Polyaniline, redox behavior

Figure 8.3 Typical cyclic voltammogram (upper), redox behavior in chemical structures (middle) and expansion and contraction of polyaniline film along the stretched direction (lower). E and E are the anodic peak and cathode peak, respectively, 1/2, ls-es = 1/2 ( + EJ.

The redox behavior of chemically and electrochemically prepared PABA in acidic solution (0.5 M HCl) is reportedly similar to that observed for unsubstituted polyaniline [41]. Two sets of redox waves are observed, at 0.18 and 0.74 V, suggesting facile conversion of leucoemeraldine to emeraldine and subsequent conversion to pernigraniline oxidation states. These results suggest that the boronic acid substituent and polymerization conditions have no detrimental influence on the electronic properties of the polymer. This is in contrast to sulfonated polyaniline where the redox couples are more closely spaced than for polyaniline due to the electronic and steric effects of the -SOa" groups on the backbone of the polymer (for details see, Chapter 2, section 2.5.3). 

The redox behavior of PABA in the presence of D-fructose and fluoride up to neutral pH values is typical of the emeraldine salt form of polyaniline [41], Under weakly acidic and neutral pH conditions, the... 

Figure 3.24 shows the redox behavior of PABA thin films observed at neutral pH in the presence of NADH and NAD" ". The PABA film was redox inactive at neutral pH (Figure 3.24,a) due to deprotonation and loss of dopant as with polyaniline [150,151). However, in the presence of NADH (Figure 3.24, b) and NAD" " (Figure 3.24, c), PABA films became redox active due to complexation of boronic acid with cis-2,3-ribose diols and subsequent formation of self-doped polymer. In the presence of NADH, the cyclic voltammogram of PABA thin film exhibited a single redox couple at pa 0.05 and pc —0.10 V. In contrast, a second redox couple was observed in the presence of NAD+ at pa 0.34 and pc... 

Hong S-F, Hwang S-C, Chen L-C. Deposition-order-dependent polyelectrochromic and redox behaviors of the polyaniline—prussian blue bilayer. Electrochim Acta 2008 53(21) 6215-27. 

Figure 12 Redox behavior of polyaniline (as proved by spectroscopic studies) [108].

Extensive studies on polyaniline (PANI) have been carried out in the last five years. Some unique features related to PANI such as electrical conductivity, highly environmental stability, magnetic and electrochromic properties, and as well as availability have attracted much attention from both academic and industrial sectors (1). Among all the potential applications the most attractive one is the use as electronic devices such as electronic capacitors (2,3), sensors (4), electrochromic displays (2,3,5-9), and photodiodes (10,11). All these applications are based on its unique reduction-oxidation (redox) behavior and the associated spectral properties. 

The instability of PANI above 0.8 V aside, the redox behavior of the first couple of polyaniline, leucoemeraldine to emeraldine, is stable and has been exploited in EC systems (9,13,24). Thin layer electrochromic devices have been reported by a number of groups, however despite the seemingly good results derived it would be desirable to address the pemigraniline (blue) form as well. If it were possible to access this state reversibly, then PANI or related materials could find utility in glare reduction applications and in smart windows since the band associated with this form best matches the solar spectrum (25). 

Polyaniline has been shown to exhibit similar redox behavior in both aqueous and nonaqueous media. However, in organic solvents, the second (more positive) oxidation response is irreversible, due to the lack of protons in such media. 

Interesting supports are the polymeric materials, notwithstanding their thermal instability at high temperatures. In the electrocatalysis field, the use of polypyrrole, polythiophene and polyaniline as heteropolyanion supports was reported [2]. The catalytically active species were introduced, in this case, via electrochemical polymerization. Hasik et al. [3] studied the behavior of polyaniline supported tungstophosphoric acid in the isopropanol decomposition reaction. The authors established that a HPA molecular dispersion can be attained via a protonation reaction. The different behavior of the supported catalysts with respect to bulk acid, namely, predominantly redox activity versus acid-base activity, was attributed to that effect. 

It is also possible to modify the deposited conducting polymer in order to change its electrical, optical and other properties. For instance, polyaniline film was modified by subsequent electrodeposition of diaminomethylbenzoate (Fig. 4.6) [10,129]. As a comparison of the spectrum of PANI— where the absorbance related to the delocalized electrons at A > 600 nm is clearly apparent— with the spectrum of the modified PANI shown in Fig. 4.7 reveals, the electronically conductive parent polymer can be transformed into a redox polymer. However, the electrochemical behavior, the color [10] and the conductivity [129] of the polymer during the modification procedure can easily be regulated, and so the required properties can be finely turned [10,129]. 

Polycarbazole is similar to polyaniline in its requirement for protonic doping. It shows interesting variation in its behavior in different media. For example, polycarbazole prepared in DMF does not show detectable redox activity but functions as an active electrode material and exhibits membrane behavior [11]. In an aqueous acidic medium (perchloric acid), it undergoes a redox reaction exhibiting a CV oxidation peak at 0.5 V versus an Ag AgCl electrode, but the redox activity in perchloric acid is not stable to extensive cycling. This behavior contrasts with that of polypyrrole, which is stable to extensive cycling, in excess of 5 x 10 cycles. 

Calvo and Etchenique summarized in their review some further in situ combinations of EQCM with non-electrochemical techniques (see [35] and references therein). For example, EQCM was also combined with ellipsometry in order to study the nucleation and growth of polyaniline films (reference 24 in [35]) or the viscoelastic behavior of poly(7-methyl-L-co y-n-octadecyl-L-glutamate) [17]. EQCM was combined with UV-visible absorption spectroscopy, in order to investigate the redox reactions of viologens. A combination of EQCM and probe beam deflection, PBD, was also reported in the literature (references 29, and 30 in [35], and [81]). PBD can discriminate between anion, cation, and solvent fluxes that might be generated on the electrode surface. 

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