Polyaniline Nanostructures: Techniques in Structure-Tailored Polymerisation-Superstructures

Abstract

Polyaniline (PANI) is one of the widely studied conducting polymers. As such it is one of the widely applied conducting polymers for laboratory bench work applications. The limitation to application in commercial work has been hampered by the inherent difficulty of a polymer being processed once synthesised. The solution to this lies in synthesising the PANI that has uniform structures ready for application in that form or creating a composite with other molecules that bring about the level of processability to acceptable processible levels. This paper seeks to outline the general synthetic underlying principles behind the synthesis of PANI that may bear the structural nature for ready-to-apply or processible to some extent for possible application. The paper outlines the general synthetic concept framework for one to manipulate for suited use.

1. Introduction

1.1 Polyaniline as conductor polymer, historical perspective and synthetic techniques

In this chapter, structured tailored polyaniline (PANI) synthesis schemes will be focused on, the body will provide various synthetic schemes that may result in a specifically shaped PANI. The structured tailored PANI is seen as ideal for applications.

Polyaniline (PANI) is one of the most highly studied intrinsic conductor polymers (C.P). This has been due in part to its conductivity [1], stability of polymer [1, 2] and ease of polymerisation process [1, 2, 3, 4, 5, 6]. Intrinsic conductor polymers are organic polymers with the ability to allow electricity to flow through them or they allow the transfer of charge from one point to another in the polymer chain. In particular, they possess the conjugations in the chain that may alter the energy gap and electronegativity through the various forms of synthesis. For polyaniline-based conductor polymers, in principle, the chain is made up of benzenoid and quinoid linked through the amine [6, 7, 8, 9, 10, 11, 12, 13]. The amine could be protonated and deprotonated by the use of acid enrich or depleting substances called mainly referred to as dopants [1, 12, 13, 14, 15, 16, 17]. The diagram in Figure 1 shows the basic backbone polyaniline is the simplest form.

Figure 1.

The benzenoid and quinoid are the basic alignments of aniline monomers in polyamine [13, 17].

The number of benzenoids and quinoids in the chain gives the distinctive colour of different PANIs that have been reported. The basic colours reported so far include emeraldine green, violet, purple and colourless. The colouration can be structurally understood, as given in Figure 2.

Figure 2.

Shows different forms of polyaniline with the expected colour.

1.2 A brief history of polyaniline (PANI)

The earliest moments of researchers working with polyaniline or its predecessor named black dye, dates back to the 1800s [18, 19]. The present name of polyaniline was identified in the 1960s, earlier to which it was named based on various colour-based names and commonly referred to as aniline black. The colour-based name code was mainly because the primary application of early polyaniline was as green, blue and black dyes for early cotton, with industrialisation in full swing, in the early days of industrialisation, dyes date far as the 1800s [19, 20]. The current momentum and interest in PANI have grown with the after-match of the 2000 Nobel Prize and subsequent accelerated desire for conductor polymers and their application [21, 22]. Generally, the drawback to PANI application is its inability to be processed after it has been polymerised, the rigidity of a polymer and unpredictable nanostructures from the easy methods to polymerise it that is chemical oxidative and electrochemical techniques.

1.3 Polyaniline (PANI) synthesis techniques

Polyaniline is generally synthesised through oxidative polymerisation techniques. The process involves polymerisation where monomers usually aniline or derivatives of aniline in the presence of the oxidising agent or electric field in an acidic medium [19, 20, 23, 24, 25, 26, 27]. Specific techniques such as (i) chemical oxidative polymerisation (ii) electrochemical polymerisation, (iii) plasma polymerisation (iv) enzyme catalysed polymerisation, (v) bio mimic catalysed polymerisation and (v) microwave-assisted polymerisation [6, 8, 10, 28, 29, 30, 31, 32]. The undertaking is done to control the type of shapes.

2. Polyaniline: structure-tailored synthesis for possible application

Structure-tailored polymerisations are the methods used to control the molecular structures to obtain the structurally predictable, consistent properties of PANI. This is generally done by adjusting reaction or polymerisation conditions to direct the reaction towards the desired goal. Various studies have been conducted when it comes to control of morphology, crystallinity and conductivity of polyaniline [10, 29, 33, 34]. What this means is that if PANI with a desired and targeted application is to be made, then it means implementing a scheme of synthesis designed with the specific outcome of structure-tailored PANI in the back of the synthesis plan. The following techniques are used to obtain structure-tailored PANI.

• Template-assisted synthesis (hard and soft templates) [33, 35].

• Molecular imprinting [10, 36, 37, 38, 39, 40, 41].

• Dopant-assisted polymerisation [42, 43, 44, 45, 46, 47, 48].

• Emulsion-based (inverted and non-inverted) techniques [48, 49, 50, 51, 52, 53, 54, 55].

• Composite polymerisation [56, 57, 58, 59, 60, 61].

2.1 Polyaniline synthesis template-assisted synthesis: soft templates

Soft templates are mainly solutions with the ability to line up the acidified or non-acidified aniline monomers. The soft templates tend to have multiple functions in PANI production. Among the roles, it may work as a surfactant for dispersing the solution of anilinium, providing a local environment that tends to affect the pH of the immediate surrounding of the solution, composites matrix for much processible PANI and in some cases work as a doping material itself [62, 63, 64, 65, 66]. Chen was able to use the suprastructures of methyl orange in a low pH solution. It was established that the effectively self-assembled supramolecular aggregation in the shape of flake and dendrite could work as soft templates for the preparation of micro-tubes of PANI the monomer was aniline the activating agent for the reaction was ammonium peroxydisulphate APS [35]. Meanwhile, Qui et al. were able to synthesise highly crystalline PANI nanofibres and nanorods by changing the concentration of sucrose octaacetate, 2.0 g and 3.0 g, respectively [62]. The solution of sucrose octaacetate was working as a soft template. It was observed that they were significantly irregularly shaped agglomerates and scaffolding formed at high concentrations. Chattopadhyay et al. [63], synthesised structure-tailored PANI by oxidative polymerisation of aniline monomers in the presence of sodium alginate. They were able to produce nanofibres, ammonium peroxydisulphate was used as an oxidising agent while sodium alginate worked as a soft template. The reaction was carried out in the presence of hydrochloric acid. For soft templates, the concept is that a molecule with some solubilities in an aqueous medium helps align the aniline or anilinium ions in the solution. When close examination of the molecules so far used, will show they show some centre in the molecule with the potential to be ionised in an acidic or basic medium. Figure 3 shows molecules of methyl orange, sucrose octaacetate and sodium alginate.

Figure 3.

Showing chemical structures of molecules used in the soft template: Methyl orange, sucrose octaacetate and sodium alginate [64, 65, 66].

The monomer is still aniline, which in an acidic medium, is protonated and becomes anilinium ion. The molecules used in aqueous medium help align the anilinium ion just before oxidising agent can activate the process of oxidative polymerisation [8, 67, 68]. The process can be understood in Figure 4.

Figure 4.

Showing the anilinium taking part in the various reaction conditions and mechanisms [68].

Therefore, for the soft template, it could be understood as in Figure 5. The soft templates help in aligning the protonated aniline molecules which then get oxidised by APS or hydrogen peroxide or another oxidising agent that is usable for the reaction.

Figure 5.

Shows the methyl orange molecule being used as a soft template for the synthesis of PANI.

This is not much different from other types of soft templates. Their role is typically that of aligning monomers. But at other times, they too help in creating the background for the composite formation and subsequent doping or points where the pH of the local environment is more strongly acidic than the solution of the reaction medium [34, 68, 69, 70]. The soft templates in synthesis of structure-tailored polyaniline means that, the use of small molecules or polymers for controlling the general growth of a highly crystalline PANI. The soft templates may also serve other purposes such as doping the PANI, solubility hence processibility and easy of post-synthesis processing.

2.2 Polyaniline synthesis template-assisted synthesis: hard templates

Hard templates could generally be understood as any substance that is solid or gel-like material that helps in aligning the other materials. In this context, even composite forming materials that intertwine with the polyaniline forming on conducting matrix still could qualify to be referred to as hard templates [70, 71]. Wei et al., synthesised the enzyme-catalysed polyanionic template based on sulfonated polystyrene. Meanwhile, a hard template may entail fabricating polyaniline in the structures of hard templates, this technique not only has high specificity, predictability and enhanced electro-activities of the PANI so synthesised, but it is easy and the throughput is relatively high [72, 73]. Other techniques in the structure-tailored polymerisation on hard templates include electrochemical polymerisation on the surface of charged hard plates, predetermined nanostructured material to resemble the PANI nanostructures, growth on the surface of other materials that have a specific design [74, 75].

Figure 6 shows electrochemical polymerisation using a conducting hard template mechanism. The electroactive surface can be graphene, metal plate or zeolite on a semi-conducting surface.

Figure 6.

Shows the technique of polymerisation using the conducting material as aligning surface.

Then, for halloysite PANI synthesis, Lui et al. [33] used typical hard structure halloysite-suited materials that were used to make nanotubes, Figure 7 depicts on concept.

Figure 7.

Illustration of halloysite for PANI synthesis [33].

The shapes of nanostructures obtained were hollow tube-like structures resembling the halloysite they were fabricated from. Their SEM images were remarkably shaped as the hard templates they fabricated from. Figure 8 shows the actual produced PANI.

Figure 8.

Typical halloysite structure tailored PANI [33].

The halloysite templates provide surfaces that provided foundation and support for the nanostructures of PANI synthesised by the technique. The polymerisation technique remains anilinium and APS or any other oxidising material that may be suitable. Figure 9 outlines the general concept of the halloysite technique for the synthesis of PANI.

Figure 9.

The general concept of halloysite hard template for PANI synthesis.

2.3 Polyaniline synthesis: molecular imprinting synthesis

Molecular imprinting (MIP) is the technique used to create artificial receptor recognition sites which have an enhanced selectivity and specificity to a specific molecule usually referred to as a target molecule [10, 36, 37]. In MIPs, the synthesis involves the synthesising of a backbone-bearing polymer in this case polyaniline. The synthesis is simultaneously done in the presence of the template or substrate molecule, which is achieved with another molecule called a cross-linker [10, 38]. Then, the template molecules or substrates are removed from the backbone and cross-linked matrix. The process leads to cross-linked matrix polymers, which have cavities that are size, shape and functional group specific. It is this high specificity to a template or substrate molecule that brings about the tailor-structured polyaniline nanostructures. The polymer produced in the final state is not just specific at the molecular level in terms of complementary sites but is still the consistency of suprastructures.

Bagdžiūnas, synthesised what he termed a supramolecular system with tailor-made sites for binding, which stood complementary to molecule templates in each of the following parameter’s size, shape and specific functional groups intending to evaluate its analytical, physical and theoretical interactions [38]. Different teams have reported nanospheres, nanofibres and nanotubes [39, 40, 41]. The synthesis technique may use any type of polymerisation available on PANI [38]. Therefore, the general principle could be understood in Figure 10.

Figure 10.

Shows a simplified synthesis in molecular in printing structure tailored PANI.

The molecular imprinting (MIPs) technique can be performed following any other standard polymerisation types [10, 38, 39, 40, 41]. In many instances for PANI due to its inherent difficulty of processing once polymerised, the in printing in molecular in printing techniques it is advisable to put in one mix the aniline monomers, oxidising agent, cross linkers and the substrate/template molecules. Based on the physical properties of a substrate/template molecules can be removed from the cavities [10].

2.4 Polyaniline synthesis: dopant-assisted synthesis

In a quest to synthesise structured tailored dopant-assisted synthesis has been used to try to control or manage the morphological makeup of the structures. In principle, dopant-assisted polymerisation is the method used in polymer synthesis by adding a small number of materials called dopants into the polymer to change monomer reactivity and influence the chain growth and structures of the polymer. The dopant used is referred to functional dopant. The functional dopant is meanwhile defined as molecules that are added to conductor polymers to enhance conductor polymers’ conductivity [43]. Hafizah et al. [43], synthesised PANI nanoparticles whose particle sizes were manipulated by the amount of sodium dodecyl sulphate SDS [44]. The same SDS or SLS has been reported as playing the duo role in PANI synthesis that of the surfactant for dispersing the aniline or anilinium ions and the dopant material. Alshareef et al. were able to synthesise morphology-controlled PANI nanostructures based on altering three parameters; (i) tunable oxidant manganese (IV) oxide (MnO₂) as a reactive template, (ii) redox-active electrolyte and (iii) porousness of the synthesised PANI [46]. The other team made multi-layered morphology-controlled PANI. The key to control of morphology is the SDS that helped control the shape and ionic atmosphere where the PANI had a morphology [45]. It has also been established that SDS can work as a dopant and also a wettability control medium (processability) [47]. Hence, to prepare some morphologically controlled PANI, dopants or redox-active chemical reagents must be incorporated into the polymer final structure. The dopant selection for versatility’s sake may need to have multiple roles in the synthesis scheme. They can follow any of the known synthesis protocols.

The general reaction equation could be understood in Figure 11.

Figure 11.

The general understanding of dopant-assisted structure tailored PANI.

Meanwhile, the specific cases where dopants were used as aligning molecules and dopants is Shen et al., and Goswami et al. in each where various substituted sulfonic acids were used in aligning oligomers for PANI synthesis for a particular application [76, 77]. The technique produced fibres of the polymers of PANI that were meant for the application. Figure 12 shows the typical application involving the structures of PANI obtained.

Figure 12.

(a) Dopants used in PANI for superconductor capacitors and (b) the SEM images for the nanofibres produced [76, 77].

2.5 Polyaniline synthesis emulsion as structure-tailored polymerisation technique

Emulsion polymerisation is the type of polymerisation technique that allows the formation of a polymer on the surfaces of the layers of two solutions that may have different densities (miscibility). The polymerisation is sometimes done in the presence of surfactants that provide stability to the reaction adduct. Emulsion polymerisation is widely applied in PANI synthesis with a view to controlling the narrowness of nanostructures, distribution of particles and reduce the residue monomers in the final product—oligomers and dimers [48]. In emulsion polymerisation, the monomer molecules are emulsified in a continuous phase of an immiscible liquid, while the activator is dispersed in another layer of an aqueous system. Then, the two solutions are brought in contact with each other and allowed to interact. Since the solutions are not miscible, they will layer up but at the boundary, the polymer will form [49]. The polymer and its oligomers may diffuse in a layer favourable to their polarity. Meanwhile, the basic synthesis of PANI is still maintained where you use aniline monomer, oxidising materials [8, 10]. The emulsion can be carried out in the following four major types of emulsion (i) microemulsion, (ii) macroemulsion, (iii) miniemulsion and (iv) inverse emulsion (inverted).

1. (i) Microemulsion is the type of emulsion where the dispersion is oil water and the particles of dispersion range from 1 nm to 100 nm, though common practice shows a range between 10 nm and 50 nm being practised in synthesis.

2. (ii) Macroemulsion is the type of emulsion where the dispersion is oil water and the particles are in the range of 1–100 μm.

3. (iii) Miniemulsion is the type of emulsion where the oil and water are the dispersing media, but the particle sizes range between 50 nm and 1 μm.

4. (iv) In inverted emulsion on the other hand the dispersion is water oil medium, especially with denser than oil/organic medium that is denser than water [50].

In any case, it is understood that the monomer is dispersed in one medium, while the activator or oxidants or initiator is set in another medium. Then, as the two molecules come in contact either of boundary or during transfer the polymer is building up at the interface of the two. Emulsion polymerisation by desire is meant to control the sizes of particles, distribution and uniformity of particles in the final product [48]. So, for PANI structured tailored suprastructures, the technique entails dispersing the aniline or anilinium ion monomers in an organic medium. Sometimes it may require stabilisation employing the addition of surfactants. Chajanovsky and Suckeveriene managed to synthesise PANI hybrid nanoparticles by inverse emulsion but used ethanol as the medium where aniline was dispersed, while APS was dispersed in water [51]. Österholm et al. reported synthesis of PANI through a weakly acidic medium but stabilised by dodecyl benzene sulphonic acid DBSA [52]. Perrin et al. reported a method of synthesis of PANI, through emulsion but the oxidant APS was dispersed in decylphosponic acid (DPA), while the aniline monomer was dispersed in a mixture of heptane and chloroform, it was found that the emulsion synthesised PANI ratio improved rate of polymerisation increased with the decrease in DPA concentration [53]. In principle, emulsion polymerisation may require aniline monomers, stabilising agents, oxidising agents and its polar solvent or medium for dispersing it. The general understanding of the PANI synthesis could be as shown in Figure 13.

Figure 13.

Showing the general understanding of emulsion polymerisation for PANI.

In general emulsion polymerisation, oil is used as dispersion molecules for monomers [50]. However, for PANI, it is other polymeric solutions such as surfactants that work well. These include dodecyl benzene sulphonic acid, sodium dodecyl sulphate (SDS), sodium dodecyl benzene sulphonate and Triton X 100, [8, 10, 51, 52, 53, 54, 55, 56, 78]. The oxidants are APS, hydrogen peroxide, peroxide salts, dichromate solutions, iron (III) oxide and manganese (IV) oxide in addition to any redox controlling agent.

2.6 Polyaniline synthesis: composite structures

Composite materials are materials that are made from a combination of two or more materials of different chemical and physical properties for synergistic abilities when combined [56]. For polyaniline suprastructures, it can be said these are structures that are hierarchically organised mixtures of PANI with other materials such as nanotubes carbon, graphene, metals coreshell, polymeric molecules, clay, silicate, cellulose whiskers and different metal oxides [57]. The PANI composite nanostructures do not only have high nanoscale dispersing abilities but they possess far better properties than individual components of the composite. In order to obtain the structured tailored nanocomposite materials for PANI, they are three basic routes the synthesis need to take. (i) In situ PANI polymerisation on the surface of nanomaterials of the other part of the composite. (ii) One-pot one-step polymerisation of PANI and its composite material—this is common for polymeric support material. (iii) The last one is the physical mixing of the two materials that are separately synthesised to create a composite [58].

1. (i) Structure tailored polymerisation—in situ. In situ polymerisation of composite materials is usually done on the surface of the already-made nanocomposite materials. The process of synthesis may follow any of the different types of polymerisations available. Where aniline monomers are activated to polymerise in presence of activator APS under controlled temperature conditions and other special focus on nanostructures techniques are used. That is emulsion, dopant-assisted polymerisation and soft template use. This process has the potential to produce structure tailor nanocomposites on the basis that the other nanomaterials added may already have their definite structure. The graphenes for example are well-structured nanomaterials such that forming PANI on their surfaces is like overlaying a sheet on them. Highly definite shapes [59].

2. (ii) Structure-tailored polymerisation—one-pot synthesis. In this polymerisation technique, what is involved in the addition of various chemicals to the very same reactor [58]. The addition can be sequential at timed intervals with each addition tackling one part of the synthesis ultimately the product is PANI composite of the other materials. Pina et al. reported a one-pot synthesis of Fe₃O₄-PANI nanocomposite for super magnetic property evaluation [60]. The synthesis involved the mixing of aniline, iron ferrofluids and hydrogen peroxide as the oxidant. The spherical-shaped composite for Fe₃O₄-PANI formed.

3. (iii) Physical mixing of the already formed materials. The technique is the simplest as the PANI and the other composite materials are prior made. Zong et al. reported the synthesis of PANI-magnetic graphene oxide composite for the removal of metal waste and phenol, by pouring already synthesised PANI in a plasma reactor at adjusted conditions [61].

The PANI composite suprastructures are the diverse ones mostly for the creation of structured tailor PANI due to the many forms that can be manipulated. The summary of the synthesis could be illustrated as shown in Figure 14.

Figure 14.

Illustrates the PANI-composite synthesis techniques [58].

The composite molecule most applied is the graphene-based PANI material. This is because graphene by itself can be conducting material, while PANI too is conducting. The synergistic conduction becomes the alluring property the composite brings out. The general synthetic scheme follows the in situ polymerisation of the graphene and aniline monomer in one pot. The final product is the graphene around the PANI. Figure 15 depicts the general reaction.

Figure 15.

Graphene-PANI-composite synthesised in situ general overview.

3. Conclusion

The synthesis techniques outlined above are the basic outline approach to conceptualise the process of designing the structure tailored PANI one desires. The actual reaction conditions and time may be adopted as per case analysis. However, every synthesis requires monomers, initiators and dopant material acids or redox-active chemicals. The actual synthesis for PANI with suprastructures that are tailored may need to follow many ways. Among the practical ways may involve the use of templates, both soft and hard ones. In either way, the template brings about the structural alignment of the monomers and subsequently the polymer formation follows the same aligned form. This in itself is beginning in the structured tailored polymerisation process. The other technique may involve the use of molecular imprinting. In this synthesis scheme for structure-tailored PANI, molecules to be imprinted are introduced in the polymer matrix to leave curvatures in a PANI, this is the highest form of tailor-made structures, but for suprastructures other forth mentioned techniques could still be used to bring about the structures of desire. In other techniques, dopants such as differently substituted sulfonic acids have shown great potential for synthesising the PANI of specific structures. While the molecules serve as dopants yet still work massively aligning molecules of PANI. The emulsion is another form of polymerisation that leads to the formation of PANI. Different shaped PANI from spheres, rods and fibres. Finally, the composites too are known to produce suprastructures that have tailored made structures. This is from the fact that by design the material to composite with may have already well-formed nature that acts as a frame for polyaniline to ride off as they form. In all the techniques mentioned above PANI is synthesised through the form of oxidation of aniline or anilinium ion.

Acknowledgments

The authors wish to acknowledge the fellow members of the faculty and department for moral support during the process of preparation. We also wish to acknowledge the Mulungushi University management for the financial support.

Conflict of interests

The authors declare they are no conflicts of interests.