Synthesis and properties of novel electroactive poly(amic acid) and polyimide copolymers bearing pendant oligoaniline groups

Abstract

A series of novel electroactive copolymers of poly(amic acid) (PAA) bearing pendant aniline tetramer groups were prepared via direct polycondensation from novel electroactive diamine (EDA), 4,4^'-diaminodiphenyl ether (p-ODA) and 3,3^',4,4^'-biphenyltetracarboxylic dianhydride (s-BPDA). The structures of the copolymers were confirmed by FT-IR and ¹H NMR measurements. Then the prepared PAA copolymers were heated at 300 °C to induce cyclization, and transformed into polyimide (PI) films. The obtained PI films exhibited excellent thermal stability and outstanding mechanical properties. The PAA copolymers films had reversible redox couples with good cycle stability. The stepwise oxidation of as-prepared PAA copolymers by ammonium persulfate is monitored by UV-vis spectroscopy. The absorption peak at 571 nm underwent a red shift when it transformed from emeraldine state to pernigraniline state. Moreover, the PAA copolymers films revealed electrochromic behaviors with a color change from yellowish at its neutral state, to green, and finally to dark blue. They also exhibited extremely high optical contrast (%ΔT) up to 63% at 700 nm, moderate coloration efficiency (CE ≈ 100 cm² C⁻¹) and low switching times of 3.9 s at 0.8 V and 1.3 s for fast bleaching.

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Introduction

As one of the most promising organic conducting polymers, polyaniline has attracted considerable attention for its potential applications in organic lightweight batteries,¹ electrochromic display devices,^2,3 photo-electronic devices,⁴ chemical sensors^5,6 and hydrogen storage,⁷ due to its straightforward polymerization and excellent chemical stability combined with relatively high levels of conductivity.^8,9 However, PANI prepared by chemical and/or electrochemical means usually exhibits structural defects and has limited solubility in many solvents. These shortcomings would impede the better understanding of the structure–property correlations and the conducting mechanism, and also restrict the practical applications of PANI. One possible approach to alleviate the problems is the incorporation of well-defined and conjugated oligoaniline^10–12 into the copolymer backbones, which could combine the properties of the specific oligoaniline and desirable polymer properties such as mechanical strength and film-forming ability. The properties of copolymers can be tailored by adjusting the length of the oligoaniline and the molar percent of the oligomer through copolymerization. In contrast to blending with ‘conventional’ polyaniline blends (polymer/polyaniline blend),^13,14 the covalent bond attachment of the oligoaniline to the polymer backbones could prevent the loss of active species from the bulk polymer, which may be caused by their phase separation, migration and extraction. Recently, much effort has been made for chemically synthesizing polymers based on oligoanilines, such as graft, alternate, and block-like polymers. Benicewicz and Chen¹⁵ synthesized poly(methacrylamide)s with oligoaniline side chains and discussed the influence of the side chains length on the redox behavior of the copolymers. Zagorska et al.¹⁶ copolymerized a dialkylbithiophene unit with the thiophene unit containing oligoaniline side chains by post-polymerization. Both of the oligoaniline side chains and the polymer main chains could be doped in different ways. Several kinds of polymers with conjugated oligoaniline in the main chains were prepared by Zhang et.al.^17–20 through polycondensation or oxidative coupling polymerization, and displayed excellent thermal stability. In addition, a series of novel electroactive block copolymers with bioactive properties were synthesized by polycondensation of electroactive carboxyl-capped aniline pentamer and bioactive bihydroxyl-capped polymers.^21–23

Aromatic polyimide (PI) is a kind of high-performance engineering plastic with outstanding properties, such as good film-forming ability, excellent thermal stability, mechanical properties and high molecular weight,^24,25 and have great potential for advanced applications in photosensitive materials,²⁶ polymer memories²⁷ and proton exchange membranes.²⁸ In our group, electroactive polyimide with oligoaniline in the main chain has been synthesized via oxidative coupling polymerization.²⁹ It has good solubility and exhibits a large enhancement in the dielectric constant when doped with HCl in comparison to traditional polymers. However, the synthesized electroactive poly(amic acid) has a low inherent viscosity, so it will never form a film, nor afford mechanical stretching attributing to the oligoaniline moieties in the main chain, which is highly intractable and infusible. Consequently, it is a very important challenge to prepare applicable electroactive polyimide with excellent film-forming ability and mechanical strength by introducing pendant conjugated oligoaniline on backbones.

In this article, we synthesized a novel electroactive diamine monomer (EDA) bearing aniline tetramer groups, and incorporated it facilely into poly(amic acid) backbones, yielding copolymers with oligoaniline side chains. Then electroactive polyimide films were successfully obtained by thermal imidization. Due to their similar molecular structures, PAA copolymers, as the precursor of polyimide, were used to investigate the molecular structure and electrochemical and optical properties. Moreover, the electrochromic behavior of these polymer thin films on the coated ITO glass was also examined.

Experimental

Materials

All chemicals, including 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA, 98%), 4,4′-diaminodiphenyl ether (p-ODA, 98%), 4-aminophenol (99%), dichloromethane (99.5%), were purchased from Shanghai Chemical Factory. N,N′-Dimethylacetamide (DMAc, 99%) and toluene (99%) were used as received without further purification. Distilled water was self-made. Anhydrous potassium carbonate was dried at 110 °C for 24 h before use. Optically transparent Indium–Tin Oxide (ITO) glass substrates (Reintech electronic technologies Co. Ltd, Beijing) with dimensions of 4.0 cm × 0.6 cm were used as electrochromic thin films electrode.

Measurements

Mass spectroscopy (MS) was performed on an AXIMA-CFR laser desorption ionization flying time spectrometer (COMPACT). Fourier-transform infrared spectra (FTIR) measurements were recorded on a Bruker vector 22 spectrometer. The nuclear magnetic resonance spectra (NMR) of EDA and poly(amic acid) in deuterated dimethyl sulfoxide (DMSO) were run on a Bruker-500 spectrometer. The elemental analyses were measured by a Flash Ea 1112 elemental analysis instrument. Inherent viscosity was determined on an Ubbelohde viscometer in a thermostatic container with the poly(amic acid) concentration of 0.5 g dL⁻¹ in DMAc at 25 °C. UV-vis spectra were performed on UV-2501 PC Spectrometer (SHIMADZU) in DMAc. Differential scanning calorimeter (DSC) measurements were performed on a Mettler Toledo DSC821e instrument. A Perkin–Elmer PYRIS 1 TGA was used to investigate the thermal stability of the copolymers. Mechanical properties of the thin films were evaluated at room temperature on SHIMADIU AG-I 1KN at a strain rate of 10 mm min⁻¹. The CV was investigated on a CHI 660A Electrochemical Workstation (CH Instruments, USA) with a conventional three-electrode cell, using a saturated calomel electrode (SCE) as the reference electrode, a platinum wire electrode as the counter electrode, and a glassy carbon electrode (GCE, Φ 3.0 mm) as the working electrode. Spectroelectrochemical measurements were carried out in a cell built from a 1 cm commercial cuvette using a UV-2501 PC Spectrometer (SHIMADZU). The ITO-coated glass was used as the working electrode, a Pt wire as the counter electrode, an Ag/AgCl cell as the reference electrode and 0.1 mol L⁻¹ H₂SO₄ was used as the electrolyte.

Synthesis of 2,6-difluorobenzoyl aniline tetramer (compound 1)

The synthetic route for the preparation of compound 1 from parent aniline tetramer and 2,6-difluorobenzoyl chloride is depicted in Scheme 1. Suitable preparations have been reported in the literature.³⁰

Scheme 1 Schematic diagram for the synthesis of electroactive diamine monomer (EDA).

MALDI-TOF-MS: m/z calculated for C₃₁H₂₄F₂N₄O = 506.5. Found 506.6. FTIR (KBr, cm⁻¹): 3369 (s, ν_NH), 3299 (s, ν_NH), 1657 (vs, ν_C=O), 1600 (s, ν_C=C of benzenoid rings), 1525 (vs, ν_C=C of benzenoid rings), 1303 (s, ν_C–N), 1008(m, δ_CF), 817 (m, δ_CH), 746 (m, δ_CH), 692 (m, δ_CH). ¹H NMR (d₆-DMSO, ppm): δ = 10.51 (s, 1H, due to H₁), δ = 7.80 (s, 1H, due to H₄), δ = 7.77 (s, 1H, due to H₃), δ = 7.65 (s, 1H, due to H₂), δ = 7.56 (t, 1H, due to H₉), δ = 7.48 (d, 2H, due to H₇), δ = 7.23 (t, 2H, due to H₆), δ = 7.14 (t, 2H, due to H₈), δ = 6.96 (m, 12H, due to H₁₀), δ = 6.68(t, 1H, due to H₅). A typical elemental analysis for C₃₁H₂₄F₂N₄O: calcd C 73.50, H 4.78, N 11.06, O 3.16; found, C 73.25, H 4.52, N 10.82, O 3.37%.

Synthesis of electroactive diamine monomer (EDA)

A mixture of 6.54 g (0.06 mol) of 4-aminophenol, 4.14 g (0.03 mol) of anhydrous potassium carbonate, 80 mL of DMAc, and 10 mL of toluene was heated at 140 °C under stirring and blowed with pure nitrogen in a 250 mL round-bottomed flask. The solution was kept refluxing for 2 h to ensure that the 4-aminophenol was totally turned into phenoxide, and then it was cooled before 15.18 g (0.03 mol) of compound 1 was added into the reaction vessel. The mixture was heated for 2 h to remove water by azeotropic distillation with toluene. After toluene was removed, the mixture was heated at 160 °C for 6 h to ensure the completion of the reaction. The solution was cooled to room temperature and poured into water (400 mL); the resulting purple precipitate was washed with water (300 mL × 3), and then dried in a vacuum oven at 50 °C for 24 h. The yield of the purple product (EDA) was 17.44 g (85%).

MALDI-TOF-MS: m/z calculated for C₄₃H₃₆N₆O₃ = 684.78. Found 685.0. FTIR (KBr, cm⁻¹): 3380 (s, ν_NH), 3037 (m, ν_CH), 1657 (vs, ν_C=O), 1602(s, ν_C=C of benzenoid rings), 1506 (vs, ν_C=C of benzenoid rings), 1294 (s, ν_C–N), 1233 (m, ν_C–O–C), 829 (m, δ_CH), 750 (m, δ_CH), 696 (m, δ_CH). ¹H NMR (d₆-DMSO, ppm): δ = 10.24 (s, 1H, due to H₁), δ = 7.71 (s, 1H, due to H₄), δ = 7.69 (s, 1H, due to H₃), δ = 7.60 (s, 1H, due to H₂), δ = 7.54(d, 2H, due to H₇), δ = 7.14 (t, 3H, due to H₆, H₉), δ = 6.93 (d, 12H, due to H₁₀), δ = 6.81 (d, 4H, due to H₁₃), δ = 6.67 (t, 1H, due to H₅), δ = 6.59 (d, 4H, due to H₁₂), δ = 6.29 (d, 2H, due to H₈), δ = 4.99 (s, 4H, due to H₁₁). A typical elemental analysis for C₄₃H₃₆N₆O₃: calcd C 75.42, H 5.30, N 12.27, O 7.01; found, C 75.28, H 5.18, N 12.24, O 7.30%.

Synthesis of electroactive poly(amic acid) and polyimide copolymers

The poly(amic acid) (PAA) copolymers were copolymerized from EDA, p-ODA and s-BPDA. The polymerization procedure was given below, as shown in Scheme 2. To a 50 mL of three-necked flask equipped with a mechanical stirrer and a nitrogen-inlet, EDA and p-ODA (total molar amount 0.005 mol) were dissolved in DMAc and then s-BPDA (1.470 g, 0.005 mol) was added into the solution slowly. The mixture was stirred at room temperature for 6 h with a nitrogen flow to give black PAA solution. The solid content of the solutions was 20 wt%. Molar content of EDA was controlled to be 20%, 40%, 60%, 80% and 100%. Then they were subsequently poured onto the supporting substrate to form the electroactive PAA films and dried under vacuum at 300 °C for 4 h. The PI films from the corresponding PAA copolymers were named as PI-20, PI-40, PI-60, PI-80 and PI-100, respectively.

Scheme 2 Synthesis of the electroactive poly(amic acid) (PAA-X) and polyimides (PI-X) copolymers (X represents the oligoanilines mole percent of the copolymers).

Results and discussions

Synthesis and characterization of the electroactive diamine monomer (EDA)

The one-step synthetic route for the electroactive diamine monomer (EDA) is shown in Scheme 1. The preparation of EDA was carried out by K₂CO₃-mediated nucleophilic reaction and toluene was used to dehydrate the reaction system. The reaction temperature was first controlled at 140 °C to remove the water generated during the phenoxide formation, and then increased slowly to 160 °C to accomplish the reaction. In the FTIR spectrum of EDA, the characteristic absorption band at 3380 cm⁻¹ was observed, arising from the terminal –NH₂ of diamine monomer. The peaks at 1602 cm⁻¹ and 1506 cm⁻¹ were assigned to the vibrational bands of benzenoid rings of diamine monomer, respectively. The characteristic absorption peak of aromatic –C–O–C– group was also observed. The structure of EDA was also confirmed by ¹H NMR spectra (Fig. 1). When compound 1 changed to the corresponding diamine form, a chemical shift at 4.99 ppm, the characteristic of amino groups, appeared in the ¹H NMR spectrum. Furthermore, the signal of H₈ exhibited a triplet at 7.14 ppm in the ¹H NMR spectrum of compound 1, but it appeared to be a doublet at 6.31 ppm of EDA, which confirmed that the fluorine groups had completely reacted with the hydroxyl groups. These results confirmed that the synthesized EDA herein was consistent with the proposed structure.

Fig. 1 ¹H NMR specta of electroactive compound 1 and diamine monomer (EDA).

Synthesis and characterization of the electroactive PAA and PI copolymers

The functional PI copolymers in Scheme 2 were prepared via the conventional two-step method by reacting EDA, p-ODA and s-BPDA in DMAc to give the precursor, PAA solutions at room temperature, followed by thermal cyclodehydration. The structures of the PAA copolymers were confirmed by FTIR and ¹H NMR spectra. The ¹H NMR stacked spectra of PAA-40 and PAA-100 in the aromatic region are shown in Fig. 2. Although the ¹H NMR spectra of the PAA copolymers appeared complex and overlapped to some extent, the assignment of each spectra could be accomplished by the comparison with fully assigned spectra of EDA. The protons located at the electron-rich ortho-ether position were strongly shielded, and their signals appeared at high-field region (7.51–6.57 ppm), while the protons located at para- or ortho-positions of carbonyl groups were deshielded due to their strong electron-withdrawing effects, and their signals appeared at low-field region (8.09–7.72 ppm). Moreover, the ¹H NMR spectra of the PAA copolymers revealed a decrease in intensity ratio of H₅ (10.51 ppm)/H₁ (10.37 ppm), indicating a decrease in the feed composition ratio of EDA/p-ODA existed in the as-prepared electroactive PAA copolymers.

Fig. 2 ¹H NMR spectra of the poly(amic acid) copolymers (PAA-40, PAA-100).

As shown in the FTIR spectra (ESI†, Fig. S1) of PAA and PI copolymers, both of them exhibited characteristic peaks at around 1600 cm⁻¹ and 1509 cm⁻¹ assigned to the stretching vibration of benzene rings. In the case of PAA (Fig. S1(a††), the characteristic peaks, for C=O in COOH and CO–NH group, could be seen at 1710 and 1658 cm⁻¹. After complete thermal imidization of PAA copolymers, all the polyimide copolymers (Fig. S1(b††) exhibited characteristic absorption peaks of the imide ring at around 1774 cm⁻¹ (asymmetric C=O stretch), 1718 cm⁻¹ (symmetric C=O stretch) and 741 cm⁻¹ (imide ring deformation).

The inherent viscosities (η_inh) of the PAA copolymers were measured by an Ubbelohde viscometer and listed in Table 1, the η_inh of the PAA copolymers were in the range of 0.84–1.19 dL g⁻¹ and decreased with the increasing of the content of side-chains. The PAA copolymers exhibited an outstanding solubility in polar solvents such as THF, DMAc, DMF and NMP (ESI†, Table S1), due to the attachment of bulky pendent groups. The crystallinity of the PI copolymers was investigated by wide-angle X-ray diffraction (WAXD, shown in the ESI†, Fig. S2†) patterns and the results indicated that the copolymers were essentially amorphous, mainly attributed to the introduction of bulky pendant aniline tetramer groups and a high steric hindrance for hydrogen bonding and π–π interactions, which disturb the close packing and regularity of the polymer chain.

Table 1 The basic properties of the electroactive poly(amic acid) and polyimide copolymers

Polymer	η PAA /dL g^−1	T g /°C	DT5^c/°C	DT10^d/°C	Char yield^e/%
a Inherent viscosity was determined on an Ubbelohde viscometer in a thermostatic container with the polymer concentration of 0.5 g dL^−1 in DMAc at 25 °C. b Glass transition temperature was measured by DSC. c 5% Weight-loss temperature was detected at a heating rate of 10 °C min^−1 in nitrogen with a gas flow of 100 mL min^−1. d 10% Weight-loss temperature was detected at a heating rate of 10 °C min^−1 in nitrogen with a gas flow of 100 mL min^−1. e Residual weight percentage at 700 °C in nitrogen.
PI-20	1.19	244	547	585	73.6
PI-40	0.95	231	511	555	73.4
PI-60	0.88	214	487	534	72.4
PI-80	0.82	206	475	520	71.2
PI-100	0.84	198	473	513	69.3

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Thermal and mechanical properties of the PI copolymers

The thermal properties of the PI copolymers were evaluated by their T_g and T_d data (Table 1) measured by DSC and TGA, respectively. The DSC thermograms of the PI films showed glass transition temperatures (T_g) ranging from 198 to 244 °C with the decrease of aniline tetramer content in the copolymers, which was attributed to the fact that the bulky pendant groups hindered the interaction of the polymer chain. Compared with the general aromatic polyimide, the as-synthesized polyimides with large side groups have low T_gs, which would bring them easy process. Thermal properties of the PI films with different contents of side-chains were evaluated by TGA under identical drying and heating conditions. As displayed in Fig. 3, all the PI copolymers exhibited excellent thermal stability and were stable at the temperature up to 380 °C. Only 10% weight loss could be observed at temperatures between 513 and 585 °C under nitrogen atmosphere. Besides, these polymers exhibited high char yield in the range of 69–73% at 700 °C in nitrogen stream. These results suggest the electroactive PI copolymers containing pendent aniline tetramer exhibit an excellent thermal resistance compared with the reported polymers with aniline oligomer in the main chain.^17,29

Fig. 3 TGA thermograms of the PI copolymers in N_2.

Concurrently, the entire synthesized PI copolymers afforded good quality films with dark color. The tensile properties of the PI films were examined. As summarized in Table 2, there was a clear tendency that the tensile modulus, tensile strength and elongation at break decreased with an increase in the aniline tetramer content. The films exhibited ultimate tensile strengths of 10.5–73.2 MPa, elongations to break of 3.6–7.2%, and initial moduli of 1.1–3.4 GPa, indicating that they were strong and tough enough to use as a film material.

Table 2 Mechanical properties of the PI films

Polymer	Tensile modulus /GPa	Tensile strength /MPa	Elongation/%
PI-20	3.4	73.2	7.2
PI-40	3.3	67.7	5.9
PI-60	2.9	58.8	5.1
PI-80	1.4	10.4	3.6
PI-100	1.1	10.5	3.9

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Spectroscopic properties

The optical properties of the PAA copolymers were investigated by UV-vis spectroscopy. It was found that they have similar variations when changing from the leucoemeraldine state to the pernigraniline state. Fig. 4 shows the stepwise oxidization process of the PAA-100 copolymer with a trace amount of (NH₄)₂S₂O₈ in DMAc. The PAA-100 copolymer in the leucoemeraldine oxidation state exhibited a single strong absorption at about 282 nm which was ascribed to the π–π* transition in the benzenoid ring. As the oxidization proceeded, a new peak at about 571 nm appeared, associated to the excitonic-type transition between the HOMO orbital of the benzoid ring and the LUMO orbital of the quinoid ring in the oxidized structural unit. Further oxidation caused an increase in the intensity of all peaks presented in the spectrum (Fig. 4(a)). When the intensity of the peak at about 571 nm reached the maximum, the PAA-100 copolymer was in the emeraldine oxidation state with the parent aniline tetramer segment containing one quinoid ring. With further oxidization of PAA-100 copolymer, all the absorption peaks decreased and the new absorption at about 571 nm underwent red shifts and extended toward the NIR region (Fig. 4(b)), which was contrary to those of polyaniline, oligoaniline and polymers with oligoaniline in the main chain.^17,19 Finally, the absorption in the range of 400 nm to 800 nm disappeared, which indicated that the PAA copolymer had reached the pernigraniline oxidation state. The excitonic-type transition absorption region indicated that the energy gap (Eg) between HOMO and LUMO decreased and effective conjugation length of the aniline tetramer increased. So we speculated that the red shift could be caused by the incorporation of the electron-withdrawing acyl group leading to delocalization of polarons and a higher intensity of free-carrier tail into the NIR region.¹⁰

Fig. 4 UV-vis spectra monitoring the chemical oxidation of the PAA-100 copolymer. (a) From the leucoemeraldine oxidation state to the emeraldine oxidation state; (b) from the emeraldine oxidation state to the pernigraniline oxidation state.

Electrochemical activity

Take PAA-100 copolymer as an example: the DMAc solution of the PAA-100 copolymer was cast on the GCE working electrode and evaporated to form a thin solid film. The cyclic voltammetry of the PAA-100 electrode was performed in 1 mol L⁻¹ H₂SO₄ at different potential scan rates (10–100 mV s⁻¹; Fig. 5). The CV of the PAA-100 electrode underwent two oxidation processes with the peaks at 300 and 430 mV. The first oxidation peak corresponded to the transition from the leucoemeraldine oxidation state to the emeraldine oxidative state, and the second peak corresponded to the transition from the emeraldine oxidative state to the pernigraniline oxidative state. A linear dependence of the peak currents as a function of scan rates in the region of 10–100 mV s⁻¹ (inset of Fig. 5) confirmed both a surface controlled process³¹ and a well-adhered electroactive polymer film. Because of the stability of the films and good adhesion between the polymer and working electrode, the PAA copolymers exhibited excellent redox stability.

Fig. 5 CV of PAA-100 electrodes in 1 mol L⁻¹ H₂SO₄ at different potential scan rates: 10–100 mV s⁻¹. Inset shows the relationships between the oxidation peaks and reduction current vs. potential scan rate.

Electrochromic performances

Electrochromism has been broadly defined as a reversible optical change in a material induced by an external voltage.³² Among the various electrochromic polymers, available PANI has multiple colored forms depending on the oxidation states, including leucoemeraldine (bright yellow), emeraldine (green), and pernigraniline (dark blue).^33,34 It is envisioned that the oxidation states transformation and color change could likewise occur at the oligoaniline linkages of polymers; hence, the PAA copolymers are subsequently analyzed for their abilities to exhibit electrochromic characteristics.

Electrochromic behaviors of PAA-100 copolymer film were examined by an optically transparent thin-layer electrode coupled with UV-vis spectroscopy. The change in optical transmittance at various applied potentials is shown in Fig. 6. The optical contrast value (%ΔT) was found to be 63% at 700 nm measured between its coloring (oxidization) and bleaching (reduction) states. The color of the copolymer thin films changed drastically from yellowish (at 0.0 V), to green (at ca. 0.4 V), and finally to dark blue (at ca. 0.8V) (inset of Fig. 6). We extracted the electrochromic parameters by analysis of absorbance change at 700 nm with respect to time, while the potential was switched between the neutral (−0.2 V) and the oxidized state at 0.8 V (vs. Ag/AgCl in 0.1 mol L⁻¹ H₂SO₄) with a residence time of 30 s. The results for the first 10 cycles were shown in Fig. 7. The switching time was defined as the time required for reaching 90% of the full change in coloring/bleaching process. Thin film from PAA-100 copolymer required 3.9 s at 0.8 V for the coloring process at 700 nm and 1.3 s for bleaching. The electrochromic CE (η = ΔOD/Q) was measured by monitoring the amount of ejected charge (Q) as a function of the change in optical density (ΔOD) of the polymer film. The electrochromic behavior of PAA-100 copolymer exhibited CE up to 105.8 cm² C⁻¹ (at 700 nm) at the first oxidation stage. After over hundreds of cyclic scans between −0.2 and 0.8 V, the performances of PAA-100/ITO electrodes declined a little, as indium–tin oxide is unstable in aqueous solutions with low pH values.

Fig. 6 Optical transmittance of PAA-100/ITO electrodes in 0.1 mol L⁻¹ H₂SO₄ at different potentials (vs. Ag/AgCl). Inset shows photographs of PAA-100/ITO electrodes at different potentials.

Fig. 7 (a) Current consumption and (b) absorbance change monitored at 700 nm of PAA-100 copolymer in 0.1 mol L⁻¹ H₂SO₄ for the first 10 cycles.

Conclusions

We have synthesized a series of novel electroactive PAA copolymers containing pendent aniline tetramer moieties by polycondensation reaction. Afterwards, the as-prepared PAA copolymers with high viscosity are transformed to tough and flexible PI films by thermal imidization. The synthesized PAA copolymers exhibit a broad and strong absorption in the visible region and good solubility in organic solvents. The PI films exhibit outstanding mechanical properties and excellent thermal stabilities with decomposition temperatures (at 10% weight loss) between 513 and 585 °C in nitrogen atmospheres. Furthermore, the PAA copolymers also reveal valuable electrochromic characteristics with extremely high contrast value, low switching time and moderate coloration efficiency. Given the good solubility, electroactivity, thermal stability, mechanical and electrochromic properties, the copolymers have great potential for foreseen applications, such as electromagnetic interference shielding materials, smart windows, and thin films in electronic applications, etc.

Acknowledgements

This work has been supported in part by the National 863 Project (no. 2007AA03Z324), National 973 Project (no. 2007CD936203), and NSFC grants (no. 20674027 and 50873045).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c1py00035g

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