COMPUTER-SIMULATION STUDIES OF ANOMALOUS DIFFUSION IN GELS - STRUCTURAL-PROPERTIES AND PROBE-SIZE DEPENDENCE

We have carried out simulations of particle diffusion through polyacrylamide gel networks, The model structures were built on a diamond lattice, in a simulation box with periodic boundary conditions. The method of structure generation consists of a random distribution of knots on the lattice and interconnection between randomly chosen pairs of knots. The structures generated by this chosen procedure approximate the topology of real polymer gels. Parameters that control the distance between knots and the degree of stretching of the chain permit us to simulate a polyacrylamide system in which the concentration of species as well as the degree of crosslinking can be compared to realistic gels as prepared by the available experimental procedures. These structures were geometrically characterized by the analysis of the pore size distribution and excluded volume. The structures thus generated are used as model networks for Monte Carlo studies of the diffusion of hard spheres in the restricted geometry. Modeling the deviations from the normal diffusion behavior as a purely geometrical phenomenon, these simulations can give us insights into the factors which lead to anomalous diffusion in gel-like systems, In these simulations a sphere of variable radius is allowed to perform an off-lattice random walk through the space between the generated structures, which are assumed to be rigid. It will be shown that the study of the influence of tracer size on diffusion is complementary to the study of the influence of obstacle concentration on the diffusion of monodispersed particles. While the latter method can give us information about the fractal nature of the internetwork space the former method provides information about the fractal nature of the network and the connectivity of the internetwork space. (C) 1995 American Institute of Physics.