Importance of glassy-to-rubbery state transitions in moisture-protective polymer coatings

The aim of this study was to better understand the mechanisms governing water transport in polymeric film coatings used for moisture protection. Tablets containing garlic powder were coated with Methocel® E5 (hydroxypropyl methylcellulose), Opadry® AMB [poly(vinylalcohol)-based formulation] and Eudragit® E PO [poly(methacrylate-methylmethacrylate)]. Their water content at different temperatures and relative humidities were determined. The polymers were characterized by X-ray powder diffraction and differential scanning calorimetry (DSC). The latter revealed significant physical changes in Opadry® AMB during storage, while Eudragit® E PO remained unchanged. For Opadry® AMB, a strong dependence of the vapor permeability on the water content of the system was observed. The water uptake drastically increased with increasing relative humidity and storage temperature due to structural polymer changes (glassy-to-rubbery state transition). Linear relationships between the initial uptake rate and the relative humidity or temperature were established. Storage below critical threshold values of 66% RH (at room temperature) and 16 °C (at 75% RH) significantly reduced water imbibition. Opadry® AMB-based film coatings undergo a glassy-to-rubbery state transition upon storage at room temperature and elevated relative humidity, resulting in significantly increased mobility of the polymer chains and, thus, increased water uptake rates.