DIFFUSION OF A POLYMER-CHAIN IN POROUS-MEDIA

Using an off-lattice Monte Carlo bead-spring model of a chain in a ran dom environment, we study chain conformations and dynamic scaling of d iffusivity and relaxation times with chain length N and density of the host matrix C-obs. Our simulational results show that with growing C- obs the mean size (gyration radius) of the polymer, R(g)(2), initially slightly decreases and then rapidly increases as the macromolecule ex ceeds the size of the average entropic wells and stretches through bot tlenecks into neighboring wells. The chain dynamics changes from a Rou se-like one into a reptational one as the permeability of the matrix d ecreases. Although at variance with some previous treatments [M. Muthu kumar, J. Chem. Phys. 90, 4594 (1989)], these findings agree well with a recent analytic approach (S. V. Panyukov, Zh. Eksp. Teor. Fiz. 103, 1287 (1993) [Sov. Phys. JETP 76, 631 (1993)]) to chain conformations in random media. We also suggest a simple scaling analysis, based on a ''blob'' representation of the polymer chain, whereby the blob size i s governed by the size of the cavities in the host matrix and yields a faithful description of our computer experiments.