COMPUTER-SIMULATION OF MOLECULAR-DIFFUSION IN AMORPHOUS POLYMERS

Diffusion of small molecules in amorphous polymers has been examined b y 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 QU ANTA, CHARMM and Cerius(2) programs. The four amorphous polymers used in our calculation were polyethylene (PE), poly(dimethyl siloxane) (PD MS), ply(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 (D-c) were compared with the exp erimentally obtained values (D-e) found in the literature. The ratio o f D-c/D-e varied from 0.04 to 24 000. In general, the close values of D-c to D-e were obtained when the system dealt with hydrophobic molecu les diffusing through hydrophobic polymers. The D-c/D-e ratio became e ither very high or very low when the experimental system included hydr ophilic diffusants and/or a hydrophilic polymer. The simulation time a nd the size of molecular models also played key roles in determining t he consistency of calculations and the correlation with experimental v alues. Our study suggests that the current computer simulation of mole cular diffusion may be useful in obtaining relative values rather than absolute diffusion coefficient values. (C) 1997 Elsevier Science B.V.