A new electrochromic copolymer which switched between neutral black and oxidized transmissive

Abstract

In this study, two monomers, 4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dio-xin-5-yl)-2-dodecyl-2H-benzo[1,2,3]triazole (M1) and 2,3-di(5-methylfuran-2-yl)-5,8-bis(2-(3,4-ethylenedioxythiophene))quinoxaline (M2), were used for the preparation of a neutral black polymer. A black polymer P(1-co-2) was successfully achieved by the electrochemical copolymerization of the two monomers with a feed ratio of 1 : 1, and then the obtained polymer was well characterized. P(1-co-1) has a wide range of absorption in the range of 400–800 nm, which makes the polymer exhibit a deep black color in the neutral state, and a transmissive light gray color could be obtained in the oxidized state. The polymer has a low band gap of 1.20 eV. Besides, a high coloration efficiency, a fast response time, as well as high transmittance changes (more than 78%) in the near-IR region were also found with this polymer, which suggested that the polymer might be a feasible candidate for the fabrication of electrochromic devices.

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Introduction

Electrochromism is broadly viewed as the phenomenon that some substances display reversible optical change in absorption or transmittance when a doping–dedoping process is performed. As an important category of conjugated polymers, electrochromic conducting polymers (ECPs) have gained extensive attention from both scientific and industrial circles, due to their possible application aspects, e.g., in information process, rear-view mirrors, as well as energy-saving windows.^1–5 Although some ECPs which can switch between various colors (such as red,⁶ blue,⁷ purple^8,9 and green^10,11) to transmissive states (which generally have a mild gray color) have been demonstrated, few black-to-transmissive ECPs have been reported so far.^12–14 Neutral black films have the unique characteristic of absorbing solar light with different wavelengths in the range of 400–800 nm in a homogeneous way, which is more difficult to achieve whether in theory or in practice.

Since the first discovery of the neutral black to transmissive ECP by Reynolds,¹² significant progress has been made on this subject by employing the subtractive color mixing theory which takes advantage of the superposition of the absorption peaks of light with different wavelengths. In practice, several strategies have been utilized, which include the copolymerization of different types of units,^12–16 the mixing of different pinks¹⁷ or the overlay of different polymer films.^18,19 The donor–acceptor approach plays an irreplaceable role in the design and preparation of cathodically coloring polymers, including neutral black polymers.¹² Donor–acceptor type polymers usually have two different absorption peaks; one is due to low-energy transitions and the other is due to high-energy transitions, and the color of the polymers is dependent on the positions and the relative intensities of the two absorption peaks²⁰. Obviously, the “merging” of the discrete absorption peaks would lead to homogeneous and broad absorption bands, which would probably give rise to a saturated black polymer film. Several neutral black polymers have been constructed by the copolymerization of electron-rich and electron-deficient units with specific feed ratios.^12–15

In addition to the chemical copolymerization method, the electrochemical copolymerization method is also feasible for the preparation of neutral black polymers, and it is simple, rapid, and time- and labor-saving.¹³ It has been reported that the absorption bands of hybrid copolymers are strongly and positively correlated to the combination of the respective absorption band of the conjugated monomers, whether they are donor–donor or donor–acceptor units, and that broadening of the resultant absorption profiles resulted from the optimization of the monomer feed ratios.¹² However, besides the work reported by Atilla Cihaner, preparation of neutral black polymers using the electrochemical strategy has remained very rare, which creates an urgent need to strengthen the work in this field.¹³

According to the previous work reported by Toppare et al.,²¹ poly(4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-dodecyl-2H-benzo[1,2,3]triazole) (P1) showed a saturated blue color in the dedoped state and a highly transparent blue in the doped sate. A recent work by our research group reported a neutral green polymer,²² named poly[2,3-di(5-methylfuran-2-yl)-5,8-bis(2-(3,4-ethylenedioxythiophene))quinoxaline] (P2), which can switch between a green color (neutral state) to a highly transmissive gray color (oxidized state). As shown in Fig. 1, we noticed that the overlay of the absorption profiles of P1 and P2 covers a homogeneous broad wavelength of 400–800 nm, covering the entire spectrum of visible light. The respective monomers of P1 and P2 could be represented by the symbols M1 and M2, respectively. According to the literature, the onset oxidation potentials of M1 and M2 are close to each other (0.84 V for M1 and 0.74 V for M2 vs. Ag-wire).^21,22 A random copolymer could be anticipated for the simultaneous electrochemical oxidation of the mixture of two monomers in the electrolyte. In the present study, a neutral black ECP was obtained by accurately tuning the concentrations of the two monomers. Furthermore, the resultant polymer also showed a transmissive light gray color in the doped state. Detailed analyses concerning the electrochemistry, spectroelectrochemistry, as well as the switching kinetics of the polymers was conducted, and the results showed that the obtained polymer has impressive electrochromic (EC) properties and would be a good candidate for EC device utilizations.

Fig. 1 Electronic absorption spectra of P1 and P2 in their neutral states in 0.2 M TBAPF₆ dissolved in ACN/DCM (a) and the colors of the polymers in different redox states (b).

Experimental

Materials and reagents

Acetonitrile (ACN), dichloromethane (DCM) and tetrabutylammonium hexafluorophosphate (TBAPF₆) were obtained from Sinopharm Chemical Reagent Co., Ltd. The monomers 4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-dodecyl-2H-benzo[1,2,3]triazole (M1) and 2,3-di(5-methylfuran-2-yl)-5,8-bis(2-(3,4-ethylenedioxythiophene))quinoxaline (M2) were synthesized according to the previous references (see ESI Fig. S1 and S2†).^21,22 Indium-tin-oxide (ITO)-coated glass (sheet resistance: < 8 Ω □⁻¹) was purchased from Zhuhai Kaivo Optoelectronic Technology Co., Ltd. and was cleaned thoroughly and dried before use. A weighing bottle with a volume of 5 ml was used as the one-compartment cell in the electrosynthesis and electroanalysis experiment. A quartz cuvette (1 × 1 × 4.5 cm) was used as the measuring cell in the spectroelectrochemical and kinetic switching measurements.²² In the electrosynthesis and the following electroanalysis experiments, a platinum wire with a diameter of 0.5 mm was used as the working electrode, a platinum ring with the same diameter was used as the counter electrode, and a silver wire (Ag wire, Φ 0.5 mm) was used as a pseudo-reference electrode.

Experimental instrument

A CHI 760 bipotentiostat controlled by a computer was used in the electrochemical experiments. Scanning electron microscope (SEM) measurements were taken by using a Hitachi SU-70 thermionic field emission SEM. The thicknesses of the polymers were tested by a KLA-Tencor D-100 step profiler. The spectrum measurement was performed using a Varian Cary 5000 UV-Vis-NIR spectrophotometer. The surface element characterization of the synthesized polymers was conducted by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) using monochromatic Al K_α (1486.6 eV) radiation. The digital photographs of the polymer films were taken with a commercial digital camera.

Test methods

The electrochemical synthesis was performed in monomer (or monomer mixture) solutions dissolved in DCM/ACN solutions containing 0.2 M of TBAPF₆ as the supporting electrolyte. The characterization of the polymer films with regards to the electrochemical and spectroelectrochemical properties was conducted in a monomer free supporting electrolyte solution. The spectroelectrochemistry and switch kinetics were conducted in the same way as reported in our recent reports.^23,24 For this purpose, the polymer films were prepared with an active area of 0.9 cm × 3.0 cm by the potentiostatic deposition method with a constant polymerization charge of 2 × 10⁻² C.

Results and discussion

Electrochemical polymerization and characterization

Cyclic voltammograms (CV) of M1, M2 and their mixture (M_mix) with a mole ratio of 1 : 1 were performed in the supporting electrolyte as stated in the former part. The occurrence of the copolymerization reaction can often be inferred from the changes in the shape of the CV curve of the M_mix from that of the respective monomers. From the first cycle of the CV curves, as recorded in Fig. 2, the onset oxidation potential (E_onset) values were calculated as 0.84, 0.74 and 0.67 V for M1, M2 and M_mix, respectively. It is apparent that the E_onset of M_mix is lower than that of both M1 and M2, which might arise from the interactions between M1 and M2, and thus copolymerization may happen. As shown in Fig. 2a, the CV curves of M1 showed asymmetrical redox peaks, and three oxidation peaks are located at 0.18, 0.57 and 0.96 V, respectively, while the corresponding reduction peaks are integrated into a broad and flat shape. The polymerization curve of M2 showed two oxidation peaks located at 0.27 and 0.79 V, which were accompanied by a flat and wide reduction peak between −0.4 and 0.8 V. As for the CV curves of M_mix, two pairs of asymmetrical redox curves were observed with oxidation peaks located at 0.47 and 1.12 V and the corresponding reduction peaks located at 0.28 and 0.71 V, respectively. In addition to the difference in the shape of the CV curves, the current densities of the M_mix are higher than that of the two other monomers, which should be another piece of important evidence for the occurrence of the copolymerization reactions between the two monomers.^25–28

Fig. 2 Cyclic voltammogram curves of M1 (a), M2 (b) and M_mix (n(M1) : n(M2) = 1 : 1) (c) in 0.2 M TBAPF₆/DCM/ACN solution at a scan rate of 100 mV s⁻¹. j denotes the current density. E denotes the potential.

The electrode diffusion kinetics are associated with the EC properties of the ECPs. The as prepared copolymer P(1-co-1) coated on a Pt wire electrode (made from the CV method for three cycles) was taken out of the electrolysis cell and rinsed with ACN before being reconstructed in the electrolysis cell with a monomer free electrolyte solution containing 0.2 M TBAPF₆ in DCM/ACN. Without specific explanations, P(1-co-1) refers to the polymer obtained from the electrochemical deposition of M_mix with a mole ratio of 1 : 1. The CV of P(1-co-1) was conducted between −1.6 V and +1.15 V (vs. Ag) at different scan rates from 25 to 300 mV s⁻¹, and the data are shown in Fig. 3a. It is obvious that the n-doping properties of the two homogenous polymers P1 and P2 are inherited by the copolymer, which was proven by a couple of redox peaks at −1.33 V and −1.43 V, respectively. In the process of p-doping, there are two irreversible redox couples located at 0.15 and −0.10 V and 0.57 and 0.46 V. Although almost all conjugated polymers have a tendency to be p-doped, only a handful of them reveal ambipolar characteristics,^29,30 which render them suitable for numerous applications such as light emitting diodes and ambipolar transistors.

Fig. 3 CV curves of the P(1-co-1) film at different scan rates (a), and the dependence of the peak currents on the scan rates for both p-doping and n-doping processes (b).

A good linear relationship between the scan rates and the magnitudes of the peak currents was observed for both the p-doping and the n-doping processes (see Fig. 3b), which suggested that the polymer was perfectly grown on the electrode and the electrochemical processes were not limited by diffusion control. In addition, the stability of the copolymer was studied by successive CV scans up to 1000 cycles, and the loss in integral area of the CV curve was only 4.4% after the completion of the scans. The data suggested the excellent stability of the P(1-co-1) film (Fig. 4).

Fig. 4 The first and the 1000th CV curve of the copolymer P(1-co-1) film at a scan rate of 200 mV s⁻¹ in 0.1 M TBAPF₆/ACN electrolyte/solvent couple.

Morphology

The SEM images provide specific information about the microstructure and bulk morphologies of the polymer films, which could also be taken as evidence for the occurrence of copolymerization between the two monomers from the differences between the SEM images of the three polymers.³¹ Fig. 5 gives the SEM images of P1, P2 and P(1-co-1), which were prepared potentiostatically on ITO electrodes with a charge of 0.1C in the electrolytes containing the relevant monomers and dedoped before characterization. As presented in Fig. 5, the SEM images of the three polymers are quite different from each other. P1 shows a coral cluster structure with scattered holes between the clusters. P2 exhibits a tiled structure and there are lots of gibbosities pervading on the surface, while the copolymer P(1-co-1) reveals a uniformly dispersed structure with many dense irregular globules on it. The different morphological features of the three polymer films confirmed the occurrence of the copolymerization reaction to a certain extent. The thickness of the copolymer P(1-co-1) film was also measured by the step profiler, and the thickness is calculated to be 896 nm for P(1-co-1), as shown in Fig. 6. Additionally, the rugged scanning curves indicated the uneven surfaces of the films, which is in good agreement with the morphologies given above.

Fig. 5 SEM images of P1 (a), P2 (b) and P(1-co-1) (c) deposited potentiostatically onto an ITO electrode.

Fig. 6 Film thickness of P(1-co-1) deposited potentiostatically onto an ITO electrode.

XPS investigation of the copolymer

To analyze the surface composition of the polymer and to testify the generation of the copolymer, the relevant films deposited on the ITO electrode were further analyzed through XPS analysis as illustrated in Fig. 7 and Table 1. It can be seen in Table 1 that the atomic weight (%) ratios of P1 and P2 are approximately the same as those of M1 and M2, respectively. In addition, the atomic weight (%) of the copolymer is between P1 and P2. Based on this, the formation of the copolymer could be initially confirmed. From the survey scans of the XPS, we can see that all three polymers express N1s, O1s, C1s and S2p spectra (see Fig. S3 in the ESI†). The formation of the copolymer could be further confirmed by the existence of different types of nitrogen atoms presented in the polymers. In P1 (Fig. 7a), the peak at a binding energy (BE) of 399.61 eV can be ascribed to the amine group (the bond of C–N) and the peak at 401.75 eV can be ascribed to the imine N (the –N= bond).^32–34 In P2 (Fig. 7b), however, there is only one peak at a BE of 399.05 eV, which belongs to the pyridinic N. In accordance with our expectation, three peaks emerging in the N1s spectra of P(1-co-1) were observed (see Fig. 7c). The peak at a BE of 399.04 eV is the same as P2 which also belongs to the pyridinic N.³⁵ The peaks at the BE of 399.70 eV and 401.94 eV are in accordance with P1 which belong to the amine group (the C–N bond) and the imine N (the –N= bond), respectively. In the end, we can draw the conclusion that the copolymer has been formed.

Fig. 7 The XPS survey of P1 (a), P2 (b) and P(1-co-1) (c) deposited on an ITO electrode.

Table 1 The atomic weights of P1, P2, P(1-co-1), M1 and M2

Atomic weight%	P1	P2	P(1-co-2)	M1%	M2%
S2p	5.06	4.82	4.97	11.3	11.82
C1s	73.96	70.3	72.85	63.46	61.98
N1s	7.07	5.03	5.7	7.4	5.16
O1s	13.9	19.84	16.48	11.27	17.69

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Optical properties of the polymer films

Polymers including P1, P2 and several copolymers were obtained through the potentiostatic polymerization method on an ITO electrode (coated area: 0.9 cm × 1.8 cm) with a polymerization charge of 2.5 × 10⁻² C and the polymeric substrates M1, M2 and M_mix (with different molar feed ratio), respectively. The polymerization potentials for the monomers were 1.1, 0.9 and 1.15 V for M1, M2 and M_mix, respectively. After the formation of the polymer film, dedoping of the polymer films was performed by causing the polymer films to change into the reduced state, more specifically, by applying electrical potentials at −0.5, −0.8 and −0.6 V for P1, P2 and the copolymers, respectively, in a monomer free electrolyte. Then, the polymers were rinsed with ACN several times before further characterization. For comparison purposes, other copolymers derived from different feed ratios of the monomers were also prepared and examined at the same time. Copolymers P(2-co-1) and P(1-co-2) refer to the polymer obtained from the M_mix with the mole ratios (M1 : M2) of 2 : 1 and 1 : 2, respectively. Comparing the three copolymer films, only the absorption spectra of P(1-co-1) could overlap with the visible region better (see Fig. 8). In this case, P(1-co-1) was used for further study.

Fig. 8 Absorption spectra of three copolymers derived from three different mole ratios (M1 : M2).

As illustrated in Fig. 9, in the dedoped state, P1 showed a broad absorption band centered at 618 nm, which gave rise to a blue color for P1, and there were two well separated absorption peaks for P2 in the visible region with a valley centered at 536 nm, which are the typical characteristics of the neutral green polymer films. As for the absorption peak of P(1-co-1), the shape of which is similar to the superposition of the absorption curves of the two homopolymers P1 and P2, a flat and broad absorption band ranging from 350–1000 nm with coverage of the whole visible light region was observed. It is fortunate that the neutral black film P(1-co-1) was conveniently obtained by the copolymerization of the two monomers in the feed ratio proportion of just 1 : 1 without further optimization in feed ratios. The optical band gap (E_g,op) values of the copolymers were calculated to be 1.20 eV, indicating a neutral black film with a low band gap.^23,24 For comparison purposes, Table 2 summarizes the spectroelectrochemical parameters of three polymer films, including E_onset, the maximum of absorption wavelength (λ_max), the absorption onset wavelength (λ_onset), E_g,op.

Fig. 9 UV-Vis spectra and corresponding colors of P1, P2 and P(1-co-1) deposited on ITO in a neutral state.

Table 2 The onset oxidation potential (E_onset), the maximum of absorption wavelength (λ_max), the absorption onset wavelength (λ_onset), and optical band gap (E_g,op) of the three polymers P1, P2 and P(1-co-1)

Compounds	Eonset vs.(Ag wire) (V)	λmax (nm)	λonset (nm)	Eg,op^a (eV)
a Eg,op = 1241/λonset, in which λonset refers to the low energy absorption edges.
P1	−0.05	618	750	1.60
P2	−0.40	436/780	976	1.23
P(1-co-1)	−0.22	401/697	1000	1.20

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The electrochromic properties of the polymers

Information on the EC properties was recorded with an ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer when gradually increasing potentials were applied to the polymer films from −0.6 V to +1.15 V (vs. Ag). As shown in Fig. 10, at −0.6 V, the P(1-co-1) film was in its neutral state and exhibited a wide and homogeneous absorption between 450 and 900 nm with two absorption peaks located at 401 nm and 700 nm, respectively. A more saturated black color was obtained due to the significant extension of the absorption band into the NIR region. An uneven but gradual decline in the absorption intensities was observed along with a continuous increase in the applied potentials on the polymer films. Compared with the wavelength in the middle region between 450 and 900 nm, the absorption of both ends of the wavelength decreases slowly, leaving a valley like absorption band with the lowest absorption point at 560 nm in the whole visible spectrum. The great tail extending into the NIR and the visible light region make the P(1-co-1) film exhibit a transparent light gray color in the full oxidized state at +1.15 V. In the process of stepwise oxidation or doping, polaronic or bipolaronic transitions occurred and led to an increase in the conductivity and stability of the polymer. At the intermediate potential of +0.5 V, a well defined new absorption band was formed with a peak centered at 900 nm, which was generally attributed to the occurrence of the polaron. Also, at the full oxidized state of +1.15 V, the generation of the bipolaron was witnessed by the presence of the maximum absorption peak beyond 1700 nm.

Fig. 10 The spectroelectrochemistry of the films' p-doping processes on an ITO electrode as applied potentials between −0.6 V and 1.15 V for P(1-co-1) in the monomer-free 0.2 M TBAPF₆/ACN/DCM solution and the colors of the P(1-co-1) in different redox states.

The CIE1976 L*a*b color space was used for the presentation of the colors of the P(1-co-1) polymer film at the different oxidative potentials. In the neutral state, the CIE1976 L*a*b values are 4, −2 and 0, pointing to a state of saturated black color, which is in accordance with the absorption peaks that cover an even and wide spectral absorption range (400–800 nm), leaving no light transmitted by the polymer film. When the polymer film switched to the oxidized state, CIE1976 L*a*b color space values are 51, −5 and 12, respectively. The large positive ΔL* value (47) suggests that the film becomes brighter and more transparent, and the negative Δa* (−3) showed that the film becomes more green. Finally, the positive Δb* (12) means that the film becomes more yellow. In a word, the color of the film becomes lighter and more transparent than that of the neutral state, or becomes a transparent light grey color in the oxidized state.

Electrochromic switching studies

The EC switching properties^36,37 of P(1-co-1) were studied, and several parameters including the percentage of optical contrasts (ΔT%) at a specified wavelength, response time (the time needed for the realization of the 95% of the full optical contrasts), as well as coloration efficiency^38,39 were recorded and analyzed. The ΔT% value of P(1-co-1) were calculated to be 32% at 700 nm and 78% at 1600 nm, respectively (Fig. 11). The relatively high ΔT% values in the NIR region suggested the potential application of P(1-co-1) in NIR devices. Fast response time is also a necessary prerequisite for the preparation of high performance EC devices, especially for the fabrication of EC display devices. The time required to complete doping from the neutral state is generally defined as the response time of the polymer at a given wavelength. In this case, the response times for P(1-co-1) were 1.15 s at 700 nm and 1.0 s at 1600 nm, which are relatively fast transition rates among all of the ECPs.

Fig. 11 Electrochromic switching and the percent transmittance change monitored at 700 nm and 2000 nm for P(1-co-1) between −0.6 V and +1.15 V.

The coloration efficiency (CE) is related to the amount of energy required to complete the unit absorbance changes. According to the formula reported previously,^22–24 the CE values were calculated to be 121.3 cm² C⁻¹ at 700 nm and 281.4 cm² C⁻¹ at 1600 nm, with much higher CE values in the NIR region than that of in the visible light region. P(1-co-1) showed sufficient stability in the CV test to be considered a promising candidate for EC materials, which must have sufficient dynamic stability during successive color conversions.^40,41 Therefore, the persistence of the optical contrast (ΔT%) in the switching dynamic measurements was also recorded for the P(1-co-1) polymer, from which excellent stability was observed for the polymer with 93% retention of the ΔT% at 1600 nm and 88% at 700 nm after 1000 cycles of EC switching (Fig. 12). Considering the above discussion, the relatively high optical contrast, fast switching time and satisfactory coloration efficiency would make the copolymer P(1-co-1) a better candidate in EC display applications, especially in the field of smart windows.

Fig. 12 Stabilities of the P(1-co-1) film deposited on ITO at 1600 nm (a) and 700 nm (b) with a square-wave potential switching between −0.6 V and +1.15 V. Switching times are 4 s.

Conclusion

In this paper, a neutral black copolymer P(1-co-1) was successfully obtained by the electrodeposition of M1 and M2 in a 1 : 1 monomer feed ratio. The CV measurements suggested that the polymer film is both p- and n-dopable and possesses a high long-term stability. Spectroelectrochemical investigations exhibited that this copolymer displayed a deep black color in the neutral state which absorbed nearly the whole visible spectrum (400–800 nm) and a transmissive gray color at the oxidized state. The considerable optical contrast and fast response time in the NIR region may render the polymer an excellent candidate for applications in NIR EC devices. The band gap of copolymer P(1-co-1) was calculated as 1.20 eV. To the best of our knowledge this is the lowest bandgap among all the reported neutral black polymers.

Acknowledgements

The work was financially supported by the National Natural Science Foundation of China (31400044, 51473074), the General and Special Program of the postdoctoral science foundation China (2013M530397, 2014T70861) and the Natural Science Foundation of Shandong Province (ZR 2014JL009).

Notes and references

1. Y. Shi and G. Yu, Chem. Mater., 2016, 28, 2466–2477.
2. C. Yang, H. Wei, L. Guan, J. Guo, Y. Wang, X. Yan, X. Zhang, S. Wei and Z. Guo, J. Mater. Chem. A, 2015, 3, 14929–14941.
3. M. Karakus, A. G. Nurioglu, H. Z. Akpinar, L. Toppare and A. Cirpan, Electrochim. Acta, 2013, 100, 110–117.
4. E. Rende, C. E. Kilic, Y. A. Udum, D. Toffoli and L. Toppare, Electrochim. Acta, 2014, 138, 454–463.
5. Y. Gao, W. Yao, J. Sun, H. Zhang, Z. Wang, L. Wang, D. Yang, L. Zhang and H. Yang, J. Mater. Chem. A, 2015, 3, 10738–10746.
6. A. L. Dyer, M. R. Craig, J. E. Babiarz, K. Kiyak and J. R. Reynolds, Macromolecules, 2010, 43, 4460–4467.
7. P. Shi, C. M. Amb, A. L. Dyer and J. R. Reynolds, ACS Appl. Mater. Interfaces, 2012, 4, 6512–6521.
8. B. D. Reeves, C. R. G. Grenier, A. A. Argun, A. Cirpan, T. D. McCarley and J. R. Reynolds, Macromolecules, 2004, 37, 7559–7569.
9. J. A. Kerszulis, K. E. Johnson, M. Kuepfert, D. Khoshabo, A. L. Dyer and J. R. Reynolds, J. Mater. Chem. C, 2015, 3, 3211–3218.
10. H. Zhao, Y. Wei, J. Zhao and M. Wang, Electrochim. Acta, 2014, 146, 231–241.
11. S. Ming, S. Zhen, K. Lin, L. Zhao, J. Xu and B. Lu, ACS Appl. Mater. Interfaces, 2015, 7, 11089–11098.
12. P. M. Beaujuge, S. Ellinger and J. R. Reynolds, Nat. Mater., 2008, 7, 795–799.
13. M. İçli, M. Pamuk, F. Algı, A. M. Önal and A. Cihaner, Org. Electron., 2010, 11, 1255–1260.
14. P. Shi, C. M. Amb, E. P. Knott, E. J. Thompson, D. Y. Liu, J. Mei, A. L. Dyer and J. R. Reynolds, Adv. Mater., 2010, 22, 4949–4953.
15. K.-R. Lee and G. A. Sotzing, Chem. Commun., 2013, 49, 5192–5194.
16. G. Ding, C. M. Cho, C. Chen, D. Zhou, X. Wang, A. Y. X. Tan, J. Xu and X. Lu, Org. Electron., 2013, 14, 2748–2755.
17. S. V. Vasilyeva, P. M. Beaujuge, S. Wang, J. E. Babiarz, V. W. Ballarotto and J. R. Reynolds, ACS Appl. Mater. Interfaces, 2011, 3, 1022–1032.
18. H. Shin, Y. Kim, T. Bhuvana, J. Lee, X. Yang, C. Park and E. Kim, ACS Appl. Mater. Interfaces, 2012, 4, 185–191.
19. E. Unur, P. M. Beaujuge, S. Ellinger, J.-H. Jung and J. R. Reynolds, Chem. Mater., 2009, 21, 5145–5153.
20. C. M. Amb, A. L. Dyer and J. R. Reynolds, Chem. Mater., 2011, 23, 397–415.
21. A. Balan, G. Gunbas, A. Durmus and L. Toppare, Chem. Mater., 2008, 20, 7510–7513.
22. Z. Xu, M. Wang, J. Zhao, C. Cui, W. Fan and J. Liu, Electrochim. Acta, 2014, 125, 241–249.
23. S. Chen, D. Zhang, M. Wang, L. Kong and J. Zhao, New J. Chem., 2016, 40, 2178–2188.
24. Y. Luo, J. Zhang, M. Wang, Y. Zhang, J. Zhao and C. Fu, Int. J. Electrochem. Sci., 2016, 11, 1581–1600.
25. C. Zhang, C. Hua, G. Wang, M. Ouyang and C. Ma, Electrochim. Acta, 2010, 55, 4103–4111.
26. P. Camurlu, Z. Bicil, C. Gültekin and N. Karagoren, Electrochim. Acta, 2012, 63, 245–250.
27. R. Yue, J. Xu, B. Lu, C. Liu, Y. Li, Z. Zhu and S. Chen, J. Mater. Sci., 2009, 44, 5909–5918.
28. J. T. Kearns and M. E. Roberts, J. Mater. Chem., 2012, 22, 25447–25452.
29. L.-T. Huang, H.-J. Yen and G.-S. Liou, Macromolecules, 2011, 44, 9595–9610.
30. Y. Liu, M. Wang, J. Zhao, C. Cui and J. Liu, RSC Adv., 2014, 4, 52712–52726.
31. H. Liu, Y. Li, J. Xu, Z. Le, M. Luo, B. Wang, S. Pu and L. Shen, Eur. Polym. J., 2008, 44, 171–188.
32. M. Tourabi, K. Nohair, M. Traisnel, C. Jama and F. Bentiss, Corros. Sci., 2013, 75, 123–133.
33. M. Finšgar, Corros. Sci., 2013, 77, 350–359.
34. D. Dontsova, C. Fettkenhauer, V. Papaefthimiou, J. Schmidt and M. Antonietti, Chem. Mater., 2016, 28, 772–778.
35. Y. Lu, X. Wang, M. Wang, L. Kong and J. Zhao, Electrochim. Acta, 2015, 180, 86–95.
36. E. Yildiz, P. Camurlu, C. Tanyeli, I. Akhmedov and L. Toppare, J. Electroanal. Chem., 2008, 612, 247–256.
37. E. Sefer, F. B. Koyuncu, E. Oguzhan and S. Koyuncu, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 4419–4427.
38. P. M. Beaujuge and J. R. Reynolds, Chem. Rev., 2010, 110, 268–320.
39. M. Deepa, A. Awadhia and S. Bhandari, Phys. Chem. Chem. Phys., 2009, 11, 5674–5685.
40. E. Karabiyik, E. Sefer, F. B. Koyuncu, M. Tonga, E. Özdemir and S. Koyuncu, Macromolecules, 2014, 47, 8578–8584.
41. E. Sefer, S. O. Hacioglu, M. Tonga, L. Toppare, A. Cirpan and S. Koyuncu, Macromol. Chem. Phys., 2015, 216, 829–836.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12596d

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