SYNTHESIS AND CHARACTERIZATION OF POLYANILINE AND
POLY-ORTHO-METHOXYANILINE. BEHAVIOUR AGAINST CARBON
STEEL CORROSION.

ROSA VERA A. , HUGO ROMERO B*, y EDUARDO AHUMADA

Laboratorio de Corrosión, Instituto de Química, Universidad Católica de Valparaíso,
Av. Brasil 2950, Casilla 4059, Valparaiso, Chile, rvera@ucv.cl
P.O.Box 462, Machala, Ecuador, cientifico80@hotmail.com
(Received: October 30, 2000 - Accepted: october 21, 2002)

ABSTRACT

In the present study the preparation of polyaniline (PANI) and poly-ortho-methoxyaniline (POMA) by chemical method, with different synthesis conditions, has been described. The obtained polymers were characterized by lnfrared and UV-Vis absorption techniques. The morphology of the resulting polymeric films was examined by scanning electron microscopy (SEM). The dissolution inhibitive action on carbon steel was determined measuring the amount of iron(II) in the solution by the o-phenanthroline method. The results indicate that modified carbon steel presents a decrease in their dissolution within the corrosive medium. Also, this method confirmed that PANI-coated samples exhibited satisfactory protection against dissolution and are much more effective than POMA coated ones.

Key Words: Chemical polymerization, polyaniline, poly-ortho-methoxyaniline, inhibition, carbon steel.

INTRODUCTION

The interest for studying conducting polymers as inhibitors of corrosion, has increased notably in the last two decades. These materials for being polyconjugated systems having alternate simple and double bonds, possess a considerable electron availability, which provide them with a rigid structure and a better capacity to be adsorbed on metallic surfaces^1,2. As with other polymeric coatings, they act as a physical barrier isolating the metal from the corrosive environment. Moreover, because they have polar groups, they can shift the potential of the substrate towards more positive values, which are located in the passive zone^1,3.

Of all conducting polymers, those synthesized from aniline have demonstrated to be the best candidates to be used as protectors of corrosion, basically because they possess interesting physical, chemical, electrochemical, optical and mechanical properties^4-6. D.E. Tallman and col.⁷, have reported that polyaniline reduces the corrosion rate of metals due to the stabilization of the oxide layer formed in the metal/polymer interface. In particular, it can be found in the literature that the utilization of polyaniline as primary electroactive material for the preparation of stainless steel coatings, has caused a decrease in the corrosion rate of this metallic sustrate from 3.1 x 10⁴ m m/year to values lower than 25 m m/year³.

On the other hand, M.M. Attar and col.⁸, have determined that the presence of PANI on or near the surface of a sweet steel plate in contact with solutions of 0.01 M NaCl saturated of polyaniline, inhibits the disolution process. Corrosion potential measurements at open circuit, exhibited shifts of approximately 100 mV.

The barrier effect of PANI films is related to the physical and structural properties of the polymer, which in turn depend on the conditions of synthesis⁹. In this regard, because of the redox properties, the passive film can be maintained on the base metal¹⁰.

Although PANI and its derivatives have been evaluated in protection against metal corrosion, the effect of the different conditions for its chemical synthesis has not yet been well explored. For this reason, it becomes important to study the synthesis of polyanilines in different electrolytic media, which will lead to a better understanding of these promising materials¹¹.

In this work, the synthesis of polyaniline and poly-ortho-methoxyaniline (Fig. 1) in different electrolytic media is presented. At the same time, its spectroscopic and morphological characterization as well as the evaluation of its protective character against corrosion to carbon steel is reported.

Fig. 1. Chemical structures of a) polyaniline (PANI), b) poly-ortho-methoxyaniline (POMA).

EXPERIMENTAL PART

Chemical Synthesis.
All the polymers were obtained following procedures found in the literature^12-15. Two solutions were prepared, one with the oxidyzing agent ammonium persulfate ((NH₄)₂ (S₂O₈)) and the other the monomer: aniline (C₆H₅NH₂) or ortho-methoxyaniline (C₇H₉NO). Both precursors were dissolved in electrolytic media of: 1M CF₃COOH; 1M HCI - 1M NaCl; and 1M H₂SO₄ - 0.5 M Na₂SO₄. Table 1 presents the different electrolytes used in the chemical synthesis carried out in this work.

Table 1. Electrolytes used in the synthesis of polymers.

Solutions were cooled at 0 ± 0.1 ^oC and stirred for 2 hours under nitrogen atmosphere. The precipate formed (polymer) was filtered under vacuum and washed with acetone. Afterward, both PANI and POMA polymers were treated with ammonium hydroxide for deprotonation and kept for 16 hours, after which were filtered and dried under vacuum.

Morphological and Spectroscopic Characterization.
The IR spectra of the polymers were registered with a Perkin-Elmer Model 1605 FT-IR Spectrophotometer, using KBr pellets. The UV/Visible spectra were obtained in a Genesis 2 UV/Visible Spectrophotometer scanning between 250 and 800 nm. Adherence and morphology were examined by SEM with a JEOL 5410 instrument.

Corrosion Essays.
After synthesis and characterization of the polymers they were used for coating of the test material. For this, the polymers were dissolved in 1-methylpyrrolidinone (NMP), and the solvent evaporated at 90 ^oC on a hot plate until dryness.

Thickness measurements of all polymeric films deposited on carbon steel, were carried out using an Elcometer 345, determining an average thickness with a 95% confidence limit. In order to validate the results previously obtained, measurements were also performed by MEB. The results showed good agreement between both techniques.

To evaluate the protective action of the obtained polymers, bare and modified samples of carbon steel (exposed area: 1.0 cm²) were immersed in a known volume (50 ml) of a 5% NaCl solution for 1 month, after which the Fe(II) concentration was determined by the 1,10-phenanthroline spectrophotometric method¹⁶.

RESULTS AND DISCUSSION.

The chemical synthesis consisted in the direct oxidation of the monomer by oxidizing agents in acid medium, where the final product was a precipitate with conducting properties named esmeraldina salt, which when treated with base was converted into non-conducting PANI or esmeraldina base (Fig. 1a).

Figure 2 shows the IR spectrum of polymer P3, which was synthesized from aniline. The most important vibrational bands which allows the identification of polyaniline are: 829 cm^-1, out-of-plane C-H bond¹⁷, 1165 cm^-1 aromatic C-N-C bond¹ ; 1495 cm^-1 aromatic C=C double bond⁶ , and 1591 cm^-1 nitrogen bond between bencenic and quinonics rings². On the other hand, polymers synthesized using o-methoxyaniline as monomer (Fig. 3), present the same bands observed in Fig. 2, but in addition there are two bands at 1118 and 1257 cm^-1, which can be assigned to vibrations of the C-O-C bonds of the ether group¹⁸ and aromatic C-O¹⁹ respectively. These signals allow to distinguish polyaniline from poly-o-methoxyaniline (Fig. 1a and 1b).

As an example, in Table 2, are summarized the main wavelengths and assignments of the obtained polymers. The spectra shown in Figure 2, are characteristic of esmeraldina base and are in agreement with those reported by different authors^{16, 20, 21}. It should be noted that in both, PANI and POMA spectra, it should appear a signal at ~ 3400 cm^-1, typical of the N-H bond of a amino group, which is not observed because the polymers are totally deprotonated.

Fig. 2. IR spectrum of polymer P3 (PANI).

Fig. 3. IR spectrum of polymer P6 (POMA).

Table II. IR absorptions of polymers.

Reference absorption bands (cm-1)	Vibrational assignments	Experimental absorption bands (cm-1)
		P1	P2	P3	P4	P5	P6
1590	Benzenic-quinonic nitrogen	1588	1553	1591	1587	1580	1590
1490	C-C aromatic	1493	1455	1495	1455	1490	1490
1330	Aromatic amine	1302	1337	1305	1330	1303	1329
1250 1118	C-O aromatic C-O-C ether		1257 1120		1260 1116		1259 1118
1190	C-N-C	1117	1117	1165	1123	1158	1165
1030	C-H in-plane	1024	1027		1020	1006	1028
920	Poly-conjugated system	950	950	952*	950	953	950
830	C-H out-of-plane	843	850	829	850	820	849

* very weak absorption band.

The UV/Visible spectrum corresponding to polymer P1 dissolved in NMP (Fig. 4) shows two absorption bands characteristic of PANI in its esmeraldina base form, at wavelengths between 328-346 and 610-643 nm. The absorption band in the UV region is attributed to the chain of the aromatic nuclei and corresponds to the p - p ^* transitions of the polymeric skeleton²². On the other hand, the absorption band appearing in the visible region, indicates an interaction between the bencenic nuclei and the quinone di-imine structure.

Fig. 4. UV-Visible spectrum of polymer P1.

In Table 3, the UV-Visible data for each one of the synthesized polymers is summarized. It can be observed from this data that there is a small difference in the absorption at 600 nm for the polymers PANI and POMA, which can be attributed to the delocalization of the electron pairs on oxygen of the methoxy group (Fig. 1b).

Table III. UV-Visible absorption bands of polymers.

On the other hand, a comparison of the surface microphotographs of the polymeric films of PANI and POMA synthesized in 1 M CF₃COOH, leads to the conclusion that the morphology of these films is dependent on the monomer used in the polymerization process. This can be observed in Figures 5 and 6, where PANI shows a fibrilar morphology, whereas POMA presents a rough surface.

Fig. 5. Microphotograph of polymer P1 deposited on carbon steel (500x)

Fig. 6. Microphotograph of polymer P2 deposited on carbon steel (500x).

As far as the adherence presented by the formed films, Figures 7 and 8 show the cross-section microphotographs of polymers P2 and P5 deposited on carbon steel, in which a good adherence can be observed in the metal-polymer contact zone, regardless of the media used in the process of synthesis.

Fig. 7. Cross-section microphotograph of polymer P2 deposited on carbon steel (5000x).

Fig. 8. Cross-section microphotograph of polymer P5 deposited on carbon steel (5000x).

It can be seen from Table 4 that the thickness values obtained by Elcometer 345, do not show significant differences between the films, which is in agreement with the results obtained by MEB. Within this context, when comparing Figures 7 and 8, it is possible to say the P2 and P5 present average thickness of approximately 3.0 m m. Moreover, in Figure 7 it is possible to observe again the surface roughness shown by the polymeric film P2, in concordance with observation presented in Figure 6. However, for P5 (Fig. 8) a more smooth deposit is evident.

Table IV. Average thickness of polymeric films (95% confidence limit).

Once the thickness of the deposited films on carbon steel were determined, its protective behaviour against metal dissolution was evaluated. The results obtained in this experiment are presented in Figure 9. From the analysis of this graph, it is possible to point out that as far as the barrier effect caused by the various polymeric films, these in general show a protective character against dissolution of carbon steel.

Furthermore, we can indicate that the polymers synthesized from aniline show a higher degree of protection than those obtained using o-methoxyaniline as monomer. For example, when comparing the metal modified with P1 (PANI) and with P2 (POMA), it could be established that the active dissolution of the bare metal with respect to the modified metal is approximately 5.5 and 2.5 times lower, respectively. A possible explanation for this is that the large volume of the substituent located in the poly-o-methoxyaniline, generate a steric impediment which perturbs the geometry of the polymeric chain. This induces the formation of a rough polymeric surface, as the one shown in Figure 6, avoiding a complete coverage of the metallic substrate^{4, 23}, which results in a decrease in the protection against its dissolution.

Another argument supporting the results presented in Figure 9, is related to the tridimensional structures of aniline and o-methoxyaniline. In the case of aniline, the amino group and the aromatic ring are in the same plane, whereas for o-methoxyaniline the methoxy group is perpendicular to such plane, increasing the torsion angle between this group and the phenyl group. This coplanar orientation respect to the metallic surface, confers to polyaniline a greater capacity to form more homogeneous films that poly-o-methoxyaniline, which in turn it translates into a better behaviour as a barrier agent against the dissolution of the base metal.

Figure 9 also shows indirectly, for PANI and POMA, that the molecular size of the dopant (counterion), affects the morphology of all the studied polymers, as shown by the small variation in the degree of protection against the dissolution of the base material. This allows to establish an order of the protective behaviour as a function of size and charge of the dopant: CF₃COO^- > Cl^-, SO₄₌ . The above is in agreement with the report by D.C. Trivedi¹⁷ who indicated that PANI undergoes substantial volumetric changes during the processes of doping and undoping, which notably affect the mechanical force of the polymer. In addition, this author indicated that the polymerisation process is influenced by the size and ionic charge of the dopant.

Another way of representing the protective behaviour of the polymeric films against the dissolution of carbon steel, is by calculating the penetration rate (V_p) for each one of the samples, from the concentration of the metal ions mentioned above (Figure 9), assuming a generalized attack. This calculation was performed using the following expression:

where D g is the weight loss in grams for each sample, A is the exposed area of the sample in cm², t is the time of exposition in years, and d is the density of the metallic species in g/cm³. The values obtained for V_p are shown in Figure 10 in increasing order with respect to its behaviour as barrier agent in millinches per year (mpy). From an analysis of this figure, it can be said that the sample of the bare metal has a higher V_p, whereas for the steel modified with the polymeric films, the rate of penetration decreases notably.

Fig. 9. Concentration of Iron (mg/L) in bare and modified samples kept for 1 month in 5% NaCl.

Fig. 10. Penetration rate (mpy) of bare and modified carbon steel, 5% NaCl.

CONCLUSIONS

In this study, the different synthesized polymers have been characterized by IR and UV-Visible spectroscopy. It was possible by IR to distinguish between the chemical structures of both polymers (polyaniline and poly-o-methoxypolyaniline). This was due to the appearance of two additional vibrational bands at 1118 and 1250 cm^-1 corresponding to the C-O bond of the ether group and the aromatic C-O respectively, present in the poly-o-methoxyaniline. The polymeric films deposited on carbon steel, show good adherence and homogeneous thickness, regardless of the monomer used in their synthesis. However, they showed differences in surface morphology, being fibrilar for polyaniline and rough for poly-o-methoxyaniline. The decrease in the rate of penetration for modified carbon steel, demonstrated the protective capacity of these films against dissolution of material in 5% NaCl. It is believed that the morphological differences of the polymeric films as a function of the chemical nature of the monomers, influenced their behaviour as barrier effect against the agresive medium. In this sense, PANI films showed better protective properties than the POMA films.

ACKNOWLEDGMENTS

The authors thank the Dirección de Investigación de la Universidad Católica de Valparaíso for financial support through Project N^o 125.753.

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