Diffusion constants in polyacrylamide gels

The stimulated spin-echo method with pulsed magnetic field gradients was used to study the self-diffusion process of the fluid in swollen and collapsing polyacrylamide gels. Restricted-diffusion effects are evidenced by the dependence on the time of diffusion Δ of the diffusion constant ${\mathit{D}}_{\mathit{f}}$. The results are interpreted in terms of the diffusion process through permeable barriers. A changeover in the time behavior of ${\mathit{D}}_{\mathit{f}}$ for the gel around the critical point is pointed out. The pseudodiffusion of the chains is investigated separately. Within the time scale of the experiment (≤100 ms) the squared dependence on the magnetic field gradient of the echo amplitude allows the definition of a local diffusion constant ${\mathit{D}}_{\mathrm{ch}}$ for the monomer units. ${\mathit{D}}_{\mathrm{ch}}$ depends on the measuring time in the form expected when restoring forces are active.