(470b) Modeling and Simulation of Nanocapsule Formation

In this work, a model of spherical nanocapsule formation is developed to describe the solvent diffusion induced phase separation process from an initial homogeneous polymer/ solvent/ nonsolvent system.

This process consists in dispersing one phase containing polymer, solvent saturated with water and a non solvent into a water phase saturated with the solvent. The water is added at this emulsion, so the thermodynamic equilibrium is displaced and the solvent is extracted of the dispersed phase. This dilution induces a phase separation and nanocapsule formation.

The model is based on multicomponent mass transfer phenomena and takes into account the moving boundary induced by solvent extraction of the nanocapsule. In this model we use the extended version of the Maxwell-Stefan model for diffusion which takes into account of different sized molecules. In effect, polymer/ solvent/ nonsolvent system presents molecules with different molar volumes. Therefore to take into account of the volume occupied by molecules, Fornasiero and al. (2005) have developed the Maxwell Stephan formulation assuming that the collision between molecules occurs only if they are of equivalent volume. Therefore the natural state variables are volume fractions. The diffusion coefficient of polymer depends on its concentration.

The developed model is applied to describe nanocapsule formation and to predict the morphology associated with the formation of the thin polymer film.

This model is solved numerically using a finite volume method based on the variable grid and with a collocation method. These methods are compared. The finite volume method gives more precise results: the global mass balance is preserved. Moreover this method is more stable.

F. Fornasiero, J. Prausnitz et C. Radke, " Multicomponent Diffusion in Asymmetric Systems. An extended Maxwell-Stephan Model for Starkly Different-Sized, segment-accessible chain molecules, Macromolecules, 38, 1364-1370 (2005)