(591c) Diffusion of Small Molecules in Rubbery Polymers

The rate of transport of small molecules in polymers, such as monomers, oligomers, plasticizers or additives plays a key role in both polymer reaction, polymer processing and in applications where sorption/desorption kinetics are involved (e.g. drug release from polymer matrixes). Despite that, and the numerous models proposed in past years, to date no fully predictive theory exists to evaluate diffusion coefficients in polymers.[1] Here we show that the Free Volume (FV) concept may be used as a bridge between kinetic and thermodynamic theories: assuming a correspondance between the FV and the unoccupied volume of pure components as given by Lattice Theories (e.g. the Sanchez-Lacombe molecular theory [2]) a new equation is obtained for the calculation of self diffusion coefficients in multicomponent mixtures.[3] A parameter evaluation scheme is proposed, which is substantially simpler compared with the conventional Vrentas-Duda approach,[4] even without losing generality. The theory is compared with experimental data of self diffusion coefficients for several solvent/polymer binary mixtures above the mixture glass transition temperature in wide ranges of physical conditions showing that good correlative ability is achieved in all cases. [1] L. Masaro, X. X. Zhu, Prog. Polym. Sci. 1999, 24, 731. [2] I. C. Sanchez, R. H. Lacombe, Macromolecules 1978, 11, 1145. [3] L. I. Costa, G. Storti, J. Polym. Sci. Part B: Polym. Phys. 2010, 48, 529. [4] J. S. Vrentas, J. L. Duda, J. Polym. Sci. Polym. Phys. Ed. 1977, 15, 403.