Emeraldine hydrochloride

Polyaniline (PANI) has complicated chemical structures that arc postulated [90,91]. To study such postulated structures Kaplan et al. [92] and Richter et al. [93] made solid state " C and NMR [92] and N NMR [93] and an emeraldine base (EB), a leucoemer-aldine base (LEB) and an emeraldine hydrochloride (ES), and revealed that EB is an alternating co-polymer... 

Troare et al. [162] performed thermogravimetric analysis of emeraldine hydrochloride under high vacuum in conjunction with thermal volatilization analysis. The initial weight loss was attributed to the... 

PANI is more commonly prepared by polymerization of aniline usiag (NH 2S2 8 HCl (112,127). As prepared, it has stmcture (16) known as emeraldine hydrochloride. In this form, PANI is highly conductive but completely insoluble. When emeraldine hydrochloride is deprotonated with NH OH, the highly soluble emeraldine base (17) is produced. It is processible from oiganic solvents such as aqueous acetic acid or DMSO. It must then be treated with HCl to regenerate the again insoluble conducting form of the polymer. 

Four-probe pressed-pellet conductivities for PANI emeraldine hydrochloride nanofibers are in the range 2-10 S cm [51], similar to conventional PANI emeraldine hydrochloride powder [1]. Maximum conductivities of PANI-NFs prepared by various template and template-free methods are given in Tables 2.1 and 2.2, respectively. 

The terms leucoemeraldine , "emeraldine and "pernigraniline , used in the following discussion will refer to the different average oxidation states of the polymer where y = 1, 0.5 and 0 respectively, either in the base form, e.g. emeraldine base or in the protonated salt form, e.g. emeraldine hydrochloride.2/3 it seems highly likely that the true average emeraldine oxidation state/ where y is exactly equal to 0.5, may never have been synthesized from aniline. 

The fundamental difference between the oxidative and non-oxidative doping methods for synthesizing the highly conducting form of polyaniline is illustrated diagrammatically below for emeraldine hydrochloride ... 

It is interesting to note that the above, partly crystalline, emeraldine base powder can be repeatedly intercon-verted to its amorphous form, and then back to its crystalline form. For example, protonation with HCl, to give the corresponding emeraldine hydrochloride (0 18 S/cm./4-probe compressed pellet), followed by deprotonation with NH4OH, results in the amorphous form. Treatment of this amorphous form with THF, in which it is essentially insoluble, results in some solvation of the polymer, giving it sufficient mobility to rearrange, at least in part, back to the more thermodynamically stable, partly crystalline form. 

The compressed pellet conductivities of powders of the polymers, after protonation in IM HCl, fall in the range 10" to 10"3 s/cm, significantly less than that of the parent emeraldine hydrochloride( l-5 S/cm) synthesized in a similar... 

Figure 46.22 presents [73] the low temperature dielectric constant for a series of emeraldine hydrochloride samples plotted against the square of the crystalline coherence length, (as measured by x-ray diffraction). For low temperatures, is proportional to independent of the direction of orientation of the sample with regard to the microwave frequency electric field. This demonstrates that the charge is delocalized three-dimensionally within the crystalline regions of these samples. Using a simple metallic box model [73,143],... 

Epstein et al. [41, 54, 71] suggested that the sulfonated polyanilines have much stronger temperature dependence conductivity than emeraldine hydrochloride due to greater electron localization. The temperature dependence of the conductivity of 50 % [41], 75 % [54] and fully ring sulfonated polyanilines [71] was best fit by the quasi one-dimensional variable range hopping model described by Equation (2.1) ... 

Electronic spectra show an exciton peak at 2.8 eV and broad absorption at 1.5 eV in emeraldine hydrochloride [225]. However, Cao et al. [226] have reported three spectral features at 1 eV, 15 eV and 3 eV for a polyaniline film spun cast from sulphuric acid solution onto sapphire on a substrate and showing intrachain absorption at 1 eV and 3 eV and interchain absorptin at 1.5 eV... 

With increased orientation and crystallinity, PANI-emeraldine hydrochloride registers an increase in conductivity by two orders of magnitude. In contrast, the POT-hydrochloride salt which has the same electronic structure as PANI-ES shows a significantly decreased conductivity [54]. The origin of this divergent results is stated to be due to increased localization of conduction electrons in POT-ES as compared to PANI-ES, and it is caused by a reduction in interchain interactions due to increased interchain... 

Figure 20.42. Emeraldine hydrochloride (I) emeraldine base (II) ring-sulfonated, protonated, polyaniline (III) and ring-sulfonated, non-protonated sodium forms of polyaniline (IV).

Figure 20.43. pH dependence of conductivities of ring-sulfonated poly anilines ( ) and emeraldine hydrochloride form of polyaniline (O). (Reprinted by permission of ref. 37)... 

Self-doping was eonfirmed by the similarity between absorption speetra of the sulfonated polyaniline and the emeraldine hydrochloride form (Figure 20.44). The effect of the sulfonate group on steric interactions between adjacent rings is evident from the blue shift in the absorption spectra of the sodium salt of the non-protonated sulfonated derivative compared to the emeraldine base (Figure 20.45). 

When we reported the first synthesis of the emeraldine base form of polyaniline 13 years ago (54) by the deprontonation (removal of HCl) from the emeraldine hydrochloride form of polyaniline by aqueous ammonium hydroxide, we were content with elemental C,H and N analyses which added up to -99%. In some cases, we reported the Cl content (as HCl) as <0.4%. Few research groups investigating conducting or non-conducting forms of electronic polymers report elemental analyses and fewer still pay attention to small amount of impurities. It has generally been believed that the properties of interest in such polymers are not particularly sensitive to traces of impurities. However, we have recently found as shown below, that selected properties of interest in the LED field are very sensitive to traces of certain impurities. 

Fig> 12-3 Loss Tangent at 6.5 GHz vs. temperature for four-folded stretched emeraldine hydrochloride parallel to the stretch direction (x), perpendicular to the stretch direction ( ), unoriented poly(orthotoluidine) hydrochloride (o) and unoriented self-doped sulfonated polyaniline (+). After Reference [190], reproduced with permission. 

Quite evidently, all states except leuco-emeraldine can be protonated. The states with no protonation are denoted as the "base" form, e.g. emeraldine base (which would be non-conductive). The reader may note tlie presence and alternation of "benzenoid" and "quinonoid" segments, as defined in an earlier Chapter, in the structures of Fig. 13-9. Upon protonation, the polymer is denoted as the salt, for example protonation of the emeraldine base form (Fig. 13-9c with HCl would yield emeraldine hydrochloride-. 

ABSTRACT. Detailed experimental procedures are given for the chemical synthesis from aniline of analytically pure emeraldine hydrochloride, a highly conducting polymer derived from the emeraldine oxidation state of polyaniline, which contains equal numbers of oxidized and reduced repeat units, the non-protonated base form of which has the composition,... 

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