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The chemical modification of polyaniline with enhanced properties: A review
Progress in Organic Coatings 126 (2019) 35–43
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
The chemical modification of polyaniline with enhanced properties: A review ⁎
Guangfu Liaoa, , Qing Lib, Zushun Xuc,
T
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a
School of Materials Science and Engineering, PCFM Lab, Sun Yat-sen University, Guangzhou, 510275, China College of Chemistry and Chemical Engineering, Guangxi Key Laboratory of Chemistry and Engineering of Forest Products, Guangxi University for Nationalities, Nanning, 530008, China c Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory for The Green Preparation and Application of Functional Material, Hubei University, Wuhan, Hubei, 430062, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Conductive polymers PANI Doped PANI Substituted derivatives of PANI Enhanced properties
Polyaniline (PANI) has recently received sustained attention due to its outstanding electrical properties, good chemical and environmental stability, simple preparation process, and extensive application in numerous fields such as chemical sensor, corrosion devices, photovoltaic cell, gas separation membranes, etc. However, the main drawback of PANI is poor solubility caused by the rigid backbone. Interestingly, the chemically modified PANI not only shows improved processability, but exhibits better conductive and anticorrosion properties than pure PANI. This review mainly highlights the developments in chemical modifications of PANI with enhanced properties over the last decades, which would be helpful for guiding the rational design of PANI in structure and property in order to further cater the practical demands.
1. Introduction During the last decades, increasing attentions are focused on the development of conductive polymers, which are applied in the fields of optics, electronics, energy, etc. [1–5]. The structural diversity of conductive polymers can be acquired via high-precision molecular designs and suitable preparation methods. Conductive polymers mainly include polyaniline (PANI), polythiophene (PTH), polypyrrole (PPY), and their derivatives. They possess many potential applications including electromagnetic interference shielding, photothermal therapy, rechargeable battery, photovoltaic cell, gas separation membrane, chemical sensor, anticorrosion coating, microwave absorption, etc [6–8]. In addition, conductive polymers can be also used as promising conducting fillers in insulating polymer substrates to obtain conducting polymer composites [9–13]. These composites possess potential applications in electromagnetic interference shields, electronic equipments, display devices, electrodes, etc [14–17]. Although conductive polymers, especially PANI shows many unique advantages and possess many applications as mentioned above, PANI also has many disadvantages. The main drawback of PANI is poor solubility caused by the rigid backbone [18,19]. To improve its processability, various methods have been tried, and two significant attempts to overcome these drawbacks are chemical modification such as doped
⁎
PANI and substituted derivatives of PANI, respectively. The chemically modified PANI not only shows improved processability, but exhibit better conductivity property and anticorrosion property than pure PANI. This review mainly highlights the developments in chemical modifications of PANI with enhanced properties during the last decades, which will act as a strategic guide to build a close connection between chemical modifications of PANI and practical application. 2. A brief introduction of PANI A milestone discovery came out that doped polyacetylene was used as a conductive polymer in 1977 by Shirakawa et al [20]. They discovered that only a few conducting polymers were stable enough under conventional processing conditions. As a leading candidate, PANI possessed many unique features when compared to other conducting polymers such as polythiophene, polypyrrole, polyphenylene, polyphenylenevinylene, polydiacetylene, etc. As well know, PANI has good thermostability and can be easily synthesized by chemical and electrochemical methods in various organic solvents or even in aqueous media [21]. Moreover, PANI also has achieved increasing attentions because of its low price, good environmental stability, excellent optical and electrical properties, good anticorrosion property, which ensures its wide use in commercial and technological aspects [22], such as
Corresponding authors. E-mail addresses: [email protected] (G. Liao), [email protected] (Z. Xu).
https://doi.org/10.1016/j.porgcoat.2018.10.018 Received 6 August 2018; Received in revised form 16 September 2018; Accepted 18 October 2018 0300-9440/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Three different types of basic PANI (0 ≤ x ≤ 1).
achieving the purpose of modification.
secondary batteries [23], electromagnetic interference shielding [24], solar cells [25], biology or chemical sensors [26], corrosion devices [27], organic light emitting diodes [28], and electrorheological materials [29]. MacDiarmid firstly proposed three different types of basic PANI in 1997 [30], including fully reduced leucoemeraldine (LEB) (x = 0), half oxidized emeraldine (EB) (x = 0.5), and fully oxidized pernigraniline (PAB) (x = 1) (Fig. 1). Among them, EB can be transformed into conductor by doping via protonic acid, but LEB and PAB cannot, so the most widely studied PANI type is the EB. PANI and its derivatives, can be converted to conductive materials from their insulating states through doping techniques, such as chemical doping by charge transfer groups, electrochemical doping by protonic acid, photodoping by charge injection at a metal/semiconducting polymer interface [31–33]. However, as with other π-conjugated polymers, the application of neat PANI is strictly limited because of its poor solubility and processability [34]. The large rigidity of PANI molecular backbone and the existence of strongly conjugated π electron system result in its poor solubility. In order to improve its solubility and processability, two methods have been tried. Firstly, the functionalized protonic acids were used in the protonation of PANI. Secondly, the use of substituted derivatives of PANI to improve their properties.
3.1.1. Modification of PANI by doping via protonic organic acids The solubility and conductivity of PANI can be greatly enhanced by doping via protonic organic acids [35]. Three reasons can possibly explain this phenomenon. First, the protonic organic acids doped into the molecular chain of PANI act as surfactant, which can improve its solubility. Second, functional groups of the protonic organic acids further improve its solubility. Third, when the protonic organic acids are doped into the PANI molecular chain, it is beneficial to the ionization of charge and meanwhile increases its conductivity. These protonic organic acids commonly contain long alkyl side chains such as camphorsulphonic acid (CSA), dodecylbenzenesulfonic acid (DBSA), p-toluenesulfonic acid (p-TSA), phytic acid (PA), carboxylic acid, acetic acid, oxalic acid, etc. Since early 2003, Viswanath’s group [36] found that a series of the properties of chemically synthesized PANI could be changed by adjusting the protonation media (acetic, citric, oxalic, and tartaric acid). In order to obtain a PANI with high conductivity, oxalic acid was found to be one of the most suitable protonic acid media among these organic acids. SaCak et al. [37] prepared oxalic acid doped PANI by electrochemical and chemical oxidation. The results showed that oxalic acid doped PANI owned good solubility in DMSO and DMF, and the polymerization rate of aniline in oxalic acid was much slower than that in H2SO4. Morgan’s group [38] reported that the thermal doping of EB form of PANI was obtained by using two different sulfonic acids including DBSA and p-TSA. They were surprised to found that EB underwent thermally induced doping with DBSA and p-TSA depending on the initial quantity of the dopant and the doping temperatures, exhibiting a growing trend with an increasing proportion of the both sulfonic acids. Interestingly, the solubility and conductivity was greatly improved when compared to neat PANI. Wu et al. [39] studied the structures, morphologies and thermoelectric properties of the obtained PANI nanostructures by doping different acids including HCl, AcOH and PSA. The prepared pTSA doped PANI nanowires showed excellent TE properties, which was four times higher than that of HCl doped PANI. This was mainly because doped bulky anions made PANI molecule backbones more regular. Moreover, the conductivity of p-TSA doped PANI was also best. AcOH doped PANI showed poorest property because of the weak doping ability of AcOH.
3. Modified methods 3.1. Modification of PANI by doping via protonic acids The emeraldine base (EB) PANI is able to dissolve in some polar organic solvents, such as N-methylpyrrolidone (NMP) and dimethyl sulfoxide (DMSO), but it is insulating. In contrast, the emeraldine salts (ES) form is conductive but commonly insoluble and non-processable. An efficient and feasible method to improve its solubility is that EB can be doped by protonic acids containing proton organic acids and proton inorganic acids. The doping mechanism of PANI by protonic acids is shown in Fig. 2. First, the protonation reaction takes place on the N atoms of quinine imide unit. Next, phenylenediamine radical cation is formed by intramolecular change transfer. The charge of N atoms is delocalized to the neighboring benzene ring and N atoms by conjugation. Thus, the transmission of the π electron is not limited between two C atom, but in the whole system, which makes the chemical environment of N atoms in the molecular chain show homogenization, and thus
Fig. 2. Doping mechanism of PANI by protonic acids. 36
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Phytic acid (PA), a cyclic acidic molecule which is saturated with six dihydrogen phosphate, can provide a strong binding affinity to PANI [40,41]. Moreover, PA molecules with higher phosphorus content show higher flame retardancy, better adsorption affinity and stronger doping ability. For these reasons, PA doped PANI has received more and more attention in flame retardant and adsorbent. Zhou et al. [42] prepared PANI-deposited electrically conductive and flame retardant paper composites by using PA as dopant or co-dopant. PA used as doping acid greatly enhanced the flame retardancy of PANI-deposited paper composites. Moreover, the conductivity was higher when 5-sulfosalicylic acid (SSA) was used as doping acid. In order to prepare PANI-deposited paper composites with both high conductivity and excellent flame retardancy, it is only necessary to increase the concentration of doping acid and decrease the temperature. In fact, the excellent flame retardancy and conductivity of PA-doped paper composites was attributed to the synergetic effect of PANI coating and H3PO4. Kim et al. [43] prepared PA doped PANI nanofibers and HCl doped PANI nanofibers, they found that the PA doped PANI nanofibers showed markedly improved the capacity of Cu2+ adsorption when compared to HCl doped PANI nanofibers, this was mainly attributed to the phosphate functional group with high ionic attraction in doped PA. Carboxylic acid has many advantages such as excellent water solubility, chemical durability and optical properties, moreover, when carboxylic acid doping with PANI, an aromatic core that will interact with PANI through π-π interaction and carboxylic acid functionalities [6,44]. Malik’s group [45] reported the synthesis of PANI doped with benzene 1,2,4,5-tetracarboxylic acid (BTCA). Here, BTCA acted as both a dopant acid and a structure-directing agent to yield nanotubes with high aspect ratio showing more or less uniform diameter. Interestingly, the morphological, spectroscopic and electrical property of PANI nanotubes could be controlled by adjusting the concentration of BTCA. Similarly, they also used another carboxylic acid, perylene tetracarboxylic acid (PTCA), as dopant acid to prepare PANI nanotubes. They found that these tubes were crystalline and fluorescent due to the controlled confinement of dopant molecules in the nanotubes [46]. As mentioned above, the conductivity of PANI is attributed to its emeraldine salt, which is obtained by protonation of its emeraldine base by protonic acids. Commonly, protonic organic acids are used for the above purpose, and the use of dopants and additives to maintain the required acidity provides an alternative method to synthetize its emeraldine salt form. Contractor’s group tried to achieve the protonation by the use of a weak protonic organic acid, named as 6-cyano-2naphthol (6CN2), which was commonly used as a superphotoacid, because its excited state pKa was obviously smaller than its ground state pKa. In this work, 6CN2 acted as a chemical dopant rather than a photodopant [47–49]. Interestingly, the protonation did not occur in NMP solutions, but occurred in solid films. This phenomenon was attributed to as follows (Fig. 3): the carbonyl groups of NMP would act as candidate in the molecule and the hydrogen bonds can be formed between NMP and PANI. The only hydrogen bond donating group in PANI was the NeH moiety. This led to the formation of “blocked structures” as reported in the case of many organic molecules in proton solvents [50], which hindered the approach of 6CN2 to the aniline moieties. In addition, 6CN2 can also hydrogen-bonded to NMP molecules. Thus, there are no proton transfers from 6CN2 to PANI in the ground state of the organic molecule, but the presence of a large concentration of basic nitrogen atoms of PANI in the films, proton abstraction by the polymer from the dopant 6CN2 took place in its ground state.
acid in solid-state synthesis reaction [52]. H4SiW12O40 is one of the most important heteropoly acid and have great potential as doped acid in PANI. Gong et al. [53] described a novel solid-state synthesis method to prepare H4SiW12O40 doped PANI. The conductivity property and fluorescence property of the H4SiW12O40 doped PANI was found to be excellent. Zhang et al. [54] studied the nanostructures of PANI doped with inorganic acids (such as HCl, H2SO4, HBF4, and H3PO4). They found that the morphology, size (150-340 nm), and conductivity (0.1–10 S/ cm) of the obtained PANI nanostructures mainly relied on the dopant structures and the reaction conditions. After that, the spectroscopic, transport, and morphology of PANI doped with inorganic acids such as HCl, H2SO4, HClO4, HNO3, and H3PO4 were also studied [55]. The HClO4 doped PANI showed the folded lamellar structure derived from the fibers. Moreover, a greater fraction of the conducting emeraldine salt phase was formed in HClO4 as a protonic acid media. Interestingly, the thermal stability of H3PO4 doped PANI was found to be improved compared to other acids doped PANI. In addition to mineral acids mentioned above, lewis acids such as SnCl salts of Li+, transition metal salts such EuCl, etc. can be used also as effective dopants. Bajer et al. [56] reported that PANI can be dissolved in nitromethane through complexation with SnCl4. Moreover, the solutions of PANI-SnCl4 complex exhibited UV–vis-NIR spectral features which were obviously different from those of mineral acids doped PANI reported by others. Interestingly, the solid-state PANISnCl4 complex exhibited the stoichiometry of PANI (SnCl4)1.0(CH3NO2)1.0, which meant that both types of PANI nitrogens (imine and amine) participated in the complexation reaction and one solvent molecule was introduced in the polymer matrix. Saprigin et al. [57] reported that LiCl doped the emeraldine base EB form of PANI and then produced PANI with unique property, doping-induced charge localization. Even at the maximum doping level, LiCl doped PANI was also a strongly localized charged insulator due to their low DC conductivity, low dielectric constant, rapid decrease of the oscillator strength to zero in the infrared, low magnetic susceptibility dominated by a weak Curie component, broad electron paramagnetic resonance linewidth and predominantly Gaussian lineshape of the electron paramagnetic resonance signal. Dimitriev et al. [58] prepared a novel PANI:EuCl3 complex and studied its conductivity, electronic absorption and luminescent spectra. The electronic absorption spectra indicated that the Eu3+ ions possessed both oxidative and pseudoprotonation activity in solution and solid films. Photoluminescence spectra indicated that the charge was easy to transfer from the polymer to Eu3+ ions and there are two kinds of Eu luminescent centers in the disordered polymer matrix. Electrical conductivity of the composite films was unconventionally high, compared to that of the protonic acid doped PANI and was directly related to the stoichiometric ratio of PANI to the Eu salt.
3.1.2. Modification of PANI by doping via protonic inorganic acids Because the size of protonic organic acids is slightly large, causing their diffusion rate slows down. Therefore, more and more researchers turn their attention to protonic inorganic acids due to its size is relatively small, which facilitate its diffusion. Heteropoly acid, as a high intensity proton acid, can afford protons in the preparation process of PANI, and can also be taken as a solid state
3.2.1. Substituted derivatives of PANI on benzene rings The introduction of substituents on the benzene ring of PANI can reduce chain rigidity and interchain force, and thus improving its solubility. Moreover, this can also prevent some side reactions, which is conducive to the formation of the whole macromolecular conjugated system. At present, the most widely studied substituents are electron donating substituent and hydrophilic substituent.
3.2. Substituted derivatives of PANI The substituted derivatives of PANI are prepared by introducing substituted groups on PANI structure, which can improve its mechanical property and processability, has received more and more attention in many fields. The substituted derivatives of PANI can be classified into two categories (substituted derivatives on benzene rings and on amino N) according to their substitutional positions.
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Fig. 3. Proposed formation mechanism of 6CN2 doped PANI. Reprinted with permission from [51].
sodium dodecyl sulfate (SDS) were respectively acted as oxidants and surfactants to controlthe size and shape of POA (Fig. 4A). The electrical conductivity was from 0.022 to 168 S/cm, a variation of 4 orders of magnitude, depending on the POA nanoparticle size and SDS concentration (Fig. 4B). Butoi et al. [64] prepared PANI and POA in a direct current glow discharge plasma. They found POA showed better solubility in various organic solvents. Moreover, the electrical conductivity of PANI was higher than that of POA in a 285–315 °C temperature range. On the contrary, the electrical conductivity of POA layers increased to higher values than that of PANI in the 315–395 °C temperature range (Fig. 5).
3.2.1.1. Electron donating substituted derivatives of PANI. When the hydrogen on the benzene ring is replaced by electron donating groups (such as methyl, methoxy, ethoxy, amino, etc), the steric hindrance between the molecular chains of the polymer becomes larger which weaken the intermolecular force, and thus improving its solubility. Ma et al. [59,60] synthesized poly(2,3-dimethylaniline) (P(2,3-DMA)) by a rapid emulsion polymerization from ammonium peroxydisulfate (APS) oxidant, ammonium peroxydisulfate and ferrous ion (APS/Fe2+) composite oxidants. The synthesized P(2,3-DMA) showed better anticorrosion, electrochemical and crystal property than pure PANI. Moreover, the P(2,3-DMA) prepared by APS/Fe2+ composite oxidants possessed higher yield, better electrochemical and crystal properties than that of prepared by APS oxidant. As early as 1992, Sathiyanarayanan et al. [61] prepared a novel polymer-ortho substituted poly ethoxy aniline (POEA) via electrochemical and chemical methods. They found that the prepared POEA showed excellent solubility and anticorrosion property, exhibiting a potential application in the fields of anticorrosion coating. Patil et al. [62] synthesized poly(o-anisidine) (POA) coatings on copper through electrochemical polymerization of o-anisidine. They found that POA coated Cu was 100 times lower than bare Cu about corrosion rate. In spite of POA possessed excellent anticorrosion property, it still showed corresponding low electrical conductivity. In order to improve its electrical conductivity, Khamngoen et al. [63] synthesized POA nanoparticles by using an anion as a dopant. POA nanoparticles were prepared by chemical oxidation polymerization. Thereinto, APS and
3.2.1.2. Hydrophilic substituted derivatives of PANI. Through introducing hydrophilic groups (such as sulfonic group, carboxyl group, and hydroxyl group) on the benzene ring of PANI, the obtained PANI derivatives showed good water solubility. These groups will be doped with imine N in the main chain, such as sulfonic group can form five membered ring or six membered ring in the molecular chain, and thus improving its thermal stability [65,66]. PANI with sulfonic acid side groups (SPAN) has achieved increasing attentions because of their self-doping nature that leads to high electrical conductivity and excellent electrical and optical properties [67,68]. The earliest discovery of sulfonic group substituted PANI (SPAN) was reported Yue [69], they found SPAN showed a huge advantage in water solubility. This high water solubility ensured it not only can be conveniently deposited on various substrates, but its self38
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Fig. 4. (A) Proposed polymerization sequences of POA: (a) o-anisidine as monomer is oxidized by APS to a cation radical; (b) o-anisidine cation radicals form dimers that subsequently get deprotonated; and (c) POA polymer is doped where dodecyl sulfate ion acts as counter ion. (B) Electrical conductivity and fiber diameter of POA at various mole ratios of SDS:o-anisidine monomer. Reprinted with permission from [63].
semiconductors thermoelectric materials. Yamaguchi et al. [73] prepared sodium salts of poly(aniline-5-sulfonic acid) (PASNa) via the reaction of PAS with NaOH in water. The PAS was prepared by electrochemical oxidation, in which the two oxidation peaks shifted to a lower potential with the increasing of the degree of sodium sulfonation. The electrochemical reaction was achieved by electrochromism. The Seebeck coefficients of the PASNa increased with the increasing of the degree of sodium sulfonation of the polymers. As early as 1994, PANI films were successfully prepared, which were electrochemically active in neutral aqueous solutions. In addition, the electropolymerization of three substituted anilines (anthranilic acid, m-aminobenzoic acid and m-aminobenzenesulphonic acid) was also studied. Among these aniline derivatives, a meta position and a carboxyl group was usually more beneficial to develop the optimal selfdoped PANI [74]. There are several advantages for carboxyl group substitution on the conjugated backbone of PANI as follows [75]. Firstly, the presence of carboxyl group can evidently improve polymers’ solubility, which promotes the practical application of this kind of polymer because PANI is typically inherently insoluble and intractable [76]. Secondly, PANI can form an electron-deficient conjugated polymer due to the electron-withdrawing effect of carboxyl group. This kind of polymer has many unique features, such as high electron affinity and low-lying conduction bands, which make it a potential candidate for n-type electrical conductors [77]. Thirdly, PANI can be prepared by electropolymerization of monomers with carboxyl groups, which provide higher electrode stability and reproducibility when
doping function speeded up the process of doping and dedoping. Thus, this kind of SPAN can cater the practical demands and have potential use in many fields. SPAN not only possessed good water solubility, but its anticorrosion property is also worth discussed. Baldissera et al. [70] reported a series of epoxy resin (EP)-based coating system, in which PANI acted as an anticorrosive agent. The corrosion propertis of mild steel samples coated with an EP/PANI-EB, EP/PANI-ES, EP/SPAN, EP/ PANI-fibers, was investigated in 3.5% NaCl solution. It was found that the addition of SPAN to the EP resin significantly improved its anticorrosion property. Wei et al. [71] prepared ring-sulfonated SPAN with a higher degree of sulfonation. The degree was up to 75% and threefourth of aniline monomer units in the polymer backbone contained sulfonate groups. This has been obtained by treatment of the reduced leucoemeraldine base form of PANI with fuming sulfuric acid. The obtained product possessed about 1.5-fold increased solubility in aqueous alkaline solution. The conductivity of 75% sulfonated SPAN was found to be about one order of magnitude higher (1 S/cm) when compared with that of 50% sulfonated SPAN. In addition, the sulfonic acid proton in SPAN plays a key role in selfdoping. Therefore, the substitution of other ions, such as those of alkaline metals for the sulfonic acid proton in SPAN, can regulate the selfdoping level in SPAN. Yamamoto illustrated that the degree of lithium sulfonation in SPAN mainly depended on the loading rate of LiOH to SPAN [72]. Illumination of relationship between the thermoelectric property and the level of doping in alkaline metal salts of SPAN will provide theoretical guidance for the development of novel polymeric 39
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Fig. 5. (A) Solubility of PANI and POA samples. (B) Dependence of PANI and POA samples’ electrical conductivity on temperature. Reprinted with permission from [64].
group [81]. They prerpared a series of N-butyl-alkylated PANI, N-hexylalkylated PANI, N-octyl-alkylated PANI, N-decyl-alkylated PANI, Ndodecyl-alkylated PANI, and N-hexadecyl-alkylated PANI (Fig. 6A) by introduction of flexible alkyl chains onto the PANI through an N-alkylation method. When carbon number in alkyl side chain was six or even more, the polymers was found to be soluble in many organic solvents (such as THF, CH3Cl3, and CH2Cl2). Moreover, the films can be prepared through simple casting from the PANI solutions (Fig. 6B) even in the presence of a protonic organic acid as dopant, such as DDBSA and CSA. When the carbon number was up to 12, the obtained PANI films were flexible. DDBSA-doped N-alkylated PANI films which were cast from polymer solutions in CH2Cl2 possessed maximum conductivity ranging from 10−2 to 10−4 S/cm, which was two or even three orders of magnitude higher than those of HClO4-doped poly(N-alkylaniline)s (Fig. 6C). Similarly, to prepare conductive polymers which showed better solubility and stability than neat PANI, some N-alkylanilines were reported by Yano [82]. They used several polymerization solutions, which possessed two different anions (SO42− and ClO4-) and two different organic solvents (acetonitrile and dimethyl sulfoxide). It was found that the obtained SO42− doped poly(N-n-heptylaniline) showed highest conductivity (1.0 × 10-3 S cm-1). The conductivity depends on the alkyl chain length of poly(N-alkylaniline)s. This phenomenon was mainly attributed to the lipophilic affinity of the organic solvents for the poly(N-alkylaniline)s. Pekmez group [83–85] prepared poly(Nmethylaniline) (PNMA) coatings, which were coated on stainless steel alloy by using potentiodynamic, potentiostatic and galvanostatic techniques. It was found that the PNMA coating possessed better anticorrosion property than that of pure PANI coating and polypyrrole (PPY)
compared to that of unsubstituted monomer [78]. Fourthly, carboxyl group functionalization of PANI films show a self-doping nature and the self-doping polymers show an anionic dopant ion covalently attached to the polymer backbone, which force predominant cation movement during the doping and dedoping processes [9]. Finally, carboxylic group functionalized PANI is very suitable to be applied as coatings for “organic” electrodes or neural probes and as scaffolds to induce tissue regeneration, providing many perspectives such as in chemo/biosensors applications, and used as substrates for the immobilization of various biologically active species and inorganic nanoparticles [79]. PANI bearing hydroxyl substituents has recently also been reported. The hydroxyl-functionalized PANI could be prepared via in-situ oxidative polymerization of two novel aniline monomers with hydroxyl groups. This simple route showed a great potential for the synthesis of PANI nanospheres with good solubility and uniform size distribution. This phenomenon was mainly attributed to the hydroxyl functionality, which enhanced solvent-particle interactions and the solubility of the PANI in various solvents i.e. water and DMSO. Moreover, the interaction of hydroxyl functionality also ensured the uniform distribution of PANI nanospheres [80]. 3.2.2. Substituted derivatives of PANI on amino N The substituent derivatives on the amino N are the polymer which is directly connected with the N atoms on the eNHe and eN] groups in the PANI structure. The substitution of N has little effect on the electronic effect of aromatic rings, but it has great effect on polymer property by steric effect. As early as in1998, a very interesting study was reported by Chen 40
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Fig. 6. (A) Structure of N-alkylated PANI. (B) Solubility of N-alkylated PANI in organic solvents. (C) Conductivity of N-alkylated PANI with various acids at room temperature. Reprinted with permission from [81].
PANI and practical application.
coating.
Acknowledgement
4. Conclusions
This paper was supported by the National Natural Science Foundation of China (Grant No. 81372369, 51573039, and 8571734).
PANI possesses many applications including electromagnetic interference shielding, photothermal therapy, rechargeable battery, photovoltaic cell, gas separation membranes, chemical sensor, anticorrosion coating, microwave absorption, etc, these applications uncover its unique property, such as good conductivity, environmental and chemical stability, simple preparation method, improved viscosity under electric field. The conductivity or color will change when exposed to acidic, basic and some neutral vapors or liquids, very high capacitance values, volume will changes at different oxidation states, etc. However, the practical application of PANI has been limited due to their rigid backbone, which results in poor processability. This review has presented two methods to improve the solubility, processability and other properties of PANI including doped PANI and substituted derivatives of PANI. These two methods are very efficient. Moreover, these methods can also improve its other properties such as conductivity property and anticorrosion property. This review mainly highlights the developments in chemical modifications of PANI with enhanced property during the last decades, which will act as a strategic guide to build a close connection between chemical modifications of
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
The chemical modification of polyaniline with enhanced properties: A review ⁎
Guangfu Liaoa, , Qing Lib, Zushun Xuc,
T
⁎
a
School of Materials Science and Engineering, PCFM Lab, Sun Yat-sen University, Guangzhou, 510275, China College of Chemistry and Chemical Engineering, Guangxi Key Laboratory of Chemistry and Engineering of Forest Products, Guangxi University for Nationalities, Nanning, 530008, China c Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry of Education Key Laboratory for The Green Preparation and Application of Functional Material, Hubei University, Wuhan, Hubei, 430062, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Conductive polymers PANI Doped PANI Substituted derivatives of PANI Enhanced properties
Polyaniline (PANI) has recently received sustained attention due to its outstanding electrical properties, good chemical and environmental stability, simple preparation process, and extensive application in numerous fields such as chemical sensor, corrosion devices, photovoltaic cell, gas separation membranes, etc. However, the main drawback of PANI is poor solubility caused by the rigid backbone. Interestingly, the chemically modified PANI not only shows improved processability, but exhibits better conductive and anticorrosion properties than pure PANI. This review mainly highlights the developments in chemical modifications of PANI with enhanced properties over the last decades, which would be helpful for guiding the rational design of PANI in structure and property in order to further cater the practical demands.
1. Introduction During the last decades, increasing attentions are focused on the development of conductive polymers, which are applied in the fields of optics, electronics, energy, etc. [1–5]. The structural diversity of conductive polymers can be acquired via high-precision molecular designs and suitable preparation methods. Conductive polymers mainly include polyaniline (PANI), polythiophene (PTH), polypyrrole (PPY), and their derivatives. They possess many potential applications including electromagnetic interference shielding, photothermal therapy, rechargeable battery, photovoltaic cell, gas separation membrane, chemical sensor, anticorrosion coating, microwave absorption, etc [6–8]. In addition, conductive polymers can be also used as promising conducting fillers in insulating polymer substrates to obtain conducting polymer composites [9–13]. These composites possess potential applications in electromagnetic interference shields, electronic equipments, display devices, electrodes, etc [14–17]. Although conductive polymers, especially PANI shows many unique advantages and possess many applications as mentioned above, PANI also has many disadvantages. The main drawback of PANI is poor solubility caused by the rigid backbone [18,19]. To improve its processability, various methods have been tried, and two significant attempts to overcome these drawbacks are chemical modification such as doped
⁎
PANI and substituted derivatives of PANI, respectively. The chemically modified PANI not only shows improved processability, but exhibit better conductivity property and anticorrosion property than pure PANI. This review mainly highlights the developments in chemical modifications of PANI with enhanced properties during the last decades, which will act as a strategic guide to build a close connection between chemical modifications of PANI and practical application. 2. A brief introduction of PANI A milestone discovery came out that doped polyacetylene was used as a conductive polymer in 1977 by Shirakawa et al [20]. They discovered that only a few conducting polymers were stable enough under conventional processing conditions. As a leading candidate, PANI possessed many unique features when compared to other conducting polymers such as polythiophene, polypyrrole, polyphenylene, polyphenylenevinylene, polydiacetylene, etc. As well know, PANI has good thermostability and can be easily synthesized by chemical and electrochemical methods in various organic solvents or even in aqueous media [21]. Moreover, PANI also has achieved increasing attentions because of its low price, good environmental stability, excellent optical and electrical properties, good anticorrosion property, which ensures its wide use in commercial and technological aspects [22], such as
Corresponding authors. E-mail addresses: [email protected] (G. Liao), [email protected] (Z. Xu).
https://doi.org/10.1016/j.porgcoat.2018.10.018 Received 6 August 2018; Received in revised form 16 September 2018; Accepted 18 October 2018 0300-9440/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Three different types of basic PANI (0 ≤ x ≤ 1).
achieving the purpose of modification.
secondary batteries [23], electromagnetic interference shielding [24], solar cells [25], biology or chemical sensors [26], corrosion devices [27], organic light emitting diodes [28], and electrorheological materials [29]. MacDiarmid firstly proposed three different types of basic PANI in 1997 [30], including fully reduced leucoemeraldine (LEB) (x = 0), half oxidized emeraldine (EB) (x = 0.5), and fully oxidized pernigraniline (PAB) (x = 1) (Fig. 1). Among them, EB can be transformed into conductor by doping via protonic acid, but LEB and PAB cannot, so the most widely studied PANI type is the EB. PANI and its derivatives, can be converted to conductive materials from their insulating states through doping techniques, such as chemical doping by charge transfer groups, electrochemical doping by protonic acid, photodoping by charge injection at a metal/semiconducting polymer interface [31–33]. However, as with other π-conjugated polymers, the application of neat PANI is strictly limited because of its poor solubility and processability [34]. The large rigidity of PANI molecular backbone and the existence of strongly conjugated π electron system result in its poor solubility. In order to improve its solubility and processability, two methods have been tried. Firstly, the functionalized protonic acids were used in the protonation of PANI. Secondly, the use of substituted derivatives of PANI to improve their properties.
3.1.1. Modification of PANI by doping via protonic organic acids The solubility and conductivity of PANI can be greatly enhanced by doping via protonic organic acids [35]. Three reasons can possibly explain this phenomenon. First, the protonic organic acids doped into the molecular chain of PANI act as surfactant, which can improve its solubility. Second, functional groups of the protonic organic acids further improve its solubility. Third, when the protonic organic acids are doped into the PANI molecular chain, it is beneficial to the ionization of charge and meanwhile increases its conductivity. These protonic organic acids commonly contain long alkyl side chains such as camphorsulphonic acid (CSA), dodecylbenzenesulfonic acid (DBSA), p-toluenesulfonic acid (p-TSA), phytic acid (PA), carboxylic acid, acetic acid, oxalic acid, etc. Since early 2003, Viswanath’s group [36] found that a series of the properties of chemically synthesized PANI could be changed by adjusting the protonation media (acetic, citric, oxalic, and tartaric acid). In order to obtain a PANI with high conductivity, oxalic acid was found to be one of the most suitable protonic acid media among these organic acids. SaCak et al. [37] prepared oxalic acid doped PANI by electrochemical and chemical oxidation. The results showed that oxalic acid doped PANI owned good solubility in DMSO and DMF, and the polymerization rate of aniline in oxalic acid was much slower than that in H2SO4. Morgan’s group [38] reported that the thermal doping of EB form of PANI was obtained by using two different sulfonic acids including DBSA and p-TSA. They were surprised to found that EB underwent thermally induced doping with DBSA and p-TSA depending on the initial quantity of the dopant and the doping temperatures, exhibiting a growing trend with an increasing proportion of the both sulfonic acids. Interestingly, the solubility and conductivity was greatly improved when compared to neat PANI. Wu et al. [39] studied the structures, morphologies and thermoelectric properties of the obtained PANI nanostructures by doping different acids including HCl, AcOH and PSA. The prepared pTSA doped PANI nanowires showed excellent TE properties, which was four times higher than that of HCl doped PANI. This was mainly because doped bulky anions made PANI molecule backbones more regular. Moreover, the conductivity of p-TSA doped PANI was also best. AcOH doped PANI showed poorest property because of the weak doping ability of AcOH.
3. Modified methods 3.1. Modification of PANI by doping via protonic acids The emeraldine base (EB) PANI is able to dissolve in some polar organic solvents, such as N-methylpyrrolidone (NMP) and dimethyl sulfoxide (DMSO), but it is insulating. In contrast, the emeraldine salts (ES) form is conductive but commonly insoluble and non-processable. An efficient and feasible method to improve its solubility is that EB can be doped by protonic acids containing proton organic acids and proton inorganic acids. The doping mechanism of PANI by protonic acids is shown in Fig. 2. First, the protonation reaction takes place on the N atoms of quinine imide unit. Next, phenylenediamine radical cation is formed by intramolecular change transfer. The charge of N atoms is delocalized to the neighboring benzene ring and N atoms by conjugation. Thus, the transmission of the π electron is not limited between two C atom, but in the whole system, which makes the chemical environment of N atoms in the molecular chain show homogenization, and thus
Fig. 2. Doping mechanism of PANI by protonic acids. 36
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Phytic acid (PA), a cyclic acidic molecule which is saturated with six dihydrogen phosphate, can provide a strong binding affinity to PANI [40,41]. Moreover, PA molecules with higher phosphorus content show higher flame retardancy, better adsorption affinity and stronger doping ability. For these reasons, PA doped PANI has received more and more attention in flame retardant and adsorbent. Zhou et al. [42] prepared PANI-deposited electrically conductive and flame retardant paper composites by using PA as dopant or co-dopant. PA used as doping acid greatly enhanced the flame retardancy of PANI-deposited paper composites. Moreover, the conductivity was higher when 5-sulfosalicylic acid (SSA) was used as doping acid. In order to prepare PANI-deposited paper composites with both high conductivity and excellent flame retardancy, it is only necessary to increase the concentration of doping acid and decrease the temperature. In fact, the excellent flame retardancy and conductivity of PA-doped paper composites was attributed to the synergetic effect of PANI coating and H3PO4. Kim et al. [43] prepared PA doped PANI nanofibers and HCl doped PANI nanofibers, they found that the PA doped PANI nanofibers showed markedly improved the capacity of Cu2+ adsorption when compared to HCl doped PANI nanofibers, this was mainly attributed to the phosphate functional group with high ionic attraction in doped PA. Carboxylic acid has many advantages such as excellent water solubility, chemical durability and optical properties, moreover, when carboxylic acid doping with PANI, an aromatic core that will interact with PANI through π-π interaction and carboxylic acid functionalities [6,44]. Malik’s group [45] reported the synthesis of PANI doped with benzene 1,2,4,5-tetracarboxylic acid (BTCA). Here, BTCA acted as both a dopant acid and a structure-directing agent to yield nanotubes with high aspect ratio showing more or less uniform diameter. Interestingly, the morphological, spectroscopic and electrical property of PANI nanotubes could be controlled by adjusting the concentration of BTCA. Similarly, they also used another carboxylic acid, perylene tetracarboxylic acid (PTCA), as dopant acid to prepare PANI nanotubes. They found that these tubes were crystalline and fluorescent due to the controlled confinement of dopant molecules in the nanotubes [46]. As mentioned above, the conductivity of PANI is attributed to its emeraldine salt, which is obtained by protonation of its emeraldine base by protonic acids. Commonly, protonic organic acids are used for the above purpose, and the use of dopants and additives to maintain the required acidity provides an alternative method to synthetize its emeraldine salt form. Contractor’s group tried to achieve the protonation by the use of a weak protonic organic acid, named as 6-cyano-2naphthol (6CN2), which was commonly used as a superphotoacid, because its excited state pKa was obviously smaller than its ground state pKa. In this work, 6CN2 acted as a chemical dopant rather than a photodopant [47–49]. Interestingly, the protonation did not occur in NMP solutions, but occurred in solid films. This phenomenon was attributed to as follows (Fig. 3): the carbonyl groups of NMP would act as candidate in the molecule and the hydrogen bonds can be formed between NMP and PANI. The only hydrogen bond donating group in PANI was the NeH moiety. This led to the formation of “blocked structures” as reported in the case of many organic molecules in proton solvents [50], which hindered the approach of 6CN2 to the aniline moieties. In addition, 6CN2 can also hydrogen-bonded to NMP molecules. Thus, there are no proton transfers from 6CN2 to PANI in the ground state of the organic molecule, but the presence of a large concentration of basic nitrogen atoms of PANI in the films, proton abstraction by the polymer from the dopant 6CN2 took place in its ground state.
acid in solid-state synthesis reaction [52]. H4SiW12O40 is one of the most important heteropoly acid and have great potential as doped acid in PANI. Gong et al. [53] described a novel solid-state synthesis method to prepare H4SiW12O40 doped PANI. The conductivity property and fluorescence property of the H4SiW12O40 doped PANI was found to be excellent. Zhang et al. [54] studied the nanostructures of PANI doped with inorganic acids (such as HCl, H2SO4, HBF4, and H3PO4). They found that the morphology, size (150-340 nm), and conductivity (0.1–10 S/ cm) of the obtained PANI nanostructures mainly relied on the dopant structures and the reaction conditions. After that, the spectroscopic, transport, and morphology of PANI doped with inorganic acids such as HCl, H2SO4, HClO4, HNO3, and H3PO4 were also studied [55]. The HClO4 doped PANI showed the folded lamellar structure derived from the fibers. Moreover, a greater fraction of the conducting emeraldine salt phase was formed in HClO4 as a protonic acid media. Interestingly, the thermal stability of H3PO4 doped PANI was found to be improved compared to other acids doped PANI. In addition to mineral acids mentioned above, lewis acids such as SnCl salts of Li+, transition metal salts such EuCl, etc. can be used also as effective dopants. Bajer et al. [56] reported that PANI can be dissolved in nitromethane through complexation with SnCl4. Moreover, the solutions of PANI-SnCl4 complex exhibited UV–vis-NIR spectral features which were obviously different from those of mineral acids doped PANI reported by others. Interestingly, the solid-state PANISnCl4 complex exhibited the stoichiometry of PANI (SnCl4)1.0(CH3NO2)1.0, which meant that both types of PANI nitrogens (imine and amine) participated in the complexation reaction and one solvent molecule was introduced in the polymer matrix. Saprigin et al. [57] reported that LiCl doped the emeraldine base EB form of PANI and then produced PANI with unique property, doping-induced charge localization. Even at the maximum doping level, LiCl doped PANI was also a strongly localized charged insulator due to their low DC conductivity, low dielectric constant, rapid decrease of the oscillator strength to zero in the infrared, low magnetic susceptibility dominated by a weak Curie component, broad electron paramagnetic resonance linewidth and predominantly Gaussian lineshape of the electron paramagnetic resonance signal. Dimitriev et al. [58] prepared a novel PANI:EuCl3 complex and studied its conductivity, electronic absorption and luminescent spectra. The electronic absorption spectra indicated that the Eu3+ ions possessed both oxidative and pseudoprotonation activity in solution and solid films. Photoluminescence spectra indicated that the charge was easy to transfer from the polymer to Eu3+ ions and there are two kinds of Eu luminescent centers in the disordered polymer matrix. Electrical conductivity of the composite films was unconventionally high, compared to that of the protonic acid doped PANI and was directly related to the stoichiometric ratio of PANI to the Eu salt.
3.1.2. Modification of PANI by doping via protonic inorganic acids Because the size of protonic organic acids is slightly large, causing their diffusion rate slows down. Therefore, more and more researchers turn their attention to protonic inorganic acids due to its size is relatively small, which facilitate its diffusion. Heteropoly acid, as a high intensity proton acid, can afford protons in the preparation process of PANI, and can also be taken as a solid state
3.2.1. Substituted derivatives of PANI on benzene rings The introduction of substituents on the benzene ring of PANI can reduce chain rigidity and interchain force, and thus improving its solubility. Moreover, this can also prevent some side reactions, which is conducive to the formation of the whole macromolecular conjugated system. At present, the most widely studied substituents are electron donating substituent and hydrophilic substituent.
3.2. Substituted derivatives of PANI The substituted derivatives of PANI are prepared by introducing substituted groups on PANI structure, which can improve its mechanical property and processability, has received more and more attention in many fields. The substituted derivatives of PANI can be classified into two categories (substituted derivatives on benzene rings and on amino N) according to their substitutional positions.
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Fig. 3. Proposed formation mechanism of 6CN2 doped PANI. Reprinted with permission from [51].
sodium dodecyl sulfate (SDS) were respectively acted as oxidants and surfactants to controlthe size and shape of POA (Fig. 4A). The electrical conductivity was from 0.022 to 168 S/cm, a variation of 4 orders of magnitude, depending on the POA nanoparticle size and SDS concentration (Fig. 4B). Butoi et al. [64] prepared PANI and POA in a direct current glow discharge plasma. They found POA showed better solubility in various organic solvents. Moreover, the electrical conductivity of PANI was higher than that of POA in a 285–315 °C temperature range. On the contrary, the electrical conductivity of POA layers increased to higher values than that of PANI in the 315–395 °C temperature range (Fig. 5).
3.2.1.1. Electron donating substituted derivatives of PANI. When the hydrogen on the benzene ring is replaced by electron donating groups (such as methyl, methoxy, ethoxy, amino, etc), the steric hindrance between the molecular chains of the polymer becomes larger which weaken the intermolecular force, and thus improving its solubility. Ma et al. [59,60] synthesized poly(2,3-dimethylaniline) (P(2,3-DMA)) by a rapid emulsion polymerization from ammonium peroxydisulfate (APS) oxidant, ammonium peroxydisulfate and ferrous ion (APS/Fe2+) composite oxidants. The synthesized P(2,3-DMA) showed better anticorrosion, electrochemical and crystal property than pure PANI. Moreover, the P(2,3-DMA) prepared by APS/Fe2+ composite oxidants possessed higher yield, better electrochemical and crystal properties than that of prepared by APS oxidant. As early as 1992, Sathiyanarayanan et al. [61] prepared a novel polymer-ortho substituted poly ethoxy aniline (POEA) via electrochemical and chemical methods. They found that the prepared POEA showed excellent solubility and anticorrosion property, exhibiting a potential application in the fields of anticorrosion coating. Patil et al. [62] synthesized poly(o-anisidine) (POA) coatings on copper through electrochemical polymerization of o-anisidine. They found that POA coated Cu was 100 times lower than bare Cu about corrosion rate. In spite of POA possessed excellent anticorrosion property, it still showed corresponding low electrical conductivity. In order to improve its electrical conductivity, Khamngoen et al. [63] synthesized POA nanoparticles by using an anion as a dopant. POA nanoparticles were prepared by chemical oxidation polymerization. Thereinto, APS and
3.2.1.2. Hydrophilic substituted derivatives of PANI. Through introducing hydrophilic groups (such as sulfonic group, carboxyl group, and hydroxyl group) on the benzene ring of PANI, the obtained PANI derivatives showed good water solubility. These groups will be doped with imine N in the main chain, such as sulfonic group can form five membered ring or six membered ring in the molecular chain, and thus improving its thermal stability [65,66]. PANI with sulfonic acid side groups (SPAN) has achieved increasing attentions because of their self-doping nature that leads to high electrical conductivity and excellent electrical and optical properties [67,68]. The earliest discovery of sulfonic group substituted PANI (SPAN) was reported Yue [69], they found SPAN showed a huge advantage in water solubility. This high water solubility ensured it not only can be conveniently deposited on various substrates, but its self38
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Fig. 4. (A) Proposed polymerization sequences of POA: (a) o-anisidine as monomer is oxidized by APS to a cation radical; (b) o-anisidine cation radicals form dimers that subsequently get deprotonated; and (c) POA polymer is doped where dodecyl sulfate ion acts as counter ion. (B) Electrical conductivity and fiber diameter of POA at various mole ratios of SDS:o-anisidine monomer. Reprinted with permission from [63].
semiconductors thermoelectric materials. Yamaguchi et al. [73] prepared sodium salts of poly(aniline-5-sulfonic acid) (PASNa) via the reaction of PAS with NaOH in water. The PAS was prepared by electrochemical oxidation, in which the two oxidation peaks shifted to a lower potential with the increasing of the degree of sodium sulfonation. The electrochemical reaction was achieved by electrochromism. The Seebeck coefficients of the PASNa increased with the increasing of the degree of sodium sulfonation of the polymers. As early as 1994, PANI films were successfully prepared, which were electrochemically active in neutral aqueous solutions. In addition, the electropolymerization of three substituted anilines (anthranilic acid, m-aminobenzoic acid and m-aminobenzenesulphonic acid) was also studied. Among these aniline derivatives, a meta position and a carboxyl group was usually more beneficial to develop the optimal selfdoped PANI [74]. There are several advantages for carboxyl group substitution on the conjugated backbone of PANI as follows [75]. Firstly, the presence of carboxyl group can evidently improve polymers’ solubility, which promotes the practical application of this kind of polymer because PANI is typically inherently insoluble and intractable [76]. Secondly, PANI can form an electron-deficient conjugated polymer due to the electron-withdrawing effect of carboxyl group. This kind of polymer has many unique features, such as high electron affinity and low-lying conduction bands, which make it a potential candidate for n-type electrical conductors [77]. Thirdly, PANI can be prepared by electropolymerization of monomers with carboxyl groups, which provide higher electrode stability and reproducibility when
doping function speeded up the process of doping and dedoping. Thus, this kind of SPAN can cater the practical demands and have potential use in many fields. SPAN not only possessed good water solubility, but its anticorrosion property is also worth discussed. Baldissera et al. [70] reported a series of epoxy resin (EP)-based coating system, in which PANI acted as an anticorrosive agent. The corrosion propertis of mild steel samples coated with an EP/PANI-EB, EP/PANI-ES, EP/SPAN, EP/ PANI-fibers, was investigated in 3.5% NaCl solution. It was found that the addition of SPAN to the EP resin significantly improved its anticorrosion property. Wei et al. [71] prepared ring-sulfonated SPAN with a higher degree of sulfonation. The degree was up to 75% and threefourth of aniline monomer units in the polymer backbone contained sulfonate groups. This has been obtained by treatment of the reduced leucoemeraldine base form of PANI with fuming sulfuric acid. The obtained product possessed about 1.5-fold increased solubility in aqueous alkaline solution. The conductivity of 75% sulfonated SPAN was found to be about one order of magnitude higher (1 S/cm) when compared with that of 50% sulfonated SPAN. In addition, the sulfonic acid proton in SPAN plays a key role in selfdoping. Therefore, the substitution of other ions, such as those of alkaline metals for the sulfonic acid proton in SPAN, can regulate the selfdoping level in SPAN. Yamamoto illustrated that the degree of lithium sulfonation in SPAN mainly depended on the loading rate of LiOH to SPAN [72]. Illumination of relationship between the thermoelectric property and the level of doping in alkaline metal salts of SPAN will provide theoretical guidance for the development of novel polymeric 39
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Fig. 5. (A) Solubility of PANI and POA samples. (B) Dependence of PANI and POA samples’ electrical conductivity on temperature. Reprinted with permission from [64].
group [81]. They prerpared a series of N-butyl-alkylated PANI, N-hexylalkylated PANI, N-octyl-alkylated PANI, N-decyl-alkylated PANI, Ndodecyl-alkylated PANI, and N-hexadecyl-alkylated PANI (Fig. 6A) by introduction of flexible alkyl chains onto the PANI through an N-alkylation method. When carbon number in alkyl side chain was six or even more, the polymers was found to be soluble in many organic solvents (such as THF, CH3Cl3, and CH2Cl2). Moreover, the films can be prepared through simple casting from the PANI solutions (Fig. 6B) even in the presence of a protonic organic acid as dopant, such as DDBSA and CSA. When the carbon number was up to 12, the obtained PANI films were flexible. DDBSA-doped N-alkylated PANI films which were cast from polymer solutions in CH2Cl2 possessed maximum conductivity ranging from 10−2 to 10−4 S/cm, which was two or even three orders of magnitude higher than those of HClO4-doped poly(N-alkylaniline)s (Fig. 6C). Similarly, to prepare conductive polymers which showed better solubility and stability than neat PANI, some N-alkylanilines were reported by Yano [82]. They used several polymerization solutions, which possessed two different anions (SO42− and ClO4-) and two different organic solvents (acetonitrile and dimethyl sulfoxide). It was found that the obtained SO42− doped poly(N-n-heptylaniline) showed highest conductivity (1.0 × 10-3 S cm-1). The conductivity depends on the alkyl chain length of poly(N-alkylaniline)s. This phenomenon was mainly attributed to the lipophilic affinity of the organic solvents for the poly(N-alkylaniline)s. Pekmez group [83–85] prepared poly(Nmethylaniline) (PNMA) coatings, which were coated on stainless steel alloy by using potentiodynamic, potentiostatic and galvanostatic techniques. It was found that the PNMA coating possessed better anticorrosion property than that of pure PANI coating and polypyrrole (PPY)
compared to that of unsubstituted monomer [78]. Fourthly, carboxyl group functionalization of PANI films show a self-doping nature and the self-doping polymers show an anionic dopant ion covalently attached to the polymer backbone, which force predominant cation movement during the doping and dedoping processes [9]. Finally, carboxylic group functionalized PANI is very suitable to be applied as coatings for “organic” electrodes or neural probes and as scaffolds to induce tissue regeneration, providing many perspectives such as in chemo/biosensors applications, and used as substrates for the immobilization of various biologically active species and inorganic nanoparticles [79]. PANI bearing hydroxyl substituents has recently also been reported. The hydroxyl-functionalized PANI could be prepared via in-situ oxidative polymerization of two novel aniline monomers with hydroxyl groups. This simple route showed a great potential for the synthesis of PANI nanospheres with good solubility and uniform size distribution. This phenomenon was mainly attributed to the hydroxyl functionality, which enhanced solvent-particle interactions and the solubility of the PANI in various solvents i.e. water and DMSO. Moreover, the interaction of hydroxyl functionality also ensured the uniform distribution of PANI nanospheres [80]. 3.2.2. Substituted derivatives of PANI on amino N The substituent derivatives on the amino N are the polymer which is directly connected with the N atoms on the eNHe and eN] groups in the PANI structure. The substitution of N has little effect on the electronic effect of aromatic rings, but it has great effect on polymer property by steric effect. As early as in1998, a very interesting study was reported by Chen 40
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Fig. 6. (A) Structure of N-alkylated PANI. (B) Solubility of N-alkylated PANI in organic solvents. (C) Conductivity of N-alkylated PANI with various acids at room temperature. Reprinted with permission from [81].
PANI and practical application.
coating.
Acknowledgement
4. Conclusions
This paper was supported by the National Natural Science Foundation of China (Grant No. 81372369, 51573039, and 8571734).
PANI possesses many applications including electromagnetic interference shielding, photothermal therapy, rechargeable battery, photovoltaic cell, gas separation membranes, chemical sensor, anticorrosion coating, microwave absorption, etc, these applications uncover its unique property, such as good conductivity, environmental and chemical stability, simple preparation method, improved viscosity under electric field. The conductivity or color will change when exposed to acidic, basic and some neutral vapors or liquids, very high capacitance values, volume will changes at different oxidation states, etc. However, the practical application of PANI has been limited due to their rigid backbone, which results in poor processability. This review has presented two methods to improve the solubility, processability and other properties of PANI including doped PANI and substituted derivatives of PANI. These two methods are very efficient. Moreover, these methods can also improve its other properties such as conductivity property and anticorrosion property. This review mainly highlights the developments in chemical modifications of PANI with enhanced property during the last decades, which will act as a strategic guide to build a close connection between chemical modifications of
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