Effect of Anions and Added Organic Solvents of Polymerizing Solutions on the Conductivity of Poly (N-methylaniline)

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Poly (N-methylaniline) (PNMA) is prepared by the electro-oxidation of N-methylaniline in aqueous acid solutions. To prepare highly conductive PNMA, N-methylaniline was electropolymerized in several aqueous acidic solutions containing different organic solvents (acetonitrile, N,N-dimethylformamide an dimethylsulfoxide) and anions (ClO₄⁻, Cl⁻, NO₃⁻ and SO₄²⁻). After the initial stage of the electropolymerization for the polymerizing solutions without the organic solvents, the anodic current linearly increased for the Cl⁻, NO₃⁻ and SO₄²⁻ solutions, while it decreased for the ClO₄⁻ solution. The decreasing current for the ClO₄⁻ solution meant that the polymerization proceeded by oligomer-coupling reactions. The linear increasing current for the Cl⁻, NO₃⁻ and SO₄²⁻ solutions implied one-dimensional nucleation growth of PNMA on the electrode surface. The polymerization rate estimated from the slope values of the i-t curves was in the order of SO₄²⁻>NO₃⁻>Cl⁻. The conductivity of the obtained PNMAs was of the same order and the highest conductivity of 2.2×10⁻³ S cm⁻¹ was seen for the SO₄²⁻ doped PNMA. The order was explained by the Hofmeister series of the anions which are based on lyophilicity. During the polymerization, both the anion release from the monomer-anion and oligomer-anion ion pairs and the anion doping of the polymer occurred most frequently for SO₄²⁻ with the lowest lyophilicity. The organic solvents were added to the SO₄²⁻ polymerizing solution and the electropolymerization was performed. The conductivity of the obtained PNMAs was further enhanced. The most conductive PNMA was obtained when dimethylsulfoxide was added (σ=1.0×10⁻² S cm⁻¹). The PNMA polymer chains were tangled and stacked with each other by electron-donating association, and the tangling and stacking prevented the anion doping reaction. The addition of the organic solvents suppressed the tangling and stacking and the anion doping reaction was effectively promoted.