Analysis of controlled release in disordered structures: A percolation model

The analysis of controlled-release kinetics in disordered models (percolation clusters) of polymer matrices is developed in detail. It is shown that the ''anomalous diffusion'' experimentally found in the release kinetics from swollen gels, M(t)/M(infinity) similar to t(n), n > 1/2, cannot possibly be related to hindered diffusive motion in disordered media, i.e, is not a consequence of the disordered structure of the matrix. A kinetic model is proposed to account for an exponent n greater than 1/2. This is based on a two-phase kinetics which takes into account both field effects (deriving from potential interaction between solute and polymeric matrix) and entrapping effects due to geometric constraints.