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

We have carried out simulations of particle diffusion through polyacry lamide gel networks, The model structures were built on a diamond latt ice, 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 knot s. The structures generated by this chosen procedure approximate the t opology of real polymer gels. Parameters that control the distance bet ween knots and the degree of stretching of the chain permit us to simu late a polyacrylamide system in which the concentration of species as well as the degree of crosslinking can be compared to realistic gels a s prepared by the available experimental procedures. These structures were geometrically characterized by the analysis of the pore size dist ribution and excluded volume. The structures thus generated are used a s model networks for Monte Carlo studies of the diffusion of hard sphe res in the restricted geometry. Modeling the deviations from the norma l diffusion behavior as a purely geometrical phenomenon, these simulat ions can give us insights into the factors which lead to anomalous dif fusion in gel-like systems, In these simulations a sphere of variable radius is allowed to perform an off-lattice random walk through the sp ace between the generated structures, which are assumed to be rigid. I t will be shown that the study of the influence of tracer size on diff usion is complementary to the study of the influence of obstacle conce ntration on the diffusion of monodispersed particles. While the latter method can give us information about the fractal nature of the intern etwork 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.