Intrinsically Stretchable Electrochromic Display by a Composite Film of

May 22, 2017 - ... iron(III) tosylate at elevated temperature. The fabricated film showed reversible electrochromism without an external conductive su...
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Intrinsically Stretchable Electrochromic Display by a Composite Film of Poly(3,4-ethylenedioxythiophene) and Polyurethane Hiroyuki Kai,* Wataru Suda, Yudai Ogawa, Kuniaki Nagamine, and Matsuhiko Nishizawa* Department of Finemechanics, Tohoku University, 6-6-1 Aramaki-Aoba, Sendai 980-8579 Japan S Supporting Information *

ABSTRACT: A stretchable, electrochromic film of a uniform composite of poly(3,4-ethylenedioxythiophene):p-toluene sulfonic acid (PEDOT:PTS) and polyurethane (PU) (PEDOT/PU) was fabricated, and its integration with a hydrogel as a free-standing, stretchable electrochromic (EC) display was demonstrated. The PEDOT/PU composite film was prepared by the spin coating of a solution containing an EDOT monomer and PU, followed by oxidative polymerization using iron(III) tosylate at elevated temperature. The fabricated film showed reversible electrochromism without an external conductive support. The color change of the film can be used to quantify the progress of the redox reactions by means of digital camera image analysis and a custom mobile phone app. KEYWORDS: electrochromic display, intrinsic stretchability, conductive polymer, digital image analysis, wearable device

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polymer materials,5,6,16−19 to the best of our knowledge, intrinsically stretchable EC materials have not been reported so far. Here, we show a composite of poly(3,4-ethylenedioxythiophene):p-toluene sulfonic acid (PEDOT:PTS) and polyurethane (PU)20 (PEDOT/PU) works as an intrinsically stretchable, freestanding EC film. The electrochemical reactions are indicated and quantified by the color of the film. PEDOT is a notable example of an EC polymer that has several advantages such as a low redox potential and high stability under ambient conditions.21−24 The reduced form of PEDOT exhibits a deep blue color due to visible light absorption that accompanies π−π* transition, whereas the oxidized form is pale blue because of the depletion of low-energy π−π* absorption bands. Polyurethane is an elastic polymer with excellent stretchability that can be used for intrinsically stretchable devices. Hansen et al. developed a molecularly uniform composite of PEDOT and PU20 as a stretchable conductor and therefore, this PEDOT/ PU composite is considered as an intrinsically stretchable conductive film. Although our group has also used this PEDOT/PU film as a stretchable electrode,25,26 its EC properties have not been investigated. In this study, we found reversible and stable electrochromism of the intrinsically stretchable PEDOT/PU film, which is suitable for wearable display applications. To the best of our knowledge, this is the first investigation on an intrinsically stretchable electrochromic film. We built a stretchable EC display with the PEDOT/PU film as well as its hybrid with hydrogel, and investigated the

earable electronics and devices have attracted much attention recently, as they offer continuous monitoring of health in a noninvasive manner.1,2 Stretchable and wearable electronics using polymeric materials are of particular interest as they comfortably conform to skin contours; this has led to the development of physical and chemical sensors, batteries, energy harvesters, light-emitting diodes (LEDs), etc.3,4 One important component of wearable electronics is the display, which shows the sensor output in the device. Stretchable LED displays that emit light when electric current is applied have been developed using elastic substrates with organic LED materials. For example, Sekitani et al. developed a printed stretchable electrode made from carbon nanotubes/ionic liquid mixture dispersed in fluorinated rubber, and fabricated stretchable organic LEDs on top of that.5 A group led by Shepherd recently reported a highly luminescent and stretchable light emitter using ZnS phosphor embedded in silicone rubber.6 In contrast to the LEDs, an electrochromic (EC) display is a nonemissive device that changes its color upon redox reactions, and is a unique option for a display device because it consumes energy only for switching unlike LEDs. Various EC materials such as metal oxides7,8 and polymer materials9−11 have been studied, and flexible and stretchable EC devices have also been realized.12−14 These flexible and stretchable EC devices, however, rely on designinduced stretchability,15 where nonstretchable electronic materials are patterned into micrometer-scale springlike structures and wavelike structures. In contrast, stretchable devices with intrinsic stretchability can eliminate the need of higher-order microstructures that involve complicated fabrication, and can dramatically simplify the fabrication process, thereby providing flexibility in device designs. Although there is increasing interest in intrinsically stretchable electronics using © XXXX American Chemical Society

Received: March 3, 2017 Accepted: May 22, 2017 Published: May 22, 2017 A

DOI: 10.1021/acsami.7b03124 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces redox reactions and the accompanying color changes of the PEDOT/PU film by electrochemical analysis, UV−vis absorption spectroscopy, and digital camera image analysis. Moreover, the digital camera image analysis was integrated into a custom mobile phone app to demonstrate the usefulness of the film as a display for electrochemical signals. The fabrication of the PEDOT/PU film with varied PEDOT ratios was carried out following a procedure that was slightly modified from a previously reported one.20,25 A mixture of an EDOT monomer, iron(III) tosylate, and PU in tetrahydrofuran was spin-coated on a glass substrate at 2000 rpm, followed by oxidative polymerization using iron(III) tosylate at 100 °C for 10 min. PEDOT ratios in the resulting film were varied by changing the ratio of the EDOT monomer (Table S1). The resulting film had thickness ranging from 0.3 to 4 μm, depending on the EDOT ratio in the starting mixture and spincoating speed (Figure S1), and could be handled either on a glass substrate or as a free-standing film. Tensile testing of the film revealed that the PEDOT/PU film achieved 266% of elongation at break with 20% PEDOT content, which significantly decreased to 21% when the PEDOT content was increased to 40% (Figure S2). The elastic modulus is higher for higher PEDOT content, and ranges from 13 to 24 MPa. The higher stiffness and lower flexibility for higher PEDOT content can be attributed to the stiffness and fragility of PEDOT, compared to the soft and stretchable PU. When voltage was applied to the PEDOT/PU film via a piece of carbon fabric on the surface held by a metal tweezer that was connected to a working electrode of a potentiostat, the film underwent reversible color change visible by eye that accompanies stable redox cycles. The reversible color change was observed even when the film was stretched (Figure 1a). The fabricated PEDOT/PU film was stacked on a double-network hydrogel (DN gel)27 to make an EC device with a built-in electrolyte (Figure 1b−d). Hydrogels in general are good ionic conductors, and it allows PEDOT to undergo electrochemical reactions. The DN gel is a class of hydrogels that is made of two interpenetrating cross-linked polymer networks, and it is mechanically tougher and more stretchable than conventional hydrogels. We adopted the DN gel that consists of poly(acrylamide) cross-linked with N,N′-methylenebis(acrylamide) and gellan gum cross-linked with Ca2+ for our device, because this DN gel can be easily prepared by single-step fabrication. The DN gel worked as a stretchable electrolyte reservoir, allowing the two separate parts of the film mounted on the single substrate to undergo a pair of reduction and oxidation reactions simultaneously, whereas the DN gel is not redox active per se. The placement of the two films with electrochemical reactions in the opposite directions made the color change more obvious, since one film became darker and the other lighter at the same time, which makes high color contrast. The EC device is stretchable, and can be wrapped around a glass rod (Figure 1c) or mounted on a human finger (Figure 1d, Video S1). The color change of the EC device became obvious within a few seconds when electric current was applied at constant voltages. The EC behavior of the PEDOT/PU film was further investigated with a PEDOT/PU film on an insulating glass slide. The use of a spin-coated film on a glass substrate ensured convenient measurement because the film maintained the flat shape, and its thickness was consistent. The film on the glass substrate was immersed in 1X phosphate buffer saline (PBS) of pH 7, and oxidized at 0.8 V (against Ag/AgCl in saturated KCl)

Figure 1. An intrinsically stretchable electrochromic (EC) film of PEDOT/PU. (a) Molecular structures of PEDOT and photographs of the stretched electrochromic film of a PEDOT/PU composite in reduced and oxidized forms. (b) Schematic diagram of an EC film/ hydrogel hybrid. (c) EC film/hydrogel hybrid wrapped around a glass rod and human finger. (d) Color change of the EC film/hydrogel hybrid device placed on a human finger.

and reduced at −0.5 V. The color change was observed by eye, as well as by UV−vis absorption spectroscopy. The color change became clearly visible by eye within a few seconds, where the reduced PEDOT/PU film was dark blue colored, whereas the oxidized film was pale blue. Consistent with visual observation, the reduced form of the PEDOT/PU film showed an enhanced absorption around 570 nm in the UV−vis absorption spectrum, compared to the oxidized film (Figure 2a). This imparted a deep blue color to the PEDOT/PU film, which is typical of PEDOT in general.28 Cyclic voltammetry (CV) of the PEDOT/PU film in PBS showed stable redox reactions over three cycles, which indicates the stability of the film upon redox reactions under ambient conditions (Figure 2b). The redox potential of the PEDOT/PU film was estimated to be approximately 0.1 V; however, since the reduction peak was very broad, a precise evaluation of the peak voltage from the CV was difficult. The oxidative current of the PEDOT/PU film was evaluated by capacitive correction by subtracting the capacitive current from the oxidation peak current in the cyclic voltammograms (Figure S3). When the PEDOT/PU film thickness was varied by changing the spin coating speed with the constant PEDOT ratio of 30%, the oxidation peak current B

DOI: 10.1021/acsami.7b03124 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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(L*, a*, and b*) and represents the perception of color by human eyes. The CIELAB color and the commonly used RGB color are interconvertible by a mathematical transformation (Supporting Information). Once the RGB color is converted to the L*a*b* color, its L* value, which corresponds to the brightness of the color, is used to quantify the color change of the PEDOT/PU film. The temporal change of L* upon redox reactions was synchronous to the visual color change, which converges over time (Figure 3a). Importantly, visible light absorption at 570 nm (A570 nm) and L* value of the film sample at various degrees of oxidation were linearly correlated (Figure 3b). Therefore, we adopted L* of the reflected light from the digital camera image in the following experiment as a facile quantitative measurement of EC reactions. Digital camera images and video offer rich information on color in space and time, and the analysis can be easily combined with other methods such as conventional electrochemical measurements. Using the L* value, the coloration and bleaching cycles of the PEDOT/PU film were observed for 100 redox cycles (0.5 V and −0.5 V) (Figure 3c). The color change indicated that degradation of the film after 100 cycles was negligible, and this reversibility demonstrates the practical usefulness of the PEDOT/PU film as an EC display. Even more cycles up to 1200 caused 20% degradation of transmittance contrast (Figure S5). The measurement also exemplifies the convenience of the digital image analysis compared to UV−vis spectroscopy for electrochemical reactions in solution, which requires special equipment. To further demonstrate the applicability of digital image analysis for easy quantification of the EC behaviors, we built a custom mobile phone app to measure and show the color of the PEDOT/PU film (Figure 3d, Video S2). This app can be used for real-time measurement of color of the images taken on a mobile phone camera, and can confirm the temporal change in L* values. The quantification of redox reactions by the L* values established above is useful for the detailed investigation of reaction kinetics. We investigated the kinetics of redox reactions of the PEDOT/PU film by color measurement. When a positive (i.e., oxidative) voltage was applied, the PEDOT/PU film color turned pale blue within a few seconds, which corresponds to an increase in L* in the digital camera image (Figure 4a). The change in L* over time could fit the exponential profile described in eq 1 well, which is characteristic of a first-order reaction.

Figure 2. (a) UV−vis absorption spectra of PEDOT/PU films in the reduced (blue solid curve) and oxidized (gray dashed curve) forms. (b) Cyclic voltammetry of a PEDOT/PU film (area: 20 mm × 20 mm, thickness: 2.9 μm) with 30% PEDOT ratio on a glass substrate with a scan rate of 10 mV/s. The second cycle (blue) and the third cycle (orange) are shown. Black arrows indicate the oxidative and reductive peaks of PEDOT. Inset: oxidation peaks adjusted by baseline subtraction were plotted for the PEDOT/PU films with varied thicknesses (5.6 μm (blue), 4.4 μm (orange), and 2.9 μm (green)).

of the film showed positive correlation (R2 = 0.565) with the thickness of the film. Moreover, scanning electron microscopyenergy dispersive X-ray spectroscopy (SEM-EDX) analysis of the cross section of the PEDOT/PU film showed the reasonably homogeneous distribution of sulfur atoms along the thickness of the PEDOT/PU film (Figure S4). These results suggest that the electroactive PEDOT is uniformly distributed throughout the film thickness, and the entire film including the interior undergoes oxidation as efficiently as the surface does. This property enables the arbitrary control of color depth of the EC film while maintaining a high contrast between the reduced and oxidized forms. The stable, reversible color changes of the fabricated film demonstrated above suggest the usefulness of the PEDOT/PU film as an EC display with electrochemical input. Therefore, we performed quantitative measurement of the EC behavior of the PEDOT/PU film. Although the UV−vis absorption spectroscopy described above is quantitative with respect to the degree of oxidation and reduction of PEDOT, it would be more convenient if one could just use the reflected color observed in the digital camera images as information on the progress of redox reactions. To examine the usability of the reflected color for such purpose, we investigated the quantitative relationship between the UV−vis spectrum and the reflected color of the PEDOT/PU film. We quantified the color change in the digital camera images of the PEDOT/PU film during the redox reactions by means of a CIELAB color scale29 as previously reported, and compared it with the UV−vis spectral measurement. The CIELAB color scale consists of three coordinates

L* = L i* + (Lf* − L i*)[1 − e−k(t − t0)]

(1)

where L*i is the initial value of L*, L*f is the final value of L*, and k is a rate constant (Figure S6). When the applied voltage (E) was varied between 0.8 and −0.5 V, the final color after convergence (L*f) changed, and the L*f versus E showed a sigmoidal profile (Figure 4b and Figure S7). The film with higher PEDOT content was darker, as shown by the L*f values that are normalized by the film thickness. The sigmoidal profile indicated that the equilibrium between the reduced and oxidized forms changed depending on the applied voltage, and the redox potential can be calculated as the center (inflection point) of the sigmoidal curve that is obtained by nonlinear curve fitting of the data points (Figure S8).29 The PEDOT/PU films with different PEDOT ratios had redox potentials of around 0.1 V, which is in agreement with the cyclic voltammetry described above. The decolorization rate was highest when the PEDOT ratio was 30 wt % (Figure 4c). C

DOI: 10.1021/acsami.7b03124 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) Color change upon redox reactions of the PEDOT/PU film with 30 wt % PEDOT content measured by UV−vis absorbances and mean L* values of the CIELAB representation of the digital camera images. (b) Correlation between the absorbance at 570 nm and the L* value of the film. (c) Repeated color change of the electrochromic film during 100 redox cycles in 25% McIlvaine buffer (pH 5). (d) Custom mobile phone app that measures the L* value of the PEDOT/PU film and plots its temporal change in a real-time manner. Measurement of the L* value of the PEDOT/PU film by the app (left), and a screenshot of the app (right).

of the reduced form of PEDOT/PU (Figure S10) and propagation of oxidation from the contact point with the electrical lead to the entire film.30 Nonetheless, the propagation of oxidation to the entire film takes only a few seconds and is fast enough for practical applications. The dependence of reaction rate on the film thickness was also investigated (Figure S12). Although the deviation between samples was large, no obvious trend of reaction rate against the film thickness in the chronoamperometric measurement was observed. This suggests that the reaction propagates immediately along the thickness of the film (∼5 μm), which is in clear contrast with the noticeable delay in color change arising from horizontal reaction propagation (for ∼1 cm) described above. In conclusion, we have demonstrated the first example of an intrinsically stretchable, free-standing EC film and showed its potential as an EC display with electrochemical input. The EC film of a PEDOT/PU composite was prepared by spin-coating, which makes the fabrication process simpler in that it eliminates the need for top-down fabrication of microstructures that was previously used for design-induced stretchability. The PEDOT/ PU film works as a free-standing EC film in electrolyte solution, and can be combined with other stretchable materials such as hydrogel as a support. The color change of the PEDOT/PU film is obvious enough to be seen by eye and can be quantified by digital camera image processing that can be integrated into a mobile phone app. The developed EC film and device are useful as a nonemissive display component in a stretchable wearable device to indicate electrochemical signals.

This maximum can be attributed to a balance between the two opposite effects: higher electrical conductivity of a PEDOT/PU film with higher PEDOT content and higher ion conductivity of the film with lower PEDOT (i.e., higher PU) content. During the electrochromic reactions of PEDOT, the polymer chain switches between the neutral state and positively charged state, and the switching is accompanied by uptake and release of counteranions. Higher PU ratio should be advantageous for ion transport from and to PEDOT molecules. The rate constant of decolorization at 0.8 V reached 0.82 s−1 for the film with 30 wt % PEDOT content, where 95% of the color change occurred within 3.7 s. In contrast, the dependence of the color change rate on the voltage upon reduction (0 to −0.6 V) was much less significant (Figure S9), which is probably because the conductivity of the film rapidly decreased upon reduction (Figure S10), leading to a significant decrease in the reaction rate that does not depend on the applied voltage. A free-standing PEDOT/PU film under 50% stretching also showed reversible switching of color with rate constant of oxidation, k = 1.1 (Figure S11). The slightly faster oxidation than that of an unstretched film can be attributed to the higher electrical conductivity of the stretched film.24 It is noteworthy that the film was able to undergo fast redox reactions without an underlying conductive substrate. To understand the underlying mechanism, we measured color changes at different points on the film by digital camera image analysis described above. Upon oxidation, a point away from the electrical lead (a metal tweezer) showed a slightly (∼2 s) delayed onset of oxidation than did the point near the electrical lead (Figure 4d). The delayed response can be attributed to the low conductivity D

DOI: 10.1021/acsami.7b03124 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Color change of the electrochromic PEDOT/PU film measured by L* values of the reflected light in the digital camera images. (a) Time course of the color of the PEDOT/PU film with 20 wt % PEDOT content for different applied voltages (filled circles; 0.2 V (blue), 0.4 V (orange), 0.6 V (green), and 0.8 V (red)) and nonlinear curve fitting of the data points according to eq 1 (solid curves). (b) Final colors of the film for varied applied voltages that are normalized to 0.5 μm film thickness. PEDOT content: 20 wt % (blue), 40 wt % (orange), 60 wt % (green), and 80 wt % (red). (c) Rate constants of oxidation versus PEDOT ratio. Voltage: 0.2 V (blue), 0.4 V (orange), 0.6 V (green), and 0.8 V (red). (d) Spatial propagation of oxidation at 0.8 V on a 20 mm × 20 mm film with 80 wt % PEDOT. Normalized ΔL* values at three different points on the film are shown.

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ASSOCIATED CONTENT

S Supporting Information *

ACKNOWLEDGMENTS

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REFERENCES

We thank Micromachining Facility Micro/Nano-Machining Research and Education Center and Toshiya Kojima for their help with SEM-EDX analysis. This work was partly supported by Center of Innovation Program (COI-Stream) and Creation of Innovation Centers for Advanced Interdisciplinary Research Area Program from Japan Science and Technology Agency (JST); Regional Innovation Strategy Support Program “Knowledge-based Medical Device Cluster/Miyagi Area” and Grandin-Aid for Scientific Research A (25246016) and Challenging Exploratory Research (K15K13315) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03124. Experimental details of fabrication and characterization of PEDOT/PU films: film thickness measurement, elastic properties, oxidative current versus film thickness, SEMEDX analysis, color change for 1200 cycles, a representative plot of L* versus time, unnormalized final L* values, redox potentials, reduction reaction rate, electrical conductivity, electrochromic kinetics under stretching, and rate of oxidation versus film thickness (PDF) Video S1, EC device working on a finger (MPG) Video S2, peration of the mobile phone app to measure L*values in a real-time manner (MPG)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.K.). *E-mail: [email protected] (M.N.). ORCID

Hiroyuki Kai: 0000-0002-3544-9196 Notes

The authors declare no competing financial interest. E

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DOI: 10.1021/acsami.7b03124 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX