Polypyrrole coating films for nanoparticles

Abstract

Using AFM, adhesion (shear stresses are applied over more than 5 orders of magnitude) and conductivity measurements, we analyze the synthesis of polypyrrole (PPy) films in water by means of the oxidation of pyrrole by Fe3+ cations. The PPy polymer chains that result, are composed of conjugated bounds which are conductive when appropriately doped, with Cl− ions here, for providing the necessary electric charge carriers. Two processes are available. At low concentrations of the reactants, below a volume polymerization threshold, a PPy film is directly synthesized on the substrates thanks to catalytic sites that reduce the activation energy of the reaction. Above the volume polymerization threshold, PPy aggregates appear everywhere in the bulk of the solution. After diffusion, they settle on the solid surfaces. The aggregates exhibit crushed irregular shapes at the AFM, that contrast with the rounded shape and the similar size of the dendrites synthesized in the surface polymerization regime. The film, being made of drop-off aggregates, is powdery, soft and labile, poorly conductive if not compressed. Below the volume polymerization threshold, conversely, the PPy film exhibits enhanced mechanical and conductivity properties thanks to numerous cross-links between the PPy chains. At lower concentrations of the reactants, below a percolation threshold, the film is isolating. The conductivity and the film thickness variations with the reactant concentrations and the reaction time appear to be consistent with a second-order kinetics in the surface polymerization regime. Except for the catalytic effects, the volume polymerization reaction works similarly. It exhibits a second-order kinetics too, which is consistent with a volume polymerization threshold obeying the relation [Py] [FeCl3]=5.5×10−4mol2L−2. The saturation of the conductivity of the PPy films at large reaction times in the surface polymerization regime, and its slowing-down in the volume polymerization regime indicate that the catalytic sites on the surface get masked after a couple of hours of synthesis time. The PPy conductivity in the surface polymerization regime is measured to be in the range 10–60Sm−1 when no compression is exerted on the film. These conditions appear to be appropriate for coating silica particles with a stiff and conductive PPy film.