Development of a long-acting therapeutic system: Part I: A method to produce silicon rubbers with well-defined...

Based on the free-volume theory of diffusion in rubbers it is shown that the diffusion coefficient D of a permeating drug can be written as the product of two probabilities D approximately w(1)w(2). The polymer chains in a rubber are subject to thermal vibrations. Hence collisions between vibrating polymer segments and permeating drug molecules can occur. w(1) describes the probability that by such a collision a drug molecule experiences an energy exchange which is large enough to overcome its interactions with its neighborhood. w(2) describes the probability that a permeating drug molecule will find in its immediate neighborhood a free volume being equal or larger than its own volume. Diffusion takes place if the drug molecule experiences a sufficiently large exchange of momentum and finds at the same time a sufficiently large free volume. As both events are independent their overall probability is given by the product of the two probabilities. For a given rubber and a given species of drug molecules in a first approximation w(1) can be considered as constant. This means the diffusion coefficient is mainly determined by w(2). This probability, however, is strongly determined by the microstructure of the rubber. In this study a procedure is developed allowing for a straightforward synthesis of silicone rubbers with defined network density.