Energy storage and surface protection properties of dianiline co-polymers

Abstract

Conducting co-polyanilines of alkyl/alkoxy substituted anilines with dianiline, namely poly(OT–DA), poly(OA–DA) and poly(DMA–DA), have been synthesized chemically from o-toluedine(OT), o-anisidine(OA), 2,6-dimethylaniline(DMA) and N-phenyl-p-phenylenediamine (DA). These co-polyanilines were characterized by spectral, thermal and electrochemical techniques. The conductivities of phosphoric acid (PA) doped co-polydianilines are 2.26 × 10⁻¹ S cm⁻¹ for poly(OT–DA), 7.94 × 10⁻² S cm⁻¹ for poly(OA–DA) and 9.25 × 10⁻² S cm⁻¹ for poly(DMA–DA) at room temperature. The thermal stability of co-polymers is higher in the doped state than in the emeraldine base state. A capacitor has been fabricated using these polymers using stainless steel electrodes. The specific capacitance of the device is 177 F g⁻¹ for poly(OT–DA)–PA, 110 F g⁻¹ for poly(OA–DA)–PA and 121 F g⁻¹ for poly(DMA–DA)–PA at 1 mA cm⁻² current density. The anti-corrosion behaviour of co-polydianilines (EB state) coated on mild steel (MS) electrode was investigated by Tafel polarization method and the corrosion rate is found to be 2.07 × 10⁻⁶ mm per year for poly(OT–DA)–EB, 1.42 × 10⁻⁵ mm per year for poly(OA–DA)–EB and 5.3 × 10⁻⁴ mm per year for poly(DMA–DA)–EB.

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1. Introduction

Conducting polymers (CPs) now enjoy a special status in the field of electroactive materials especially after the pioneering and noble prize winning contributions by the Shirakawa–McDiarmid–Heeger group.¹ Particular attention has been given to polyaniline (PAni) due to its environmental stability, thin film-forming property and tunable conductivity due to controlled acid–base doping and most importantly due to its commercial viability. Although the insoluble and intractable nature of mineral acid-doped polyaniline played a retarding role for its applications in the beginning, over the years, attempts by several research groups have considerably enriched the field in terms of synthesis, processability and device applications.^2–5 The most important milestone in the chronological history of conducting polyaniline is the success of making the intractable polymer into fibers and films.⁶ The main strategies for achieving processability for conducting polyaniline are (i) synthesizing functionalized organic sulphonic acids for doping,^7–13 (ii) to introduce substituent groups on the ring,^14,15 sulphonic acid ring-substituted polyanilines (SPAN)^16,17 and (iii) blending or compositing with other conventional processable polymers such as poly(vinyl alcohol) (PVA),^18,19 poly(methyl methacrylate) (PMMA),²⁰ poly(ethylene oxide) (PEO)²¹ and poly(diallyldimethylammoniumchloride) (PDDMAC).^22,23

As mentioned above, ring substituted polyanilines are soluble in most of the common organic solvents and hence are processable.¹⁴ Conductivity in the systems is lowered owing to the loss of planar-conjugation due to substituents on the phenyl ring. Improved conductivity may be achieved in copolymers of aniline and substituted aniline. Many copolymers of aniline with substituted anilines are known in literature.²⁴ Dianiline is important building block for synthesising various oligomeric aniline units.^25,26 Few reports on electrochemical synthesis of polyanilines from dianiline is also known.^27,28 There are no reports available on chemical synthesis of copolymers of dianiline or their applications.

Polyanilines have been studied extensively for applications to practical devices for energy storage,^29,30 electrochemical sensors,^31,32 electrochromic devices,³¹ electromagnetic interference (EMI) shielding³³ and corrosion protection.³⁴ Conducting polyaniline is attractive candidate for electrode material for super capacitors.³⁰ Most of the reports explored charge–discharge behaviour (for specific capacitance) of the PAni/SS single electrode or its capacitor device^35–37 with scant reports on substituted polymers or their copolymers.³⁸

There are very few reports available on synthesis and applications of phosphoric acid doped polyaniline. Some applications require specific method of protonation. Phosphoric acid provides the counter-ion of choice in the corrosion protection of metals with polyaniline.^39,40 Our previous investigation on phosphoric acid doped polyanilines on SS electrodes as super capacitor material gave very encouraging results.⁴¹ Stainless steel (SS) is cost effective substrate for super capacitors. Many researches^42–44 use SS sheet or mesh for the fabrication. Munichandraiah et al. studied the beneficial effects of using SS as electrode substrate for super capacitor.⁴⁵ Their study showed that the capacitance of PANI/SS electrode is higher (∼250 F g⁻¹) than that of PANI/Pt (∼75 F g⁻¹) electrode. Conducting polymers are used in coatings for protection of the metal from corrosion environments and PAni has been investigated since last few years.^46–49 The corrosion protection property of emeraldine base (EB) when coated on mild steel and cold rolled steel was discussed by Jasty and Epstein⁴⁷ utilizing XPS and corrosion protection studies. The emeraldine base form polyanilines were found to be good corrosion protection for metals.

There are no reports available on PA doped co-polyanilines to the best of our knowledge. In the present article PA-doped co-poly(dianiline) are synthesized and their capacitor and anti-corrosion properties are investigated and reported.

2. Experimental part

2.1. Materials

The chemicals used were obtained from different sources; o-toluedine, o-anisidine from (Alfa Aesar 99%), 2,6-dimethylaniline, N-phenyl-p-phenylenediamine from (Sigma-Aldrich grade 98%), hydrochloric acid (Merck ACS, 37%), ammonium peroxodisulfate (APS) (S.D fine, LR grade 98%), ammonium hydroxide (FINAR 25% NH₃) and phosphoric acid (Qualigens grade 88%). These chemicals were used as supplied. Other solvents used in the characterization experiments were N-methyl-2-pyrrolidinone (NMP) (Alfa Aesar grade 98%), N,N-dimethylformamide (DMF) (S.D fine AR grade 99.5%), dimethyl sulfoxide (DMSO) (FINAR grade 99%), methanol (Merck ACS grade 99.8%), tetrahydrofuran (THF) (S.D fine AR grade 99.5) were used as supplied.

2.2. Synthesis of co-polydianilines

A mixture consisting of 1.07 g (10 mmol) of (OT) and 1.84 g (10 mmol) of (DA) was dissolved in 60 ml of 1 M HCl. The reaction mixture was magnetically stirred at 0–5 °C in ice-salt bath for 30 minutes. To this reaction mixture was added 4.56 g (20 mmol) of ammonium peroxodisulfate in 20 ml of aqueous 1 M HCl solution. The whole reaction mixture was magnetically stirred at 0–5 °C for 4 h. The controlled low temperature is chosen to prevent high increase in temperature due to the exothermic reactions. The co-polymer obtained P(OT–DA) was in the conducting state. The reaction product was separated from the solution by vacuum filtration and washed with distilled water several times. The green precipitate was treated with 50 ml of 1 M ammonium hydroxide at room temperature for 2 h, filtered, washed with distilled water and dried at 40 °C for 24 h. Yield: 1.94 g.

¹H NMR (300 MHz, DMSO-d₆) δ 7.40 (s, 2H), 7.17 (dd, J = 13.4, 7.2 Hz, 1H), 7.05–6.71 (m, J = 36.5, 12.8, 7.9 Hz, 4H), 6.65 (d, J = 16.4 Hz, 1H), 6.01 (s, 4H), 5.90 (s, 1H), 5.72 (s, 1H), 3.65 (s, 2H), 2.73 (s, 3H), 2.15 (s, 3H).

Similarly other co-polymers P(OA–DA) and P(DMA–DA) were also synthesized from ortho-anisidine and 2,6-dimethyl aniline with N-phenyl-p-phenylenediamine, following the same procedure described above.

¹H NMR (300 MHz, DMSO-d₆) δ 7.46 (s, 2H), 7.35–6.59 (m, 1H), 6.26 (s, 4H), 6.08 (s, 1H), 5.87 (s, 1H), 3.82 (d, J = 9.3 Hz, 1H), 3.73 (d, J = 10.0 Hz, 1H), 2.89 (s, 1H), of P(OA–DA) and ¹H NMR (300 MHz, DMSO-d₆) δ 7.36–6.58 (m, 2H), 2.86 (s, 1H), 2.16 (s, 1H), 2.08 (s, 1H), 2.02 (s, 1H), of P(DMA–DA).

2.3. Preparation of PAni/SS electrode for capacitor

Polyvinyledene fluoride (PVDF) 5% was taken in a motor with 0.3 ml of NMP and ground for 5 minutes and 15% of carbon was added to the PVDF solution and further grounded for 15 minutes. Then 80% of PA doped polyanilines was added to the above mixture and the total mixture was ground for 40 minutes, by which time the polymer became homogeneous paste. The resulting composite was coated on to 2 × 2 cm² stainless-steel (SS) electrodes (2 numbers) and dried at room temperature for 24 h. The weight of the active mass on each SS electrode is 1 mg.

2.4. Preparation of PAni/mild steel electrode for corrosion study

For corrosion study on mild steel, similar homogeneous paste was prepared using EB–dianiline polymer (90%) and PVDF (10%) and applied on mild steel panel (1 inch × 1 inch). The thickness of the film coated is 22 μm.

2.5. Characterizations

The molecular structure of co-polyanilines was characterized by infrared spectroscopy (FT-IR), ¹H NMR spectroscopy and UV-visible absorption spectroscopy. The FT-IR spectrums of the polymers as KBr-pressed pellets have been recorded on a Nicolet FT-IR (spectrum 100, PerkinElmer, USA) spectrometer with a frequency range of 450 to 4000 cm⁻¹. ¹H NMR spectra was recorded on a 300 MHz NMR Spectrometer, (AVANCE-300, Bruker, Switzerland). The UV-visible spectrum of co-polyanilines–EB and co-polyanilines–PA (in situ) was recorded in N,N-dimethylformamide (DMF) solutions using Shimadzu UV-1800 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). TGA experiments were performed on TA instruments, (model. no Q500, TA Instruments, USA) by heating the sample under air at a rate of 10 °C min⁻¹ from room temperature to 700 °C. DSC studies were performed using TA instruments (model. Q100, TA Instruments, USA) in presence of N₂ atmosphere from −80 to 100 °C with a heating rate of 10 °C min⁻¹. The morphology of co-polymer powders was examined by field emission scanning electron microscopy (FE-SEM), using JEOL 7610F, Japan. The samples were prepared by depositing little amount of polymer on carbon surface, followed by gold sputtering for 1 minute. Conductivity of the samples were measured by four-probe method using KEITHLEY nanovoltmeter after pressing the samples into 1 cm dia, 1.5 mm thick pellets under 3 ton pressure. We measured the conductivity of each pellet two times on either side of the pellets and averaged it. Cyclicvoltammetry was performed on a AUTOLAB (Potentiostat & Galvanostat, AUT85984, NOVA 1.10.1.9) electrochemical system using three electrode assembly consisting of a platinum disc for P(OT–DA) and P(OA–DA) polymers and glassy carbon disc for P(DMA–DA) polymer as working electrode, a platinum wire auxiliary electrode (counter electrode) and SCE (Hg/Hg₂Cl₂) as reference electrode.

3. Results and discussion

3.1. Synthesis of co-polydianilines

The chemical structures of co-polydianilines are schematically shown in Scheme 1. These polymers are green in color when synthesized and bluish when undoped. These polymers are redoped using 1 M PA for characterizations such as FT-IR spectroscopy, ¹H NMR spectroscopy, UV-vis absorption spectroscopy and DSC and use as supercapacitor material. As shown in Scheme 1 the structure of co-polyanilines (structure-a) have aniline and substituted aniline rings alternatively linked at 1,4-positions. This arrangement brings good solubility to the copolymer with decreased conductivity. In case of co-polydianiline (structure-b), the substituted aniline rings are separated by two aniline rings (dianiline). This is expected to bring enhanced conductivity retaining the property of good solubility in common organic solvents. This is indeed observed; the base polymers (dedoped polymers) poly(OT–DA), poly(OA–DA) and poly(DMA–DA) are completely soluble in polar solvents such as acetonitrile, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide and N-methyl-2-pyrrolidone but partially soluble in methanol, chloroform and dichloromethane.

Scheme 1

3.2. FT-IR spectral characteristics

The FT-IR spectra of doped and undoped co-polymers are shown in Fig. 1 and the data is collected in Table 1. N–H stretching vibration of secondary amine ([double bond splayed left]NH) was obtained as a broad strong band in the region of 3500–3000 cm⁻¹ with peak maxima at 3377 cm⁻¹ in P(DMA–DA)–EB, which shifted and split into two peaks centred at 3357 cm⁻¹ and 3225 cm⁻¹ after doping with PA. The split of N–H stretching into two peaks after doping suggests lowering of site symmetry and deformation of quinoid ring in the polymer chain. The intensity and broadness of the peaks also suggests existence and strengthening of the hydrogen bonding on protonation with PA.^50,51

Fig. 1 FT-IR spectra of the co-polydianilines (a) P(OT–DA)–EB, (b) P(OT–DA)–PA, (c) P(OA–DA)–EB, (d) P(OA–DA)–PA, (e) P(DMA–DA)–EB, (f) P(DMA–DA)–PA recorded as KBr pellets.

Table 1 FT-IR data of undoped and 1 M PA doped co-polydianilines

Polymer	FT-IR (cm^−1)
	C–Hop	C–N	N–B–N	N=Q=N	C–Harom	N–H
P(OT–DA)–EB	816	1245	1500	1595	3022	3375
P(OT–DA)–PA	812	1255	1490	1572	2923	3175
P(OA–DA)–EB	817	1247	1503	1595	3021	3365
P(OA–DA)–PA	801	1256	1487	1570	2927	3417
P(DMA–DA)–EB	828	1261	1494	1592	3026	3377
P(DMA–DA)–PA	825	1245	1492	1582	2917	3351

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Spectral region between 2000–1000 cm⁻¹ contained fundamental and combination vibrational bands of benzenoid (B) and quinoid (Q) rings and ν₃ antisymmetrical stretching modes of phosphate ion. The C=C stretching vibration of the Q ring bending mode appeared as a strong band between 1590 and 1595 cm⁻¹ in EB polymers while for benzenoid segments it appeared 1494–1503 cm⁻¹. The intensity of the latter peaks are so high compared to the quinoid peaks suggesting predominance of B units over Q segments expected from general representation of B₃Q₁ ⁵² structure known in polyaniline chemistry. The ν₃ anti-symmetrical stretching mode of phosphate ion is observed at 1108 cm⁻¹, 1122 cm⁻¹ and 1125 cm⁻¹ for P(OT–DA), P(OA–DA) and P(DMA–DA) respectively. The ν₄ bending mod of phosphate appeared as low intense band between 595 and 598 cm⁻¹ for the doped polymers.

The ¹H NMR spectra of co-polymers are shown in Fig. 2a–c with assignments. For P(OT–DA)–EB, the bunch of signals from 7.2 to 6.5 are from aromatic protons of toluedine and dianiline rings. The –NH– protons give broad absorption at 3.65 ppm. The methyl group present on benzenoid ring gave sharp signal at 2.73 ppm while methyl group on quinoid ring gave signal at 2.15 ppm. Similar NMR pattern is observed P(OA–DA)–EB and P(DMA–DA)–EB polymers. The –OCH₃ groups on benzenoid and quinoid ring in P(OA–DA) gave two doublets at 3.83 and 3.75 ppm. The –NH– peak is broad intense at 2.89 ppm. The two –CH₃ groups on benzenoid and quinoid rings in P(DMA–DA) gave two sharp, intense signals at 2.17 and 2.08 ppm. The overall spectrum showed the formation of co-polymers, with structures as shown in Scheme 1.

Fig. 2 The ¹H NMR spectra in DMSO-d₆ solution of (a) P(OT–DA), (b) P(OA–DA), (c) P(DMA–DA).

3.3. UV-visible spectroscopy

UV-vis spectra of polyanilines give valuable information about their doped or undoped states. The steric factor on the phenyl ring is further supported by the electronic absorption spectra of these polymers. As shown in Fig. 3, the spectra of the base forms of P(OT–DA), P(OA–DA) and P(DMA–DA) in DMF all consist of two absorption bands. The absorption bands are observed at 308 nm, 562 nm; 308 nm, 547 nm and 306 nm, 566 nm respectively for the above polymers (Fig. 3). The first absorption band at 308 nm is assigned due to the π–π* transition on the basis of the studies of polyanilines,^53,54 which is related to the extent of conjugation between the adjacent rings (benzenoid ring) in the polymer chain, while the transition at longer wavelength (562 or 547 or 566 nm) is due to the quinoid (quinonimine) segments.

Fig. 3 UV-vis spectra in DMF of (a) P(OT–DA)–EB, (b) P(OT–DA)–PA, (c) P(OA–DA)–EB, (d) P(OA–DA)–PA, (e) P(DMA–DA)–EB, (f) P(DMA–DA)–PA.

When the polydianilines were doped with 1 M PA a dramatic change in the electronic spectrum is observed (Fig. 3). Doping with 1 M PA does not alter the position of the peak at about 308 nm which is due to the benzenoid units, but a new band is observed at 424 nm for P(OT–DA)–PA; 424 nm band for P(OA–DA)–PA and the band at 409 nm for P(DMA–DA)–PA respectively. It is associated with the presence of radical cations on the polymer chain and is assigned to polaron–π* transitions (quinoid ring).

3.4. Thermal characteristics

The thermal stability of the polymers was examined by TG analyses. The data and the profiles are given in Table 3 and Fig. 4a and b. All EB-polymers showed a two step decomposition pathway (Fig. 4i). The first decomposition (T_1on: onset of 1st degradation temperature) for P(OT–DA) starts at 178 °C and ends by 290 °C. The second decomposition (T_2on–T_2F) is (T_2on: onset of 2nd degradation temperature; T_2F: final degradation temperature of second stage) between 298 and 477 °C. The residual remains at 600 °C are 17%. The PA doped copolymer P(OT–DA) gave a TGA profile with one step decomposition (Fig. 4ii). There is a moisture loss of 3% at 100 °C. The decomposition starts at 186 °C which is slightly higher than pure-EB. The decomposition is complete at 461 °C leaving about 53% material at 600 °C. Similar trends are observed in the decomposition profiles of P(OA–DA) and P(DMA–DA) doped and undoped forms. For doped P(OA–DA), moisture loss 2.5% occurred at 100 °C with T_on starting at 184 °C and decomposition ending at 490 °C. The final residual material at 600 °C is 67%. The moisture loss, T_on, T_final and residual mass for doped P(DMA–DA) are 3.8%, 180 °C, 458 °C and 60% respectively. Overall, the study suggests that the doped forms are more stable with decomposition starting at 180 °C than undoped (the decomposition starts in the range 156–178 °C) polymers. The TGA data is collected in Table 3.

Table 2 Conductivity and UV-vis data of undoped and 1 M PA doped co-polydianilines

Polymer	Conductivity (S cm^−1)	UV-vis (DMF) (λmax) nm
P(OT–DA)–EB	—	308, 562
P(OT–DA)–PA	2.26 × 10^−1	303, 424, 558
P(OA–DA)–EB	—	308, 547
P(OA–DA)–PA	7.94 × 10^−2	301, 424, 561
P(DMA–DA)–EB	—	305, 566
P(DMA–DA)–PA	9.25 × 10^−2	302, 409, 580

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Table 3 TGA data of undoped and 1 M PA doped co-polydianilines^a

Polymer	T1on	T1final	T2on	T2final	Weight% remaining at 600 °C
a T1on: first step onset degradation temperature; T1final: final or end temperature for the first stage degradation. T2on: second step onset degradation temperature; T2final: final or end temperature for the second stage degradation.
P(OT–DA)–EB	178	290	298	477	16.8
P(OT–DA)–PA	186	461	—	—	53.0
P(OA–DA)–EB	156	402	402	536	38.7
P(OA–DA)–PA	184	490	—	—	67.0
P(DMA–DA)–EB	176	353	374	512	32.0
P(DMA–DA)–PA	180	458	—	—	60.0

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Fig. 4 (i) TGA profile of undoped polymers (a) P(OT–DA)–EB, (b) P(OA–DA)–EB, (c) P(DMA–DA)–EB (ii) TGA profile of doped polymers (a) P(OT–DA)–PA, (b) P(OA–DA)–PA, (c) P(DMA–DA)–PA.

The thermal characteristics of the dedoped polymers were evaluated by differential scanning calorimeter (DSC) under nitrogen atmosphere. Fig. 5 shows DSC thermograms of co-polydianilines in the undoped form. As reported⁵⁵ polyaniline is not sensitive to thermal cooling/heating cycles between −100 °C and 175 °C in DSC experiment. However the present dianiline co-polymers exhibited thermal transitions as shown in Fig. 5. There is a clearly visible T_g observed for all three polymers which increased in the order P(DMA–DA) (45.7 °C) < P(OA–DA) (46.5 °C) < P(OT–DA) 48.4 °C. Apart from this, P(OA–DA) showed a sharp melting point curve at 64 °C. The coexistence of a melting point and glass transition for P(OA–DA) suggests that the polymer is semi-crystalline in nature. The property of showing a T_g in the range 45–48 °C by all undoped polymers is comparable to poly(o-hexylaniline) (41 °C) and other substituted homopolymers in the literature.⁵¹ The report also revealed that there is no direct relation between alkyl chain length on aniline and the T_g temperature. In similar way, the present set of T_g values also suggest that there is no correlation exist on the nature and number of substituents on the aniline ring. Thus unlike polyaniline, the present set of polymers show some degree of thermoplastic nature in undoped state.

Fig. 5 DSC profile of (a) P(OT–DA)–EB, (b) P(OA–DA)–EB, (c) P(DMA–DA)–EB.

3.5. Morphology and conductivity

The FE-SEM pictures of PA doped co-polymers are shown in Fig. 6. The morphology of P(OT–DA)–PA showed sponge like appearance at 5 k (or 10 k) which exist as clusters. At higher magnifications [i.e., 15 k and 20 k, Fig. 6a], these sponge-clusters are glued together with irregular shape and appears to be non-porous. The polymer P(OA–DA)–PA (Fig. 6b) showed similar morphology of sponge like structure, with lower particle agglomeration (gluing). Overall the surface morphology of P(OT–DA)–PA and P(DMA–DA)–PA are closely related, with more particles glued together, giving sponge like surfaces.

Fig. 6 FE-SEM images of PA doped co-polydianilines (a) P(OT–DA)–PA, (b) P(OA–DA)–PA and (c) P(DMA–DA)–PA at various magnifications.

The conductivity of doped polydianilines was measured by four probe method. The conductivity of P(OT–DA)–PA, P(OA–DA)–PA and P(DMA–DA)–PA (Table 2) was found to be in higher than few long chain substituted polyaniline⁵⁶ or comparable with some substitute polyaniline.^57–59 The emeraldine bases of the polymers are insulators (σ ≤ 10⁻¹⁰ S cm⁻¹).⁵⁹ The conductivities of these polydianilines decreased as compared to pure PAni, due to the methyl or methoxy group on the phenyl ring which is expected to increase torsional angle between adjacent rings to relieve steric strain and result in non-planar structure. This effectively reduces the conjugation length, hence conductivity. Overall the conductivity variation among the three polymers is not very high. The conductivity of P(OT–DA) is maximum in the series (2.26 × 10⁻¹ S cm⁻¹). The conductivity of P(DMA–DA) (9.25 × 10⁻² S cm⁻¹) is higher than P(OA–DA) (7.94 × 10⁻² S cm⁻¹). Fig. 7 shows the range of conductivity shown by the present set of polymers.

Fig. 7 Conductivity range (S cm⁻¹) plot of the 1 M PA doped copolymers.

3.6. Redox properties of polyanilines

Fig. 8a–c shows the cyclic voltammogram of polyanilines coated on Pt or glassy carbon disc from DMF solution. The cyclic voltammetry curves of polydianilines are obtained at various scan rates between 0.2 to 0.9 or 1.0 V (vs. SCE) in 1 M PA electrolyte solution. The polymer P(OT–DA) (on Pt disc) showed one oxidation peak at 0.44 V in forward scan and one reduction peak at 0.28 V in reverse scan with respect to scan rate of 50 mV s⁻¹. Similarly the polymers P(OA–DA) (on Pt disc) and P(DMA–DA) (on GC disc) showed one oxidation and one reduction peaks. The peak potential shifted to higher potential with the increase of scan rate. The currents of redox couple also increased with increasing scan rate and gave linear relation with scan rate (Fig. 8d). The single oxidation and reduction peak can be assigned to the leucoemeraldine → emeraldine and emeraldine → leucoemeraldine transition, while the emeraldine/pernigraniline transition is missing. This is probably because of steric hindrance caused by the substituents on the phenyl rings which probably retards reorganisation of benzenoid ring to quinoid rings and hence pernigraniline state is not achievable. The exhibition of single reversible transitions: emeraldine ↔ leucoemeraldine in CV is also observed for some chemically prepared poly(substituted anilines) in literature,⁵⁷ which was ascribed to the above mentioned steric hindrance by substituents.

Fig. 8 Cyclic voltammograms of (a) P(OT–DA)@Pt disc, (b) P(OA–DA)@Pt disc, (c) P(DMA–DA)@GC disc at various scan rates in 1 M PA electrolyte solution (d) plot of current versus scan rate of the copolymers.

3.7. Charge storage properties of PA doped polymers

One of the aims of the present investigation is the study of the charge storage property of the copolymers by fabricating a supercapacitor device. For this purpose, the copolymers were coated on two electrodes as described in experimental section. These electrodes were tightly placed one above other with a fresh cotton cloth as separator and tied with nylon tag to form the device. This symmetrical capacitor was subjected to charge–discharge tests from 0.0 to 1.0 V in 1 M PA acid and the specific capacitance (SC), specific power (SP), and specific energy (SE) values were calculated from discharge times^42,60 by the following relationships:

S.C (F g⁻¹) = I(A) × t(s)/ΔE × m (1)

SP (W kg⁻¹) = I(A) × ΔE/m (2)

SE (W h kg⁻¹) = I(A) × t(s) × ΔE/m (3)

where I = current (A), t = discharge time (s), m = mass of the electroactive material on two electrodes and ΔE is the potential window scanned.

The charge–discharge curves at current density of 1–5 mA are shown in Fig. 9. The capacitor of P(OT–DA) showed a specific capacitance of 176.7 F g⁻¹ at 1 mA discharge current. This value reduced to 123 F g⁻¹, 106 F g⁻¹, 101 F g⁻¹ and 96 F g⁻¹ upon increasing the charging–discharging currents to 2 mA, 3 mA, 4 mA and 5 mA respectively. The specific capacitance decreased to 121 F g⁻¹ when we used P(DMA–DA) as electrode material. The P(OA–DA) device showed still lower specific capacitance of 110 F g⁻¹ at 1 mA current density. The device cycleability was checked to 100 limited cycles and the data is collected in Table 4. It is observed that there is 57% retainability (43% fall) of specific capacitance for P(OT–DA) device after 100 charge discharge cycles. The retainability increased for the devices of P(OA–DA) and P(DMA–DA) to 67% and 83% respectively suggesting that P(OT–DA) is more prone for electrochemical degradation than other two polymers.

Fig. 9 Galvanostatic charge–discharge curves of device (a) P(OT–DA)–PA, (b) P(OA–DA)–PA, (c) P(DMA–DA)–PA at various current densities (1–5 mA cm⁻²) in 1 M PA (d) variation of specific capacitance with current density.

Table 4 Specific capacitance data of 1 M PA doped co-polydianilines

Polymer	Specific capacitance of 2 × 2 cm^2 device (F g^−1)					Specific capacitance after 100 cycle
	1 mA cm^−2	2 mA cm^−2	3 mA cm^−2	4 mA cm^−2	5 mA cm^−2	1 mA cm^−2
P(OT–DA)–PA	176.7	123.2	106	101	95.9	100
P(OA–DA)–PA	110	83.1	76.7	75.4	67.3	74
P(DMA–DA)–PA	121	80.8	63.4	59.2	53.8	101

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In general, polyaniline offer higher capacitance than the substituted-polyanilines. The value of capacitance depends on the morphology of the polyaniline particles and also on the nature of the substrate (current collector). As evidenced from literature, stainless steel (SS) is very good substrate known for enhancing the capacitance. Globule shaped polyaniline/SS system³⁶ gave as low as 70 F g⁻¹ capacitance where as when the morphology is changed to nanofiber network,³⁶ the capacitance increased to 742 F g⁻¹. The cycleability is higher for globule type polyaniline and lower for nanomaterials due to their lower molecular weight. The presently investigated dianiline copolymer showed 170–110 F g⁻¹ which is better than literature value of 73 F g⁻¹. However the retention of the capacitance is slightly on lower side for P(OT–DA) and P(OA–DA) (57% and 67%) but is good for P(DMA–DA), 83%.

The specific power (SP) and specific energy (SE) [from the above eqn (2) and (3)]^42,60 of the device was calculated based on the active mass on the two electrodes and the data is collected in Table 5. Polymer (OT–DA) exhibited about 0.5 kW kg⁻¹ specific power and 24 W h kg⁻¹ specific energy at 0.5 A g⁻¹ discharge current density. The SP increased to 2.48 kW kg⁻¹ and SE decreased to 13 W h kg⁻¹ at 2.5 A g⁻¹ discharge current. The other two polymers also showed similar trend. Over all, P(OT–DA) showed higher specific energy at any given discharge current compared to P(OA–DA) or P(DMA–DA). The specific power is marginally varied among P(OT–DA), P(OA–DA) and P(DMA–DA). The maximum SP of 3.3 kW kg⁻¹ was exhibited by P(OA–DA)–PA and P(DMA–DA)–PA at 2.5 A g⁻¹ current density.

Table 5 Specific power and specific energy data of capacitor in 1 M PA

Polymer	Discharge current density (A g^−1)	Specific power (kW kg^−1)	Specific energy (W h kg^−1)
P(OT–DA)–PA	0.5	0.5	24
	1	0.99	16.77
	1.5	1.49	14.43
	2	1.98	13.75
	2.5	2.48	13
P(OA–DA)–PA	0.5	0.66	15
	1	1.32	11.36
	1.5	1.98	10.45
	2	2.64	10.26
	2.5	3.3	9.16
P(DMA–DA)–PA	0.5	0.66	16.5
	1	1.32	11
	1.5	1.98	8.63
	2	2.64	8
	2.5	3.3	7.33

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3.8. Electrochemical impedance analysis

Electrochemical impedance spectroscopy is a powerful tool for mechanistic analysis of interfacial processes and evaluation of double layer capacitance. The Nyquist plots obtained from the impedance study of the device made from coated stainless steel electrodes are represented in Fig. 10. These plots are fitted by various elements shown in equivalent circuit given in Fig. 11. The equivalent circuit parameters are solution resistance (R_s), charge transfer resistance (R_ct), double layer capacitance (C_dl), Warburg impedance (Z_w) and constant phase element (CPE). Fig. 10a–c depict the typical Nyquist diagrams for the device at 0.2, 0.4 and 0.6 V in 1 M PA electrolyte solution. In Fig. 10a–c impedance (dotted lines) spectra showed a distorted semi-circle in the high-frequency region and a vertically linear spike in the low-frequency region. The high frequency intercept of the semi-circle on the real axis yields the solution resistance (R_sol) and the diameter of the semi-circle provides the (R_ct) of the electrode/electrolyte interface. The values of R_ct increased with the applied potentials as inferred from the diameter of the semi-circle. The impedance data obtained after fitting (Fig. 10, solid line) is collected in Table 6 and this circuit data is in the range with the data available in literature³⁶ on conducting polymer electrodes/devices.

Fig. 10 The Nyquist plots of (a) P(OT–DA)–PA (b) P(OA–DA)–PA and (c) P(DMA–DA)–PA in the range 100 kHz to 1 mHz of capacitors at various (0.2, 0.4 and 0.6 V) potentials. Dots denote the experimental values and the line represents the curve fitting of the equivalent circuit of Fig. 11.

Fig. 11 Equivalent circuit of Nyquist plot for the capacitor. R_s, solution resistance; R_ct, charge transfer resistance; C_dl, double layer capacitance; Z_w, Warburg impedance; CPE, constant phase element.

Table 6 Circuit parameters of the co-polydianilines device capacitor after 100 cycles at 0.2, 0.4 and 0.6 V obtained from curve fitting of the impedance (Nyquist) plots

	Polymer
	P(OT–DA)–PA			P(OA–DA)–PA			P(DMA–DA)–PA
	0.2 V	0.4 V	0.6 V	0.2 V	0.4 V	0.6 V	0.2 V	0.4 V	0.6 V
Rs (mΩ)	525	491	461	399	419	426	421	438	436
Rct (Ω)	17.7	17.5	18	28	27	25.5	10.5	10	10
Cdl (μF)	150	140	150	130	120	130	150	135	80.2
Zw (mMho)	50	35	35	50	35	30	45	30	20
CPE YO (mMho)	143	121	114	35.2	33.8	82.3	111	210	292
N	0.92	0.97	0.96	0.84	0.87	0.98	0.95	1.05	1.1

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3.9. Anti-corrosion properties of polyanilines–EB coated on mild steel (MS)

It has been well known in literature^61,62 that conducting and electroactive polymers are excellent materials for corrosion protection for various metal surfaces. In particular, the corrosion protection by conducting polyaniline was first proposed by MacDiarmid⁶³ and was further explored by many research groups on both polyanilines and substituted polyanilines.^64–69 The present co-polydianilines were also investigated for their ability for protection of mild steel surfaces by conducting Tafel polarization experiments in 3.5% NaCl solution. The coatings were applied on the mild steel specimen as described in experimental section to the thickness of about 22 μm. The values of the corrosion potential (E_corr), polarization resistance (R_p), and corrosion current density (I_corr) are given in Table 7. These values were directly obtained from NOVA software from Tafel plots (Fig. 12).

Table 7 Tafel data of co-polydianilines on pure MS and coated MS in 3.5% NaCl

Electrodes	Icorr (μA cm^−2)	Ecorr (V)	Corrosion rate (mm per year)	Polarization resistance (kΩ)
Pure MS	150.24	−1.05	1.75	340.62 × 10^−3
P(OT–DA)–EB/MS	178.55 × 10^−6	−0.63	2.07 × 10^−6	170.4 × 10^3
P(OA–DA)–EB/MS	1.22 × 10^−3	−0.23	1.42 × 10^−5	35 × 10^3
P(DMA–DA)–EB/MS	51.33 × 10^−3	−0.53	5.3 × 10^−4	657.2

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Fig. 12 Tafel plots of (a) pure MS, (b) P(OT–DA)–EB coated MS, (c) P(OA–DA)–EB coated MS, (d) P(DMA–DA)–EB coated MS in 3.5% NaCl solution.

In general the E_corr values shifted to more anodic potentials upon applying polymer coatings to the mild steel (MS) panels, compared to pure MS panel. The corrosion currents also greatly reduced (nano to pico amperes, Table 7) compared to literature⁴⁴ values suggesting excellent performance of polymers as anti-corrosion layers. The highest corrosion protection was offered by P(OT–DA) fallowed by P(OA–DA) and P(DMA–DA). The corrosion rate of P(OT–DA) is decreased to 2.07 × 10⁻⁶ mm per year with polarization resistance (R_p) = 170.4 × 10³ (kΩ). The corrosion rate increased for P(OA–DA) and P(DMA–DA) to 1.42 × 10⁻⁵, 5.33 × 10⁻⁴ mm per year respectively. Polarization resistance also decreased to 35 × 10³ and 657 kΩ.

4. Conclusions

PA doped conducting co-polymers of o-toluedine, o-anisidine and 2,6-dimethylamine with dianiline have been synthesized for the first time to explore into the energy storage and anti-corrosion properties. The undoped polymers showed good solubility in common organic solvents. The doped polyanilines have good thermal stability. The performance and stability of electrochemical capacitor fabricated with PA doped polymers on stainless steel (SS) electrode was studied by constant current (1–5 mA cm⁻² and in between 0 to 1.0 V) charge–discharge experiments in 1 M PA. The capacitor exhibited a specific capacitance of 176.76 F g⁻¹ for P(OT–DA)–PA, 110 F g⁻¹ for P(OA–DA)–PA and 121 F g⁻¹ for P(DMA–DA)–PA. The specific capacitance of the new co-polydianilines, is though lower compared to nanostructured polyaniline (which is usually so), it can still be improved to higher side by optimising the electroactive compound and its thickness on the electrode. The present study used only 1 mg on each electrode with a thickness of 10 μm. The undoped polymers have been studied for their anti-corrosion property in NaCl (3.5% wt) electrolyte on MS surfaces. These polydianilines showed excellent corrosion resistance with corrosion rate 2.07 × 10⁻⁶ for P(OT–DA)–EB, 1.42 × 10⁻⁵ for P(OA–DA)–EB and 5.3 × 10⁻⁴ mm per year for P(DMA–DA)–EB respectively. Over all, the copolymers showed very good charge storage and anti-corrosion properties.

Author contributions

All authors contributed to the writing of the manuscript. All authors approved the final version of the manuscript.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

Authors sincerely acknowledge the financial support from the CSIR-network project INTELCOAT (CSC-0114).

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