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Three-Dimensional Compressible and Stretchable Conductive Composites You Yu, Jifang Zeng, Chaojian Chen, Zhuang Xie, Ruisheng Guo, Zhilu Liu, Xuechang Zhou, Yong Yang, and Zijian Zheng* Flexible, compressible and stretchable conductors that feature both high conductivity and large deformation stability can enable a wide variety of new applications in wearable displays, deformable antenna and capacitors, stretchable solar cells, electronic skins, camera eyes, and biological actuators.[1–10] One critical step in the realization of these devices is how to retain high conductivity of interconnects under substantial degrees of bending, shearing, stretching and compression deformation, and even abrasive damage. Several materials strategies have been reported to date to address the some of the above-mentioned challenges, which include: i) the formation of buckled interconnects by so-called “pre-strain” methods,[11–16] ii) the fabrication of two-dimensional (2D) conductive networks, mats, serpentines and rough structures,[17–31] and iii) the development of three-dimensional (3D) conductive composite materials. Compared with thin film conductors, 3D conductive composites have attracted increasing attention in recent years because they are potentially more mechanical durable, lower cost, and are more promising for large-scale applications. Current state-of-the-art demonstrations have shown that very high conductivity (>103 S/cm) and large mechanical deformation (strain>10%) can be achieved by infiltrating highly conductive nanomaterials such as single-wall carbon nanotubes (SWNTs), graphene, and silver nanowires into elastic matrix materials such as poly(dimethylsiloxane) (PDMS) and polyurethane (PU).[32–39] However, these nanomaterials are too expensive for most practical applications. In some other reports, less expensive liquid metal alloys (Eutectic Gallium-Indium, EGaIn) were filled into 3D nano/micro elastic channels to make stretchable conductors.[9,40–42] However, this method requires complicated nano-/microfabrication and the use of liquid metal can be hazardous to electronic devices due to leakage. Indeed, it is still highly desirable in the research front to develop a simple, Dr. Y. Yu, C. Chen, Z. Xie, R. Guo, Dr. Z. Liu Dr. X. Zhou, Prof. Z. J. Zheng Nanotechnology Center, Institute of Textiles and Clothing, The Hong Kong Polytechnic University Hong Kong, China E-mail: [email protected] Dr. Y. Yu, C. Chen, Z. Xie, Dr. Z. Liu, Dr. X. Zhou, Prof. Z. J. Zheng The Hong Kong Polytechnic University, Shenzhen Research Institute Shenzhen, 518000, China Dr. J. Zeng, Prof. Y. Yang Centre for Advanced Structural Materials, Department of Mechanical and Biomedical Engineering, City University of Hong Kong Tat Chee Avenue, Kowloon Tong, Kowloon, Hong Kong, China
DOI: 10.1002/adma.201303662
810
wileyonlinelibrary.com
low-cost, and scalable method for fabricating high-performance compliant 3D conductive composites. To address this need, we report a new kind of cost-effective, yet high-performance stretchable, compressible, and abrasionresistant conductive composite on the basis of commercially available PU sponge and solution-processed thin metal coating. The key novelty to prepare this composite lies in the conformal solution deposition of metal thin films (Cu, Ag, Au) onto chemically functionalized porous PU sponges to form robust macroscopically continuous 3D conductive metal networks. Our mechanical simulation proves that this unique 3D structure can effectively release the strain on the metallic layer by transferring the strain deformation to rotation movements, so as to prevent the formation of cracks. We show that the 3D stretchable composites can remain stable conductivity at >1 × 103 S/cm under repeated bending, shearing, twisting, stretching and compression deformation, and even serious surface damage. Importantly, this low-cost solution-processed method can be readily scaled up to fabricate 15.5 inch conductive samples in ambient conditions, and is compatible with a high-throughput screen printing technique to fabricate patterned 3D conductive interconnects for stretchable arrays of light-emitting diodes (LEDs). The fabrication process is schematically illustrated in Figure 1a. In a typical experiment, commercially available, precleaned PU sponges were dipped into an ethanol solution of poly[2-(methacryloyloxy)ethyl trimethylammonium chloride-co3-(trimethoxysilyl)propyl methacrylate] [P(METAC-co-MPTS), METAC to MPTS molar ratio (φMETAC/MPTS) = 3.5:1], which was synthesized by a “one-pot” radical polymerization process following the literature procedures (see Experimental Section for details).[43] After hydrolysis and curing steps, the copolymercoated PU sponges were immersed into an aqueous solution of (NH4)2PdCl4, where PdCl42− moieties were loaded onto the copolymer layer (Figure 1b). The samples were then immersed into electroless deposition (ELD) baths of Cu and Au for 60 min to prepare Cu-coated and Au-coated PU sponges (PU-Cu and PU-Au), respectively. We also prepared binary-metal-coated PU sponges by a second electroless deposition of Ag on top of PU-Cu for 30 min to form PU-CuAg (Figure 1c). Finally, all PUmetal samples were infiltrated with precursors of polydimethylsiloxane (PDMS) (Sylgard 184, 10:1) and cured at 70 °C for 2 h to obtain the conductive composites, namely PU-metal-PDMS (Figure 1d). From scanning electron microscopy (SEM), we found that all metals were coated uniformly on PU surfaces (Figure S2, Supporting Information and Figure 1e,f). Elemental analysis showed that the Cu to Ag ratio of the PU-CuAg-PDMS sample was ca. 1:1. All composite samples were found to be highly conductive,
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 810–815
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It should be noted that the P(METAC-coMPTS) layer plays a critical role in the fabrication and final properties of the conductive composite. It acts not only the reservoir of the PdCl42− catalyst for promoting the ELD process, but also the adhesion layer between the thin metal film and the substrate surface.[11,21,44] Cu was as a model metal to investigate this issue (Table S1, Supporting Information). Without using the P(METACco-MPTS) coating, only thin Cu films (
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Three-Dimensional Compressible and Stretchable Conductive Composites You Yu, Jifang Zeng, Chaojian Chen, Zhuang Xie, Ruisheng Guo, Zhilu Liu, Xuechang Zhou, Yong Yang, and Zijian Zheng* Flexible, compressible and stretchable conductors that feature both high conductivity and large deformation stability can enable a wide variety of new applications in wearable displays, deformable antenna and capacitors, stretchable solar cells, electronic skins, camera eyes, and biological actuators.[1–10] One critical step in the realization of these devices is how to retain high conductivity of interconnects under substantial degrees of bending, shearing, stretching and compression deformation, and even abrasive damage. Several materials strategies have been reported to date to address the some of the above-mentioned challenges, which include: i) the formation of buckled interconnects by so-called “pre-strain” methods,[11–16] ii) the fabrication of two-dimensional (2D) conductive networks, mats, serpentines and rough structures,[17–31] and iii) the development of three-dimensional (3D) conductive composite materials. Compared with thin film conductors, 3D conductive composites have attracted increasing attention in recent years because they are potentially more mechanical durable, lower cost, and are more promising for large-scale applications. Current state-of-the-art demonstrations have shown that very high conductivity (>103 S/cm) and large mechanical deformation (strain>10%) can be achieved by infiltrating highly conductive nanomaterials such as single-wall carbon nanotubes (SWNTs), graphene, and silver nanowires into elastic matrix materials such as poly(dimethylsiloxane) (PDMS) and polyurethane (PU).[32–39] However, these nanomaterials are too expensive for most practical applications. In some other reports, less expensive liquid metal alloys (Eutectic Gallium-Indium, EGaIn) were filled into 3D nano/micro elastic channels to make stretchable conductors.[9,40–42] However, this method requires complicated nano-/microfabrication and the use of liquid metal can be hazardous to electronic devices due to leakage. Indeed, it is still highly desirable in the research front to develop a simple, Dr. Y. Yu, C. Chen, Z. Xie, R. Guo, Dr. Z. Liu Dr. X. Zhou, Prof. Z. J. Zheng Nanotechnology Center, Institute of Textiles and Clothing, The Hong Kong Polytechnic University Hong Kong, China E-mail: [email protected] Dr. Y. Yu, C. Chen, Z. Xie, Dr. Z. Liu, Dr. X. Zhou, Prof. Z. J. Zheng The Hong Kong Polytechnic University, Shenzhen Research Institute Shenzhen, 518000, China Dr. J. Zeng, Prof. Y. Yang Centre for Advanced Structural Materials, Department of Mechanical and Biomedical Engineering, City University of Hong Kong Tat Chee Avenue, Kowloon Tong, Kowloon, Hong Kong, China
DOI: 10.1002/adma.201303662
810
wileyonlinelibrary.com
low-cost, and scalable method for fabricating high-performance compliant 3D conductive composites. To address this need, we report a new kind of cost-effective, yet high-performance stretchable, compressible, and abrasionresistant conductive composite on the basis of commercially available PU sponge and solution-processed thin metal coating. The key novelty to prepare this composite lies in the conformal solution deposition of metal thin films (Cu, Ag, Au) onto chemically functionalized porous PU sponges to form robust macroscopically continuous 3D conductive metal networks. Our mechanical simulation proves that this unique 3D structure can effectively release the strain on the metallic layer by transferring the strain deformation to rotation movements, so as to prevent the formation of cracks. We show that the 3D stretchable composites can remain stable conductivity at >1 × 103 S/cm under repeated bending, shearing, twisting, stretching and compression deformation, and even serious surface damage. Importantly, this low-cost solution-processed method can be readily scaled up to fabricate 15.5 inch conductive samples in ambient conditions, and is compatible with a high-throughput screen printing technique to fabricate patterned 3D conductive interconnects for stretchable arrays of light-emitting diodes (LEDs). The fabrication process is schematically illustrated in Figure 1a. In a typical experiment, commercially available, precleaned PU sponges were dipped into an ethanol solution of poly[2-(methacryloyloxy)ethyl trimethylammonium chloride-co3-(trimethoxysilyl)propyl methacrylate] [P(METAC-co-MPTS), METAC to MPTS molar ratio (φMETAC/MPTS) = 3.5:1], which was synthesized by a “one-pot” radical polymerization process following the literature procedures (see Experimental Section for details).[43] After hydrolysis and curing steps, the copolymercoated PU sponges were immersed into an aqueous solution of (NH4)2PdCl4, where PdCl42− moieties were loaded onto the copolymer layer (Figure 1b). The samples were then immersed into electroless deposition (ELD) baths of Cu and Au for 60 min to prepare Cu-coated and Au-coated PU sponges (PU-Cu and PU-Au), respectively. We also prepared binary-metal-coated PU sponges by a second electroless deposition of Ag on top of PU-Cu for 30 min to form PU-CuAg (Figure 1c). Finally, all PUmetal samples were infiltrated with precursors of polydimethylsiloxane (PDMS) (Sylgard 184, 10:1) and cured at 70 °C for 2 h to obtain the conductive composites, namely PU-metal-PDMS (Figure 1d). From scanning electron microscopy (SEM), we found that all metals were coated uniformly on PU surfaces (Figure S2, Supporting Information and Figure 1e,f). Elemental analysis showed that the Cu to Ag ratio of the PU-CuAg-PDMS sample was ca. 1:1. All composite samples were found to be highly conductive,
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 810–815
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It should be noted that the P(METAC-coMPTS) layer plays a critical role in the fabrication and final properties of the conductive composite. It acts not only the reservoir of the PdCl42− catalyst for promoting the ELD process, but also the adhesion layer between the thin metal film and the substrate surface.[11,21,44] Cu was as a model metal to investigate this issue (Table S1, Supporting Information). Without using the P(METACco-MPTS) coating, only thin Cu films (
