Thermal degradation of polyaniline films prepared in solutions of strong and weak acids and in water – FTIR and Raman spectroscopic studies
Introduction
Polyaniline (PANI) is one of the most investigated conducting polymers. It has been studied frequently because of its low cost, its ease in preparation, good thermal and environmental stabilities, and its versatile applications [1], [2], [3], [4]. Polyaniline is typically prepared by the oxidation of aniline with ammonium peroxydisulfate in aqueous solutions of strong acids (Scheme 1) [5], [6]. We have recently reported that the courses of aniline oxidation in aqueous solutions of strong (sulfuric) or weak (acetic) acids, as followed by temperature and pH changes, are substantially different [7]. The granular morphology is typical for PANI prepared in the strongly acidic solutions. The oxidation of aniline under mildly acidic conditions produces nanotubes. The morphology of PANI, granular or tubular, depends on the acidity conditions during the reaction rather than on the chemical structure of the acid [7], [8]. Nanotubes or nanofibres of conducting polymers have attracted interest because of their novel properties and wide potential application in nanoscale engineering. Many applications of PANI, such as sensors, antistatic and anticorrosive coatings, require thin films. The technique of in situ surface polymerization, giving rise to thin PANI films, has often been used in the coating of various materials with a conducting polymer overlayer [9]. For applications, the stability of the nanostructured films is crucial and good comprehension of the degradation mechanisms is necessary.
The thermal stability of various forms of PANI (doped and undoped) has been studied recently, and most of these investigations have been carried out on powders using differential scanning calorimetry or thermogravimetric analysis [10], [11], [12], [13], [14], [15]. These techniques can provide only macroscopic information, such as melting point, phase transition, or decomposition temperature profile. Structural variations cannot be observed by these methods.
Thermal degradation of the electrical conductivity of doped PANI powders has been studied in combination with vibrational spectroscopies [15], [16], [17]. The degradation of conductivity of protonated PANI depends on the surrounding atmosphere and on the temperature. The decrease in the conductivity of doped PANI appears as a result of a combination of structural changes, such as the deprotonation, loss of conjugation, oxidative processes, crosslinking, and other chemical reactions on PANI chains (e.g., chlorination and sulfonation) [15].
There are only a few studies [18], [19], [20], [21], [22], [23], [24], [25] concerning the conductivity stability of doped PANI films grown on various surfaces. The changes at the molecular level, manifested by gradual conversion of the PANI salt to the PANI base, is the main process controlling the level of conductivity [19]. The thermal stability of PANI films doped by various inorganic salts has been studied using FTIR spectroscopy [20]. The conductivity decrease and the spectral shifts induced by thermal ageing at 150 °C have been examined and discussed in terms of deprotonation. Other chemical changes, and the nature of the structural and organizational modifications introduced during the thermal treatment, have also been interpreted with the help of FTIR spectroscopy [21]. The FTIR characterization of films aged at 135 °C in air suggests a complex mechanism involving dedoping, oxidation, and chemical crosslinking by the formation of inter-chain tertiary amine bonds [22]. An in situ Raman spectroscopic study of the degradation kinetics of self-doped PANI films showed that the degradation proceeds faster in more acidic solutions [23].
Less is known about the thermal behaviour of deprotonated PANI, a PANI base. Polyaniline base is non-conducting but its stability is of importance for applications not directly corresponding to conductivity, such as those depending on the redox properties of PANI. The thermal and morphological stabilities of PANI base powders have been studied in combination with FTIR and Raman spectroscopies in several studies [10], [12], [14], [26], [27], [28], [29], [30], [31]. The evidence from X-ray diffraction and FTIR analyses showed that the thermal transition at about 250 °C is due to a crosslinking reaction rather than due to recrystallisation [14]. Ageing experiments of the powder form of undoped PANI at 140 °C, with a special emphasis on the effects of the atmosphere, have been carried out [27]. It has been shown that the emeraldine base (EB) exhibits two different degradation mechanisms. The first mechanism of degradation is inherent to the polymer and occurs independently of the ageing conditions, in vacuum or air. It consists of crosslinking through tertiary amine groups that are created from imine nitrogens after breaking the double bond. The second mechanism depends on external conditions that occur concomitantly upon ageing in air. It is based on the incorporation of oxygen as carbonyl groups and on oxidative chain scission [27]. A different chemical mechanism of the degradation, such as the oxidation, the crosslinking and the deprotonation of PANI powders heated from 50 to 200 °C, has been proposed [30]. The protonated PANI was significantly less stable than the EB form, and the stability of the former polymer varies with the nature of the protonic acid. NMR data together with FTIR data clearly show the conversion of quinonoid to benzenoid rings upon heating at about 200 °C in vacuum. NMR reveals the presence of tertiary amine nitrogens that are generated from the crosslinking of the annealed chains, yielding an N,N′-diphenylphenazine structure at the site of crosslinking [32].
We have recently studied the changes in the molecular structure of PANI hydrochloride and of the corresponding deprotonated films deposited in situ on silicon windows using FTIR spectroscopy and conductivity measurements during the heating [21]. A notable change in the character of the time-dependence of resistivity at a fixed temperature has been observed when the temperature of ageing exceeded 85 °C. Ageing was much faster above this limit. This observation is reflected in FTIR spectroscopic measurements on both protonated and deprotonated samples. The FTIR spectral variation may be explained by a conformational transition of the polymer chain at about 85 °C. The fact that a similar transition has also been found with a deprotonated sample indicates that this feature is an inherent property of PANI, and is not caused by the effect of an acid constituting a PANI salt. The glass-like transitions at ∼70 °C in EB powder were earlier observed using modulated differential scanning calorimetry [12]. These results clearly demonstrate that the conformational structure is changed by thermal treatment.
The main goal of this work is the study of the effect of ageing at 80 °C in air on the morphology and molecular structure of thin PANI films produced in situ on silicon windows by FTIR and Raman spectroscopies. The stability of the resulting PANI films deposited on silicon windows during the nanotubular growth of PANI in solutions of weak acids is now reported and is compared with that of the PANI films produced in the conventional way in solutions of strong acids.
Section snippets
The preparation of PANI films
Polyaniline was prepared by the oxidation of 0.2 M aniline (Fluka, Switzerland) with 0.25 M ammonium peroxydisulfate (APS) (Lach-Ner, Czech Republic) in 0.1 M sulfuric acid and 0.4 M acetic acid, or in water [9]. The films were deposited in situ on silicon windows, 24 mm in diameter, used for the FTIR spectroscopic measurements. Some films were deprotonated with an excess of 1 M ammonium hydroxide to the PANI base, which was then rinsed with acetone. The dry films were kept at 80 °C for three months
Results and discussion
It is assumed that the aniline oligomers produced in the early stages of aniline oxidation adsorb at available surfaces due to their hydrophobic character [9]. They initiate the propagation of PANI chains [8], i.e. the growth of PANI film, in the later stages of oxidation. Depending on the initial acidity and the acidity profile during the oxidation, PANI is produced in the surrounding aqueous phase as granules (at high acidity) or nanotubes (at moderate initial acidity). It is anticipated that
The possible mechanism of degradation
The protonated conducting PANI is less stable with respect to molecular changes during ageing at elevated temperature than the corresponding non-conducting PANI base. The correlation between stability of the molecular structure and conductivity may certainly be coincidental, yet a possible explanation can be offered.
Redox reactions are based on the transfer of electrons from a reductant to an oxidant. It has been reported that an oxidant and a reductant separated with a PANI membrane can react
Conclusions
The thermal stability and structural variation of films produced on silicon windows during the polymerization of aniline in 0.1 M sulfuric acid, 0.4 M acetic acid, or water have been studied by FTIR and Raman spectroscopies. The morphology of PANI, granular or tubular, depends on the acidity conditions during the reaction. The granular morphology was produced in a solution of a strong acid, nanotubes prevail in the solution of a weak acid or in water. FTIR and Raman spectroscopies demonstrated
Acknowledgments
The authors thank the Czech Grant Agency (202/06/0419 and 203/08/0686) for financial support. A secondary school student, Ms. Šárka Gregorová, participated in this work in the framework of the Open Science Project supported by the Academy of Sciences of the Czech Republic.
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