Conducting Polymers: Polyaniline - Wiley Online Library

CONDUCTING POLYMERS: POLYANILINE 1. Introduction Polyaniline is one of the most important conducting polymers, along with polypyrrole and poly(3,4-ethylenedioxythiophene). Polyaniline is prepared by the oxidation of the respective monomer, aniline salt. The resulting polymer has a typical conductivity of the units S cm−1 and behaves as an organic semiconductor. The conductivity, however, is important but often not the most valued property. For example, the responsivity, that is the ability to respond to external stimuli by the change in physical parameters, is exploited by various applications. The electrochemical behavior may be mentioned as next most important feature of this polymer, which is used in analytical sciences and in energy conversion devices. Polyaniline is colored, and optical properties are also of importance. All these aspects combined with easy and economic preparation, and nanoscale morphologies polyaniline produces, make this polymer an object of ever-increasing number of research papers and patents. The research papers on polyaniline have been reviewed at various times in a general manner (1–5) or have been oriented on the formation of nanostructures (6–13), chemical modification of polyaniline (14), materials comprising this polymer (15–18), or on applications (9,11,19–21). The reader is referred to these and other reviews and original papers for more detailed information.

2. Historical Background The story of polyaniline starts at 1830–1840s. At that time, attempts to elucidate the molecular structure of indigo by dry distillation led to the discovery of aniline (22). The studies of the properties of this new chemical included its oxidation that led to a green product (23) (Fig. 1) that would be regarded as polyaniline in today’s terminology. This reaction leading to colored products has later been used in analytical chemistry for detection of aniline (24). The oxidation of raw aniline, containing in addition also methylanilines, led to the discovery of the first industrially produced synthetic violet dye, mauveine (25) (Fig. 2), used also for printing of Victorian postage stamps (26). Mauveines are aniline tetramers of various molecular structures. Depending on the reaction conditions, the oxidation route may prefer the formation of mauveine-like products or polyaniline, but the oxidation chemistry of aniline is even more complex. Various forms of oxidation products were recognized at about 1910. Depending on the degree of oxidation, 1 c 2015 John Wiley & Sons, Inc. All rights reserved. Encyclopedia of Polymer Science and Technology. Copyright

DOI: 10.1002/0471440264.pst640

2

CONDUCTING POLYMERS: POLYANILINE

Fig. 1. Front page of the paper by J. Fritzsche published in Bull. Sci. Acad. Imp. Sci. St. Petersburg 7, 161 (1840). H3C

N

H2N

N

CH3

+

CI

–

NH

Fig. 2. One of the mauveine structures.

CONDUCTING POLYMERS: POLYANILINE Salts

Bases

+

+

NH

NH

–

–

A

3

A

N

N

N

NH

Pernigraniline

+

NH A

NH

–

Emeraldine

NH

NH Leucoemeraldine

Fig. 3. Polyaniline exists primarily as an emeraldine salt with an acid, HA. It can be oxidized to pernigraniline salt or reduced to leucoemeraldine. Both emeraldine and pernigraniline salts (left) can be converted to corresponding bases (right).

they were called as leucoemeraldine, emeraldine, and pernigraniline (Fig. 3). At that time, they were assumed to be linear aniline octamers. The most important conducting form, emeraldine, received its name due to green color typical of emeralds. The polymer character of oxidation products have been recognized only later, at about 1960, but the names originally proposed for octamers, have been used in the contemporary science also for polymers. The modern history of polyaniline started in 1980s. The fundamental features of polyaniline have been described in the pioneering work by MacDiarmid and Epstein (27), and the future role of conducting polymers was outlined in the Nobel lecture delivered by Professors A. G. MacDiarmid (28) and A. J. Heeger (29). Since that time conducting polymers, including polyaniline, have become an integral part of the polymer science.

3. Molecular Forms Polyaniline exists in three fundamental forms differing in the degree of oxidation (30) (Fig. 3). The intermediate form, emeraldine salt, is produced directly by the oxidative polymerization. One half of nitrogen atoms in the polymer chain are present as secondary amines, the other as protonated imines. The completely oxidized form, blue pernigraniline salt, is observed as an intermediate during the oxidation, which takes place in the presence of excess oxidant and converts to the emeraldine form after the oxidant has been consumed and the reaction completed.

4


Fig. 4. The oxidation of aniline, for example, with ammonium peroxydisulfate, under acidic conditions yields the polyaniline salt (emeraldine hydrogen sulfate). Sulfuric acid and its ammonium salt are by-products.

When transferred to alkaline media, emeraldine salt transforms to a corresponding blue emeraldine base. This important transition, when a conducting form converts to a nonconducting one, is discussed in detail below. The pernigraniline salt yields similarly a violet pernigraniline base. The reduction of emeraldine produces a yellowish leucoemeraldine base. Some studies indicate that leucoemeraldine may also exist in salt and base forms (31). The salt would be produced, however, only under strongly acidic conditions (32). Polyaniline is a true polymer. Weight-average molecular weights determined by gel-permeation chromatography in N-methylpyrrolidone are of the order of tens thousands (33,34) and exceptionally even higher. The lower is the polymerization temperature, the higher is the molecular weight (33). The polymer fraction is often accompanied by oligomers that are produced at the beginning of aniline oxidation. One has also to realize that the chromatography refers only to the soluble part of the oxidation product, which typically amounts to 40–50 wt% (35). Any product of aniline oxidation is thus composed of aniline oligomers, soluble polyaniline fraction, and insoluble component, and only their mutual proportions differ depending on reaction conditions (35).

4. Preparation of Polyaniline 4.1. Chemical Oxidation. Polyaniline is typically prepared by the oxidation of aniline with ammonium peroxydisulfate in acidic aqueous medium (Fig. 4). This is often referred to as the chemical polymerization to make a distinction with the electrochemical polymerization. Ammonium peroxydisulfate is preferred over the potassium salt because of faster and easier solubility. Other oxidants have been frequently used, and they include salts of iron(III), cerium(IV), silver(I), or dichromates. Their oxidation potentials should be sufficiently high, >0.7 V, to obtain polyaniline (36). The highest yields and conductivity of


5

10 0.2 M NH4OH

40

8 Mainly neutral aniline 0.2 M NH4OH

6 0.1 M H2SO4

pH

Temperature, °C

35

30

Mainly anilinium cations

4 0.4 M HAc

25

0.4 M HAc

2 0.1 M H2SO4

20 0

10

20

Time, min (a)

30

40

0

10

20

30

40

Time, min (b)

Fig. 5. (a) Temperature and (b) acidity profiles during the oxidation of 0.2 M aniline with 0.25 M ammonium peroxydisulfate started in media of high acidity (0.1 M sulfuric acid), low acidity (0.4 M acetic acid), and in alkaline solutions (0.2 M ammonium hydroxide). (Adapted from Ref. 38.)

polyaniline are obtained with peroxydisulfate as a rule. The starting acidity of the reaction medium should be sufficiently high, pH < 2.5. Polyaniline is obtained as an insoluble conducting salt, for example, polyaniline hydrogen sulfate. 4.2. “Standard” Polymerization. For routine preparation of polyaniline, the following protocol, sometimes referred to as “standard” polymerization, was proposed (37): 2.59 g of aniline hydrochloride is dissolved in water to 50 mL of solution. 5.71 g of ammonium peroxydisulfate is also similarly dissolved to 50 mL of solutions. Both solutions are mixed in a beaker at ambient temperature, 20°C. The concentrations of reactants are 0.2 M aniline hydrochloride and 0.25 M ammonium peroxydisulfate, which corresponds to the stoichiometry given in Figure 4. The colorless reaction mixture turns lightly then deeply blue and finally converts to greenish slurry within about 10 min. After 1 h, or next day, the solids are separated on a filter, rinsed with copious amounts of 0.2 M hydrochloric acid, then with acetone or methanol, and dried in air at room temperature, and if need to be, in a desiccator over silica gel. The reaction yields 2 g of polyaniline hydrochloride powder in a conducting emeraldine form. 4.3. The Course of Chemical Oxidation. The oxidation of aniline is exothermic, and the temperature of the reaction mixture increases during its course (Fig. 5a). This can conveniently be used to monitor the progress of polymerization. “Standard” polymerization is the safe process how to prepare polyaniline

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at a laboratory scale. If the concentrations of reactants or the volume of reaction mixture or both are substantially increased, the temperature may exceed the boiling point and the reaction mixture can explode. Two hydrogen atoms are released as protons when an aniline molecule is added to the growing polymer chain, and sulfuric acid is the by-product of the reaction (Fig. 4). For that reason, pH drops during the reaction and can be used to monitor the reaction progress as well (Fig. 5b). The reaction profile depends on the acidity of the medium at the start of oxidation. For the oxidation starting at acidic pH and leading to polyaniline, an initial or intermediate induction period, where the temperature changes are low, is typical (38). The oxidation started under alkaline conditions leads mainly to aniline oligomers but may contain a polyaniline fraction because, due to the generation of sulfuric acid, the pH become acidic at the end of reaction (Fig. 5b). The fact that pH of reaction mixture changes during the oxidation and, the mechanism of aniline polymerization may change accordingly, is crucial for the understanding of oxidation chemistry. Aniline has pK 4.6. While it is neutral aniline, which is oxidized under alkaline and low-acidity conditions, the anilinium cation undergoes the oxidation in highly acidic media (6,38). 4.4. Electrochemical Oxidation. Electrochemical synthesis of polyaniline involves oxidation of the aniline in electrolyte solutions by applied electric potential. Compact electroactive polyaniline films are obtained on conducting substrates (eg, electrode arrays). Electrochemical methods include potentiostatic (39,40), galvanostatic (40) oxidations at constant potential or current, respectively, or potentiodynamic cyclic voltammetry polymerization (40–42) using a two- or three-electrode systems. Two-electrode system contains a working electrode (indium tin oxide, noble metals such as gold, silver or platinum, glassy carbon, etc.) and a reference electrode (saturated calomel electrode or silver chloride electrode). Three electrode systems include in addition a counterelectrode from electrochemically inert materials such as gold, platinum, or carbon, which together with a working electrode provides a circuit over which current is either applied or measured. The electropolymerization of aniline has usually been carried out in aqueous solutions containing inorganic acids, for example, sulfuric acid (41–43), perchloric acid (41), nitric acid (43), or phosphoric acid (44), or in the solutions of organic acids, for example, p-toluenesulfonic acid (45). It can also be carried out in the aqueous solutions of polymeric acids, such as poly(styrenesulfonic acid) (46) or poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (39,40,47,48). The sufficient acidity is a prerequisite of the polyaniline formation, similarly to the chemical oxidation of aniline. Organic solvents have also been used for the electropolymerization, for example, acetonitrile (49,50) or dichloromethane (50). The reaction progress and result depend on the concentration of aniline, current density, properties of solvent used, and so on. The polyaniline films produced at the electrode contain counterions incorporated from the solution to polymer the matrix during the electropolymerization. The method is limited to the preparation of polyaniline films on conducting surfaces but may be of choice for specific applications. 4.5. Mechanism of Aniline Polymerization. There is a general consent that the oxidation of aniline to polyaniline is a chain process. Such reactions have initiation step, chain propagation, and termination reactions. There is a


NH2

7

HN + Aniline

NH2 o-Semidine

+ Aniline

Oxidation

HN

HN N

N

NH2

+

+ NH2

Fig. 6. The oxidation of aniline yields a dimer, semidine, and subsequently a trimer. Such trimer is expected to serve as an initiation center of chain growth.

vast literature on this topic but fundamental principles may be summarized as follows (51). Many authors assume that the first product of aniline oxidation is an aniline cation radical, and this approach is accepted in the present text. Other assume the formation of both the aniline cation radicals and nitrenium cations depending on reaction conditions (type of oxidant, anode potential, and so on), and the reader is referred to the literature (5,52) for details. The first product of aniline oxidation is an aniline dimer, semidine. The para-isomer is usually anticipated, but the ortho-isomer (Fig. 6) better explains the formation of phenazine heterocycle (38,53), also known from the chemistry of mauveine dye (Fig. 2). Whether the aniline trimer is produced via a phenazine intermediate is still open to discussion. Aniline trimers were proposed to serve as nucleates (38) in the morphology formation (see below). They also act as initiation centers. After the oxidation to a cation-radical form, the trimers are believed to react with the aniline cations radical, and this step is repeated as the polyaniline chains starts to grow (Fig. 7). The propagation proceeds in a fully oxidized protonated pernigraniline form (Fig. 3) and it is reflected by a blue color of the reaction mixture. It has recently been proposed (51) that the reason for the diversity of the molecular structures, morphologies, and properties of products in aniline oxidation is associated with the existence of two basic reactants, the monomer and the growing chain, in both nonprotonated and protonated forms. The different forms are oxidized with two various oxidation mechanisms: (1) the chain reaction of electrophilic substitution and (2) the coupling of cation-radical centers. The

8


R

+ NH

+ NH2 +

+ H2N

coupling

R

+ NH

+ NH2

+ NH2 rearrangement

R

+ NH

NH2 + 2 H

NH

+

oxidation

R

+ NH

+ NH

+ NH2

Fig. 7. The terminal aniline cation-radical unit recombines by coupling with aniline cation radical. The benzidine (semidine) rearrangement followed by the reoxidation recovers the aniline cation-radical terminal unit.

proportion of the two reactions is dependent on the reactants’ protonation states and, therefore, the pH of the reaction medium. At pH > 2.5, the mechanism of electrophilic substitution takes place mainly with nonprotonated forms of the reactants that proceed at low oxidation potentials. The reaction leads to the formation of nonconducting aniline oligomers with heterogeneous molecular structures. At pH 2.5–4, electrophilic substitution is reduced because of the protonation of aniline and a consequent increase in its oxidation potential. The cyclic dimer, phenazine, becomes the main product of the oxidation. At pH < 2.5, where the reactants are protonated, the oxidation proceeds in two stages. At the initial phase, the phenazine, the intermediate in aniline polymerization is slowly formed by electrophilic substitution. The polymer chains begin to grow after overcoming the limiting step of addition of the first monomer unit to phenazine that proceed at the oxidation potential 1.05 V (vs reference hydrogen electrode). The growth of the chains proceeds at 0.7 V through the coupling of the cation radicals: the chain-end (terminal) cation radical and monomer cation radical with the formation of π-complex. The transformation of π-complex into para-substituted monomer unit is proposed to occur through an intramolecular semidine rearrangement (Fig. 7). The regular structure of growing chains is due to the high regioselectivity of the sigmatropic rearrangements and significant energy benefit of protonated polyconjugated chains in the agglomerated state. After monomer addition, the terminal cation radical is recovered by reoxidation.


9

The termination has not virtually been discussed in the literature. The chains may grow as far as the oxidant is available to recover the terminal cationradical state. It was indeed proposed the process has a living character, and the chains stop to grow simply because the oxidant was depleted (54). Alternatively, the chain end may be made inactive by the hydrolysis to quinones (55).

5. Morphology Both aniline oligomers and polyaniline are insoluble in aqueous reaction medium and separate during the synthesis. However, the precipitation is not the disordered agglomeration of insoluble polymer. Complex morphology of nanoparticles and layers of polyaniline suggests that the organization of macromolecules proceed as they grow, during the synthesis. It is believed (6,8,56) that a random or regular organization of insoluble phenazine nucleates in the volume of the reaction phase or on the template surfaces is the primary basis of self-organization. The subsequent heterophase growth of polymer chains from the phenazine aggregates, accompanied by the formation of hydrogen bonds between existing and growing chains, is the reason for self-organized and often crystalline areas. This model can from a single position, explaining the formation of all types of supramolecular structures, as well as to understand the reasons of morphological changes depending on the experimental conditions. Polyaniline is known to produce characteristic morphologies (38), namely globules, nanofibres (7,11,57), and nanotubes (8,10,53). Globular powders (Fig. 8a) are produced when the oxidation of aniline is carried out under strongly acidic conditions. Its formation is explained by following scenario: At first, the oxidation of aniline yields phenazine-like oligomers, the nucleates. They are insoluble in the reaction medium, separate, and form random aggregates. Then they act as initiation centers and subsequently start the growth of polyaniline chains that produce a body of the globule (58). The dilution of the reaction mixture allows for the better organization of the nucleates (7) that tend to aggregate to columns due to π–π stacking of flat phenazine-like nucleates. The growth of polyaniline chains from the stacked nucleates generates polyaniline nanofibres (or nanowires) (6,8). Nanotubes are produced when the oxidation of aniline starts in neutral or mildly acidic conditions (59–61). Aniline oligomers are produced at first and were proposed to organize to needles. They serve as templates for the growth of polyaniline chains after the acidity becomes sufficiently high (and pH low; Fig. 5b). The spiral-like model has been proposed (38,62), but other ways, such as the growth on soft templates (63) or self-curling of polyaniline sheets (64), have been suggested. A comment on aniline oligomers should be made at this place. The oxidation of aniline does not need to proceed to the formation of polyaniline (65) and may stop at the oligomer stage. This is especially the case when the oxidation proceeds under alkaline conditions. The microspheres often displaying an opening or small-and-large sphere snowman-like morphology (Fig. 8c) are produced (38,66). Aniline in a base form is not completely insoluble in the medium, and the oxidation to the oligomers takes place on the surface of droplets and produces

10

CONDUCTING POLYMERS: POLYANILINE (a)

1 μm

500 nm

(b)

1 μm

200 nm

(c)

1 μm

500 nm

Fig. 8. Scanning (left) and transmission (right) electron microscopy: (a) the globular morphology is typical of polyaniline prepared in strongly acidic solutions (0.1 M sulfuric acid). (b) The oxidation of aniline under mildly acidic conditions (0.4 M acetic acid) produces nanotubes. (c) Oligomer microspheres are obtained when the oxidation is initiated in alkaline medium (0.2 M ammonium hydroxide). (Reprinted with permission from Ref. (38). Copyright (2008) American Chemical Society.)

microspheres. After the acidity of the medium becomes higher (Fig. 5b), the aniline inside droplets starts to convert from the insoluble base to a soluble salt. The osmotic pressure is thus responsible for the rupture that creates an opening and expelled monomer may produce a secondary sphere. The molecular structure of such oligomers is likely to contain quinone moieties in addition to aniline constitutional units (65,66).


11

Fig. 9. Flower-like morphology of aniline oligomers. (Unpublished results.)

When the oxidation takes place under mildly acidic conditions but the oxidation is not allowed to proceed to the growth of polyaniline chains, the aniline oligomers of phenazine type self-assemble to a variety of spectacular flower-like morphologies (67) (Fig. 9) or hairy rambutan-like microspheres (68,69).

6. Application Forms 6.1. Powders and Pellets. In standard oxidative polymerization of aniline, polyaniline is produced as a powder (Fig. 10). Powder alone is difficult to apply except to become a part of conducting composites, similarly to, for example, carbon black. The powder is easily compressed to pellets, typically at 70 kN force. Such pellets are conveniently used for the characterization of electrical properties. The mechanical parameters of the pellets are comparable to many commodity polymers (70). 6.2. Thin Films and Coatings. Thin polyaniline films are more interesting application form of polyaniline. Any surface in the contact immersed in the mixture used for the preparation of polyaniline becomes coated with a thin polyaniline film (Fig. 10). Its formation follows the principle of morphology formation. In the present case, the aniline oligomers adsorb at available surfaces. The subsequent growth starts from adsorbed nucleates and produces a film (Fig. 12d). Thin films are assumed to have a brush-like structure (71); film thickness is 100–400 nm depending on reaction conditions (72). The films are more uniform

12


Polyaniline forms

Colloidal dispersions

Pellets Powder

Thin films

Fig. 10. Polyaniline is obtained by the chemical oxidation of aniline as powder, which can be compressed to pellets for conductivity measurements. Any surface in the contact with reaction mixture becomes coated with a thin polyaniline film. In the presence of a water-soluble polymer, colloidal dispersions may be obtained. Polyaniline salt is green and changes its color to blue when exposed to alkaline conditions.

on hydrophobic surfaces; on hydrophilic substrates they tend to have a globular structure. Similar films can also be produced electrochemically but only on conducting surfaces. In this respect, the chemical polymerization represents a more general approach. The formation of film on planar surfaces, such as polystyrene Petri dish (Fig. 10), can been generalized virtually to any surfaces represented, for example, by carbon nanotubes (59), silica particles (73,74), metal particles (18,75), ferrites (76), montmorillonites (77–79), and many other objects. In this case, we are talking about the coatings (Fig. 11e). 6.3. Colloidal Dispersions. If the oxidation of aniline takes place in the aqueous medium containg a water-soluble polymer, such as poly(Nvinylpyrrolidone) or poly(vinyl alcohol) (80,81), colloidal polyaniline dispersions are often produced. They look like “soluble” forms of polyaniline (Fig. 10), but this is not a molecular but a colloidal type of solubility. The polyaniline particles have a typical diameter of 200–400 nm. Colloidal dispersions are well suited for, for example, printing of conducting patterns (82). In the contrast to powders and films, colloidal dispersions represent a composite form, which includes a watersoluble stabilizer. Concerning the mechanism of particle formation, the oligomer nucleates are expected to adsorb at the chains of water-soluble steric stabilizer (Fig. 11f), and followed by subsequent polymerization that forms a particle body.


Globules

13

Nanofibre

Nanotube (a)

(b)

(c) Colloidal particle

Thin film

Coating (d)

(e)

(f)

Fig. 11. The formation of polyaniline (a) globules, (b) nanofibres, (c) nanotubes, (d) thin films, (e) coatings, and (f) colloidal dispersion particles. The phenazine-like nucleates are depicted as triangles, from which polyaniline chains grow. Various surfaces available for adsorption of nucleates are depicted in red.

N

N

N

N

n

Pernigraniline

N

NH

NH

N

n

Emeraldine

NH

NH

NH

NH

Leucoemeraldine

n

Fig. 12. Redox forms of polyaniline bases.

7. Electrochemical Properties The green conducting emeraldine salt is obtained after polymerization. It can be oxidized chemically or electrochemically to pernigraniline, or reduced in a similar way to leucoemeraldine (30,83) (Figs. 3 and 12).

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NH

NH HSO4 NH

NH

HSO4

n – 2n H2SO4

N

N

NH

NH

n

Fig. 13. Polyaniline salt (here emeraldine hydrogen sulfate) converts to the corresponding base under alkaline conditions.

These conversions make the basis for many polyaniline applications, such as sensors, electrochromic devices, corrosion protection, energy conversion in batteries, or noble metal recovery. Even though polyaniline is primarily rated as a conducting polymer, its electrochemical properties are also valued.

8. Salt–Base Transition The transformation of polyaniline salt to the polyaniline base (Fig. 13) that takes place under alkaline conditions is also of fundamental importance for polyaniline applications. It is associated with the change both in electrical and optical properties. The green conducting emeraldine form of polyaniline converts to a blue nonconducting polyaniline base when immersed in, for example, 0.1–1 M ammonium hydroxide (Fig. 10).

9. Spectroscopic Properties 9.1. UV–Vis Spectra. The change in color (Fig. 10) is reflected in the UV–vis spectra (84–87) (Fig. 14). Polyaniline salt exhibits the local absorption maximum at 334 nm, which corresponds to π–π* transition of benzenoid ring. The bands observed in the visible region at 426 and 846 nm are associated with the presence of polaron states (charged cation radicals) and assigned to π−polaron and polaron−π transitions (88,89). In the spectrum of the polyaniline base, the maximum at 323 nm is connected with π–π* transition of the benzenoid ring (90). The broad band with the maximum at 603 nm has been assigned to the n–π* transition between the

CONDUCTING POLYMERS: POLYANILINE 2.0

Absorbance

1.5

323 334

UV–vis spectra films on quartz 422

838

608

15

PANI salt

1.0

PANI base 0.5 400

600

800

1000

Wavelength, nm

Fig. 14. UV–vis spectra of original polyaniline salt film obtained after polymerization, and spectrum of corresponding polyaniline base. (Adapted from Ref. 86.)

nonbonding isolated electron pair of a nitrogen bonded to a benzenoid ring and the π* of the quinonoid ring (89,91,92). Please note that the positions of absorption maxima are not strictly fixed and depend on the type of polyaniline salt and its morphology. The deprotonation of polyaniline salt to the base thus manifests itself by the shift of absorption maximum (84) from 800 nm (polaron–π transition) to 600 nm (π–π* transition in quinonediimine units) and by the disappearance of the shoulder at 420 nm (π–polaron transition) (Fig. 14). 9.2. Transition Control. In more detail, the conversion of the polyaniline salt to the base is gradual and takes place in broad pH interval, typically pH 4–6 (Fig. 15) for a polyaniline hydrochloride film. In practice, this means that, at neutral conditions, polyaniline loses most of its conductivity. This is a serious drawback for any application operating at physiological pH. Various strategies have therefore been offered to shift the salt–base transition to the alkaline region. The use of polyaniline salts with hydrophobic organic acids, such as perfluorooctanesulfonic acid (93), or polymeric acids, for example polyvinylsulfonic (94) or polystyrenesulfonic acids (95), has been demonstrated to shift the transition above pH 8. For the use of polyaniline in medicine, the control of the transition is of importance. 9.3. FTIR Spectra. The vibrational spectra are also sensitive to the changes in the electronic structure of polyaniline (86,87). The infrared spectrum of a “standard” polyaniline film deposited on silicon windows in its salt form corresponds to the typical spectrum of the emeraldine salt (Fig. 16) (87,96,97). Absorption bands in the region 3400–2800 cm–1 are connected with nitrogen-containing groups, such as the secondary amine –NH– and protonated imine –NH+ = . They reflect the organization of polyaniline chains within the film by hydrogen bonding involving these groups (98–101).

16

CONDUCTING POLYMERS: POLYANILINE 2.0

12

2

Absorbance

1.5

6

7 1.0

0.5

0.0

400

600

800

1000

Wavelength, nm

Fig. 15. UV-vis spectra of the polyaniline hydrochloride film in buffer solutions in pH 2–12 range. (Adapted from Ref. 93.)

2936 3234 3062 2835

FTIR spectra of films on Si

Absorbance

1495 1147 1303 1583

PANI salt 1245 808 1508 1592

707 1310

1380 3266 3055 3384

4000

3500

3000

1171

831

PANI base 2500

2000

Wavenumber,

1500

1000

cm–1

Fig. 16. Infrared spectra of original polyaniline hydrochloride film obtained after standard polymerization (37), and spectrum of corresponding polyaniline base. (Adapted from Ref. 86.)

A broad absorption band typical of the conducting form of polyaniline is observed at wave numbers above 2000 cm–1 (102,103). Two main bands with maxima situated at 1583 and 1495 cm–1 , assigned to quinonoid and benzenoid ringstretching vibrations, respectively, dominate the spectrum in the region below 2000 cm–1 . The absorption band situated at 1303 cm–1 corresponds to π-electron delocalization induced in the polymer by protonation (103). The band of CN+• stretching vibrations in the polaronic structure (where denotes the bond intermediate between the single and the double bond) is observed at 1245 cm–1 (104).


1589 1337

1506

1171

17

Raman spectra of films on Si Si

Intensity

1260 1470

810

PANI salt

578

423

970 1560

1417

PANI base

1335

1161 1220

Si 780 838 746

417

exc. 633 nm 2000

1500

1000

500

Wavenumbers, cm–1

Fig. 17. Raman spectra of original polyaniline hydrochloride film obtained by standard polymerization (37), and of corresponding polyaniline base. (Adapted from Ref. 86.) Excitation wavelength 633 nm.

The prominent band situated at 1147 cm–1 , which is formed during protonation, has been assigned to the vibrations of the –NH+ = structure (4,105,106). The region 900–700 cm–1 corresponding to the aromatic ring out-of-plane deformation vibrations is represented by a band located at 808 cm–1 , which belongs to C–H deformations in the para-substituted ring (107). The structure of polyaniline salt changes after the conversion to the polyaniline base (87,96). The broad polaron band above 2000 cm–1 disappears (Fig. 16). The conversion to the polyaniline base is revealed by a shift of the bands of quinonoid and benzenoid ring vibrations to higher wavenumbers, 1592 and 1508 cm–1 . The band at 1380 cm–1 is attributed to a C–N stretching vibrations in the neighborhood of a quinonoid ring. The 1310-cm–1 band is assigned to the C–N stretching of a secondary aromatic amine whereas, in the region of 1010– 1170 cm–1 , the aromatic C–H in-plane bending modes are usually observed. Outof-plane deformations of C–H on 1,4-substituted rings are located in the region of 800–880 cm–1 (108). 9.4. Raman Spectra. Raman spectra depend on the excitation wavelength because polyaniline absorbs them in a different manner (Fig. 14). Depending on the energy of the laser excitation line, the electronic transition of either benzenoid or the quinonoid constitutional units is resonance enhanced (109). It can be expected that vibration bands originating from the quinonoid oxidized units are enhanced with the excitation line at 633 nm. The Raman spectrum of the polyaniline salt deposited on silicon corresponds to the typical spectrum of the protonated emeraldine form of polyaniline (86,87) (Fig. 17). The peak observed at 1589 cm–1 is connected with C=C stretching vibrations in a quinonoid ring (106,110,111). The peak with maximum at about 1506 cm–1 corresponds to the N–H deformation vibrations associated with the semiquinonoid structures (112,113). The contribution of the C=N stretching

18


vibrations in quinonoid units at 1480 cm–1 is possible (111,112,114). The band at 1337 cm–1 provides the information on the CN+• vibrations of delocalized polaronic structures (112,114). Benzene-ring deformation vibrations are connected with the band at 1260 cm–1 (106,114). The band at 1171 cm–1 corresponds to the C–H in-plane bending vibrations of the semiquinonoid or benzenoid rings (106,111). The band situated at 810 cm–1 is linked to the benzene-ring deformations (106), and the band observed at 578 cm–1 can be linked to the amine deformation vibrations (in-plane) of the emeraldine salt structure (115). Out-of-plane deformations of the ring are connected with the bands situated at 520 and 423 cm–1 (88). The first is overlapped by a strong band at about 519 cm−1 , which comes from the silicon substrate. The spectrum of the film of standard polyaniline base corresponds to the spectrum of a typical emeraldine base (86,87) (Fig. 17). The intensity of the peak of C=C vibrations in a quinonoid ring vanished after deprotonation, and the intensity of the band assigned to C=N vibrations in the quinonoid units at 1470 cm–1 dramatically increased and this band dominates the spectrum. This band is in resonance with a broad range of energy including the energy of the excitation line used (633 nm) (116). The peak situated at 1417 cm–1 connected with phenazine-like structures appears in the spectrum. The intensity of the band of CN+• stretching vibrations at 1335 cm–1 , which is connected with charge delocalization, and of the band at 1260 cm–1 decreased. The band of C–N stretching vibrations is observed at 1220 cm–1 , and the band at 1161 cm–1 of C–H bending vibrations of quinonoid rings is narrower in the spectrum of the polyaniline base. A broad structural band with local maxima at 838, 780, and 746 cm–1 , which reflect benzene-ring deformations of variously substituted aromatic rings, is observed in the spectrum. This band also includes phenazine-like structures (117).

10. Conductivity Electrical conductivity of polyaniline is the property of interest. It varies from 10−10 to 102 S cm−1 depending on its oxidation or protonation degree, morphology, and method of synthesis. In the latter parameter, especially the acidity level during the synthesis and the type of counterions play an important role. The conductivity of polyaniline prepared in the solutions of various acids differs substantially (Table 1). The conductivity exhibits usually a semiconducting behavior but in some cases also the metallic one. The transition from semiconducting through critical to metallic states can be reached for instance by increased doping. The crossover from critical to semiconducting or metallic regime can be induced by external magnetic field or pressure (118). The conjugated molecular structure of polyaniline enables effective charge transport along chain provided that charge carriers are available. An electronic structure of pristine conjugated polymers, such as the polyaniline base, is that of an insulator with a band gap due to the Peierls instability in one-dimensional systems (119). Upon conversion to salt in acidic medium, the so-called protonation (the number of electrons is conserved), charged defects in a form of polarons are introduced (120) (Fig. 18). The introduction of the charge carriers is referred


19

Table 1. Conductivity and Density of Polyaniline Salts Produced by the Oxidation of 0.2 M Aniline with 0.25 M Ammonium Peroxydisulfate Started at 20°C in the 1.2 M Aqueous Solutions of Various Acids Acid Hydrochloric Sulfuric Methanesulfonic Phosphoric Hydrobromic Camphorsulfonic Picric Citric Succinic Formic Acetic No acida

Conductivity, S cm−1

Density, g cm−3

11.9 10.1 9.7 4.8 4.7 3.1 2.3 1.0 0.28 0.21 4.2 × 10−2 4.4

1.386 1.397 1.385 1.466 1.526 1.345 1.461 1.375 1.348 1.344 1.375 1.333

Data are taken from Ref. 34. a “Standard” polymerization (cf Ref. 37).

NH

NH

HSO4 NH HSO4

NH n Polaron formation

NH

NH

HSO4 NH HSO4

NH n Delocalization

NH

NH

NH HSO4

NH HSO4

n

Fig. 18. Two electrons from two double bonds are injected to quinoneiminoid ring, which converts to a benzenoid one. Two unpaired electron left on nitrogen atoms produce cation radicals, polarons. These may be delocalized over the polyaniline chain.

20


to as doping in analogy with inorganic semiconductors, although its nature is different, this term is commonly used in the literature. The conductivity thus strongly depends on the degree of protonation, and extends from 10−10 S cm−1 for emeraldine base up to units of S cm−1 for fully protonated emeraldine salt (cf below). A higher conductivity, up to 400 S cm−1 , was achieved in the form of thin films casted from solution and protonated with camphorsulfonic acid (121) or by stretching films protonated with 2-acrylamido-2-methyl-1-propanesulfonic acid, which exhibit anisotropy in conductivity and value as high as 670 S cm−1 (122). The orientation of polymer chains thus may also improve the conductivity. 10.1. Charge Transport. Polyaniline and other conducting polymers are often classified as amorphous semiconductors, and the mechanism of charge transport is therefore interpreted within models originally developed for inorganic amorphous semiconductors, such as the variable-range hopping (VRH) model. When the heterogeneity determines transport properties, models for granular metals embedded in the insulating matrix are used instead (123). A comparative experimental study on conducting and dielectric properties of various systems, including polyaniline, has recently been reported (124). The presence of the midgap states generated by protonation leads to the charge transport via a hopping mechanism. This phonon-assisted process is thermally activated, and temperature dependence of conductivity takes a form of the stretched-exponential function, σ(T) exp(−(T0 /T)n ), where n and T0 are parameters. Such dependence is mostly referred to as VRH (charge carriers can hop to the states varying both in the distance or the energy) with exponent n depending on the dimensionality D; n = 1/(1+D). The optimum hopping distance decreases with temperature, and, at high temperatures, only the nearest-neighbor hops prevail. The exponent in this situation changes to 1. On the other hand, at very low temperatures, several or tens of K, electron–electron interactions can limit the conductivity; and the exponent n changes to 1/2 in such case (which coincides with D = 1) (125). Various regimes thus can be found when measurements are conducted over a sufficiently wide range of temperatures (Fig. 19). Since the heterogeneity is inherently present in polyaniline (eg, heterogeneity of protonation distribution, the existence of well and poorly ordered phases), homogeneous models often fail to describe the transport properly. Polyaniline is often regarded as the one-dimensional conductor (the charges move along polymer chains), but occasional barriers and structural defects limit intrachain transport. Interchain transport, which is considerably lower, then becomes essential and determines macroscopic conductivity. In this case, quasi-one-dimensional VRH takes place with exponent n = 1/2 (126). Heterogeneity also manifests itself in a granular-like structure, for example, when regions of high conductivity, the so-called metallic islands, exist and are separated by disordered less conducting areas. For small islands, the charging energy is the limiting factor for the charge transport via tunneling. For large islands, separated by only thin insulating layers, the conductivity is due to tunneling modified by thermal fluctuations of charge carriers in the area of junction. The former exhibits the VRH-like temperature dependence with n = 1/2 (127), and the latter can take a different form, σ(T) exp(−T1 /(T0 + T)) (128). Owing to similar temperature dependence for various scenarios, the assignment of the theoretical model is not straightforward. Polyaniline often exhibits VRH-like temperature dependence with n between 0.4


21

σ , S cm–1

100

10–2

10–4

0

100

T, K

200

300

Fig. 19. Temperature dependence of conductivity for polyaniline doped with methanesulfonic acid fitted to equation for the VRH model with the change in the exponent n = 0.5 (solid line) and 1 (dashed line).

and 0.5 (Fig. 19) and quasi-one-dimensional VRH (126), the charging-energylimited tunneling between polaronic clusters (129) or VRH for coupled metallic rods with a linear Coulomb gap (130,131) are assumed to explain data. Besides semiconducting behavior, the transition to the metallic state through the critical regime was observed and analysed for polyaniline camphorsulfonate (118). The charge transport can be very sensitive to technological variations during or after the synthesis. The charge transport varies significantly with subsequent change in exponent n for different protonation levels (131) or for lower polymerization temperatures (132). Even such marginalized postsynthesis treatment, such as the compression of polyaniline powder into pellet, can modify the charge transport, at least on the level of variations in the rest of fitting parameters (133). A detailed analysis of the charge transport, namely of parameter T0 , which reflects the disorder in the material, can bring information about physical quantities that determine the conductivity, such as the localization length, the density of states at the Fermi level, the size of conducting domains, thickness of insulating barriers, and so on, provided that at least some of them are independently determined. 10.2. Reprotonation. The salt–base transition (Fig. 13) is reversible. This means that polyaniline base can be again reprotonated with any sufficiently strong acid. The polyaniline base is simply immersed in the aqueous solution of acid, but its acidity has to be sufficiently high, pH < 3, for the formation of salt. For that reason, salts are produced with most common inorganic acids and strong organic acids, such as sulfonic acids (134,135). The carboxylic acids, however, do not produce analogous salts with polyaniline. The physical properties of polyaniline salts, including the conductivity, depend both on the polyaniline backbone and on the acid that constitutes a salt with polyaniline. For example, the conductivity, density, and hydrophilicity

22

CONDUCTING POLYMERS: POLYANILINE Table 2. Properties of Polyaniline Salts Produced by the Reprotonation of Polyaniline Base in the Solutions of Various Acids

Reprotonation medium 50% tetrafluoroboric acid 5 M methanesulfonic acid 1 M hydrochloric acid 1 M hydriodic acid 5% perfluorooctanesulfonic acid Water (no acid)


Density, g cm−3

Contact angle, deg

1.22 0.48 0.33 0.14 7.3 × 10−3 1.8 × 10−10

1.418 1.414 1.337 2.056 1.448 1.206

44 32 52 87 102 55

Data are taken from Ref. 134.


100 10–2 10–4 10–6 10–8 10–10

0

20

40

60

80

100

Mole fraction of aniline, %

Fig. 20. Conductivity of poly(aniline-co-p-phenylenediamine) copolymers in dependence on the mole fraction of aniline in the mixture with p-phenylenediamine. (Adapted from Ref. 137.)

(water contact angles) depend on the nature of acid that make a salt with polyaniline (Table 2). Polyaniline thus is not a single material because various polyaniline salts represent numerous materials widely differing in properties. 10.3. Conductivity Control. The samples with a specific conductivity are sometimes required. There are two ways how to prepare such samples. The first is based on the copolymerization of aniline with p-phenylenediamine (136–138). Depending on the composition of starting reaction mixture, the conductivity can be varied from 10−10 S cm−1 for poly(p-phenylenediamine) to the units S cm−1 for neat polyaniline (Fig. 20). For o-phenylenediamine or mphenylenediamine such approach is not viable because the dependence of conductivity on composition of a monomer mixture is not sufficiently smooth (136). The conductivity can alternatively by controlled by partial reprotonation, that is by the immersion of polyaniline base in the solutions varying in the acid concentration and, consequently, in their pH (Fig. 21).

CONDUCTING POLYMERS: POLYANILINE 100

σ , S cm–1

10–2

23

log σ [S cm–1] = 0.70 + 2.34 log C [mol L–1]

10–4 10–6 10–8 10–10

10–4

10–3

10–2

C, mol

10–1

100

L–1

Fig. 21. The conductivity σ of the polyaniline base after reprotonation in solutions of phosphoric acid of various molar concentrations, C. (Adapted from Ref. 139.)

10.4. Ionic Conductivity. The bound acid is present in solid polyaniline salts (Fig. 13). The protonated emeraldine form contains one molecule of the acid per two aniline constitutional units. This leads to the appearance of the ionic conductivity component in polyaniline (140). The acids are an integral part of the conducting form of polyaniline as they interact with the imino group and maintain a high level of conjugation. A polaron interaction with the anion of the acid stabilizes the positive charge (Fig. 18). At the same time, the proton of the acid becomes relatively free. The presence of “free” protons causes the proton conductivity of the material (141). The proton transport in a polymer has a hopping mechanism. The proton, by activation jump, moves from one polaron to another (Grothuss mechanism). This mechanism of transport, in the contrast to the conventional diffusion of the solvated proton (vehicular mechanism), provides much higher carrier mobility and a high level of proton conductivity (142). As the acids saturation and the degree of wettability of polymer increase, both the electronic and proton conductivity of the material grow. The highest level of proton conductivity up to 10−2 S cm−1 is reached in the protonated emeraldine form of polyaniline (143). Thus, polyaniline in the protonated emeraldine state is a mixed conductor, electrons and protons being simultaneously the charge carriers (140,144). The conductivity of polyaniline is determined in a dry state as a rule. The fact that ambient humidity increases the conductivity compared to the dry state (145) suggests that the contribution of ionic conductivity may be important. In many applications, however, this polymer is used directly immersed in aqueous media, and biomedicine, corrosion protection, or batteries may serve as examples. It has been proposed (146) that under such conditions the apparent conductivity of polyaniline may be several orders of magnitude higher, because of enhanced proton mobility in aqueous media and better contact between metallic islands.

24


This is an important issue in the design of applications, which has been neglected so far.

11. Magnetic Properties Polyaniline charge carriers are positive polarons (Fig. 18) or in the chemical terminology, cation-radical centers. They contain electron with unpaired spins, and that is why polyaniline is a paramagnetic material (9). The numbers of unpaired spins in the protonated emeraldine reaches 1020 –1021 spin g−1 . In the emeraldine base and in leucoemeraldine form, they are reduced by 4–5 orders of magnitude; however, polyaniline always contain paramagnetic centers (147–149). There are two types of paramagnetic centers, Curie-localized spins and Pauli-delocalized spins. The fraction of the Pauli spins increases during the protonation along with the conductivity of the polymer. At the high level of conductivity, the measured values decrease due to the skin effect. Polyaniline has high level of spin–spin interactions. The model of the polaron lattice was proposed for their description (90,149,150). In a number of studies, the phenomenon of ferromagnetic spin ordering in emeraldine and pernigraniline was observed (151). This manifests itself in the form of a small hysteresis of the magnetization in weak magnetic fields (9,152, 153) (Fig. 22). Further studies of magnetic properties of polyaniline are needed.

12. Stability of Polyaniline 12.1. Elevated Temperature. The stability of polyaniline conductivity at elevated temperature is one of the most important parameters (97,154). Many applications require such stability, that is the conductivity should be maintained at a reasonable level. Conducting polyaniline salts are prone to deprotonate to bases upon heating. While polyaniline hydrochloride loses its conductivity easily (Fig. 23), polyaniline sulfate is considerably more stable (135) and its conductivity is reduced only four times after 125 h at 125°C. It has been proved that the stability depends on the ability of the respective counterions to be hydrogenbonded to polyaniline (135). The oxygen-containing counterions, such as sulfates or sulfonates, thus endow polyaniline with higher stability, compared with, for example, chlorides. Electrical conductivity of polyaniline depends strongly on water content, which can amount to about 8–10 wt% at the humidity of ambient atmosphere. The reduction in conductivity at elevated temperature can also be associated with the loss of water (145). Also after the removal of moisture in dynamic vacuum, the conductivity of polyaniline hydrochloride drops (132,133) (Fig. 24). Finally, the mechanical integrity of polyaniline pellets used in the experiments has to be considered. Various polyaniline salts reduce their volume due to the loss of protonating acid and water during the thermal deprotonation, and the formation of defects, such as cracks, rather than the deprotonation itself may be the main reason for the reduction of macroscopic conductivity.


25

0,006 0,0010

0,004

0,0000

Magnetization (emu/g)

–0,0010

0,002 –2000

–1000

0

1000

2000

0,000

–0,002

–0,004

–4000

–2000

0

2000

4000

6000

Magnetic field (Oe)

Fig. 22. Hysteretic behavior of the magnetization curve of pernigraniline. The inset shows, on an enlarged scale, the hysteresis part of the magnetization curve in weak magnetic fields (