Time scale dependence of diffusion in porous material: Dynamic light scattering and computer simulation

Diffusion of polymer chains in liquid-filled pores of controlled pore size silica glasses was studied experimentally by using dynamic light scattering (DLS), and by a computer simulation based on a hydrodynamic theory of hard spheres in cylindrical pores. Dynamic light scattering at fixed scattering wave vector revealed faster apparent diffusion at short times (corresponding to diffusion within a single pore) followed by a slower relaxation which we attribute to macroscopic diffusion over distances large enough to average over the microscopic nonuniformities of the glass-pore matrix. The measured time at which the behavior crosses over from faster to slower diffusion was found to be independent of the light scattering wave vector, and is roughly equal to the time required for a polymer molecule to diffuse a distance comparable to the pore radius. At small ratio of polymer to pore radius, the crossover time was found to be proportional to the polymer radius, both in DLS measurements and in the computer simulation.