Synthesis and electronic properties of comb-like polyamides bearing different contents of tetraaniline pendant groups

Abstract

Comb-like polyamides bearing different contents of tetraaniline pendant groups were synthesized via nucleophilic polymerization coupled with a post-functionalization method, and exhibit excellent solubility, good thermal stability and interesting spectroscopic properties. Their electronic properties, including conductivity, dielectricity, electroactivity, electrochromism and anticorrosion, were effectively adjusted and controlled by importing different numbers of tetraaniline pendant groups.

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Polyaniline has been researched extensively and exhibits great potential for advanced applications such as sensors,¹ supercapacitors,² photovoltaic cells,³ and electrochromic windows,⁴ due to its excellent environmental stability, ease of synthesis and reversible acid/base doping/dedoping chemistry.⁵ However, obstacles still remain because polyaniline is neither soluble nor fusible under normal conditions, which not only impedes its academic research, but is also unfavourable for industrial applications. Thus new strategies for increasing its processability are required.

As model compounds of polyaniline, oligoanilines are attractive as a means to impart electroactivity and stimuli-responsiveness into a series of polymeric scaffolds. Compared to their parent homopolymer, oligoaniline containing copolymers feature superior solubility, processability and tunability, while retaining much of polyaniline's functional capabilities.^6,7 Most recently, there has been concerted effort to design and synthesize oligoaniline containing copolymers in our lab. By exploiting the inherent characteristics of oligoaniline and enhancing their capabilities through the synergistic action with other functional groups, we have prepared several of promising materials for advanced applications,^7b,8 such as anticorrosion coatings, ammonia sensors, electrochemically responsive diffraction gratings, multiple stimuli fluorescent sensors, and multicolours electrochromic devices. Although these materials disclosed excellent properties and attractive application prospect, there remains significant room for improvement and variation in the performance of these materials. Undoubtedly, the content of oligoanilines in the polymer architecture is one of the most important factors in determining their performance. Therefore, we synthesized a few electroactive copolymers with different numbers of tetraaniline pendant groups in the repeat unit. A comparative study of their performance, in particular electronic properties, was carried out, which will provide useful information for developing anticorrosion coatings, electrochromic devices, and so on.

Scheme 1 shows the synthetic route of the comb-like polyamide (PXs, where X is the number of tetraaniline pendant in repeat unit) prepared by nucleophilic polymerization coupled with post-functionalization method. PXs bearing high content of tetraaniline pendants could not be prepared using common one-step nucleophilic polymerization due to the ultra-difficult synthesis of electroactive monomers with multiple tetraanilines.

Scheme 1 Synthetic route of the comb-like polyamide bearing tetraaniline pendants. Reagents and conditions: (i) DMAc/RT; (ii) and (iii) tetraaniline, DCC, DMAP/DMAc.

However, the post-functionalization strategy could achieve this aim readily, which can insert three tetraaniline pendants into one repeat unit. The number of tetraaniline pendants in PXs could be controlled from 1 to 3 by importing different amount of tetraaniline during the post-functionalization process. Here, the PXs bearing integral tetraaniline pendants such as P1, P2 and P3 were used as the typical specimen for the performance investigation, which bearing one, two, or three tetraaniline pendants in one repeat unit, respectively. The structures of P1, P2 and P3 were confirmed via nuclear magnetic resonance (NMR), Fourier-transform infrared spectra (FTIR), elemental analysis and gel permeation chromatography (GPC) (see the ESI†). With the increase of tetraaniline pendants, the proton signal of carboxylic acid around 13.2 ppm decreases gradually. In the GPC measurement, the weight-average molecular weight of PXs increase from 66 280 (P1) to 90 320 (P2) and 110 880 (P3), which also confirm the molecule structure and the degree of reaction as expected. Here, the weight percent of tetraaniline in P1, P2 and P3 is calculated about 37%, 55% and 64%, respectively. In the solubility test, the PXs are soluble in a variety of common polar solvents including tetrahydrofuran, dimethyl formamide, dimethyl acetamide (DMAc), dimethyl sulfoxide, and N-methyl-2-pyrrolidinone due to the bulky pendants in the molecular architecture. The visual difference was obvious and was presented in the ESI (Fig. S2†). Moreover, the thermal stability of PXs were probed via thermogravimetric analysis (TGA). Without regard of the weight loss from imine cyclization dehydration, PXs shows good thermal stability with high degradation beginning temperature (300 °C) in the air atmosphere (Fig. S3†). Their efficient synthesis, good solubility, and outstanding thermal stability indicate that this kind of polymers could be attractive in a number of advanced applications.

As an important consideration of electroactive polymer, the conductivity of PXs were firstly measured using a standard four-probe method. Contrast to the parent doped polyaniline (1–100 S cm⁻¹),⁹ the obtained PXs reveal lower conductivities of 0.1 × 10⁻⁷ S cm⁻¹ (P1), 0.8 × 10⁻⁷ S cm⁻¹ (P2), 1.7 × 10⁻⁷ S cm⁻¹ (P3), which should be ascribed to their unconjugated structure. Furthermore, the intrinsic dielectric properties of PXs were also investigated in the range of 0.1 kHz to 1 MHz at room temperature. Fig. 1 reveals that P3, P2, and P1 give high dielectric constant values of 1221, 82.1, and 35.5 detected at a frequency of 0.1 kHz, and then decreased to 21.4, 18.1 and 16.1 as the frequency was increased to 1 MHz, respectively. The phenomenon of dielectric constant decreasing significantly with the increase of frequency indicates that more and more dipoles and charge carriers within the polymer material can no longer keep up with the increasing AC frequency. Overall, PXs show higher dielectric constant (>16 at the frequency of 1 MHz) compare with commodity polymers;¹⁰ and the dielectric constant grows drastically with the increasing of tetraaniline content due to the characteristic high dielectric constant of parent polyaniline materials.¹¹ Meanwhile, P3 present much higher dielectric loss than that of P1 and P2, due to the increase of delocalized charges. Moreover, a dielectric loss maximum peak, presented in the diagram of P3, also reveals the existence of interfacial polarization.

Fig. 1 Frequency dependence of (a) dielectric constant and (b) dielectric loss of PXs.

The electrochemical activity of PXs was studied by cyclic voltammetry (CV) using traditional three-electrode system in 0.5 M H₂SO₄ solution. The platinum wire electrode, Ag/AgCl electrode, and PXs/ITO electrode were combined and used as counter electrode, reference electrode, and working electrode, respectively. The spin-formed PXs layer from a 0.01 g mL⁻¹ DMAc solution were measured about 150 nm thickness by profilometer. Under these conditions, the CV curves of P1 reveal two pairs of redox peaks at 0.27 V/0.17 V and 0.47 V/0.36 V (Fig. 2a), which are assigned to the transition of leucoemeraldine base (LEB)/emeraldine base (EB) and EB/pernigraniline base (PNB),^8a respectively (Scheme S1†). A linear dependence of the peak currents, as a function of scan rates in the region of 10–100 mV s⁻¹, confirmed a surface controlled process of P1 in the electrochemical measurement. The CV curves for P2 and P3 were also collected and keep similar graph shape except for the peak currents and peak potentials, which could be found in the ESI.† The representative CV curves of PXs/ITO electrode with the potential scan rates of 100 mV s⁻¹ are presented in the Fig. 2b. With the increasing of tetraaniline content, the redox peak potentials of PXs shift regularly, while the peak currents also rise observably. These results indicate that the electroactivity of PXs has been enhanced markedly through importing more tetraaniline pendants.

Fig. 2 (a) CV of P1/ITO electrode in 0.5 M H₂SO₄ at different potential scan rates: 10–100 mV s⁻¹. Inset shows the relationships between the oxidation and reduction current vs. potential scan rate. (b) Representative CV curves of PXs/ITO electrodes with the potential scan rates of 100 mV s⁻¹.

In light of its unique spectroscopic properties under three different oxidative states (Fig. S5†) as well as the characteristic electroactivity, the obtained PXs should be promising electrochromic material. Therefore, the spectroelectrochemical experiments was performed to explore their electrochromic performance. The applied potentials were fixed at −0.2, 0, 0.2, 0.4, 0.6, 0.8 and 1.0 V. The spin-formed PXs layer from a 0.05 g mL⁻¹ DMAc solution were measured about 350 nm thickness by profilometer. All the PXs layers are found flat without any other nanostructures, which was confirmed by SEM (Fig. S6†). As shown in Fig. 3a, the P1/ITO electrochromic layer exhibits the varied UV-Vis absorption spectra at different applied potentials. The optical contrast value (% ΔT) was calculated to be about 35% at 700 nm measured between its colouring and bleaching states. Meanwhile, the colour of electrochromic layer changed drastically from transmissive gray (at −0.2 V), to green (at 0.2 V), to dark green (at 0.4 V), and finally to absorptive black blue (at 0.8 V) (inset of Fig. 3a), due to the transformation of oxidative states of tetraaniline moieties. In the spectrochronoamperometry measurements, the switching time is calculated about 19 s at 0.8 V for the colouring process and 13 s at −0.2 V for bleaching; the Coloration Efficiency (CE) was found about 71 cm² C⁻¹ at the oxidation stage (Fig. 3b). With the increasing of tetraaniline content, P2 and P3 exhibit an improvement in electrochromic performance with higher contrast value (45% for P2, 49% for P3), shorter switching time (14 s/11 s for P2, 8 s/7 s for P3), and better coloration efficiency (90 cm² C⁻¹ for P2, 117 cm² C⁻¹ for P3), which certifies the effectiveness of importing more tetraaniline pendants for the enhancement of electrochromic performance. For these nonconjugated polymer materials, the conjugated structure of tetraaniline segments were used as the conducting moieties. With the increasing of tetraaniline content in the polymer architecture, the corresponding electronic properties, including conductivity, and electroactivity, should be enhanced automatically. Although the switching times of PXs electrochromic layers are not as good as these of common polyaniline^12a and nanostructural polyaniline material,¹² they are still acceptable values as the electrochromic material. Moreover, the calculated CEs of PXs layers are up to the level of 100 cm² C⁻¹, which are higher than the common polyaniline film (CE: 55 cm² C⁻¹)^10a and 3D ordered macroporous polyaniline (CE: <20 cm² C⁻¹).^10b All the results indicate that PXs are the qualified electrochromic polymer material.

Fig. 3 (a) The spectral changes of the P1/ITO electrode in 0.5 mol L⁻¹ H₂SO₄ at different potentials, and the inset shows photographs of electrochromic electrode at different potentials; (b) transmittance changes monitored at 700 nm of PXs/ITO when the applied potential was switched between −0.2 V and 0.8 V with a residence time of 30 s.

Considering their good electroactivity and film-forming property, the obtained PXs were expected to be potential anticorrosion material. Therefore, their anticorrosive ability for stainless steel (SS) was studied by polarization technique and electrochemical impedance spectroscopy (EIS) measurements in 3.5 wt% NaCl solution. After the SS substrates coated by P1 with the thickness of 10 μm, 20 μm, and 40 μm, the corrosion potential of P1/SS electrode undergoes a significantly positively shift from −0.610 V to −0.396 V, −0.142 V and 0.161 V (Fig. 4a). The corresponding protection efficiency is calculated 97.95%, 98.61%, and 99.24% for the P1/SS samples with increasing of the coating thickness. In addition, P2 and P3 exhibit an enhanced anticorrosive ability with high protection efficiency (Fig. 4b and Table S1†), which certifies that the anticorrosion performance could be effectively improved by importing more tetraaniline pandents into the polymer architecture. Contrast to the polyaniline-type anticorrosive materials,¹³ the synthesized PXs show better corrosion protection behavior. Because the general polyaniline-type anticorrosive materials consist of soluble materials and low ratio of polyaniline, due to its poor solubility and processability.

Fig. 4 (a) Tafel plots for P1/SS corrosive electrodes with various thicknesses (10 μm, 20 μm, and 40 μm) in 3.5 wt% NaCl solution; (b) Tafel plots for P1/SS, P2/SS, P3/SS corrosive electrodes with the thickness of 20 μm in 3.5 wt% NaCl solution; (c) EIS for P1/SS corrosive electrodes with various thicknesses (10 μm, 20 μm, and 40 μm) in 3.5 wt% NaCl solution; (d) EIS for P1/SS, P2/SS, P3/SS corrosive electrodes with the thickness of 20 μm in 3.5 wt% NaCl solution.

In the EIS measurements, the real film resistance for PXs/SS can be exactly read from the Nyquist plots, which have been fitted by Randles type equivalent circuit model. As shown in Fig. 4c, the R_f of P1/SS samples increases from 10 742 Ω to 18 752 Ω, 47 329 Ω, 150 975 Ω as the thickness of P1 coating increase from 0 μm to 10 μm, 20 μm, and 40 μm. Moreover, the Nyquist plots of P2 and P3 with 20 μm thickness are also contrasted with that of P1 (Fig. 4d). An obvious increasement has been found for the resistance with increasing of tetraaniline content. All the detailed electrochemical corrosion parameters could be found in the ESI.† In addition, the mechanical stability of these anticorrosion coatings has also been investigated. After immersion test in 3.5 wt% NaCl solution for twenty days, all the anticorrosion coatings remain unchanged, which indicated that the PXs coatings reveal great potential of anticorrosive protection for the practical application. The study of their long-term anticorrosive protection is currently undergoing in our lab. All these results indicated that the anticorrosion performance of PXs coatings could be adjusted significantly by importing different content of electroactive segments into the polymer architecture.

In summary, we have synthesized comb-like polyamides bearing different content of tetraaniline pendant by nucleophilic polymerization coupled with post-functionalization method, which exhibit excellent solubility, good thermal stability and interesting spectroscopic properties. The conductive and dielectric properties of the PXs were studied and determined. Electrochemical properties of PXs layers, including electroactivity, electrochromism and anticorrosion, were confirmed by multiple electrochemical techniques. The improvement of electrochemical performance could be accomplished by adjusting the content of tetraaniline pendants in the molecular architecture. These attributes not only pave the way for their practical application, but has also inspired us to explore new characteristics for polymer materials with various architectures. Further investigation on the stimuli-responsive aspects of these materials for possible sensing capabilities are currently carried out in our laboratories.

Acknowledgements

We graciously acknowledge the National Natural Science Foundation of China for funding (grant no. 21104024), Jilin Province Science and Technology Department project (20130522139JH), the Functional Research Funds for the Central Universities (JCKY-QKJC07) and the Open Project of State Key Laboratory of Superhard Materials (Jilin University), China (Grant No. 201403).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04759a

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