu,[a, b] Zhuang Xie,[a, b] Zhifeng Huang,[c] and Zijian Zheng*[a, b]
Abstract: This paper describes a fully aqueous- and air-compatible chemical approach to preparing high-performance conductive textiles. In this method, the surfaces of textile materials are first modified with an aqueous solution of double-bond-containing silane molecules to form a surface-anchoring layer for subsequent in situ free-radical polymerization of [2(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) in the air. Thin layers of poly-METAC (PMETAC) are therefore covalently
grafted on top of the silane-modified textile surface. Cu- or Ni-coated textiles are finally fabricated by electroless deposition (ELD) onto the PMETACmodified textiles. Parameters including polymerization time, temperature, and ELD conditions are studied to optimize the whole fabrication process. Keywords: conducting materials · electroless deposition · polymerization · radicals · wearable electronics
Introduction
terference protection. At the moment, conductive textiles can be readily fabricated by depositing a layer of conductive metals on the textile surface by means of techniques such as galvanic deposition, atomic layer deposition, solution process of Al precursor composite, and electroless deposition (ELD).[6–17] Among these techniques, ELD is particularly attractive because it does not require expensive fabrication devices and can be carried out under ambient conditions on a large scale.[18–20] Previously, we have demonstrated the fabrication of conductive textiles by ELD of Cu and Ni onto various textile surfaces that have been modified with a thin layer of polyelectrolyte brushes.[21–27] The polyelectrolyte brushes that covalently tether one end on the surface served as a seeding layer for ELD as well as an adhesion layer between the metal coating and the textile surface, thereby providing metal coatings on textiles with remarkable durability against many cycles of bending, stretching, and even washing tests. However, two major drawbacks exist in our previous approach. Firstly, surface-initiated atom-transfer radical polymerization (SI-ATRP) was employed to prepare the polyelectrolyte brushes on the textile surface. Although ATRP is recognized as a green chemical approach for polymer synthesis,[28–31] it is very time-consuming, offers low yields, and requires N2 protection during the synthesis process. Therefore, it is difficult to scale up the fabrication with ATRP. Secondly, the synthesis requires the use of trichlorosilane chemicals in toluene, in which the HCl released in the reaction can dramatically degrade the textile performance.
Wearable electronics, one of the emerging areas of electronics, are in high demand, especially in on-the-go entertainment, military, sports, wearable displays, solar cells, and real-time health-monitoring devices.[1–5] With increasingly large applications that require high-performance flexible conductive interconnects, contacts and electrodes, wearable electronics rely heavily on metal-coated textiles not only just to provide desirable wearing comfort, but also functions and applications such as electrical conductivity, electrostatic discharge, electromagnetic shielding, and radio-frequency in[a] Dr. X. Wang, C. Yan, Prof. H. Hu, Dr. X. Zhou, R. Guo, X. Liu, Z. Xie, Prof. Z. Zheng Nanotechnology Centre, Institute of Textiles and Clothing The Hong Kong Polytechnic University Hung Hom, Kowloon, Hong Kong SAR (China) Fax: (+ 852) 27731432 E-mail: [email protected] [b] Dr. X. Wang, C. Yan, Prof. H. Hu, Dr. X. Zhou, X. Liu, Z. Xie, Prof. Z. Zheng Advanced Research Centre for Fashion and Textiles The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen (China) [c] Prof. Z. Huang Department of Physics Hong Kong Baptist University Kowloon, Hong Kong SAR (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402230.
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The as-made conductive textiles exhibit sheet resistance as low as 0.2 W sq 1, which makes them highly suitable for use as conductive wires and interconnects in flexible and wearable electronic devices. More importantly, the chemical method is fully compatible with the conventional “pad-dry-cure” fabrication process in the textile manufacturing industry, thus indicating that it is very promising for high-throughput and roll-to-roll fabrication of high-performance metal-coated conductive textiles in the future.
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Moreover, toluene is undesirable in textiles owing to its adverse effects on the human body. To overcome the challenges mentioned above, this paper describes a facile and versatile in situ free-radical polymerization approach to synthesizing thin layers of polyelectrolyte brushes on top of various textile materials including cotton, nylon, polyester, Kevlar, and spandex. In this proposed fabrication method, double-bond-containing silane molecules are first attached to the target textile by a condensation reaction in an aqueous solution, and subsequent in situ freeradical polymerization of monomer in water yields a layer of polymer brushes. Such a polyelectrolyte brush layer has been demonstrated to be an effective seeding and adhesion layer for subsequent ELD of Cu and Ni to yield conductive textiles in which the metal-deposited textiles show excellent mechanical and electrical properties. Polymerization and metal-deposition conditions including polymerization time, polymerization temperature, and ELD time have been studied. Importantly, the whole chemical process is carried out in aqueous and air environments, and this in situ polymerization method is also compatible with the conventional rollto-roll fabrication process in textile finishing, therefore allowing a high-throughput fabrication in the textile industry in the future.
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surfaces with double-bond-containing silane molecules, 2) synthesis of a polyelectrolyte brush by means of in situ free-radical polymerization, and 3) deposition of metal on the polyelectrolyte brush by ion exchange and ELD. Various textiles including cotton, nylon, polyester, Kevlar, and spandex were used as substrates. Apart from cotton, all textiles were exposed to an oxygen plasma before solution reactions. In a typical experiment, textile fabrics were immersed in 10 % (v/v) vinyltrimethoxysilane (VTMS) in deionized (DI) water to allow the reaction of silane molecules with hydroxyl groups on the surface of textiles through a condensation reaction. Subsequently, the VTMS-modified fabrics (VTMStextile) were immersed in a 10 % (v/v) [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) aqueous solution for 60 min at 80 8C to carry out in situ free-radical polymerization on the textile surface using potassium persulfate (KPS) as initiator (see the Supporting Information for detailed chemical reactions). As a result, PMETAC-coated fabrics (PMETAC-textile) were yielded. PMETAC textile was then immersed in a 5 mm (NH4)2PdCl4 aqueous solution for 15 min to immobilize PdCl42 by ion exchange. With thorough rinsing with deionized (DI) water, textile fabrics were finally immersed in a Cu ELD plating bath for 30 min at room temperature, which resulted in Cu-coated fabrics (Cu textile). It should be noted that thorough rinsing with DI water was carried out at the end of each step to avoid physisorption of any unattached chemicals.
Results and Discussion 1. Preparation of Conductive Textiles
2. Comparative Study on Si Wafer
Figure 1 outlines the chemical fabrication of metal-coated textiles, which typically involves 1) modification of textile
It is worthy of note that the detailed study of the in situ free-radical polymerization process directly on textiles is difficult because of their rough surfaces and the existence of physisorbed chemicals. Therefore, we carried out comparative experiments in which we synthesized PMETAC brushes on flat Si wafers by following the same procedures and conditions. Prior to any chemical modifications, Si wafers were exposed to oxygen plasma to render the surface hydrophilic. Then they were immersed in an aqueous VTMS solution (pH 4.5) for 15 min to yield VTMS-deposited Si wafer (VTMS-Si). Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used to confirm surface dep-
Figure 1. Schematic illustration of the fabrication of conductive textiles by the formation of polyelectrolyte brushes by means of in situ free-radical polymerization and subsequent ELD of the metal layer on top of the polyelectrolyte brushes. Drawing is not to scale.
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Figure 2. ATR-FTIR spectra of raw Si, VTMS-Si, and PMETAC-Si.
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osition of VTMS (Figure 2). Strong peaks at 1408 and 1599 cm 1, which are attributed to stretching vibrations of C=C of VTMS, indicate successful deposition of VTMS onto Si wafer. Peaks at 1004 and 1026 cm 1 are attributed to the Si-O-Si stretching vibrations of VTMS, which suggests cross-linking between VTMS. Another peak at 752 cm 1 is ascribed to the Si C stretching vibration in VTMS. Subsequently, in situ free-radical polymerization on VTMS-Si was carried out by immersing the substrates in an aqueous monomer solution under air for a certain amount of time at 60–80 8C. PMETAC-grafted silicon wafer (PMETAC-Si) is evidenced by the carbonyl peak at 1700–1800 cm 1. Successful grafting of PMETAC could also be confirmed by X-ray photoelectron spectroscopy (XPS; Figure 3). The unmodified Si wafer is mainly composed of Si and O signals. After silanization and in situ free-radical polymerization of PMETAC, distinctive signals at 404.1 (N 1s) and 200.5 eV (Cl 2p) indicate the successful polymerization of PMETAC on the Si wafer. Importantly, without using VTMS as an anchoring layer, no polymer coating was found after the polymerization process. The polymerization kinetics were then studied by measuring the PMETAC brush thickness at different polymerization temperatures and times. It was found that the thickness of the PMETAC brushes proportionally increases from approximately 4–20 nm when the polymerization time increases from 15 to 60 min, depending on polymerization temperature (Figure 4). After 30 min polymerization, the thickness of the PMETAC brushes does not show rapid increase when the polymerization time is prolonged further. When observed by atomic force microscopy (AFM), the obtained PMETAC brushes (40 min polymerization) are dense and uniform with a surface roughness of 6.2 nm (see the Supporting Information). PMETAC-Si was then immersed in PdCl42 solution for ion exchange, in which PdCl42 ions absorbed on the positively charged quaternary ammonium groups.[32] Through XPS analysis, prominent Pd signals at binding energy values of 532.4 (Pd 3p3/2), 343.3 (Pd 3d3/2), and 337.8 eV (Pd 3d5/2) confirm the successful immobilization of PdCl42 moieties on PMETAC during the ion-exchange process (Figure 3C). Finally, after brief rinsing with DI water, ELD of Cu was carried out by immersing the PdCl42 -loaded substrate into a Cu plating bath for 5–30 min. Loaded palladium moieties act as effective catalytic sites for ELD of Cu.[33–36] It was found that the Cu film reaches a thickness of approximately 50 nm rapidly after 5 min of ELD. After that, its thickness increased proportionally with ELD time at a lower deposition rate of approximately 3 nm per min (Figure 5). After 30 min of plating, the thickness of the Cu film on the Si wafer increased to approximately 140 nm. The AFM topography of Cu coating is shown in the Supporting Information. The size of the Cu particle aggregates is estimated to be approximately 100 nm, and the resistance of the as-deposited Cu layer measured by a conventional four probe method is approximately 0.45 W sq 1.
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Figure 3. A) XPS spectra of modified silicon wafers. Detailed scan of the PMETAC-Si sample B) before and C) after ion exchange.
3. Fabrication of Metal-Coated Cotton Fabrics After knowing the polymerization details, we then performed similar procedures on cotton fabrics to yield Cucoated cotton fabrics (Cu cotton). Surface modification of the cotton fabric was also characterized by ATR-FTIR. As shown in the Supporting Information, strengthened peaks at 1410 and 1602 cm 1 are attributed to the stretching vibration of C=C, whereas another peak at 752 cm 1 is ascribed to the
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layer, Figure 6C) look similar to that of unmodified cotton (Figure 6A). However, water contact angle measurements reveal their dramatic difference in surface wettability. After VTMS treatment, the cotton surface becomes largely hydrophobic owing to the presence of dense vinyl groups (Figure 6B, inset). After growing the water-absorbing PMETAC brushes, the cotton surface becomes hydrophilic again (Figure 6C, inset). After ELD, the surface of Cu cotton is covered with a layer of densely packed Cu particles (Figure 6D), which are further confirmed by cross-sectional SEM and energy-dispersive X-ray spectroscopy (EDS) (see the Supporting Information). The Cu film uniformly surrounds the cotton fiber with almost the same thickness all around. Notably, the Cu film coats the surface only instead of smearing into the depth direction. No cracks are found on the surface of Cu cotton. The substrate surface becomes hydrophobic again, which is a typical property of Cu thin films in the air. As cotton fabrics are very light in weight, it is possible to observe the weight gain during the ELD process. As shown in Figure 7A, the weight of Cu cotton increases as a first-order function of ELD time, and reaches approximately 112 % after 30 min of ELD plating. This is consistent with our previous finding that the thickness of the Cu coating on Si increases linearly during the ELD process. We then studied the electrical resistance of Cu-cotton samples. It should be noted that the conventional four-probe measurement used for Si wafer samples is not suitable for Cu cotton because the fabric surface is extremely rough and its resistance primarily comes from the fiber-to-fiber and yarn-to-yarn contact resistances. Therefore, we adopted a homemade four-probe method based on a recent report by Jur et al.[12] This modified method takes into account of the contact resistance of textile structures and is especially designed for measuring the resistance of fabrics (see the Supporting Information, or the Experimental Section for details). Before ELD on 20 nm-thick PMETAC cotton, the sample is electrically insulated. With 15 min of ELD, Cu cotton becomes electrically conductive, yet its resistance remains as high as approximately 50 W sq 1. The high resistance of the Cu cotton can be explained by the poor Cu coverage on the surface, as shown in SEM images in the Supporting Information. When the ELD time is prolonged to 30 min, as a consequence of the increased weight ( 12 %) imparted on the cotton fabric by Cu deposition, the resistance of Cu cotton at 30 min of ELD decreased dramatically to approximately 1.2 W sq 1. The resistance decreased slowly to 0.2 W sq 1 when the ELD time was further extended to 90 min. The thickness of the PMETAC brushes is important in determining the sheet resistance. When using thinner brushes (e.g., 5 nm), the resistance increased to approximately 80 W sq 1 under the same ELD conditions after 30 min (Figure 7C). The increase in resistance can be explained by the fact that thinner PMETAC brushes immobilize less catalytic palladium moieties, thereby leading to a slower rate of ELD and thinner Cu film. Without the PMETAC brushes, Cu cotton can also be obtained. However, the Cu coating is dis-
Figure 4. Thickness of PMETAC brushes as a function of polymerization time on Si wafers.
Figure 5. Thickness of Cu layer fabricated on Si wafers as a function of ELD time.
stretching vibration of Si C in VTMS. All these peaks confirm the deposition of VTMS on the cotton surface. PMETAC cotton shows a strong C=O peak at 1716 cm 1, which suggests successful polymerization of PMETAC on cotton fabric. The surface morphology of the as-made PMETAC cotton and Cu cotton were directly visualized by scanning electron microscopy (SEM). Typical surface morphologies of VTMS cotton (Figure 6B) and PMETAC cotton ( 20 nm polymer
Figure 6. SEM images of A) unmodified cotton fabric, B) VTMS cotton, C) PMETAC cotton obtained at 40 min polymerization, and D) Cu cotton obtained at 30 min ELD. Insets are the corresponding images of water contact-angle measurements.
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Figure 7. A) Weight increase of Cu cotton at different ELD plating times. Resistance of Cu cotton at different B) ELD plating times and C) PMETAC brush thicknesses. D) Electrical resistance aging test of the as-made Cu cotton.
continuous and only small portions of the cotton fibers are covered with Cu (see the Supporting Information). The color of the Cu-coated cotton without PMETAC brushes is dark brown, and the resistance is typically > 200 MW sq 1, even when the ELD time is 90 min. To evaluate the conductivity of the as-made Cu cotton under aging, the resistance of Cu cotton (40 min polymerization and 30 min ELD) was recorded when it was stored under air for a period of time (Figure 7D). It was found that the sheet resistance increased by 128 % after one day and then remained almost constant for the rest of the week. This significant increase in resistance is due to the partial oxidation of Cu into Cu2O, which was shown in our previous study.[24] The instability can be solved by capping a layer of air-stable metals or by directly depositing air-stable metals. For example, Ni-coated cotton fabrics (Ni cotton) were fabricated with a similar process (see the Supporting Information). The as-made Ni-coated (thickness of approximately 320 nm) cotton fabrics exhibited excellent electrical conductance with a resistance of approximately 7 W sq 1 after being freshly made, and this resistance did not show a significant increase after the sample was stored for one year in the air. Finally, we tested the electrical resistance change of Cu cotton upon bending. First of all, the sample was bent at different radii of curvature from 30 nm down to 0.5 mm. Surprisingly, the electrical resistance decreased as the bending angles decreased (Figure 8A). This can be explained by the fact that when Cu cotton is bent to a small radius, higher tensile strain is applied to the sample. In this case, the
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cotton fibers and yarns of the fabric are actually stretched to form a more compact contact between each other, thus leading to a lower contact resistance of the whole fabric. We further bent and unbent this sample at r = 5 mm for 1000 times as illustrated in the inset of Figure 8B. It was found that the resistance increased 3.5-fold after the cyclic tests. We believe that this is due to the continuous abrasion among the cotton fibers and yarns of the fabric. As discussed above, when the fabric is repeatedly stretched and relaxed during the bending tests, fibers and yarns are continuously switching between compact and loose forms of each other. As such, friction among them occurs, which might lead to partial removal of the surface Cu. 4. Applications on Other Conductive Textiles and Feasibility of Scale Production
As this chemical approach is solution processable, it can be readily extended to fabricating metal-coated conductive fibers, yarns, and fabrics on various materials. For proof-ofconcept purposes, we fabricated Cu-coated spandex monofi-
Figure 8. A) Electrical resistance change of Cu cotton at different bending angles. B) Cyclic tests of electrical resistance of Cu cotton with bending radius = 5 mm.
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Figure 9. SEM images of A) Cu-coated nylon fabric, B) polyester fabric, C) Kevlar yarns, D) and spandex monofilaments. The insets are SEM images of fibers in high magnification.
laments, cotton and Kevlar yarns, and nylon and polyester fabrics by the same procedures and under the same conditions (20 % monomer concentration, 40 min polymerization, 30 min ELD; shown in Figure 9). For nylon, polyester, Kevlar, and spandex, a treatment of oxygen plasma for 5 min prior to silanization is needed to coat their surfaces with functional hydroxyl groups for further reactions. Apart from spandex, all these Cu-coated textiles possess resistance as low as approximately 20 W sq 1, which is low enough to be used as conductors in many electronic devices. For example, Cu-coated cotton yarns were used as conducting wires in an integrated circuit to power a light-emitting diode (LED). The highly conductive cotton yarns are flexible and robust enough to be wrapped around a pencil while still functioning as conductors for the LED (Figure 10A). Very importantly, the Cu-coated cotton yarns can be sewn onto a piece of fabric to form a simple circuit pattern. Figure 10B shows an example on a Lycra knit. The patterned circuit works even under large uniaxial or biaxial stretching, which is highly desirable for wearable electronics (Figure 10C, D). Importantly, because whole chemical fabrication process is carried out in aqueous solutions and in the air, it is fully compatible with industrial roll-to-roll-type “pad-dry-cure” textile finishing process. As a proof-of-concept, VTMSmodified cotton fabrics pre-dipped in METAC aqueous solution were passed through a pair of padding mangles (Figure 10E) by using a “three-dip, three-nip” method at room temperature. Subsequently, the cotton fabrics were cured for 40 min at 80 8C to allow polymerization. Finally, the PMETAC-grafted cotton fabrics were passed to 15 min of ion exchange and then 30 min of ELD. The resulting Cucoated cotton fabrics show an average resistance of 14 W sq 1, thus indicating that the chemical approach is very promising for roll-to-roll fabrication of highly conductive metal-coated textiles.
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Figure 10. Digital images of electrical circuits containing a commercial LED, a 9 V battery, and the as-made Cu-coated cotton yarns. The conductive yarn is wrapped A) around a pencil and B–D) sewn onto a piece of Lycra fabric. C) Uniaxial or D) biaxial stretching of the Lycra fabric does not show a significant effect on the performance of the circuit. E) Digital image of a typical padding machine.
Conclusion In conclusion, we have reported an aqueous- and air-compatible approach to preparing highly conductive textiles based on the high-throughput formation of PMETAC brushes by means of in situ free-radical polymerization and subsequent ELD of metal onto the brushes. This approach possesses several features that are superior to previous studies that use SI-ATRP methods, which are summarized in Table 1. First, VTMS anchoring molecules can react with textiles effectively in aqueous medium, and do not need to use anhydrous toluene, which is undesirable in any textile processing or products. Second, the use of VTMS avoids the generation of HCl, which is a significant drawback when using trichlorosilane in SI-ATRP. Therefore, the tensile strength shows significant improvement over SI-ATRP methods. Third, in situ free-radical polymerization is much faster than SI-ATRP, in which the grafting of PMETAC brushes only takes approximately 30 min, whereas SI-ATRP
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Table 1. Comparison of properties of Cu cotton fabricated by means of an in situ free-radical polymerization method and SI-ATRP.
silane modification polymerization environment polymerization time sheet resistance tensile strength
In situ free-radical polymerization
SI-ATRP
VTMS in water air 30 min 10 1 W sq 1 130 % cotton
Br-terminating trichlorosilane in anhydrous toluene N2 24 h 10 1 W sq 1 89 % cotton
tion B was 9.5 mL L 1 formaldehyde in water. The electroless plating of Ni was performed in the following steps in the plating bath that consisted of 40 g L 1 nickel(II) sulfate hexahydrate, 20 g L 1 sodium citrate, 10 g L 1 lactic acid, and 1 g L 1 dimethylamine borane (DMAB) in water: A nickel stock solution of all components except the DMAB reductant was prepared in advance. A DMAB aqueous solution was prepared separately. The stock solutions were prepared for a 4:1 volumetric proportion of nickel-to-reductant stocks in the final electroless plating bath. All ELDs were carried out at room temperature.
takes approximately 24 h to achieve similar PMETAC brush thickness. The resulting metallic textiles show similar performance. The efficiency is hence 48-fold improved when using in situ polymerization. Fourth, the whole fabrication process can be carried out under ambient conditions, which overcomes the drawback of N2 protection and use of expensive metal catalyst that is required in SI-ATRP. Therefore, the approach is fully compatible with conventional roll-to-roll fabrication processes in the industry. The chemical approach reported herein shows remarkable potential towards largescale production in the future.
Control Experiments Control experiments were carried out on Si wafers. Si wafer was used as substrate to investigate the details of PMETAC and Cu films. The Si wafer was first treated by oxygen plasma for 5 min, silanized, then followed by in situ free-radical polymerization and Cu ELD. The surface morphology and thickness of as-made PMETAC and Cu films were investigated.
Experimental Section Materials
Characterization
All chemicals were purchased from Aldrich. The inhibitor in the monomer, METAC (80 % (w/v) in H2O), was removed by elution through a neutral alumina plug before use. Potassium persulfate (KPS) was used as received. Cotton fabric used in the experiments was pretreated by singeing, desizing, scouring, bleaching, and mercerizing, followed by washing with toluene, ethanol, and water to remove any possible impurities; it was then dried at room temperature and cured at 105 8C for 24 h. Spandex and Kevlar fiber, and nylon and polyester fabric were used as received.
ATR-FTIR were recorded with a Perkin–Elmer Spectrum 100 spectrometer. Chemical composition information about the samples was obtained by XPS. The measurement was carried out with a Sengyang SKL-12 spectrometer using MgKa radiation. The morphology of the fibers was investigated with a scanning electron microscope (TM3000, Hitachi). Atomic force microscopy was performed with a XE-100 (Park Systems) instrument in tapping mode to characterize the surface morphology of modified Si wafer.
Silanization
A homemade four-probe measurement method was used to quantify the effective conductivity of metal coatings on fibrous structures. One-sided copper tapes (3M) were used to prepare the probe electrodes, as shown in the Supporting Information. The metal-coated fabrics were cut into strips of 30 mm long and 10 mm wide. The sample was placed on the central region of the four-probe apparatus, sufficiently far from the edge of the parallel electrodes. A glass slide with dimensions of 25 mm 75 mm 6.25 mm was placed on top of the sample, and measurements were conducted by loading weights (500 g) to the top of the glass slide. In the measurement, current was supplied to the outer electrode pair and voltage was measured between the inner electrodes.
Resistance Measurement
Silanization of textiles was carried out by immersing in 10 % (v/v) vinyltrimethoxysilane (VTMS) for 15 min to allow reactions with the hydroxyl groups of fibers and fabrics through a condensation reaction. Cotton was directly immersed in the solution for silanization because there are plenty of activated hydroxyl groups on its surface, whereas other materials—for example, spandex and Kevlar fiber, and nylon and polyester fabric—were freshly treated with oxygen plasma for 5 min to form a chemically activated surface before immersion. The silanized textiles were finally rinsed with water and dried at 110 8C for 15 min. In Situ Free-Radical Polymerization The silanized textiles were immersed in a mixture of 20 % (v/v) METAC aqueous solution (100 mL) and potassium persulfate (KPS; 60 mg) to carry out the polymerization for 60 min (or other specific time) at 80 8C with stirring under ambient conditions. Finally, the textiles were washed with plenty of water and dried under vacuum at 45 8C for 2 h.
Acknowledgements Z.J.Z. acknowledges GRF of Hong Kong (project no. 5030/12P) and The Hong Kong Polytechnic University (project no. A-SA74) for financial support of this work. X.W. and C.Y. contributed to this work equally.
Electroless Deposition (ELD) ELD was carried out following the procedures in our previous reports.[19–25] PdCl42 was loaded by immersing PMETAC-coated textiles into a 5 mm (NH4)2PdCl4 aqueous solution for 15 min. After thorough washing with water, the textiles were coated with a metal film by immersion in a homemade ELD plating bath under stirring, followed by water rinsing and blow drying. The Cu ELD was performed in a plating bath that contained a 1:1 mixture of freshly prepared solutions A and B. Solution A consisted of 12 g L 1 sodium hydroxide, 13 g L 1 copper(II) sulfate pentahydrate, and 29 g L 1 potassium sodium tartrate tetrahydrate. Solu-
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