Computer simulation of molecular diffusion in amorphous polymers

Diffusion of small molecules in amorphous polymers has been examined by computer simulation. Diffusion coefficients of small molecules with molecular weights ranging from 16.04 (methane) to 452.50 (fluocinolone acetonide) Da in four amorphous polymers were calculated using the QUANTA, CHARMM and Cerius2 programs. The four amorphous polymers used in our calculation were polyethylene (PE), poly(dimethyl siloxane) (PDMS), poly(methyl methacrylate-co-hydroxyethyl methacrylate) (P(MMA-co-HEMA)), and ethyl and benzyl esters of hyaluronic acid (HA-E, HA-B). The calculated diffusion coefficients (Dc) were compared with the experimentally obtained values (De) found in the literature. The ratio of Dc/De varied from 0.04 to 24 000. In general, the close values of Dc to De were obtained when the system dealt with hydrophobic molecules diffusing through hydrophobic polymers. The Dc/De ratio became either very high or very low when the experimental system included hydrophilic diffusants and/or a hydrophilic polymer. The simulation time and the size of molecular models also played key roles in determining the consistency of calculations and the correlation with experimental values. Our study suggests that the current computer simulation of molecular diffusion may be useful in obtaining relative values rather than absolute diffusion coefficient values.