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Fully Elastic Conductive Films from Viscoelastic Composites Sunghwan Cho, Jun Hyuk Song, Minsik Kong, Sangbaie Shin, Young-Tae Kim, Gyeongbae Park, Chan Gyung Park, Tae Joo Shin, Jae-Min Myoung, and Unyong Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14504 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Materials & Interfaces

Fully Elastic Conductive Films from Viscoelastic Composites

Sunghwan Cho,† Jun Hyuk Song,† Minsik Kong,‡ Sangbaie Shin,‡ Young-Tae Kim,‡ Gyeongbae Park,‡ Chan-Gyung Park,‡ Tae Joo Shin,§ Jaemin Myoung,†* and Unyong Jeong‡*

†

Department of Materials Science and Engineering, Yonsei University, 50, Yonsei-ro,

Seodaemun-gu, Seoul, Korea ‡

Department of Materials Science and Engineering, Pohang University of Science and

Technology, 77, Cheongam-Ro, Nam-Gu, Pohang, Korea §

UNIST Central Research Facilities & School of National Science, Ulsan National Institute of

Science and Technology, 50, UNIST-gil, Banyeon-ri, Eonyang-eup, Ulju-gun, Ulsan, Korea

* Corresponding authors: [email protected] , [email protected]

KEYWORDS: stretchable electronics, SBS block copolymer, Au nanosheets, composite film, thermoplastic polymer, stretchable electrode, stretchable electrochemiluminescence displays.

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Abstract We investigated, for the first time, the conditions where a thermoplastic conductive composite can exhibit completely reversible stretchability at high elongational strains (ε =1.8). We studied a composite of Au nanosheets and SBS block copolymer as an example. The composite had an outstandingly low sheet resistance (0.45 Ω/sq). We found that when a thin thermoplastic composite film is placed on a relatively thicker chemically-crosslinked elastomer film, it can follow the reversible elastic behavior of the bottom elastomer. Such elasticity comes from the restoration of the block copolymer microstructure. The strong adhesion of the thermoplastic polymer to the metallic fillers is advantageous in the fabrication of mechanically robust highly conductive stretchable electrodes. The chemical stability of the Au composite was used to fabricate high luminescence stretchable electrochemiluminescence displays with a conventional top–bottom electrode setup and with a horizontal electrode setup.

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Introduction Stretchable devices are an attractive format for next-generation electronics such as wearable healthcare sensors,1-2 electronic skin,3 and stretchable displays.4 The realization of such devices necessitates the development of stretchable electrodes capable of maintaining stable performance under large deformations. Various stretchable electrode materials have been intensively studied;5-8 most of these materials are composites of conductive fillers embedded in chemically-crosslinked elastomers such as polydimethylsiloxane (PDMS) or Ecoflex.9-10 Recently, conductive composites in physically-crosslinked elastomers have attracted increased attention because the elastomers can be dissolved in solvents, enabling direct printing of the composites.11-13 Thermoplastic block copolymers such as polystyrene-block-polybutadieneblock-polystyrene (SBS) and polystyrene-block-poly(ethylenebutylene)-block-polystyrene (SEBS) are physically crosslinked elastomers commonly used in this application.7, 14 They comprise a soft matrix (polybutadiene and poly(ethylenebutylene)) and hard microdomains (polystyrene). The solid microdomains function as the physical crosslinker in the soft matrix. A stretchable conducting polymer based on the physical crosslinking of poly(3,4ethylenedioxythiophene) nanofibrils in a soft matrix has also been recently reported.6, 15 Such thermoplastic block copolymer composites are viscoelastic. They have elastic property but they can be deformed permanently under external mechanical force because the stress in the composites can be relaxed under strained states. Once a critical yield strain (ε c ) is applied the viscoelastic composites cannot shrink back to the initial dimension when the stress is removed, leaving a residual strain in the material. The critical yield strain of block copolymer is typically less than 10%, hence the viscoelastic composites are not intrinsically stretchable. In spite of the limitation in the stretchability of the viscoelastic composites, their uses for 3



stretchable electrodes have been increasing without clear understanding on the conditions to obtain a large degree of stretchability. To acquire a mechanical reliability under repeated stretch cycles, the changes in the microstructure of the block copolymers also should be understood. In this study, we also investigate the conditions to obtain large stretchability without any residual strain from viscoelastic composites. We demonstrate that a thin-film composite comprising the Au nanosheets (NSs) and a thermoplastic block copolymer placed on a chemically-crosslinked elastomer substrate is mechanically stable and highly stretchable (ε = 1.8), with outstandingly low sheet resistance (0.45 Ω/sq). The microstructural stability of the composite upon repeated stretching cycles is analyzed via synchrotron small angle X-ray scattering (SAXS). We use the stretchable Au composite electrode to fabricate a stretchable display that operates on the basis of electro-generated chemiluminescence (ECL).

Results and Discussion Previously, we suggested a multilayer of Au nanosheets (NSs) piled on a chemicallycrosslinked elastomer substrate (PDMS, Ecoflex) as a new stretchable composite electrode.5, 16 Unfortunately, the Au NSs on top of the chemically-crosslinked elastomer substrates were easily delaminated from the substrate and the contacts between the Au NSs were unstable at large strains, which are critical drawbacks for practical applications. We modified the process to produce a composite of the Au NSs in the block copolymer matrix.

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Figure 1. (A) Schematic of the procedure for fabricating the Au nanosheets (NSs)/polystyreneblock-polybutadiene-block-polystyrene (SBS) block copolymer composite film on a polydimethylsiloxane (PDMS) substrate. (B) A camera image of showing the transfer of the Au NSs composite film from Si wafer to a PDMS substrate. The numbers indicate the weight percent (wt%) of SBS in the solution containing the Au NSs

Figure 1A illustrates the process used to fabricate the thin-film composite of the Au NSs and the SBS block copolymer (M w = 140,000, volume fraction of polystyrene = 0.3). The Au NSs were synthesized following the process described in a previous report.5 The Au NSs dispersed in butanol were floated on deionized (DI) water. We previously reported the floating process.17 We used the floating process to fabricate stretchable Au NS electrode on a PDMS substrate and published the results.5 The floating process includes spraying an alcohol 5



suspension of the Au NSs on a fountain of water. The spreading of the alcohol suspension on the water surface forms a monolayer of Au NSs during evaporation of alcohol. This technique allows monolayer assembly of hydrophilic nanomaterials dispersed in hydrophilic solvents. Transferring the Au NSs monolayer floated on water onto other substrates was done by simple contact on the top surface of the floated monolayer with the new substrate. The floated Au NSs film was transferred to a PDMS film (40 nm) coated onto a Si wafer. A SBS block copolymer solution (10 wt% in toluene) was spin-coated onto the Au NSs film and annealed at 160 °C for 1 h under vacuum to form the microdomains.18 It is notable that the interface of the composite with the Si wafer was used as the top surface of the stretchable electrode because the composite film on the Si wafer was transferred onto a PDMS substrate. The block copolymer chains penetrated through the Au NSs film and bonded with the Au NSs to form a mechanically stable composite. The thickness of the polymer layer was controlled by the polymer concentration (Figure S1, Supporting Information). The composite film maintained its low sheet resistance when a 7 wt% SBS solution was coated onto the Au NSs film, which indicates that the Au NSs were not completely covered by the block copolymer under the experimental deposition conditions. When a 10 wt% SBS solution was used to coat the Au NSs film (2 µm in thickness), the sheet resistance increased to 1.3 MΩ/sq because the Au NSs were covered by the block copolymer layer (Figure S2, Supporting Information). Complete coverage with SBS was required when the composite film was to be transferred onto other surfaces. For the thickness measurement, we compared the values obtained by ellipsometry and TEM. We found the values were in reasonable accordance within 3 % in error. Later then on, we used the measured values by ellipsometry. Repeated measurement for each sample showed errors less than 3 %.

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The composite film was treated with oxygen plasma (50 W, 45 s, 22 sccm) and transferred to a plasma-treated PDMS substrate via a gentle finger-touch and lift-off (Movie S1, Supporting Information). It can be noted that clean transfer was achieved when a 10 wt% solution was coated, but only local areas (edges of the Au NSs composite film) were transferred when the polymer concentration was less than or equal to 5 wt% (Figure 1B). The composite film was pressurized under 0.2 MPa at 180 °C for 5 min to help the polymer molecules fill the pores between the Au NSs.

Figure 2. Sheet resistances of the bare Au NSs layers according to the number of transfer from the floated bare Au NSs layer (black lines), and the Au NSs composite film with the same number of transfer (red lines). Solid lines are for the sheet resistance and the dotted lines are for the conductivity.

It can be noted that the sheet resistance of the transferred composite film was dependent on the thickness of the Au NSs layer, which was controlled by the transfer number of the floated Au NSs. Figure 2 compares the sheet resistances of the bare Au NSs layers with the sheet resistances of their SBS composites. The composite was prepared by spin-coating a 10 wt% 7



SBS solution and transferring onto a PDMS substrate. The sheet resistance (red solid line) of the composite maintained that of the bare Au NSs layer. The conductivity was shown as a dashed red line. The bare Au NSs film are the corresponding black lines. The sheet resistance of the composite film with three transferred layers was 2.5 Ω/sq, and it decreased to 0.45 Ω/sq in the case of the film with six transferred layers, corresponding to 1.7 × 105 S/cm. The conductivity and sheet resistance are comparable to the values of the sputtered Au thin film (100 nm) (Figure S3, Supporting Information).

Figure 3. (A) Representative scanning electron microscopy image of the Au NSs composite film. (B) AFM phase image showing the overlapped Au NSs lying well parallel to the substrate. (C) X-ray diffraction diffractograms of the Au NSs composite film (red) and the Au NS powder (black). The inset is a magnified diffractogram of (111) peaks normalized by the intensity of the (200) peak. 8


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Figure 3A shows a scanning electron microscope (SEM) image of the tilted composite film. The lateral dimension of the Au NSs ranged from 20 to 50 µm and their thickness was about 20 nm. As a result of the transfer process, the Au NSs in the composite film were exposed at the surface. The Au NSs did not protrude into the air, which is important for films used as electrodes for transistors.19 Figure 3B shows an atomic force microscope (AFM) image in the phase mode for the composite film. Many Au nanosheets were exposed to the surface. The inset is the height profile of the dotted line. The AFM image in the height mode is shown in the Supporting Information (Figure S4). The root mean square surface roughness of the composite film was about 50 nm and the maximum peak-valley distance was about 120 nm or less. For comparison, the root mean square surface roughness of the transparent flexible Ag NW electrodes deposited on a glass substrate are typically about 30 nm.20 The surface roughness (50 nm) of the Au NS composite film is not problematic for printed stretchable electronic devices. The orientation of the Au NSs were investigated by the X-ray diffraction (XRD) (Figure 3C). The patterns of the Au NS powder with random direction and the composite film are presented as a black line and a red line, respectively. The basal plane of the Au NS is the (111) facet.21 The inset is a magnification of the (111) and (200) peaks. The diffraction peak of the (111) plane in the pattern of the composite film was much more intense than the peaks of the other planes, indicating increased parallel orientation of the Au NSs to the substrate. The orientation factor (𝑓𝑓) of the Au NSs in the composite film was calculated on the basis of the Lotgering factor (LF),22 𝐿𝐿𝐿𝐿 = (𝑃𝑃 − 𝑃𝑃0 )/(1 − 𝑃𝑃0 ), where P is the relative intensity of the (111)

peak, i.e., 𝑃𝑃 = ∑ 𝐼𝐼 (111)/ ∑ 𝐼𝐼(ℎ𝑘𝑘𝑘𝑘), and 𝑃𝑃0 is the value of P in the isotropic standard value

from the Au NS powder. On the basis of the XRD diffractograms, the 𝐿𝐿𝐿𝐿 of the Au NSs

powder was typically 0.6, whereas the 𝐿𝐿𝐿𝐿 of the Au NSs in the composite film was 0.97, 9 ACS Paragon Plus Environment


indicating that the block copolymer coating and pressurizing process resulted in parallel orientation of the Au NSs to the substrate.

Figure 4. (A, B) Cross-sectional transmission electron microscopy images of the Au NS composite film. (C) The microdomains of the SBS block copolymer matrix in the narrow space (marked by a red square in Figure 4A). (D) The microdomains of the SBS block copolymer matrix in the wide space (marked by a blue square in Figure 4B). (E, F) Schematically describe the orientation of the SBS microdomains corresponding to Figures 4C and D, respectively. 10


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The shapes of the Au NS in the composite (six transfer layers) were visualized by crosssectional transmission electron microscope (TEM) images as shown in Figures 4A, B. The TEM sample was prepared using a focused ion beam. The bright parts represent the block copolymer matrix. Most Au NSs located on the top and bottom surfaces of the composite film were parallel to the surface, although the NSs with tilted angles and perpendicular orientations were also observed. Some NSs were bent and made contacts with other NSs with parallel orientation; hence, they formed a unique structure resembling a hollow cylinder. We speculate this structure was formed when the standing NSs were pressurized. The standing NSs and the hollow cylinders in the middle region electrically bridge the parallel layers on both surfaces, and they can be readily deformed upon bending and stretching, which is beneficial for maintaining high electrical conductivity upon large mechanical deformation. The bulk SBS block copolymer used in this study has hexagonally close-packed cylindrical microdomains (Figure S5, Supporting Information). Figure 4C shows a TEM image of the block copolymer filled in the narrow space between the parallel Au NSs (marked by a red square in Figure 4A). The cylindrical microdomains are clearly observed in the narrow space. The dark and bright parts are the polybutadiene matrix stained with OsO 4 and the polystyrene microdomains, respectively. The TEM images indicate that the block copolymer even penetrated into the narrow spaces and the butadiene block formed the contacts with the Au NSs. Although SBS penetrated into the narrow spaces between the Au NSs, the percolation network of the Au NSs was maintained. It will be discussed in Figure 6 later. SBS could penetrate into the pores but could not delaminate the interfacing NSs because the highly flexible NSs could deform with the penetrating block copolymer so that the mechanical stress transferred to the interfacing regions was not enough to delaminate the NSs. In contrast to the block copolymer in the narrow parallel Au NSs, the block copolymer microdomains in the wide spaces 11



surrounded by many Au NS surfaces (marked by a blue square in Figure 4B) were randomly oriented, as shown in the TEM image (Figure 4D). This random orientation is attributed to the multiple surface directions from the surrounding Au NSs. Figures 4E and F schematically describe the microdomains of the SBS block copolymer corresponding to Figures 4C and D, respectively. Block copolymer microdomains develop from an interface with a substrate or the air.23 The polybutadiene block contacts on the Au NSs surfaces and the microphase separation takes place to form the cylindrical microdomains. In the narrow space formed by two parallel Au NS surfaces as shown in Figure 4C, the PS microdomains are aligned parallel to the Au NS surfaces, hence they can have the same parallel orientation to the Au NSs in the whole narrow space. The microdomains in the narrow space commensurate with the inter-microdomain spacing. However, in the space surrounded by many Au NS surfaces of different directions as shown in Figure 4D, the developing microdomains near the NSs surfaces have different orientations, so the central part in the space cannot have a preferred orientation, resulting in randomly oriented microdomains. Our objective in this study is to produce highly stretchable conductive composites. Unfortunately, the intrinsic stretchability of the block copolymer elastomer is limited to small strain (ε) ranges.24 The composites have residual strain after a cycle of large strain because of the viscoelastic stress relaxation of the thermoplastic physical elastomers. The pure SBS block copolymer exhibited yielding at ε = 0.15 at room temperature and severe stress relaxation (Figure S6, Supporting Information). Therefore, the block copolymer itself cannot be used as a highly stretchable elastic substrate. To overcome this limited stretchability, we placed the composite film onto a PDMS substrate. A strong interface between the composite and the PDMS is critical for the transfer of shear stress from the PDMS to the composite film. We used 12


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oxygen plasma bonding,25 where both surfaces were treated lightly with O 2 plasma and then brought into contact with each other at 90 °C. The increased surface energies resulting from the hydroxyl and carbonyl groups generated by the plasma resulted in strong binding at the interface.

Figure 5. (A) 2-D synchrotron small-angle X-ray scattering (SAXS) patterns of the composite film during a uniaxial tensile test. (B) Schematic of the morphological changes in the block copolymer microdomains under uniaxial tensile strain. Three grains of cylindrical microdomains are shown to represent the random orientation in the non-stretched state. The arrows indicate the stretch direction.

The changes in the lattice spacing and orientation of the block copolymer microdomains were investigated with a series of two-dimensional SAXS profiles of the composite film (10 µm in thickness) on a PDMS substrate (150 µm in thickness) under uniaxial tensile test.26-28 13



The SAXS profiles are shown in Figure 5A. The X-rays passed through the film in the thickness direction when the specimen was stretched in the up–down direction. The initial film at ε = 0 showed a concentric ring pattern, which indicates non-preferential orientation of the microdomains in the lateral direction of the composite film (q = 0.255 nm−1 ). At ε = 0.5,

the ring pattern deformed into an ellipsoidal shape with a compressed meridional axis (q =

0.177 nm−1 ) and an expanded equatorial axis. At ε = 1.0, the diffraction pattern observed

was parallel to the tensile direction (q = 0.117 nm−1 ) . Whereas SAXS patterns obtained from in situ measurements of the block copolymer film under the uniaxial stretching showed an ellipsoidal ring pattern when the stress was released to ε = 0,29 the concentric diffraction

ring of the composite film was recovered. The morphological changes in the block copolymer microdomains are schematically illustrated in Figure 5B. Three grains of cylindrical microdomains are shown to represent the random orientation of the non-stretched state: parallel (ǀǀ), perpendicular (=), and randomly tilted (//) relative to the tensile direction. The parallel microdomains break at ε = 0.5. The randomly tilted grains are shear-aligned, becoming parallel to the tensile direction at ε = 1.0, hence, the diffraction from the parallel microdomains (left and right sides of the SAXS pattern) are diminished with increasing strain. The perpendicular microdomains are deformed to increase the inter-domain spacing; therefore, the diffraction peak appears at a smaller wave vector (q). The isotropic ring pattern was recovered during repeated cycles of stretching under

ε = 1.0. This result indicates that the compressive force in the PDMS substrate enabled the microdomains to recover their initial morphology; hence, the block copolymer composite film on the PDMS substrate becomes mechanically reversible during stretching cycles, with no remnant strain. 14


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Figure 6. (A) Stress–strain curves of the composite films on a PDMS substrate with various thickness ratios (h c /h s ) between the composite thickness (h c ) and the substrate thickness (h s ). (B) Stress–strain curves indicating the elastic behavior of the composite film on a PDMS substrate at different strain rates and different temperatures. (C) Changes in sheet resistance of the composite film under uniaxial tensile strain. The error bars are too small to be discernible. (D) The linear resistance of the composite film, as evaluated via repeated cycle tests at various strains. (E) Dynamic test to monitor the phase angle difference between the mechanical stimulation and electrical resistance of the composite electrode. The test was done at uniaxial elongation at ε = 1.2 and frequencies of 0.002 Hz and 6.0 Hz. 15



The mechanical reversibility should depend on the compressive force in the PDMS versus the viscoelastic resistance in the composite layer. Therefore, the thickness ratio (h c /h s ) between the composite thickness (h c ) and the PDMS substrate thickness (h s ) is a governing variable for mechanical reversibility. Figure 6A shows the stress–strain (S–S) curves for the composite films on a PDMS substrate at different h c /h s ratios: 0 (PDMS, green), 0.06 (blue), 0.1 (red), and 0.2 (black). The measurements were conducted at room temperature and at a strain rate of 0.1 mm/s. The composite film was prepared by 6-times transfer, and its thickness was fixed at 10 µm. The films were plasma bonded with the PDMS substrates with various thicknesses. When h c /h s ≤ 0.1, the film followed the mechanical behavior of the PDMS elastomer with no hysteresis. When h c /h s = 0.2 (h c = 10 µm, h s = 50 µm), the hysteresis caused by the composite layer was prominent and the crazes generated in the composite film caused fracture at high strains (ε ≈ 0.8). Therefore, the composite film bonded with a thin PDMS lacks the mechanical reversibility. It can be noted that the S–S curve showed no hysteresis at h c /h s = 0.06 during the first measurement, whereas it showed a hysteresis at the first measurement and then disappeared when h c /h s = 0.1 (Figure S7, Supporting Information). To ensure reliability of the specimen, we fixed h c /h s at 0.06 for the device fabrication. The strain rate and temperature are pivotal experimental parameters in the case of viscoelastic materials. Figure 6B compares the S–S curves collected at different strain rates (10−2 and 1.0 mm/s) and different temperatures (room temperature and 100 °C). The curves for the specimen were typical of the PDMS elastomer under all conditions, indicating that the elastic behavior is not sensitive to temperature and strain rate (Figure S8, Supporting Information). We obtained composite films with excellent electrical stretchability by taking advantage of the mechanical reversibility of the composite films on a PDMS substrate. 16


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Structural uniformity of the composite depends on the uniformity of the Au NSs layer. Because we started with a Au NSs layer of a uniform thickness, the resultant composite was uniform in the Au NS composition. The double bonds in the polybutadiene block in SBS can be broken by UV irradiation, but the antioxidant mixed already in the commercial product prevents the reaction. SBS is chemically stable at 150oC in the air, but it decomposes at 200oC which is typical in most polymers. We found that SBS in the composite form endures better than the bare SBS. Thermal treatment on the composite for a few min did not cause any degradation at 200oC. The mechanical property of SBS did not degrade by thermal annealing up to 200oC. Figure 6C shows the change in the sheet resistance of the composite film upon uniaxial elongation. The sheet resistance linearly increased from 0.45 Ω/sq at ε = 0 to 5.2 Ω/sq at ε = 1.0 and to 9.4 Ω/sq at ε = 1.6, which are small changes for a conductive electrode at such high strains. The changes in the sheet resistance during repeated stretching cycles followed the traces with negligible errors (the small error bars are not discernible). Figure 6D shows the changes in the linear resistance upon repeated stretching (200 times at each strain) at various strains. The composite film showed reliable performance at each strain. Figure 6E shows the dynamic test of the composite electrode during repeated stretching cycles at ε = 1.2. The frequency was varied in the range from 2.0 × 10-3 Hz to 6.0 Hz. The results indicate that the electrical response of the composite electrode was synchronized with the mechanical response of the PDMS substrate, without any phase angle difference caused by the relaxation time of the composite that is typically observed in viscoelastic conductors.30 The movies of the stretching test are shown in the Supporting Information (Movie S2).

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Figure 7. Scanning electron microscopy images of the Au NS film and the Au NS/SBS composite film. Both films were placed on a thick PDMS substrate. Each SEM image is initial, strained at ε = 1.0 after 100 cycles and recovered. (A-C) the Au NS film, (D-F) the Au NS/SBS composite film, respectively.

The high electrical reliability is attributed to the adhesive character between the polybutadiene block and the Au NSs, which is consistent with the TEM observations (Figure 4C). The strong adhesion allows affine movements of the Au NSs with the deformation in the block copolymer matrix. The formation of cracks and their growth in the composite films were investigated to understand the excellent electrical stability at large elongations. The scanning electron microscope images in Figure 7 exhibit the electrode film before stretching (ε = 0), in the stretched state (ε = 1.0) after 100 times stretching, and in the stress-relaxed state (ε = 0). To compare the role of SBS in the composite film, we investigated the crack behaviors of the Au NS film without SBS in Figures 7A-C and the Au NS/SBS composite film in Figures 7D-F. Both films were placed on a thick PDMS substrate. When the Au NSs film without SBS was 18


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stretched by ε = 1.0, the film was exfoliated due to the poor adhesion with the PDMS substrate. The cracks in the Au NS film caused the formation of large-size plates (1-2 mm). Many of the large plates were delaminated from the substrate surface, which made the electrode surface too rough and increased the resistance steadily during stretch cycles (Supporting Information, Figure S9). The vertical orientations of the large plates were kept even after the stretch was released. On the other hand, the SBS composite film formed microcracks without any increase of surface roughness. The morphology of the composite film at the stress-released state was identical to that of the pristine film observed before stretching test. Hence, the electrical conduction could be preserved upon repeated stretching cycles at ε = 1.0. Although PDMS and Au NSs could also make a composite film through the same process, the SBS-based composite had superior advantages. The Au NSs in the PDMS-based composite were pulled out upon the taping test. The resistance of the composite was much higher than the SBS-based composite because the Au NSs were delaminated during the coating process of the PDMS precursor solution. Additionally, the surface roughness of the PDMS-based composite was not readily controllable as shown in the supporting information (Figure S10).

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Figure 8. (A) Change in the sheet resistance of the Au NSs composite film during repeated peeling test with a 3M Scotch tape. These photographs showing the adhesion difference (B) The strong adhesion between the Au NSs composite film and the ECL gel layer. (C) The ECL devices before stretching and after stretching (ε = 0.3). The composite electrode was used as the bottom electrode. (D) Dependence of the luminance on the applied voltage at various AC input frequencies from 20 to 500 Hz. (E) Changes in the luminance at the stretched states of the ECL device during repeated stretching cycles (50 Hz, 6.0 V PP ).

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The adhesive property of the composite electrode is highly useful in preventing detachment of the Au NSs from the composite. During repeated peeling tests with Scotch tape, no Au NSs were removed from the composite film and the sheet resistance of the composite film did not change (Figure 8A). A movie of the peeling test is shown in the Supporting Information (Movie S3). It is notable that the Au NSs deposited on a PDMS substrate were completely peeled off by a contact with a Scotch tape, and the Au NSs in the PDMS composite also were removed by repeated contacts. The adhesion was measured using the peeling test (Table S1, Supporting Information). The adhesion between the PVDF-HFP ECL gel and other substrates (PDMS, SBS, Au NSs/SBS composite film) was compared. The PVDF-HFP ECL gel was coated on the substrates, then the gel film was peeled off. The test setup is shown in the supporting information (Figure S11). The load during the peeling test was recorded as shown in the supporting information (Figure S12). The data was normalized for relative comparison of the adhesion and is provided in the supporting information (Table S1). The film adhesion tests revealed that the SBS composite film had higher adhesion than the PDMS composite film. The adhesive character of the viscoelastic block copolymer composite is a unique advantage for stretchable electrodes with excellent chemical, mechanical, and electrical stabilities. We utilized the uniqueness to fabricate a stretchable ECL display. The light-emitting gel was prepared by mixing poly(vinylidene fluoride-co-hexafluoropropylene), a dye ((Ru(bpy) 3 )(PF 6 ) 2 ), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide in a weight ratio of 1:1:6.31-32 The gel was coated onto the Au NSs composite electrode on a PDMS. The contact adhesion between the ECL gel and the composite electrode was sufficiently strong to prevent peeling at the interface during large strain in Figure 8B.

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Figure 8C exhibits the ECL device working at V pp = 6.0 and 50 Hz in the initial (ε = 0) and stretched states (ε = 0.3). The composite electrode was used at the bottom, and a transparent Ag nanowire electrode or an ITO glass was used as the top electrode. For the light emission in the stretched state, the SBS/Au NSs composite electrode with the ECL gel was stretched and the ITO glass was placed on it. Since the light intensity of ECL devices is highly dependent on the operating voltage and frequency, optimal device conditions were investigated.33-36 The results are shown in Figure 8D. The luminance of the ECL device was measured by using luminance meter (CS-200, Konica Minolta). An AC square wave was generated from an arbitrary waveform generator (33220A, Agilent). All measurements were performed in ambient air. The ECL device began to emit light at V pp = 3.5 V and reached a maximum at V pp = 6.0 V at various frequencies. To obtain a quantitative change of the luminescence during dynamic stretching test, a small dot (radius = 2 mm) of ECL printed by a nozzle printer (Image Master 350PC, Musashi) on the Au NS composite electrode was used. The inset shows luminescence of the ECL dot before stretching and after being stretched by ε = 0.3. Figure 8E compares the luminescence from the ECL dot after stretching 1, 50, and 100 times. The measurement at the stretched states (ε = 0.1, 0.2, 0.3) was performed after stretching the Au composite electrode with the ECL dot and placing an ITO glass as a top electrode. The device showed the same luminescence values at the strains during stretch test, which indicates that repeated stretching does not degrade the ECL device.

22


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Figure 9. (A) OM image of the Au NSs composite comb pattern with the ECL gel. The overall electrode structure is shown in the inset. (B) The linear resistance of the comb structure. (C) Camera images of a ECL display with a horizontal electrode setup at ε = 0 and ε = 0.7.

Taking advantage of the simple device setup of the ECL display, we fabricated a display with a horizontal electrode setup of a comb structure made of the SBS/Au NSs composite film. Patterning of the Au NSs film was conducted through the lift-off process. An uncured PDMS was printed by a nozzle printer directly on the Au NSs film transferred on a Si substrate. After curing, the printed PDMS lines were peeled off. The Au NSs underneath the printed PDMS were also peeled off together with the PDMS, so a pattern of the Au NSs was left on the Si wafer. SBS was spin-coated on the patterned Au NSs and the composite electrode was transferred onto a plasma-treated PDMS substrate (Figure S13, Supporting Information). The device setup is shown in Figure 9A. The inset is a camera image of the entire electrode setup. The ECL gel was coated on the composite electrode. The line width of the electrode was 750 23



µm and the distance between comb structure was 250 µm. Figure 9B shows the changes in the linear resistance of the patterned Au NSs composite film at various elongational strains. The ECL gel was coated on the electrode setup. Light emissions from the horizontal electrode at the initial (ε = 0) and stretched states (ε = 0.7) are shown in Figure 9C. The ECL device was operated at V pp = 6 V and 50 Hz. The luminance of the horizontal ECL device was changed by the increased distance between the comb structures due to the applied strain.

CONCLUSION We investigated the conditions where a thermoplastic conductive composite, Au NSs and SBS block copolymer composite as an example, can exhibit excellent stretchability at high elongational strains (ε =1.8). We found that a viscoelastic composite on a chemically crosslinked elastomer substrate can follow the elastic behavior of the substrate. The strong adhesion of the thermoplastic composite was advantageous in the fabrication of mechanically robust highly conductive stretchable electrodes. The chemical stability of the SBS/Au composite was used to fabricate high luminescence stretchable ECL displays with a conventional top–bottom electrode setup and with a horizontal electrode setup.

EXPERIMENTAL METHODS

Materials. SBS (polystyrene-block-polybutadiene-block-polystyrene, M w = 140000, volume fraction of polystyrene = 30%), hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, 99%), L-arginine, tris(2,2′-bipyridine)ruthenium(II) hexafluorophosphate ((Ru(bpy) 3 )(PF 6 ) 2 , 24


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M w = 859.55), poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP), M w = 400,000)

were

purchased

from

Sigma-Aldrich.

1-Ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) was purchased from Merck Co. The silicon prepolymer (Sylgard 184) and its crosslinking agent were purchased from Dow Corning. Preparation of Au NSs composite film. Au NSs were synthesized following the process described in our previous publication.5 The Au NSs floated on the water surface was transferred 6 times on a thin PDMS film prepared on a silicon wafer. A concentrated SBS solution (10 wt% in toluene) was spin-coated on the multi-layer film of the Au NSs at 2000 rpm for 60 s. The SBS coated Au NSs composite film was then soft-baked at 80 °C to remove residual solvent for 10 min. The composite film was treated by oxygen plasma (50 W, 45 s). A fresh PDMS substrate was also treated with plasma. The plasma-treated SBS coated Au NSs composite film was transferred to the plasma-treated PDMS substrate by simple contact and lift-off. The Au NSs composite film on the PDMS substrate was thermally annealed at 160 oC for 2 h in a vacuum to apply thermal energy for microphase separation of the block copolymer matrix. Test peel-off force. Three kinds of substrates coated with the ECL gel were prepared, and the peel-off force was measured using a universal measurement probe system (UMP100, TERALEADER Co. LTD., Korea). A portion of the coated ECL gel on each substrate was peeled to hold with the clamp connected the load cell. The peel-off force was measured while detaching the ECL gel from the substrate at a constant velocity of the load cell. Fabrication of stretchable electrochemiluminescence device with top-bottom electrode setup. The ECL gel solution was obtained by mixing P(VDF-HFP), [EMIM][TFSI], and a dye molecule (Ru(bpy) 3 )(PF 6 ) 2 at weight mixing ratio of 1:6:1 in acetone.31 The solution was coated using automatic film applicator (AB3220, TQC) on the Au NSs composite film 25



bonded to a PDMS substrate. The coated ECL gel was dried in a vacuum oven for 12 h at 45oC to remove residual acetone in the gel. The composite electrode was used at the bottom, and a transparent Ag nanowire electrode or an ITO glass was used as the top electrode. The luminance of the ECL device was measured by using luminance meter (CS-200, Konica Minolta). An AC square wave was generated from an arbitrary waveform generator (33220A, Agilent). All measurements were performed in ambient air. Fabrication of stretchable electrochemiluminescence device with horizontal electrode setup. An uncured PDMS was printed on the Si substrate 6-times transferred with Au NSs by a nozzle printer (Image Master 350PC, Musashi). After curing at 100°C and removing the printed PDMS with a tweezer, the Au NSs underneath the printed PDMS were also peeled off to prepare the patterned Au NSs film. After that, the patterned Au NSs on a Si substrate was coated with SBS solution (10 wt% in toluene) and transferred to a plasma treated PDMS substrate. The patterned Au NSs composite film was prepared through this process. Finally, the P(VDF-HFP) solution was coated using automatic film applicator (AB3220, TQC) on the patterned Au NSs composite film (the width of the electrodes = 750 µm, the gap between electrodes = 250 µm). The coated ECL gel was dried in a vacuum oven for 12 h at 45oC to remove residual acetone in the gel. The ECL device was operated at V pp = 6 V and 50 Hz. Characterization and Measurements. The morphology was investigated using SEM (SNE4500M, SEC) and cross-sectional TEM (JEM-2100F, JEOL) at 200 kV. X-ray diffraction (XRD) was obtained with D/MAX-2500/PC, RIGAKU with Cu Kα radiation (λ = 0.1542 nm). Sheet resistance was measured with a four-point probe (CMT-100S, AiT). Stress-strain curves were obtained with prepared sample (width: 8 mm, length: 26 mm) using a tensile stress tester (TST350E, Linkam Scientific Instruments). The SAXS measurements were conducted at the 26


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Pohang Light Source (PLS) 6D SAXS beamline of the Pohang Accelerator Laboratory in Korea. The SAXS patterns were recorded with tensile strain at every 30 seconds. Resistance measurements were performed using a Keithley 2400 source meter. The luminance of the ECL device was measured by using luminance and color meter (CS-200, Konica Minolta). An AC square wave was generated from an arbitrary waveform generator (33220A, Agilent). All measurements were performed in ambient air.

ASSOCIATED CONTENT The supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed data and figures (PDF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI)

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected],

[email protected]

Notes The authors declare no competing financial interest.

27



ACKNOWLEDGMENTS This work was supported by the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CASE-2015M3A6A5072945) and Pohang Steel Corporation (POSCO) through the Green Science Program (Project No. 2015Y060). Experiments at PLS-II 6D UNIST-PAL beamline were supported in part by UNIST Central Research Facilities, MSIP and POSTECH.

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