Diffusion of a Flexible Polymer in a Random Porous Material

We report the direct measurement of the diffusion of polystyrene in porous silica by dynamic light scattering. The effective diffusion coefficient is smaller than the diffusion coefficient in bulk solution ($D_0$), and depends on the scattering wave vector $q$. At low $q$, macroscopic diffusion coefficients ($D_{\infty }$) are obtained; the ratio $\frac{D_{\infty }}{D_0}$ depends on the size of the polymer. At higher $q$, where $q^{-1\mathrm{}}$ is comparable to pore dimensions, more complicated "transitional" behavior is seen.