Intrinsically Stretchable Electrochromic Display by a Composite Film of Poly(3,4-ethylenedioxythiophene) and Polyurethane

Transcription

1 Supporting Information 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, Aramaki-Aoba, Sendai, Japan. * kai@biomems.mech.tohoku.ac.jp (H.K.) * nishizawa@biomems.mech.tohoku.ac.jp (M.N.) S-1

2 General methods Polyurethane (PU; Gumthane AR650) was a kind gift from Okada Engineering. 3,4- Ethylenedioxythiophene (EDOT; Clevios M V2) and iron (III) tosylate (Fe(III)(TsO) 3 ; Clevios C- B 40 V2) were purchased from Heraeus. All other reagents were purchased from Sigma Aldrich and Wako Pure Chemical, and were used as received. All the electrochemical potentials are reported as the voltage against Ag/AgCl in saturated KCl. All the graphs were generated by Python scripts ( and edited using the Adobe Illustrator CC 2015 software. Error bars in a graph indicate standard deviation between specimens unless otherwise stated. Fabrication of a stretchable PEDOT/PU film Glass slides (S1127, Matsunami Glass Ind., Ltd.) were cut into half and cleaned by sonication three times in acetone, denatured ethanol, and distilled water, and then kept in isopropanol. PU (4 g) was dissolved in tetrahydrofuran (THF; 35.5 ml) by stirring overnight at room temperature. EDOT, 1-butanol, and iron tosylate (Fe(III)(TsO) 3 ) were added to the resulting THF solution of PU at 0 C in varied ratios, as presented in Table S1, and the solution was spin-coated on a glass slide at 2000 rpm for 30 sec with acceleration for 7 sec. The spin-coated solution was heated on a hot plate at 100 C for 10 min to evaporate THF and polymerize EDOT. The resulting film was washed in distilled water to remove an unreacted monomer and iron tosylate. The fabricated PEDOT/PU film was subjected to electrochemical measurement as it was, or peeled off from the glass substrate for use as a free-standing film. Fabrication of a double-network hydrogel A double-network hydrogel (DN gel), which was composed of gellan gum crosslinked with Ca 2+ and poly(acrylamide) crosslinked with N,N -methylenebis(acrylamide), was prepared according to literature. 1 Acrylamide (450 mg/ml), N,N -methylenebis(acrylamide) (10 mg/ml), gellan gum (7.5 mg/ml), calcium chloride dihydrate (35.3 mg/ml), and Irgacure 184 (10 mg/ml) were mixed in water. The resulting mixture was heated in a microwave oven to boiling, and poured between glass slides with a silicone rubber spacer of 0.5 mm thickness, followed by irradiation with 365 nm UV light for 1 hour under nitrogen atmosphere. The polymerized hydrogel was washed in water on a shaker overnight, and further immersed in a 1X phosphate buffer saline solution (ph 7) on the shaker overnight. S-2

3 Fabrication of an electrochromic device made of PEDOT/PU and hydrogel An aqueous dispersion of polyurethane (PERMARIN UC-20, Sanyo Chemical) was poured onto the PEDOT/PU film fabricated on a glass substrate, and cured at 70 C for more than 3 h to prepare a PEDOT/PU-PU layered film. The DN gel fabricated above was cut into a rectangular piece, and placed on the PEDOT/PU film to fabricate the self-contained electrochromic device. Tensile testing Stress-strain curves were measured by a force gauge (FSA-1KE-5N, Imada) using a piece of PEDOT/PU film (15 mm width and 10 mm length). The PEDOT content was varied between 20, 30, and 40 wt%. The spin coating speed for the fabrication was 500 or 2000 rpm. The elastic moduli were obtained from the slope of a stress-strain curve between 0% and 5% strains. Cyclic voltammetry The PEDOT/PU film was wrapped within a folded piece of carbon fabric and connected to the electrode of a potentiostat (ALS model 7082E, ALS Co., Ltd.), and cyclic voltammetry was carried out in 1X PBS with a scan speed of 10 mv/s. Evaluation of oxidative current Oxidative current was evaluated by subtracting a capacitive baseline from the oxidative peak current. The capacitive baseline was calculated by a linear interpolation between -0.2 V and 0.5 V (Fig. S3(a)). Elemental analysis of the cross section of the PEDOT/PU film A carbon film of 1 µm thickness was deposited on the PEDOT/PU film, followed by etching with gallium by a focused ion beam (SMI500, SII Nanotechnology) to make the cross section of the film. Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy (SEM-EDX) was performed on the cross section of the film to obtain a line profile of elements along the thickness of the film. S-3

4 Quantification of color change with a CIELAB color scale The PEDOT/PU film fabricated above was held with a tweezer with reverse action with a carbon fiber fabric as an electrical contact, and connected to a potentiostat. The reduction and oxidation were carried out by applying a constant voltage, during which the current was measured. The progress of the reaction was recorded as a movie using a digital camera (Canon EOS Kiss X7i) with a frame rate of 30 frames/sec. The image intensity of the chosen area with a constant time interval (0.1, 1, or 10 sec) was quantified by the L* values of the CIELAB color scale with a Python script with NumPy and SciPy libraries ( The CIELAB scale consists of the three coordinates (L*, a*, and b*), which were calculated by the following two-step transformation of (R, G, B) to (X, Y, Z) and (X, Y, Z) to (L*, a*, b*) (with illuminant = D65 and observer = 2A).! # A B = = &(!) &(#) &($), 0! 1, 0 # 1, 0 $ 1 where & 2 = &(2) = > (otherwise)! # $ G = 116I A/ L = 500 I A/100 M = 200 I A/100 I(B/ ) where I 2 = 2 N/O (2 > ) I 2 = (otherwise) The difference of illumination was adjusted by the L* values of a Macbeth ColorChecker, where the L* values from the color checker were collected and a constant scalar value was multiplied to them to minimize the difference of the L* values for different images. The rate constant was calculated by nonlinear curve fitting of data to an equation of a first-order reaction. G = G P + G Q G P 1 R S TUT V, S-4

5 where L* i is the initial value of L*, L* f is the final value of L*, and k is a rate constant. The L* f values in Fig. 4b were normalized to the film thickness of 0.5 µm by the following equations for the calibration curve in Fig. 3b, since the absorbance is proportional to the film thickness and the thickness depends on the PEDOT content. W = & G W norm = W 2 G norm = & UN W norm where & G = G & UN (W) = 80.83W Custom mobile phone app A prototype iphone app ( was written in Xcode (Apple Inc.). Video snapshots were captured with a frame rate of 10 frames/s, and the RGB values were averaged in the region of interest in each frame. The averaged RGB values were then converted to L*a*b* values. L* versus time was plotted on the screen in a real-time manner. The picture of the iphone frame was overlaid with a screenshot of the app using S-5

6 Supporting table, figures, and videos PEDOT [wt%] 10 wt% PU solution in THF [ml] 1-Butanol [ml] EDOT [ml] 40 wt% iron(iii) tosylate solution in 1-butanol [ml] Total [ml] Table S1. Compositions of starting materials in solution for preparing the PEDOT/PU films with varied PEDOT contents. Figure S1. Dependence of the film thickness on PEDOT content and spin coating speed. PEDOT content: 20 wt% (blue), 30 wt% (orange), 40 wt% (green), 60 wt% (red), and 80 wt% (purple). Average thicknesses of two samples are shown, in each of which three points were measured and averaged. S-6

7 Figure S2. Elastic properties of the PEDOT/PU films with varied PEDOT contents. (a) Representative stress-strain curves of the films. (b) Elastic moduli and elongation at break. Four to six samples for each condition were measured. S-7

8 Figure S3. Oxidative current of the PEDOT/PU film. (a) An example of cyclic voltammogram and illustration of a method of capacitive correction. Linear interpolation between -0.2 V and 0.5 V was used as a baseline, and oxidative current was calculated by subtracting the baseline from the peak current of oxidation around 0.2 V. (b) Correlation between the film thickness and oxidative current. PEDOT/PU films with 30 wt% PEDOT content (20 mm 20 mm) were used. Figure S4. SEM-EDX analysis of the cross section of a PEDOT/PU film with 20 wt% PEDOT content. (a) Scanning line and direction. (b) Atomic ratios of carbon (blue), oxygen (orange), and sulfur (green). The plot on the bottom is a magnified plot for sulfur of the same data as the plot on the top. S-8

9 Figure S5. The color changes of the PEDOT/PU film for longer redox cycles. Voltages of 0.5 V and -0.5 V were alternatingly applied to the PEDOT/PU film with 20 wt% PEDOT content in a 0.25X McIlvaine buffer solution (ph 5). Every cycle consists of 0.5 V for 30 sec and -0.5 V for 30 sec. (a) The L* values of the film over redox cycles up to the 1210th cycle. (b) The maximum and minimum L* values (top), and transmittance contrast (i.e., the ratio of transmittance) (bottom) over 1200 redox cycles. S-9

10 Figure S6. A representative plot of L* versus time and parameters of a fitting curve with Eq. 1. The equation represents a first-order reaction. Eq. 1 has four parameters: t 0, k, L* i, and L* f, where k corresponds to the reaction rate. Figure S7. Unnormalized final L* values (L* f ) of the PEDOT/PU film. PEDOT content: 20 wt% (blue), 40 wt% (orange), 60 wt% (green), and 80 wt% (red). S-10

11 Figure S8. Redox potentials calculated from the sigmoidal profiles of L* f values in Fig. 4(b). Error bars indicate error in the non-linear fitting of two samples in each condition. Figure S9. Reduction reaction rate of the film versus applied voltage, measured by the L* values of the digital camera images. PEDOT content: 20 wt% (blue), 40 wt% (orange), 60 wt% (green), and 80 wt% (red). S-11

12 Figure S10. Electrical conductivity of the PEDOT/PU films in the oxidized and reduced forms measured by a two-terminal method. Data from two samples with 20 wt% PEDOT content were averaged for each condition. Figure S11. Electrochromic reactions of the PEDOT/PU film (30 wt% PEDOT content) under 50% stretching. (a) Photographs of the oxidized form at 0 sec and the reduced form at 60 sec with applied voltage of -0.5 V. (b) Electrochromic kinetics of the stretched PEDOT/PU film measured by the L* values of the digital camera images. Applied voltage was switched between 0.8 V and -0.5 V every 60 sec. S-12

13 Figure S12. Oxidation reaction rate of the film versus film thickness, measured by the L* values of the digital camera images. Data from two samples with 30 wt% PEDOT content were averaged. Video S1. The EC device working on a finger. Video S2. Operation of the mobile phone app to measure the L* values in a real-time manner. Reference [1] Bakarich, S. E.; Pidcock, G. C.; Balding, P.; Stevens, L.; Calvert, P.; in het Panhuis, M. Recovery from Applied Strain in Interpenetrating Polymer Network Hydrogels with Ionic and Covalent Cross-Links. Soft Matter 2012, 8, S-13