Predicting the diffusion coefficient of water vapor through glassy HPMC films at different environmental conditions using the free volume additivity approach.

Prediction of diffusion coefficient of polymer materials is important in the pharmaceutical research and becomes the aim of this paper. This paper bases the prediction method on the estimation of the polymer fractional free volume at different environmental conditions. Focussing on glassy polymers, the free volumes of polymer films were estimated using the model of Vrentas et al. [J.S. Vrentas, J.L. Duda, H.-C. Ling, Antiplasticization and volumetric behavior in glassy polymers, Macromolecules 21 (1988) 1470-1475]. The required data are the moisture sorption and glass transition temperature data, which were measured on various hydroxypropyl methylcellulose (used as a model material) free films at different water activities. The temperature and molecular weight particularly determine the free volume of the polymer, while the sorbed water can either decrease or increase the specific free volume of the polymer. At high water activity, the amount of water sorbed in the film increases to such level that the direct free volume addition by water becomes proportional to the contribution of the polymer itself. This confirms the importance of considering the environmental effect on the diffusivity of polymer during coating material selection. The presented approach enables the prediction of the diffusivity at any given relevant material variable and therefore has the potency to be used as a formulation development tool.