STOCHASTIC MODELING OF CONTROLLED-RELEASE FROM POLY-STYRENE-CO-4-VINYLPYRIDINE MICROSPHERES

The aim of this study was to establish a stochastic approach for model ing controlled release of a dye as a function of the composition of a group of polymer microspheres in various pH environments. The microsph eres were polymers based on styrene, 4-vinylpyridine and divinylbenzen e. The composition of the polymers were characterized by the ratio of styrene to 4-vinylpyridine and the mole% of divinylbenzene. Based on t he release profiles in a series of preliminary experiments, a compartm ental model that conceptually divided the sources of release from micr ospheres into shell and core regions was used. Two separate exponentia l models, each with two unknown constants, were used. The modeling pro cess consists of two steps. The first step was to determine the unknow n parameters for both models. The second step was to fit the estimated parameters in a multiple linear model of the three factors: monomer r atio, mol% of crosslinking agent and pH of the medium for release. The results showed that the general model described the release well for most cases. The discrepancy between the release pattern predicted by t he model and the experimental data could be caused by the transitional swelling points existing in pH-sensitive polymers and modification of crosslinking network by the swelling-de-swelling cycle of the dye-loa ding process.