Water sorption, relaxation and crystallization in polymer-based pharmaceutical formulations

In this work, water sorption in films made of polyvinylpyrrolidone(PVP)-based polymers, indomethacin (IND), and their respective ASDs was measured. Then, the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) was used to model and predict water-sorption equilibria and was extended with the Non-Equilibrium Thermodynamics of Glassy Polymers (NET-GP) to describe even glassy mixtures. First, a less rigorous approach, based purely on diffusion and the free-volume theory, was developed that predicted the time constants of water-sorption kinetics in the ASDs. This way, combining PC-SAFT, NET-GP, and this approach provided a methodology that predicts the extent and rate of water uptake in ASDs based solely on water uptake of the respective polymer and the API. In addition, a more rigorous approach was developed that describes the shapes of water-sorption curves in greater detail and explains their curvatures by the slow relaxation of the polymer. Finally, a diffusion-crystallization model was developed that describes the simultaneous water sorption and crystallization kinetics based on the classical nucleation theory. The diffusion-crystallization model reproduced the simultaneous water sorption and crystallization kinetics with high quality while predicting reasonably sized crystals verified via microscopic images.