A mass spectrometric investigation of polyaniline
using photoionization

Volker Sauerland and Ralph N. Schindler

Institut für Physikalische Chemie, Universität Kiel,
Ludewig Meyn-Str. 8, 24098 Kiel, Germany

Abstract

Chemically and electrochemically generated polyaniline (PANI) was analyzed mass spectrometrically using multiphoton ionization at wavelengths of λ = 262 nm and λ = 249 nm, respectively. The mass spectra obtained are characterized by a pattern of six fragmentation pathways which repeated themselves with every additional C6H4NH building block in the oligomer ions. The number of chinoide units in the polymer chain can be clearly identified.

Spectra from PANI coated stainless steel probes revealed intensive iron signals upon ionization with λ = 249 nm radiation. It is argued that complexation of Fe to the aniline moiety is needed for iron evaporation. Such complexation appears to exist already in the surface film and might be related to the corrosion protection ability reported for PANI.

Introduction

Conducting polymers have attracted considerable attention because of their possible applications in corrosion protection, electrochromic displays, batteries, or foil covers in packing materials for sensitive electronic devices [1, 2]. For corrosion protection chemically synthesized polyaniline (PANI), e.g. Versicon&tm;, seems to be a very useful material [3, 4 ]. A better resistance of PANI coated ferritic stainless steel against sulfuric acid solution was already reported in 1985 by DeBerry [5]. He deposited the PANI film electrochemically from a solution containing 1 M aniline in perchloric acid at pH 1.0 using potential sweep technique. The protection was attributed to the formation of a passive oxide film which was stabilized by a PANI layer on top of it. However other authors [6,7] observed only little or no corrosion protection at all of electrochemically deposited PANI films on mild steel.

An improved understanding of the mechanism of corrosion protection by PANI is expected to be reached through a characterization of surface films by spectroscopic analysis [8]. Mass spectrometry was found to provide very valuable information about molecular arrangements at the surface of complex systems [9]. Laser desorption [10, 11, 12], field desorption [13] and bombardment with accelerated ions [14, 15] were employed successfully to make the polymers available for mass spectrometric investigations.

Compared to other conductive polymers like polythiophene or polypyrrole, PANI is a very interesting polymer for mass spectrometric studies, because doping and undoping, i.e. the change between its conductive oxidized states and its isolating leucoemeraldine form is related to a change between chionoide and benzoide structures in the polymer chain and thus to a change in mass [16, 17]. Mass spectrometric characterization of electrochemically produced PANI has been described by Ogawa et al. [13]. For the synthesis they applied a voltage of 1.5 V between anode and cathode, using 1 M aniline in 2 N perchloric acid. The obtained field desorption mass spectra were characterized by signals on masses corresponding to intact oligomer species of aniline with masses up to 3100 a.m.u. In addition they observed fragment ions on masses 15 a.m.u. below and 15 a.m.u. above the mass of the oligomer species. Similar results in the mass range < 400 a.m.u. were obtained by Comisso et al. [18]. Chemically synthesized PANI has been investigated by Chan et al. [19] using static secondary ion mass spectrometry. Doping of PANI with HF, HCl and HBr allowed in addition to positive also negative ion spectra to be obtained with masses up to 290 a.m.u. From the detected masses, they could clearly distinguish between benzoide and chinoide structures, where the imine as well as the amine unit was involved in the interaction with the halogen in the protonation processes. Mass spectra of chemically as well as electrochemically synthesized polydiphenylamine generated in aqueous medium and pyrolysed at 500 C showed an ionic fragment at 488 a.m.u., which revealed the presence of the tetramer. Electrochemical synthesis in acetonitrile led to a degree of polymerization of 10-11 monomer units [18] as obtained from IR data. Pyrolysis of chemically synthesized poly-N-methylaniline at 350 C led to a mass spectrum similar to the one of pure PANI [18] with ionic fragments up to 513 a.m.u., indicating the presence of the pentamer. However, IR investigations showed, that the degree of polymerization in this material should be higher because of the absence of signals of terminal phenyl groups.

Using gel permeation chromatography (GPC) values for the molar mass of PANI are reported to be Mn  80000 for electrochemically produced [20] and Mn  25000 for chemically synthesized PANI [21]. In other investigations the molar mass of the polymer was determined for substituted monomers, e. g. o-aminobenzonitrile [22], N-methylaniline and diphenylamine [18] or the dimer p-aminodiphenylamine [23]. Poly-o-aminobenzonitrile showed a mass spectrum where the highest mass ion signal appeared at 1430 a.m.u., corresponding to 12 monomer units [22]. This result was supported by GPC, where a peak at Mn  1400 was observed. For electrochemically produced poly-p-aminodiphenylamine a molar weight of 2800 was reported [23].

In this contribution for the first time mass spectra are presented of CO2 laser desorbed PANI using multiphoton ionization (MPI). Mass spectra of chemically as well as of electrochemically produced PANI were investigated. MPI mass spectra of the polymer contain to a high degree specific information on the structure of the desorbed material. They also suggest chemical interaction of the polymer with stainless steel probes.

Experimental

Chemically synthesized PANI was obtained from Zipperling in the form of dispersions. PANI used in these dispersions was Versicon [24]. The dispersion was put on stainless steel or Pt probes and the solvent was evaporated at 70 C. This coating process was repeated for 3-5 times until the probe appeared visually completely covered by PANI. Prior to applying the dispersion, the probe was polished in 4 steps with diamond paste and Al2O3-powder (Leco) with particle sizes down to 0.05 m. The experimental setup allowed the direct transfer of the probe with the deposited PANI film into the mass spectrometer through a vacuum lock. Generally mass spectra were recorded about 20 minutes after the first contact of PANI with the metal surface. In the chemical as well as in the electrochemical synthesis PANI was obtained in its green, protonated form of the emeraldine base.

Electrochemical polymer preparation was carried out in a three-electrode compartment cell from 0.1 M aniline in aqueous 0.5 M H2SO4 using potential sweep technique in the potential range -0.2 to 0.9 V vs. SCE at 50 mV/s. A change of the upper switching potential between E = 0.9 and 1.1 V had no significant influence on the obtained mass spectra. The thickness of the films obtained after 20 cycles by passing a total anodic charge of about 0.6 C/cm2 was greater than 2 m [25]. The working electrode was of identical shape as the probes used in experiments with dispersions. Its surface area was 25 mm2. The counter electrode had a surface of 2.5 cm2.

For polymer analysis a reflectron time-of-flight mass spectrometer type Bruker TOF1 was used (see Fig. 1).

The probe was positioned in the radiation field of a CO2 laser in the differential pumping stage of the spectrometer. From the probe the deposited polymer was evaporated by pulses from this laser. The pulse energy was about 60 mJ with a duration of 10 s. The laser beam was slightly focused with a ZnSe lens (f = 100 mm) to give a calculated power density of  6105 W/cm2. Lowering the power density led to a decrease of the signal intensities of PANI, while the masses of the observed ion signals did not change. Evaporation took place 1 mm below the nozzle of a pulsed supersonic Ar jet which transported the evaporated polymer and possible fragments through a skimmer into the ion source of the mass spectrometer. This technique of CO2 laser desorption in combination with jet cooling is documented in the literature through reports by Schlag et al. [26] and Lubman et al. [27]. Besides being an effective vehicle to transport evaporated material into the ion source the expanding supersonic Air jet provided for an effective cooling of the molecules [28]. Thus, decomposition of large, thermally unstable molecules is greatly reduced. Most important for mass spectrometric analysis is the translational cooling, i.e. the narrowing of the velocity profile of the molecules in the jet which enhances mass resolution. Using multiphoton ionization (MPI), the mass analysis could be carried out at a resolution greater than 2500. Due to its positive potential of 620 V no desorbed material carrying a positive charge could enter the ion source of the spectrometer.

For photoionization of the evaporated polymer in the ion source a system was available which consisted of a Lambda Physik FL3002 dye laser pumped by a Lambda Physik EMG 150EC excimer laser. The output of the dye laser was frequency doubled to reach the wavelength range around 250 nm. The laser pulse energy was about 0.3 mJ with a duration of 5 ns. A repetition rate of 10 s-1 was used in most experiments. Thus to register 100 complete MPI mass spectra for averaging an experimental time of 10 s was required. The UV radiation was moderately focused with a plane convex lens of 10 cm focal length, giving a calculated power density of about 107 W/cm2 assuming Gaussian laser profile. With this power density, good signal-to-noise ratio for masses up to 1000 a.m.u. was observed. By further focusing of the laser beam, strong fragment ion signals on the masses of PANI were obtained and the intensity of background signals became stronger. Under focal conditions the base peak in all mass spectra was found at 12 a.m.u. corresponding to atomar carbon. Defocusing the laser beam led to a drop in signal intensities.

EI mass spectra were registrated with an electron energy of 20 eV to minimize fragmentation.

Results

Laser MPI of PANI at the wavelengths of 262 nm or at 249 nm, carried out as a two-quanta process [29], can be considered as soft ionization. The combined energy of two quanta in this wavelength range is roughly 10 eV. This energy is high enough to ionize even monomer aniline molecules, the ionization potential of which is reported to be below 10 eV [30]. For fragmentation, absorption of a third photon would be required. Photoionization constitutes a resonant step function process [26]. Furthermore, wheras in EI only the kinetic energy of the electrons can be adjusted, in MPI in addition to variation of the energy of the ionizing quanta also their density can be varied over orders of magnitude. With high power densities excessive fragmentation and strong signals in the low mass range are obtained. Upon attenuation of the power density of the laser output, situations can be reached to obtain acceptable signal intensities in the high mass range and to minimize at the same time fragmentation [31]. MPI mass spectra of PANI can be obtained much poorer in fragments than the corresponding EI spectra.

Mass spectra in this contribution are presented as stick spectra. They were calculated by integrating signal intensities over each mass peak. Generally 100 single spectra were averaged for the presentation. Individual spikes that showed up occasionally only in the record and all signals with an intensity less than 0.5 % of the base peak were rejected. Mass spectra were recorded up to masses > 8000 a.m.u., but with the present settings signal intensities in the mass range > 1000 a.m.u. became low and exhibited too much scatter for a reliable analysis. From an investigation of several dozens of different PANI samples a general pattern emerged that shall be presented below.

In Fig. 2 is shown a MPI survey mass spectrum of chemically synthesized PANI obtained at an ionization wavelength  = 262 nm for the range up to 1000 a.m.u. At this wavelength a virtually identical spectrum was obtained from a electrochemically produced sample. Nine groups of ion signals can be identified in this mass range with decreasing intensities towards higher masses. In each group one main ion is marked to assist in the interpretation of the fragmentation pattern as outlined below. The mass difference between the main signals in these groups is 91 a.m.u., corresponding to a building block C6H4NH in the polymer chain. In the following, such ion signals will be denoted as series. For clarity in Fig. 2 only one series containing one chinoide structure (see below) is marked with a triangle. No such series could be recognized in EI spectra.

For the analysis of PANI MPI mass spectra in Fig. 3

is shown a magnification of Fig. 2 for the mass range 150 - 300 a.m.u. This range contains two groups of signals. Intense signals with a spacing of 91 a.m.u. belonging to one series are always marked with the same symbol. Six series are identified and marked (see Table 1). Four of them start at masses < 200 a.m.u., i.e. 167 (), 182 (),184 (), and 197 a.m.u. (), respectively, and reach up to masses in the signal group around 900 a.m.u. (not explicitly shown). The residual two series start at 256 a.m.u. () and 271 a.m.u. (). They can be followed up to 802 a.m.u. and 743 a.m.u., respectively. Another information that can be extracted from Fig. 3 is the preference for a spacing of two mass units between the more intensive ion signals. This characteristic feature is observed throughout the whole mass spectrum, i.e. also in the high mass range above 500 a.m.u.

Fig. 4 shows a survey mass spectrum of chemically synthesized PANI recorded at an ionization wavelength of  = 249 nm. The power density of the ionizing laser was nearly the same as in the experiments at  = 262 nm but the photon energy was higher. By analogy to EI in which ion fragmentation increases when the electron energy is raised also in MPI ion fragmentation increases with photon energy. Again groups of signals with a spacing of 91 a.m.u. are observed. In analogy to Fig. 2 one series is marked with triangles. The decrease of signal intensities towards higher masses is more prominent at  = 249 nm than in the experiments at 262 nm.

Most dominant in the spectrum is an ion signal at the mass of 56 a.m.u. In Fig. 4 this signal is downscaled by a factor of two. This signal was only obtained with stainless steel probes. It was missing in experiments with the Pt sample holders. It was also absent in all experiments at 262 nm irradiation. The intensity of the signal at 56 a.m.u. depended mainly on two parameters, i.e. the thickness of the polymer film and the time between the coating process and the analysis of the polymer. A signal at 56 a.m.u. was already observed with very thin polymer layers but increased until the probe was completely covered by PANI. Very thick polymer films (thicker than 10 m) led to a different desorption/ablation process in which the whole film was evaporated in one single laser shot [32]. All spectra shown in this contribution were registered about 20 minutes after the first contact of PANI with the stainless steel surface. After storing a coated probe for twelve hours at room temperature, the signal at 56 a.m.u. was about a factor of two larger than in the fresh one.

The spectrum of electrochemically synthesized desorbed PANI photoionized at a laser wavelength of  = 249 nm is given in Fig. 5.

For the preparation of the polymer, a stainless steel working electrode has been used instead of a Pt electrode. As in Fig. 4 all characteristic groups of ion signals were detected with the highest masses being in the 600 a.m.u. region. The strongest signal in the spectrum was observed at 93 a.m.u., i.e. the mass of aniline. Additional signals at < 200 a.m.u., were obtained at 27, 44 and 173 a.m.u. (see discussion). These signals did not show up in the MS of chemically synthesized PANI.

Discussion

All photoionization mass spectra obtained from PANI showed characteristic ion signal groups with a spacing of 91 a.m.u. These signals could be attributed to fragment ions which are based on aniline oligomers with different numbers of C6H4NH-units. Table 1 summarizes the assignments of the six most prominent ion series. The given structures reflect only their stoichiometry and possible arrangements. It is reported from MPI experiments, that the azepinium cation is involved in the aniline fragmentation process [29].

Marked with a square () is the series of oligomer anilines ions, which starts with the fragment C6H5NH2+ at 93 a.m.u. The second signal in this series corresponds to an ion of p­aminodiphenylamine stoichiometry (184 a.m.u.). This series of signals was also observed by Comisso et al. using electron impact ionization [18] and by Ogawa et al. [13] using field desorption ionization. While in our arrangement spectra showed the highest masses in the 900 a.m.u. range, in ref. [13] signals of high intensity were detected up to 3000 a.m.u. In a recent paper Duic et al. [17] showed that the production of PANI in relation to dimer formation is mainly dependent on the concentration of the aniline radical cation near the electrode. In the chemical synthesis for a change of the aniline concentration from 10-2 M to 10-1 M under the same experimental conditions they observed a PANI/p-aminodiphenylamine ratio that was about six times higher. A ten times higher aniline concentration as used by Ogawa et al. [13] might therefore have resulted in the formation of a polymer with a distribution reaching higher masses.

Duic et al. [17] also described that in the electrochemical polymerization of aniline a much better PANI/p-aminodiphenylamine ratio was registrated when upper switching potentials E < 0.8 V were used. Unfortunately no polymerization on a stainless steel surfaces could be registered by us at these potentials.

By oxidation of one benzoide unit in the oligomer chain to a chinoide structure, two H-atoms are lost. The second series marked () starts at a mass of 182 a.m.u. and can be attributed to oligomer ion species which contain one single chinoide unit. None of the MPI mass spectra showed a signal at 91 a.m.u., i.e. a chinoide species C6H4NH+ which would be the origin of this series. The absence of this signal indicates that for the stabilization of a chinoide ion system the presence of at least one benzoide system is necessary. Fragmentation processes that correspond stoichiometrically to the loss of a terminal NH-group from ions of the -series led to the series marked with a circle (). This series has also been observed before by Comisso et al. [18] and Ogawa et al. [13].

Adding a second chinoide structure to the aniline oligomer ions leads to the two series marked () and () in Fig. 3. In general the same fragmentation is observed as for the molecules with one chinoide unit. However, these two series start at ion masses of 271 and 256 a.m.u. indicating again that for stabilization at least one benzoide structure must be present in ions with two chinoide units. Addition of further chinoide units gives rise to new ion series each with a mass of 2 a.m.u. lower than ions containing benzoide units. These ions show up preferentially in the high mass range (cf. Fig. 2).

Ions of the series marked () starting at 197 a.m.u. contain one chinoide structure, but the number of amine/imine groups was higher by one than the number of benzoide/chinoide ring systems. A series starting at 199 a.m.u. as observed by Comisso et al. [18] and Ogawa et al. [13] was also found, but did not belong to the main fragmentation pathways.

Upon ionization at the wavelength of  = 249 nm and using stainless steel probes an additional strong signal at 56 a.m.u. was observed (Fig. 4). This signal was unequivocally identified as iron by its observed isotopic distribution of 54Fe = 6.8 %, 56Fe = 100 %, 57Fe = 2.6 % and 58Fe = 0.5 %. No iron signals were detected at  = 249 nm from dispersions evaporated from a Pt-probe, demonstrating that the chemically synthesized Versicon did not contain any iron impurity that might have been introduced during chemical synthesis. In addition, no iron could be observed in CO2 laser irradiation experiments at the given laser power using uncovered stainless steel probes. The signal at 56 a.m.u. was only detected in the  = 249 nm experiments in the presence of iron in contact with PANI. This signal may indicate formation of an interfacial layer formed by a chemical interaction of PANI with the stainless steel. The fact that the iron signal intensities increased with the duration of contact time between PANI coating and steel surface lends strong support to this suggestion, i.e. the interfacial layer builds up in a slow reaction in the time between coating process and analysis. Weaker iron signals were detected already with very thin coatings, in which not the complete probe surface was coverage by PANI. The iron signal intensity seems to be correlated with the percentage of the probe covered by PANI until thick layers were produced (see results).

It has been reported that an interfacial layer exists between PANI and the steel surface [3] being exposed to air. Explicitly this layer consists of two parts, a layer of -Fe2O3 in direct contact to PANI on the outside and a layer of Fe3O4 in contact to the steel surface. In these oxide layers iron will be in oxidation states +3 and thus due to the positive potential of the ion source of the mass spectrometer, iron ions are not able to enter the ion source. Also, desorbed iron oxides ions have not been observed upon CO2 laser irradiation of stainless steel probes that have been exposed to air but were uncovered by PANI. On the other hand experiments with the present setup have demonstrated repeatedly that metal organic complexes can easily be evaporated and transported into the ion source to give metal ion signals [33]. From these observations it is suggested, that the iron carried into the ion source was complexed to PANI. The observed increase in iron signal intensities with PANI/steel probe contact duration dismisses the argument such complexes are formed during the desorption process at badly defined situations in which high temperatures might induce chemical reactions. Existence of such complexes in the interface on the steel probe may, however, contribute to the reported corrosion protection and may offer a plausible mechanism for electrode modification and corrosion behavior of stainless steel as observed in ref. [5].

In the mass range > 200 a.m.u. the spectrum of electrochemically produced PANI at  = 249 nm (Fig. 5) is very similar to the spectrum of chemically produced polymer at the same wavelength (Fig. 4). However, besides the iron signal at 56 a.m.u., which was again obtained with steel probes only, an intense signal on 173 a.m.u. is detected. Surprisingly the mass spectrum of 4-amino-benzenesulforic acid (PTS) registered under the same experimental conditions fits perfectly to the additional signals in the mass spectrum of the polymer. To the authors' knowledge there is no report in literature describing production of PTS during the polymerization process. Therefore it is suggested that PTS may build up during the desorption process from PANI and the imbedded SO42- counterions. The signal at 93 a.m.u., corresponding to monomer aniline ion appeared much stronger in the mass spectrum of the electrochemically synthesized PANI compared to the mass spectrum of Versicon. Due to the amorphous structure of the polymer, some aniline seems to have stayed included in the polymer matrix.

The mass spectrometric results obtained by the different authors reflect different experimental approaches. While Chan et al. [19] detected only masses < 290 a.m.u., Ogawa et al. [13] reported signals up to 3100 a.m.u. Substituted anilines like diphenylamine were found to have a chain length of only 3-4 monomer units when synthesized in water, while the same polymer produced in acetonitrile revealed a chain length that was 3 times larger [18]. These results already show, that the composition of the polymer depends sensitively on synthesis conditions. Changes in the used solvent as well as in monomer concentration can lead to very different chain length distributions in the polymers [17].

Conclusion

It is shown that MPI of laser desorbed PANI in combination with mass resolved analysis of the ions formed, offers detailed information on PANI itself and on the interaction of PANI with the probe surface. The mass spectra of Versicon obtained with an ionization wavelength of  = 262 nm showed groups of signals with characteristic masses up to 900 a.m.u. Most of the strong signals could be classified using a pattern of six fragmentation pathways. The number of chiniode systems in the observed fragment ion species could be clearly identified by the loss of 2 a.m.u. per aniline unit when going from a benzoide to a chinoide structure.

MPI mass spectra of Versicon obtained at a photoionization wavelength of  = 249 nm showed a higher degree of fragmentation than at 262 nm, however, the masses of the strong ion signals were the same. In addition PANI mass spectra from stainless steel probes revealed the presence of intensive iron ion signals that could not be found in experiments using uncovered stainless steel probes or from PANI coated Pt probe. These signals are suggested to arise from an interfacial layer formed by a chemical interaction of PANI with the stainless steel leading to iron complexation.

The MPI mass spectrum of electrochemically synthesized PANI at an ionization wavelength of  = 249 nm was nearly identical to the one of Versicon, except for an intense ion signal at 173 a.m.u., that is proposed to result from the formation of 4-amino-benzenesulfonic acid during the desorption process.

Acknowledgements: The authors acknowledge financial support by theTechnologiestiftung Schleswig-Holstein and the Fonds der Chemischen Industrie, Germany.

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