Ionic ingress and charge-neutralization phenomena of conducting-polymer films.

Ionic ingress and diffusion through a conducting-polymer (CP) film containing embedded charges under potential and concentration gradients is studied. Electroneutrality, a common assumption in modeling of similar systems, is not justified in this case or similar diffusion-limited processes, as the timescale of ionic diffusion in the solid matrix is quite large. Counter ions therefore cannot move instantaneously for effective neutralization of excess charges. Poisson-Nernst-Planck (PNP) equations have to be solved for such cases without any simplifying assumption. Analytical solution shows the existence of a charge boundary layer, which limits and strongly influences the ionic flux. A general numerical method for solution is also developed for the dynamic modeling, analysis, and design of these types of systems. The numerical results are validated by comparison with analytical solutions as well as with some experimental results available in the literature. With this modeling framework, the basic features of controlled release of molecules across a CP film under an applied electrical potential can be explained quantitatively.