Glass transitions in binary drug + polymer systems

To evaluate miscibility, glass transition temperatures T g have been determined for two binary polymer (Plasdone S-630 copovidone or Eudragit® E) + drug systems as a function of composition. Each polymer serves for encapsulation of the anti-HIV drug Efavirenz. In both systems the T g vs. drug concentration diagrams are s-shaped. T gs of Efavirenz + Plasdone mixtures with drug mass fraction below φ drug = 0.6 are above linear values. This implies enhanced thermal and mechanical stability—an advantage for the drug encapsulation. In the other system, a strong negative deviation of T gs is observed over the entire compositional range and explained by positive excess mixing volumes. Several equations are used to represent T g vs. composition diagrams, but only one (Brostow et al. Mater Lett 2008; 62:3152) provides reliable results.