Polyaniline thermoset blends and composites

Reactive and Functional Polymers 114 (2017) 86–103

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Reactive and Functional Polymers journal homepage: www.elsevier.com/locate/react

Review

Polyaniline thermoset blends and composites F.X. Perrin ⁎, C. Oueiny Laboratoire MAPIEM EA 4323, SeaTech-Ecole d'ingénieurs, Université de Toulon BP 20132, 83957 La Garde Cedex, France

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Article history: Received 15 November 2016 Received in revised form 15 March 2017 Accepted 19 March 2017 Available online 22 March 2017 Keywords: Thermosetting resin Conductive composites Polyaniline Smart materials Electrical properties

a b s t r a c t The blending of polyaniline (PANI) with insulating polymers is an active area of research which has been driven by the possibility to combine the good mechanical properties and processability of the polymer matrix with low electrical resistance. This review will focus on thermosetting polymer matrix composites or blends. A good dispersion of PANI in the thermoset matrix is essential for the enhanced mechanical and electrical properties of the material. Much effort has been exerted in attempts to improve the compatibility of PANI with thermoset matrices. Attention will be given to describe some of the processing parameters that affect the morphology of PANI thermoset blends and composites. In recent years, there has been renewed interest in PANI thermoset composites with the emergence of multifunctional ternary composites. The different approaches for the design of ternary composites will be reviewed. Additionally, promising applications of PANI thermoset composites in different fields will be described such as electromagnetic shielding and microwave absorption, static electricity dissipation, flame-retardant materials, conductive adhesives, coatings for anticorrosion protection, sensor materials and electro-stimulated drug delivery systems. © 2017 Published by Elsevier B.V.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation methods of binary blends and composites . . . . . . . . . . . . . . . . . . . . . 2.1. Basic aspects of the physical blending method . . . . . . . . . . . . . . . . . . . . . . 2.2. In-situ polymerization (ISP) of aniline method. . . . . . . . . . . . . . . . . . . . . . 2.3. Considerations of the composition of reaction medium and precursors of polymer thermoset 2.3.1. Conductive PANI epoxy composites using amine hardener . . . . . . . . . . . . 2.3.2. Reactive solvent approach. . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3. Organic solvent-free process. . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4. Thermally latent curing agent . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5. PANI as a crosslinking agent . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.6. Dopant as a co-curing agent . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.7. Dopant with a cationic polymerization initiator role . . . . . . . . . . . . . . .

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Abbreviations: DMSO, dimethylsulfoxide; DMF, dimethylformamide; NMP, N-methyl-2-pyrrolidone; DBSA, dodecylbenzenesulfonic acid; DSA, dodecylsulfonic acid; CSA, camphorsulfonic acid; TSA, p-toluene sulfonic acid; PA, phosphoric acid; DPA, n-decylphosphonic acid; DESSA, dialkoxyester of sulfosuccinic acid; DEHSSA, di(2ethylhexyl)sulfosuccinic acid; TA, tartaric acid; CP, conductive polymer; PANI, polyaniline; EB, emeraldine base; ES, emeraldine salt; ACAT, amine-capped aniline trimer; TANI, tetraaniline; POTOE, copoly(o-toluidine/o-ethylaniline); POTOEPTS, copoly(o-toluidine/o-ethylaniline)-p-toluenesulfonate; OAN, oligoaniline; PAPTS, polyaniline-p-toluenesulfonate; EP, epoxy resin; DGEBA, diglycidyl ether of bisphenol A; PU, polyurethane; DPI, dopable polyimide; CFRP, carbon fiber-reinforced polymer; PAMPS, poly(2-acrylamido-2methylpropane sulfonic acid); PAAm, polyacrylamide; DVB, divinylbenzene; EPDM, ethylene propylene diene monomer; NR, natural rubber; PVA, polyvinyl alcohol; PMMA, poly(methyl methacrylate); POE, polyoxyethylene; POA, poly(ortho-anisidine); TETA, triethylenetetramine; DAB-AM-4, N,N,N′,N′-tetrakis(3-aminopropyl)-1,4-butanediamine; TMDA, trimethylhexanediamine; DPA, 4-diphenylamine diazonium; B-DPA, 4-diphenylamine diazonium-modified bentonite; NC, nanocomposite; IPN, interpenetrating polymer network; Semi-IPN, semi-interpenetrating polymer network; HG, hydrogel; ISP, in situ polymerization; SIP, surface initiated polymerization; P, particles; W, wires; G, globules; NP(s), nanoparticle(s); NS, nanospheres; NR, nanorods; FL, flake-like; NF, nanofibers; GF, glass fiber; SCF, short carbon fibers; MWCNT, multiwall carbon nanotube; rGO, reduced graphene oxide; CN(s), cellulose nanowhisker(s); DMA, dynamic mechanical analysis; Tg, glass transition temperature; TCP, cloud point temperature; RL, reflection loss; HRR, heat release rate; MAM, microwave absorbing materials; RAM, radar absorbing materials; LSP, lightning strike protection. ⁎ Corresponding author. E-mail address: [email protected] (F.X. Perrin).

http://dx.doi.org/10.1016/j.reactfunctpolym.2017.03.009 1381-5148/© 2017 Published by Elsevier B.V.

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2.4. Oligoaniline based thermoset composites. . . . . . . . . . . . . . . . . . Preparation methods of ternary systems . . . . . . . . . . . . . . . . . . . . . 3.1. Physical blending . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Surface initiated polymerization method (SIP method) . . . . . . . . . . . 3.3. Thermoresponsive organic composite from sterically stabilized PANI dispersion 3.4. Multi-branched PANI ternary composite . . . . . . . . . . . . . . . . . . 4. Electrical and mechanical properties of PANI thermoset composites . . . . . . . . . 4.1. Thermomechanical properties . . . . . . . . . . . . . . . . . . . . . . . 4.2. Electrical conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Applications of PANI thermoset composites . . . . . . . . . . . . . . . . . . . . 5.1. Microwave or radar absorbing materials (MAM/RAM) . . . . . . . . . . . . 5.2. Lightning damage suppression . . . . . . . . . . . . . . . . . . . . . . 5.3. Conductive adhesives. . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Pressure sensor and chemical sensor applications . . . . . . . . . . . . . . 5.5. Anticorrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6. Flame-retardant conductive composites . . . . . . . . . . . . . . . . . . 5.7. Metal ions removal. . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.

1. Introduction Among the intrinsically conducting polymers, polyaniline is one of the most widely studied because it combines ease of synthesis, low cost, stability, conductivity and a unique protonic doping process. Two major limitations of polyaniline are difficulties regarding its processing since the polymer is insoluble in its doped form in common organic solvents and its mechanical properties are poor. The interest for composites or blends of PANI with common polymers started from the early 1990′s with the discovery of solution processability of polyaniline. The continuously growing interest in the study of PANI polymer blends and composites over the years is driven by the need to replace traditional inorganic conductive fillers (metallic particles, carbon black) and to improve the mechanical properties and processability of PANI. The preparation method and properties of PANI blends and composites with organic polymers have been the subject of an excellent review by Pud et al. in 2003 [1], with a focus on polymer thermoplastic matrices materials. To the best of our knowledge, there is still no review on PANI blends and composites with thermoset polymer matrices. A thermosetting resin is a prepolymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. Curing is generally induced by the action of heat or a suitable radiation (UV, electron beam, etc.). Reactive combinations that cure at room temperature can be used in two-package thermoset systems, in which one package contains a resin with one of the reactive groups and the second package contains the component with the other reactive group or a catalyst for the reaction. The packages are mixed shortly before use. For thermoset coatings, several approaches have been developed to permit cure at room temperature. For example, by using a cross-linking reaction that requires an atmospheric component as a catalyst or reactant (such as oxygen or water vapor) or by using a volatile inhibitor that works when the coating is stored in a closed container but volatilizes after application as a thin film, permitting the cure to proceed. For the preparation of PANI thermoset, polyaniline (or aniline, when in situ polymerization method is used), is always added in the prepolymer stage, i.e. before curing. In other words, curing of the thermoset polymer matrix, is always the last step in the preparation of PANI thermoset composites. There are basically two approaches for the synthesis of PANI thermoset blends (1) blending methods which correspond to the mixing of a previously prepared PANI with the matrix polymer and (2) in-situ polymerization methods which correspond to the chemical in situ polymerization of aniline in the matrix polymer. Homogeneous dispersion of PANI in the polymer matrix is a prerequisite for obtaining a material with high conductivity and low percolation threshold. Using one of the two methods above, many efforts have been made to increase the compatibility of PANI with thermoset matrices.

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Recently, the design of PANI thermoset ternary composites has attracted much attention. The introduction of an organic component or inorganic filler into the thermoset matrix together with PANI can provide matrix with specific properties (magnetic [2,3], thermal sensitivity [4], etc.), decrease the percolation threshold by serving as a bio [5,6] - or hard [7] template or enhance mechanical [8,9], thermal [10] or barrier [11] properties. This review covers the design principles being applied to synthesize binary and ternary PANI thermoset composites. In addition, some of the emerging applications of these materials will be outlined. Chemically crosslinked hydrogels are covalently connected network which swell but do not dissolve in water. Smart conductive hydrogels have received increasing attention in recent years [12–14] but they will not be discussed further as the fundamentals and recent advances in electroconductive hydrogels design and applications have been already reviewed [15–17]. 2. Preparation methods of binary blends and composites 2.1. Basic aspects of the physical blending method PANI decomposes before melting and it is insoluble in common solvents. PANI is thus generally considered an intractable material. Different synthetic strategies are used to assist dispersion of the conductive polymer in the matrix polymer. The most typical and simple method to prepare PANI thermoset is to blend PANI with polymer matrix in solutions. One way to improve the dispersion of PANI into polymer matrix is to include substituents (Nor ring substituents) on the polymer backbone of PANI that make it more soluble and/or sterically hinder interchain interactions. However, the chain torsion generally causes significant decreases in conductivity (b10−5 S/cm for the N-alkylated PANIs [18]) and the ring-substituted PANIs have poor stability towards hydrolysis [19]. This is certainly why only a few papers have been published reporting the preparation of ring-substituted PANI thermoset composites. Copoly(o-toluidine/oethylaniline)-p-toluenesulfonate (POTOEPTS) were incorporated into a photocurable system consisting of poly(urethane-acrylate) resin [20]. The different components were dispersed in DMF. The authors noted that POTOEPTS dissolves in DMF while PANI-TSA does not dissolve to an appreciable degree. The absence of a percolation region for PU loaded with POTOEPTS was explained by the molecular dispersion of PANI into the PU matrix. The literature also reports the formation of poly(ortho-toluidine)/nano-ZnO/epoxy coating but the conductivity values of the composites are not given [11]. Alkoxy derivative of polyaniline, poly(ortho-anisidine), POA have also been recently used for the preparation of PANI thermoset coatings [21]. TA–DBSA doped POA nanofibers with high conductivity (~2.1 S/cm) were synthesized

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Fig. 1. Schematic representation of EP/PANI-DBSA blend (a) near the percolation threshold, (b) with 5–7 wt% PANI-DBSA and (c) with N10 wt% PANI-DBSA [24].

via emulsion polymerization and these NFs were dispersed in epoxy-siloxane coatings by the solution blending method. Counter-ion induced solubilization has been more frequently reported to assist the dispersion of PANI in the polymer matrix. The insertion of anionic surfactant (CSA, DSA, DBSA,…) as a dopant for PANI plasticize the material and increase the compatibility between PANI and the polymer matrix [22,23]. PANI-DBSA/epoxy resin blends prepared by direct mixing of PANI-DBSA with EP resulted in poor dispersion of PANI-DBSA [24]. This was related to the large difference of the solubility parameters for the two components (δ 20.1 and 27.6 J/cm3 for PANI-DBSA and EP, respectively). These blends containing 3 wt% PANI were insulators (conductivity b10−13 S/cm). In contrast, the use of toluene as a solvent for PANI-DBSA in the preparation of the blend resulted in the formation of a fine PANI-DBSA network with a low percolation threshold (around 2.3 wt%) (Fig. 1). With PANI-DBSA content above 2.3 wt%, PANI-DBSA rich regions were observed by TEM and SEM analysis. The atomic force colloidal probe technique can be useful for selecting proper and compatible components while designing PANI based composites [25]. This technique revealed a short-range purely repulsive interaction between hydrophilic PANI (doped with phosphoric acid) surfaces in polyester acrylate resin. This contrasts to the attractive interaction between hydrophobic PANI (doped with decylphosphonic acid) surfaces which results in a higher tendency for aggregation of these particles in the liquid polyester acrylate resin. Thermoplastic polyurethane –based conducting polymers have been reported in many studies (see the review in 2012 by Gurunathan et al. [26] and more recent papers such as [27–31]). In contrast, thermoset PU-PANI blends and composites have been less widely reported. Malmonge et al. [32] prepared conductive PU-PANI by post-doping a film prepared from a NMP dispersion of PU components based on castor oil and PANI EB. The doping efficiency was significantly improved when DMF was used as the doping medium instead of aqueous solution. It was suggested that DMF swells the blend structure which makes easier the dopant ions diffusion. A conductivity as high as 10−2 S/cm was reached by doping in DMF solution with TSA or HCl for a PANI content of about 10%. An alternative blending procedure consists of the mixing of a PU water dispersion with a PANI aqueous dispersion [33]. Crosslinking in the PU matrix was introduced by addition of a small amount of trimethylol propane during the synthesis of the aqueous PU dispersion. 2.2. In-situ polymerization (ISP) of aniline method In-situ polymerization of aniline salt protonated with camphorsulfonic acid, CSA [34] and DBSA [35] within epoxy matrix was employed to prepare EP-PANI composites. Compared with the traditional simple solution blending method by mixing the doped PANI

and epoxy resin, the in situ polymerization method rendered the possibility of better miscibility between PANI and epoxy resin. The maximal electric conductivity of epoxy/PANI·CSA composite was 5 × 10−6 S/cm with 25 wt% PANI while a conductivity of 10−3 S/cm with 12 wt% PANI was achieved in epoxy/PANI·DBSA composites. PANI.DBSA is dispersed in the epoxy matrix in the form of microtubules while PANI·CSA is in the form of particle clusters. To our opinion, the higher conductivity values for epoxy/PANI.DBSA composites could be ascribed to the high aspect ratio of the conducting phase in the epoxy matrix. PANI was also grafted onto radiation crosslinked chitosan films by ISP [36] (Fig. 2). Grafting percentage was found to increase as the aniline concentration increases and decreases with increase in crosslinking density. This was ascribed to the decrease in the penetration of aniline onto the chitosan matrix with increase in crosslinking density.

2.3. Considerations of the composition of reaction medium and precursors of polymer thermoset 2.3.1. Conductive PANI epoxy composites using amine hardener The conventional basic amine hardeners of epoxy resins are generally not used for the preparation of PANI salt/epoxy composites as they induce dedoping of the latter [34]. Consequently, anhydride and Lewis acid such as a boron trifluoride complex are the preferred hardener and catalyst for the preparation of conductive PANI-epoxy composites. The two following examples show that it is still possible to prepare conductive PANI epoxy composites using amine hardener as a curing agent for epoxy. Yang et al. [37] prepared conductive PANI/epoxy composites in two steps. Oligomeric polyaniline in EB form was first blended with epoxy resin and the composite was cured with an aliphatic amine to form interpenetrating network. Then, doping was effected by dipping the cured film in acetic acid solution of p-toluene sulfonic acid, TSA. The fibrillary microstructure of OAN was able to form a continuous network in the epoxy matrix at very low concentration (b1 wt%) and electrical conductivities in the range 10−5–10−2 S/cm were achieved. Epoxy/triethylenetetramine (TETA) systems containing PANI doped with DBSA presented two exotherm peaks in DSC, one at 90 °C due to the cure by TETA as a hardener and one at 236 °C related to PANI.DBSA as the curing agent [38]. To achieve the stoichiometric proportion of the epoxy/mixed hardener, the TETA concentration had to be decreased to compensate for the increase amount of PANI.DBSA. An electrical conductivity of 1.99 × 10−7 S/cm was achieved whereas with a fixed amount of TETA, the maximal conductivity was found to be 3.35 × 10−10 S/cm because the deprotonation occurred to a lower extent in the former case.

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Fig. 2. Grafting of PANI onto crosslinked chitosan [36].

Fig. 3. Schematic illustrations of the two routes for the preparation of PANI-EB/epoxy composites and structure of the aminic hardeners [39].

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2.3.2. Reactive solvent approach The use of organic solvent to dissolve PANI and thermosetting resin facilitates the preparation of PANI conducting composites. However, the use of a solvent complicates the curing process as the solvent has to be removed in a controlled manner. Tiitu et al. [39] proposed a reactive solvent approach where aminic oligomers N,N,N′,N′-tetrakis(3aminopropyl)-1,4-butanediamine (DAB-AM-4) and trimethylhexanediamine (TMDA) are at the same time solvents of the emeraldine base form of PANI and hardeners for DGEBA epoxy resins (Fig. 3). In Route A, aminic hardeners DAB-AM-4 and TMDA efficiently disperse EB. The EB/hardener mixture is subsequently crosslinked by adding the epoxy resin. In Route B, mixing EB first in epoxy resin leads to macroscopic phase separation unless an additional solvent is used. The macroscopically phase separated morphology remains after adding the basic hardener to EB/ epoxy mixture and cross-linking. These results illustrate the importance of the sequence of the addition of reagents to the reaction mixture on the morphology and properties of the resulting PANI thermoset composite. 2.3.3. Organic solvent-free process PANI/epoxy hybrids have been prepared using an absorption-transferring process in which no organic solvent is involved [15]. An epoxy prepolymer cured by aniline monomer (DGEBA-aniline) was added to a freshly prepared PANI NPs aqueous solution with vigorous agitation and heating. The PANI NPs were adsorbed on the surface of epoxy droplets and then transferred into the whole droplet (Fig. 4). It was shown that PANI NPs reacts with the epoxy ring when it is transferred to the epoxy droplet. Jafarzadeh et al. achieved to prepare a homogeneous dispersion of PANI doped with phosphoric acid (PANI-PA) and a high solid polyester acrylate (PEA) resin [40]. PANI-PA was first dispersed in acetone and left in an ultrasonic bath to let the acetone evaporate. The PANI dried from acetone aggregated in loose cauliflower-like structures and was more easily redispersed in liquid PEA resin than PANI dried from any of the other solvents tested (NMP, chloroform or water). 2.3.4. Thermally latent curing agent The decomposition of PANI is generally initiated over ~110 °C, leading to decrease in electrical conductivity. Conductive epoxy-PANI composites can thus be prepared by using a thermally latent reagent initiating the curing reaction below 100 °C. Such composites were prepared by use of N-tert-butyl-5-methylisoxazolium perchlorate as a thermally latent curing reagent [22]. 2.3.5. PANI as a crosslinking agent PANI with its amine functional groups show the characteristics of curing agent of epoxy for the preparation of PANI epoxy composites [41–43]. The covalent bond between PANI nanorods and liquid crystalline epoxy matrix prevented macrophase separation of PANI [41].

One elegant way to prepare PANI and polyurethane blends was to interconnect the two polymers via condensation of a NCO terminated PU prepolymer and the amine groups of PANI EB [44,45] (Fig. 5). Doping was made by adding CSA before curing. The incorporation of only 10% PANI in the networks was able to raise the conductivity by around four orders of magnitude.

2.3.6. Dopant as a co-curing agent Phenol-formaldehyde resin, melamine formaldehyde resin or polyester resin were converted into electrically conductive materials by using a PANI protonated by a protonic acid containing at least one hydroxyl group, such as phenol-4-sulfonic acid. Such a PANI was able to act in the thermoset composition not only as a conductive component, but also as a curing agent, resulting in homogeneous and uniform thermoset compositions [46].

2.3.7. Dopant with a cationic polymerization initiator role A simple yet elegant approach to prepare PANI/crosslinked divinylbenzene (DVB) composite was reported by Kumar et al. by mixing a roll milled [47] or a centrifugally mixed [48] complex of PANI and DBSA with DVB followed by thermal treatment at 120 °C. In this approach, DBSA acts both as a dopant of PANI and a cationic initiator for the polymerization of DVB (Fig. 6). It is noteworthy that DVB can't undergo radical polymerization because PANI acts as a radical scavenger.

2.4. Oligoaniline based thermoset composites A promising alternative to substituted PANI for improved processability is the use of aniline oligomers which possess optical and electronic properties similar to those of PANI. Oligoanilines (OAN) are more soluble than PANI and have the further advantage of being chemically pure materials. An intrinsically dopable polyimide (DPI) membrane containing an amine-capped aniline trimer (ACAT) was prepared through a conventional thermal imidization reaction [49] (Fig. 7). The permeability and permselectivity of the DPI membrane can be tuned by doping with a dopant, the doping behavior being similar to PANI. The phenyl/amine end-capped tetramer, often referred as tetraaniline (TANI). In its emeraldine oxidation state, TANI comprises three benzenoid and one quinoid unit and, thus represents the repeating unit of PANI-EB (Fig. 8) [50]. Dialkoxyester of sulfosuccinic acid was used as a dopant of TANI which is known to increase solution processability of TANI. Above the percolation threshold, doped TANI-based epoxy blends show at least three orders of magnitude higher resistivities than their doped PANI analogues. They are however technologically interesting because they are not very sensitive to the processing/curing conditions and show lower percolation thresholds.

Fig. 4. Schematic representation of the absorption-transferring process [61].

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Fig. 5. Chemical structure of (a) PU model compound (prepared by end capping the NCO terminated PU with isopropanol) and (b) PU/PANI network [44].

3. Preparation methods of ternary systems Recent years witnessed a number of important reports on the preparation of ternary PANI thermoset composites incorporating an organic or inorganic third component. A brief description of some of the most important methods is discussed below.

enhancement of their electrical and mechanical properties [8]. Functionally graded materials were effectively manufactured by the centrifugation method. The local SCF concentration can be tailored by cautious selection of the centrifugation parameters such as the rotation speed. 3.2. Surface initiated polymerization method (SIP method)

3.1. Physical blending The most direct approach to prepare PANI based ternary composites is to mix the three different components efficiently. A solvent is generally used to aid mixing and ultrasonic agitation is frequently used to ensure a good dispersion of the three components. In that way, short carbon fibers (SCF) were added to epoxy/PANI blends leading to the

Magnetic epoxy nanocomposites reinforced with PANI functionalized magnetite (f-Fe3O4) nanoparticles (NPs) were prepared using the surface initiated polymerization method [2]. It was shown that the amine functional groups of PANI in the f-Fe3O4 NPs can copolymerize with the epoxy resin to form the polymer nanocomposite (Fig. 9). DMA results showed that the polymer chains have become stiffer after

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Fig. 6. Cationic polymerization of DVB initiated by DBSA [47].

adding the functionalized Fe3O4 NPs and the Tg had shifted to a higher temperature (~ 5–7 °C) compared with that of cured pure epoxy. Epoxy [3,10,11] and polyurethane/epoxy graft IPN [51] nanocomposites loaded with polyaniline-functionalized silica [10], barium ferrite [3], zinc oxide, ZnO [11] and carbonyl iron [51] nanoparticles have also been prepared by surface initiated polymerization method. The interactions between amine groups of the PANI functionalized silica NPs and the epoxy resin result in enhanced mechanical properties (tensile strength) in the nanocomposite compared to the cured pure epoxy [10]. Epoxy composites using clay/PANI as nanofillers have been recently prepared [9]. The “hairy” bentonite nanofillers were prepared through the in situ polymerization of aniline in the presence of 4-diphenylamine diazonium (DPA)-modified bentonite (B-DPA) resulting in a highly exfoliated bentonite-DPA/polyaniline (B-DPA/PANI) (Fig. 10). The exfoliated clay has NH groups from either DPA or PANI which react with epoxy by ring opening therefore ensuring covalent attachment of the resin to the clay sheets. The incorporation of a small amount of BDPA/PANI nanofiller (0.1 and 0.5 wt%) resulted in a dramatic improvement (210–220%) in fracture toughness and in a significant increase in the dielectric constant (from 7.3 to 14.3 by introduction of 0.5 wt%). The authors did not report results about the electrical conductivity of the ternary composites. However, it can be assumed that it must be relatively low since an amine curing agent (4,4′-diaminodiphenylsulfone) was used to crosslink epoxy. The above examples report functionalization of inorganic particles by PANI. Non-soluble organic particles or fibers (coconut fibers [52]) can also be PANI-functionalized by the surface initiated polymerization method. Wu et al. recently prepared PANI-cellulose nanowhiskers (CNs)/natural rubber (NR) nanocomposites with 3D hierarchical multiscale structure [5]. PANI was synthesized in situ on the surface of CNs biotemplate to form PANI-CNs nanohybrids with good dispersity. The NR latex was introduced into PANI-CN suspension. During the subsequent coagulation process, PANI-CN nanohybrid was selectively located in the interstitial space between the NR latex microspheres and formed a 3D hierarchical multiscale conductive structure. The electrical conductivity of PANI-CN/NR composite containing 5 phr PANI showed

11 orders of magnitude higher than that of the PANI/NR at the same loading fraction and a lower percolation threshold (3.6 vol% against 8.0 vol%). An epoxy – silane resin was loaded with PANI-functionalized CNs to obtain nanocomposite coatings with a good dispersion of PANICN filler [6]. 3.3. Thermoresponsive organic composite from sterically stabilized PANI dispersion Thermoresponsive PANI NPs were used as additives to improve the anticorrosion performance of a waterborne epoxy coating [4]. Stable dispersions of PANI-NPs were prepared by oxidative polymerization of aniline in water with thermoresponsive poly(vinyl alcohol) (PVA) conjugated with 2-isobutyramidopropanoate moieties (PVA-AI) as the dispersing agent (Fig. 11). The PANI NPs/PVA-Al epoxy composite films showed much better protection efficiency in NaCl aggressive solution than the PANI NPs/PVA epoxy films. This was explained by the thermoresponsive properties of the PANI NPs/PVA-Al additives. These additives are hydrophilic below the cloud point temperature, Tcp, but become hydrophobic when the temperature is above Tcp or in the presence of an inorganic salt, such as NaCl which dehydrates the hydration waters around PVA-Al. This salt-triggered hydrophobic effect would restrict water molecules from accessing the interaction sites of PVA-AI and the metal surface. That would enhance the water-resisting ability of the epoxy coating. 3.4. Multi-branched PANI ternary composite Multi-branched PANI/MWCNT/epoxy were prepared in three successive steps [7]: first, the multi-branched polyaniline was prepared by (i) synthesis of substituted PANI, (ii) ring-open reaction between PANI and 3-glycidoxypropyltrimethoxysilane and (iii) controlled hydrolysis of the PANI-containing silane PA-K to give MSiPA; second, the PANI/MWCNT composite was prepared by simply blending the two components suspended in DMSO (Fig. 12); lastly, the ternary composite was obtained by blending PANI/MWCNT with the epoxy resin. The

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Fig. 7. Synthesis of ACAT and dopable polyimide [49].

addition of PANI/MWCNT hybrid with a small ratio of PANI into epoxy resin can increase the dielectric constant and decrease the dielectric loss which opens the opportunity for high-k dielectric materials applications. The biggest advantage of these composites is the extremely low percolation threshold (0.51%) which is attributed to the unique multibranched structure of PANI.

4. Electrical and mechanical properties of PANI thermoset composites 4.1. Thermomechanical properties

Fig. 8. Structure of TANI and of the protonating agent: dialkoxyester of sulfosuccinic acid [50].

Almost all literature survey reports a decrease of glass transition temperature, Tg, of the epoxy network with the addition of PANI. For example, decreased Tg values were noted with PANI-DBSA/epoxy resin blends prepared by direct mixing which suggested that PANI-DBSA interfere with the curing of EP [24]. The influence of PANI salt on both the thermal and mechanical properties of the blends is negligible for PANI contents up to 5–7 wt% [37]. For higher amounts of PANI salt, a decrease in flexural modulus and flexural strength has been noticed

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Fig. 9. Proposed formation mechanism for the Fe3O4-PANI-epoxy ternary composite [2].

together with a significant reduction of Tg, indicating that PANI salt has a significant influence on the quality of the crosslinked network. A singular result was obtained by Soares et al. [35] with PANI (2 wt%)/epoxy composites prepared by in situ polymerization of aniline: the Tg of the PANI/epoxy composite (~ 142 °C) was significantly shifted towards higher values compared to the Tg of the neat epoxy (~ 107 °C). While the mechanical properties are generally

determined by destructive methods, it was recently demonstrated that the non-destructive ultrasonic pulse-echo-overlap method can be useful for determining the elastic properties of epoxy/PANI composites [53,54]. A small increase in micro-hardness and elastic moduli was observed at low PANI loadings (5 wt%) compared to the pure epoxy but for higher PANI loadings a decrease in mechanical properties occurred [53].

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Fig. 10. Preparation of clay/PANI nanofillers with and without diazonium cation intercalation step [9].

4.2. Electrical conductivity An overview of the literature on the electrical properties of PANI thermoset composites indicates that many parameters are responsible for the properties of the composite, such as the dopant of PANI, the chemical structure of the matrix and the mixing procedure (Table 1). PANI salts as conductive additives exhibit smoother percolation curves compared to the abrupt changes observed with traditional fillers, such as carbon fibers or carbon black. Thus, there is the possibility with PANI to tailor the electrical conductivity in the antistatic and charge dissipative applications range (10−2 to 10−9 S/cm). Another advantage of PANI salts compared to traditional fillers is their low density which allows the preparation of conductive polymer blends with low density.

Fig. 11. Structure of the thermoresponsive dispersing agent PVA-Al prepared through a one-step esterification reaction between some of the hydroxyl groups of PVA and the isobutyryl chloride modified alanine [4].

PANI-paste exhibits lower percolation threshold in epoxy resin than PANI-powder (40 wt% against 14 wt% of PANI) [55,56]. This is mainly due to the existence of excess DBSA in the paste, which acts as a plasticizer, thus much better dispersion is obtained. Lower percolation threshold, based on PANI content, can be achieved by using inorganic particles encapsulated by PANI as a filler (around 6 wt% in the epoxy/ PANI-mica paste composite [55] and around 16 wt% in the epoxy/ PANI-glass fiber composite [56]. The low percolation threshold is due to the core-shell morphology of the filler, which results in a continuous network of PANI shells. By comparing the electrical conductivity of epoxy composites containing PANIs with different morphologies (particles, fibers and wires), Jia et al. [57] proved that PANI with high aspect ratio is more efficient to achieve high electrical conductivity with low percolation threshold. The same effect of aspect ratio of PANI was obtained more recently by Oyharçabal et al. [58] and by Zhang et al. [59]. The results by Tsotra et al. [60] suggest that the mixing procedure affect the quality of the PANI dispersion in the thermoset matrix, and, therefore, the electrical properties of the developed polymer composite. PANI-DBSA was first dispersed in toluene and treated in ultrasonic bath to form a PANI solution which was then mixed with the epoxy resin in a dissolver. The developed composite showed a low percolation threshold (about 2.5 wt%). A percolation threshold well below 1% was obtained with oligomeric polyaniline (OAN)/epoxy resin composites [37]. The low percolation threshold was ascribed to the much better compatibility of OAN with the epoxy resin compared to the high molecular weight counterpart. This results in a fibrillary microstructure of the OAN phase in the epoxy matrix at a very low concentration. Polyaniline-p-toluenesulfonate (PAPTS) and copoly(o-toluidine/oethylaniline)-p-toluenesulfonate (POTOEPTS) were incorporated into a photocurable system consisting of poly(urethane-acrylate) resin

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Fig. 12. Synthesis of multi-branched PANI/MWCNT composite. Pristine (non-oxidized) MWCNTs were used and, consequently, MWCNT interacts with MSiPA through π-π interactions between PANI and MWCNT [7].

[20]. The system loaded with PAPTS exhibit the percolation behavior (at about 10 vol% PAPTS) usually observed in the case of filler-polymer matrices. In the system loaded with POTOEPTS, the percolation region is

absent which was ascribed to a molecular dispersion of POTOEPTS (Fig. 13). The maximum conductivity achieved for the POTOEPTS system is 100-fold higher than that of the filler. This anomalous behavior

Polymer matrix

Filler

Synthesis methodi

%CP

CP dopant

Conductivityn S/cm

Epoxya

PANI W P PANI PANI + SCF PANI PANI Or OANg PANI

B

0-16

HCl

10−14–10−4

OAN PANI NS NF PANI PANI PANI

Epoxy/BF3 complex catalyst Epoxy/BF3 complex catalyst

Epoxy/BF3 complex catalyst Epoxy/BF3-amine

Epoxy- Latent curing agentd

Epoxy-amine Epoxy - amine

Epoxy-amine (TETA) Epoxy-amine Epoxy-amine Epoxy-anhydride Epoxy-anilineb Epoxy/PANIc Epoxy-anhydride

[57]

[60] [8]

[24] [50]

0–10 0–10

DBSA DBSA

10−14–2 × 10−7 10−14–2 × 10−7 (no SCF) 10−14–10−4 (10% SCF)

B B

0–10 0–10 0–100

DBSA DESSA

5.10−13–2.10−7 10−3 (20%)

~2 25

B

0–50

10−8 (20%) 10−1 (DSA, 10)

Bk B

0–5 0–10

15 ~5 ~3 N10 b1

DBSA DSA DEHSSA TSA TSA

10−5-10−2 −11

10 10−8 3 10−12–10−5 – 5.10−6 (25%) 10−2 10−5–10−3 10−16–10−4

4–20 0–1 0–25

DBSA undoped CSA

PANI PANI NRh GF-PANI +PANI PANI and PANI-mica as powder or paste

Bj B B

0–40 0–25 0–60

DBSA HCl DBSA

B

0–60

DBSA

10−16–10−4 (PANI powder and PANI-mica paste) 10−16–10−2 (PANI paste)

Epoxy-anhydride

PANI

0–30

DBSA

Epoxy-anhydride

PANI

ISP B B

0–40

TSA

5.10−2 8.85.10−4 10−10–~10−1

Ref

1 5 2.5 2.5

B B

B B ISP

Epoxy-anhydride

Percolation threshold (%CP)

[22]

[37] [59]

5 3 – – –

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Table 1 Electrical properties of PANI thermoset composites.

[38] [39] [34]

28 5–10 16 (with 80% PANI in GF-PANI) 40 (PANI powder) 14 (PANI paste) 6 (PANI-mica paste) 2 10 10

[61] [41] [56] [55]

[35] [20]

97

(continued on next page)

98

Table 1 (continued) Polymer matrix

Filler

Synthesis methodi

%CP

CP dopant

Conductivityn S/cm

Epoxy-anhydride

PANI G NF FL PANI PANI

B

0–15

Sulfonic organicm

10−3

B ISP

0–12.5 22–160

HCl HCl

~10−6 (12.5) 10−11–10−5

PANI PANI

B B

~1% 0–50

H2SO4 DBSA

–

Hyperbranched polyester amide Chitosan γ-ray and EB crosslinked PVA-glutaraldehyde EPDM rubber 2 crosslinked methods - Phenolic resin (PR) - EB irradiation PAMPS and PAAm hydrogels PU PUe

PU-acrylate UV-crosslinked Polyester acrylate UV-crosslinked PU/PMMA IPN a

ISPl B B

0–50 0–25

PANI PANI PANI PANI PANI-coated coconut fiber PANI POTOE PANI

B B Bk B

0–30 0–10 0–50 0–25

HCl – HCl TSA TSA H2SO4 HCl HCl

B

0–40

TSA

PANI

B B

0–10 0–12.5

PA CSA

[58]

10−12–10−6 10−12–10−5 0.21 4.10−4 (30) 2.10−7–2.10−4 10−6–10−3 1.3.10−3 (30) 10−12–10−7 10−14–10−2 10−13–5.10−6 10−13–10−4 10−10–~10−1 10

−12

10

−14

Ref

−3

–~10

−2

–~4.10

3.9 1.7 1.35 – –

[62] [36]

– –

[63] [64]

– 5 ~17 ~15 2.5 – b5 14 4.2 10 No percolation b3

[65] [44] [66] [67] [68] [32] [52]

–

[23]

[20] [69]

2,4,6-Tri(dimethylaminomethyl)phenol catalyst. b Prepolymer prepared by reacting DGEBA with aniline at 80 °C for 4 h with aniline; DGEBA ratio of 0.5. c Liquid crystalline EP. d Cationic polymerization. e Hyperbranched OH terminated polyester crosslinked with HDI. f Polyester polyol based PU. g TANI. h PANI as the curing agent. i The synthetic methods are reduced here to two distinct groups: blending method, B, corresponds to the mixing of a previously prepared PANI with the matrix polymer and in-situ polymerization method, ISP, corresponds to the chemical in situ polymerization of aniline in the matrix polymer. j Absorption-transferring process method. k Blending with EB followed by post-doping. l Sequential semi-IPN method. m Prepared by esterification of 5-sulfosalicylic acid with butyl carbitol. n Maximal value (CP % in brackets) or conductivity range.

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PUf PU PU PU

PANI PANI PANI

Percolation threshold (%CP)

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was explained on the basis of molecular orientation and consequent reticulation. 5. Applications of PANI thermoset composites 5.1. Microwave or radar absorbing materials (MAM/RAM) Microwave absorbing materials are composite materials in which dielectric and/or magnetic fillers are dispersed in a polymer matrix. Electrical conductivity and complex permittivity are the two critical parameters when formulating an absorbing material. Classically, MAM show electrical conductivity varying between 10−4 and 10− 1 S/cm. PANI provides materials with high levels of electromagnetic shielding performances at microwave frequencies with a low mass by unit of surface. The nature of doping agent, PANI content, PANI morphology and thickness of the composite film affect the dielectric and microwave absorbing properties of PANI thermoset composites [58,66,67]. PANI can be combined with other types of absorbers such as ferroelectric (BaTiO3 [70–72]), ferromagnetic (Ni0.5Zn0.5Fe2O4 [72]) or ferrimagnetic (Fe3O4 [73]) materials in order to enhance the absorbing properties. PANI-TSA/PU composite films exhibit higher dielectric constant in the X-band compared to PANI-HCl/PU films [66]. The complex permittivity ε* = (ε′r-jε″)ε0 determines the absorption properties of the composites at a given frequency. The real and imaginary parts of the complex permittivity are proportional to filler concentrations and type of doped PANI. The minimum reflection loss RL(dB) values for the PANI-PTSA/ PU are −37 dB at (20% PANI and 11.6 GHz) and −30 dB at (15% PANI and 11.3 GHz) for thicknesses of 1.2 and 1.6 mm, respectively [66]. Thus, the microwave absorbing properties can be modulated simply by controlling the PANI content and thickness of the composite films for the required frequency bands. Globular, fibrillar and flake-like PANIs were dispersed into an epoxy resin to study the effect of PANI morphology on the microwave absorption properties [58]. It was found that flake-like PANI with its high aspect ratio improves the microwave absorption properties of the composite in the micrometer range 2.4–8.8 GHz. Higher values of dielectric losses (ε″) were obtained for composites containing flake-like PANI. This was attributed to the highly polarizable structure of flake-like PANI which contributes to more important polarization mechanisms. 5.2. Lightning damage suppression Carbon fiber-reinforced polymer (CFRP) that are used in large structures such as airframes, wind turbine blades and automobiles are generally protected of lightning damage by a lightning strike protection (LSP) system comprising a metallic mesh or metallic foil. One drawback of LSP is that it increases the total structural weight as well as the manufacturing costs. Yokozeki et al. [74,75] recently proposed an alternative approach to LSP based on the loading of insulating matrix resin with a conductive polymer to improve the electrical conductivity of CFRP. The CFRP was prepared by impregnation of a plain-woven carbon fiber sheet with a PANI-based conductive thermosetting resin, comprising PANI as a conductive polymer, DBSA and TSA as dopant and cationic initiators and DVB as a crosslinking polymer. The CF-PANI composite showed a much higher resistance to lightning damage compared with a conventional CF-epoxy composite. 5.3. Conductive adhesives Electrically conductive adhesives are widely used in various application fields, including automotive, aerospace, optical, and medical electronics. These conductive adhesives are generally prepared by the incorporation of metallic fillers or carbon black into an organic matrix. In such conventional conductive adhesive system, however, relatively large amount of a metallic filler (60–90 wt%) is needed to reach a desired high conductivity level. Although metals have a good electrical

Fig. 13. Volume conductivity versus volume loading for poly(urethane-acrylate) systems loaded with Δ, PAPTS; ∇, POTOEPTS [20].

conductivity, most of them are readily covered with an insulating oxide layer, leading to a decreasing conductivity. Carbon black-based adhesives suffer from an inadequate electrical conductive stability on thermal cycling. PANI-thermoset composites can compete with the above general approaches to prepare conductive adhesives with good electrical and mechanical properties [20,22,35]. The in situ polymerization method was found to be superior to the physical mixing method in terms of conductivity and adhesion properties of the epoxy/PANI cured system [35].

5.4. Pressure sensor and chemical sensor applications Several examples of PANI conductive composites show a variation in electrical resistivity according to the compressive stress applied, indicating that these materials can be applied for pressure-sensitive applications. These composites can be based on hydrogel [76,77], rubber [78,79] or thermoset [52] matrix. Martinez et al. [76] compared the properties of PANI hydrogel composites with segregated nanodomains of PANI inside the hydrogel (NC) with those of a semi-IPN with PANI chains directly interacting with the hydrogel chains (semi-IPN). Both composites showed electronic conductivity which increases when a compressive force is applied on a composite cylinder. The NC appears more sensitive to external pressure than semi-IPN. This is explained by the fact that in NC, it is likely that some conductive PANI nanodomains are isolated in the unpressed NC. When the NC is compressed, the conductive PANI domains contact with each other creating more conductive paths and markedly decreasing resistance. This is qualitatively represented in Fig. 14. It is noteworthy that the interpenetrating binary network of PANI and poly(N-isopropylacrylamide) matrix composite structure resulted in a totally different behavior during the compressing test with only a small decrease of resistance (b 1%) under mechanical deformation [77]. Apart from HG matrix, a rubber matrix also permits to control interparticle separation between PANI aggregates by applied pressure, due to the high compressibility of rubber matrix [78]. PANI-coated coconut fibers were used as conductive additives for thermoset polyurethane derived from castor oil [52]. PU composites containing a conducting fiber concentration above 20 wt% showed a decrease in the electrical resistivity when compressive stress was applied and the initial resistivity value almost returned to its previous value after the sample loading was removed (Fig. 15). Conductive polymer composites are also attractive as a chemical sensing materials: the swelling of the polymer matrix when exposed

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5.5. Anticorrosion

Fig. 14. Representation of spatial distribution of PANI inside HG matrix, before and after external pressure applied [76].

to a specific organic solvent disconnects the continuous conducting network and leads to the increase of electric resistance. Low interactions between particles forming the conductive network, very low percolation threshold and high swelling capacity of the polymer network (i.e. low crosslinked density) are needed to achieve good chemical sensing properties, as those demonstrated with the PANI@rGO/natural rubber blends prepared by Zhou et al. [79].

PANI has been found to be a very effective material for corrosion protection of metals (steel, aluminium, magnesium and their alloys). There is still much debate regarding the exact mechanism of protection of polyaniline and the mechanism that operates depends on many experimental factors: coating type, emeraldine environment, etc. [80,81]. Common mechanisms of corrosion protection are interpreted as physical barrier, adsorption, anodic protection, and shift of electrochemical interface. Besides, Barisci et al. [82] were the first to propose an inhibition effect by dopant anions released upon reduction of emeraldine salt to its leucoemeraldine counterpart. PANI is generally directly incorporated as pigment [4,11,21,39, 83–86] (typically b few wt%) in a thermoset resin. Among these studies, Zhang et al. reported the high protective performance of PANI nanofibers combined with the self-healing effect of epoxy microcapsules as additives of an epoxy/polyamide coating applied on mild steel [86]. The effect of PANI on the kinetics of UV-initiated radical polymerization of UV-curable acrylate formulations have been investigated [87]. PANI UV absorption and possible radical quenching of PANI result in a delayed and slow polymerization but the effect is negligible for PANI contents up to 3 wt%. In a second approach for the preparation of protective coatings, PANI is used as a primer in PANI-thermoset stratified assembly. Fig. 16 summarizes the main strategies to process PANI/thermoset stratified coatings with anticorrosion properties. Poor solubility of ES has been a problem in coating development. Thus, PANI ES primer are generally prepared by electrochemical deposition or from a solution of undoped PANI EB, followed by post-doping of the EB film with a suitable dopant [88]. PANI primer obtained by casting a PANI ES dispersion in water is an interesting environment-benign alternative process. Recently, long chain alkyl phosphonic acid stabilized colloidal polyaniline dispersions were prepared and used as a primer on steel substrate. It has been demonstrated that the alkyl phosphonic acid (n-decylphosphonic acid, DPA) acts simultaneously as a surfactant and as a corrosion-inhibitive dopant [89].

5.6. Flame-retardant conductive composites Nitrogen-based flame retardants function in two ways: (i) leading to endothermic reactions which will release ammonia gases at high temperature to dilute oxygen and combustible gases and (ii) promoting the formation of carbonaceous char. A flame-retardancy study on epoxy resins loaded with PANI nanofillers demonstrated that the introduction of PANI can decrease the heat release rate (HRR) of epoxy resin and increase the char residue [59]. The HRR with PANI nanofibers was lower than that with PANI nanospheres. This was ascribed to the larger surface area of nanofibers compared to nanospheres which implies that more amine groups of PANI nanofibers can react with epoxy resin. The HRR peak of epoxy resin reinforced with PANI-stabilized silica nanoparticles was observed to decrease dramatically with increasing silica loadings, indicating a flame retardant performance from the phosphoric-acid doped PANI [10].

5.7. Metal ions removal

Fig. 15. Electrical resistivity as a function of compressive loading-unloading stress cycles applied to PU/CF-PANI composites with CF-PANI content of 25 wt% [52].

PANI grafted crosslinked chitosan beads have been used for the removal of cadmium and lead from contaminated water [90]. Chitosan beads were first crosslinked by treatment with glutaraldehyde. The crosslinked chitosan beads were subsequently grafted with PANI chains by in situ polymerization of aniline. The amine groups of chitosan are the main reactive sites for metal ion adsorption but the role of the grafted PANI chains on the adsorption process is not discussed.

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Fig. 16. Preparation of PANI/thermoset assemblies with anticorrosion properties.

6. Conclusion From the reviewed literature on PANI thermoset blends and composites, it appears that much effort has been devoted to enhance the dispersion of the conducting phase in the thermoset matrix. The situation is complicated by the fact that a great number of factors affect the morphology and properties of PANI thermoset composites. Specifically, it is apparent that a small change in processing conditions can result in substantially different conductive properties of the resulting composite. Some of the factors and specific methods that improves the dispersion of conducting phase have been identified and can be summarized as follows: nature and morphology of the conducting phase (substituted PANIs or short-chain PANI with much better solubility rather than PANI, high aspect ratio PANI), dopant type (counterion induced solubilization approach or reactive dopant), cure conditions (ratio between curing agent and thermoset resin, PANI as a co-curing agent), specific processing methods such as the reactive solvent approach (using specific amine hardeners as a solvent of EB) or the absorption transferring process. Besides, post-doping cure PANI EB composite films in an acid solution allow to prepare composite films with a high level of conductivity from basic curing agents (such as amine hardeners in epoxy thermoset). Despite these advances, much work remains to be done to understand the factors that govern the properties of the conducting composites. Additionally, multifunctional ternary composites based on PANI conductive fillers have recently emerged as promising candidates for advanced materials in a broad range of electronic device applications. The method usually referred to as Surface Initiated Polymerization is the preferred method for the preparation of PANI thermoset ternary composites. By using a high ratio nanometric filler (such as clay, MWCNT, graphene…) as a template for the polymerization of aniline, SIP is definitely a promising approach to prepare conductive thermoset composites with a low percolation threshold. In these composites, the amount of PANI, which has low mechanical properties, is low because

it is deposited as a thin film embedding the rigid inorganic filler. Such composites are thus expected to have superior mechanical properties compared to the conventional PANI thermoset binary composites.

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