Physical chemistry behavior of enteric polymer in drug release systems

We report an analysis based on the electrical impedance (EI) spectrum of the samples of enteric random copolymer poly-methacrylic acid-co-methyl methacrylate as a function of pH of media. Important aspects of the charge transport and conformational processes in enteric polymer can be identified by mapping the complex impedance as a function of the frequency, which allows that some parallelism between titration and EI measurements can be obtained. However, the latter technique reveals details of this complex equilibrium that not appear using common titration methods. The relaxation frequency observed in the impedance spectrum act as a probe for the detection of phase transitions and conformational changes of the polymeric chains, once the distribution of size of particles can be related with this parameter. The progressive introduction of the alkali and the variation of pH between 4 and 10 are associated with a three steps process, related to the equilibrium shift from a precipitated solid or suspension, to a colloidal-like dispersion and to a complete solubilization of the copolymer. All those experimental features were reflected simultaneously as a turning point in plots of impedance, relaxation frequency and visible absorption with alkali addition giving a better and detailed insight to these processes.