EFFECT OF QUATERNIZATION ON ELECTRON-DIFFUSION COEFFICIENTS FOR REDOX HYDROGELS BASED ON POLY(4-VINYLPYRIDINE)

The effect of partial quarternization of pyridine rings on the electron transport characteristics in redox-conducting hydrogels of cross-linked poly(4-vinylpyridine), partially complexed with [Os(bpy)(2)Cl](+/2+), was studied using steady state voltammetry at interdigitated array (IDA) electrodes. The apparent electron diffusion coefficients, D-app, and their pH dependence were mapped for redox hydrogels varying in the (a) extent of quaternization of their pyridine rings, (b) nature of quaternizing groups, (c) degree of cross-linking, and (d) loading with [Os(bpy)(2)Cl](+/2+) centers. In gels made with polymers that were not quaternized, high electron diffusion coefficients were observed only when the pyridine rings were protonated. However, in gels made with substantially quaternized redox polymers, high and pH independent electron diffusion coefficients were observed. When one-half of the pyridine rings were quaternized, the electron diffusion coefficients were completely independent of pH and reached a value of 3.9 x 10(-8) cm(2) s(-1). Excessive cross-linking decreased the electron diffusion coefficients. At a fixed level of quarternization the electron diffusion coefficients increased with the density of redox centers. This increase resulted, however, from the added charge; when the density of electrostatic charge per macromolecule was fixed, i.e., when the degree of quaternization was increased to compensate for a lower loading of redox centers, the electron diffusion coefficients were the same. It was concluded that the rate of electron transport is determined by the segmental motion of the polymer chains in cross-linked redox hydrogels.