- Solution: 0.05 M Aniline, 0.05 M 2-ABSA (ortanilic acid) in 1 M HCl
- Potentiodynamic growth: 25-30 cycles between -0.2 V and 0.9 V (vs SCE), at a scan rate of 50 mV s-1
Characterization of SPAN after synthesis:
1) Quantification of the amount of deposited electroactive polymer: i) preconditioning (reduction of the whole film) during 180 s at -0.2 V in 1 M HCl, followed by ii) potential sweep up to 0.6 V at 50 mV s-1. The integration of the first oxidation peak of SPAN can give an idea of the total amount of polymer deposited, which can in turn give an estimation of the thickness of the film.
2) Studying the effect of the pH on the electrochemical behaviour of SPAN in different electrolytes with pH values ranging from 0 to 9. Previous studies in Alicante showed that both peaks (L?ES, ES?P) are pH dependent up to pH 5. Up from pH 5 only the 2nd peak is pH dependent. Electroactivity (and we tend to think that conductivity as well) is well preserved up to pH 8 for this SPAN copolymer. At pH 9 electroactivity is suddenly lost.
It is important to verify that this behaviour does not differ to much when SPAN is grown on a platinum wire (suitable for EPR), since my previous results were all obtained using glassy carbon discs as a substrate. It would also be useful to set the right potential limits for the cycling at each pH value.
To perform this study buffer solutions (i.e. 0.1 M) are preferred, in order to avoid local pH shifts in the surrounding of the electrode. A common salt such as KCl can be added to all buffers in order to prevent particular behaviours aroused from the diffusion of bulky anions such as ftalate, phosphate or borate.
3) Performing voltammetry (or step potential polarization) of the polymer inside the EPR cavity at selected pH values (in the range 0-9). Main interest would be focused on neutral pH where, due to the decreased potential range of stability of emeraldine, several authors have risen the question of whether is the oxidation of the polymer still proceeding in two steps or just in one step. Detection of EPR signal would point in the direction of semiquinoid structure formation as an intermediate, and therefore a 2 step process.
Alternative conditions for copolymerisation and subsequent study:
Changing the monomer ratio without modifying the potential program
1) High 2-ABSA to Aniline ratio. The concentration of 2-ABSA can not be much increased at acidic pH since it reaches saturation around 0.08 M. High ratios can, thus, only be achieved by decreasing the concentration of Aniline to the order of few milimolar units, which results in very slow polymer growth unless a substantial increase of the upper limit during the polimerization is selected. Many papers have used this approach to generate a highly sulfonated polymer. However, high sulfonation is not necessary for the self-doping effect and, in addition, it increases the solubility of the polymer film, that in most cases will be lost from the electrode even at neutral pH. Therefore, I would discard to go on in this direction.
2) Low 2-ABSA to Aniline ratio. It is interesting to observe what happens in the reverse situation: when the polymer is generated from a solution with a fixed concentration of Aniline (i.e. 0.05M) and decreasing concentrations of 2-ABSA, down to few milimolar units. Since Aniline is far more reactive than the sulfonated monomer, one may think that the growing polymer would be practically the same as common PANI. However, testing the so-obtained copolymers at neutral pH results in a remarkable electroactivity,
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which is not easy to explain. In the extreme case of a 10 fold difference in concentration, the redox process observed in the voltammogram seem highly irreversible but, nevertheless, the peaks are very intense and persistent. Therefore, this could be an interesting case where to apply a combined EPR/spectroscopic-in situ study.