Dynamics of swollen gel layers anchored to solid surfaces

We employ a dynamic micro light scattering technique to probe the thermal concentration fluctuations in surface-attached poly-N-isopropylacrylamide (PNIPAAm) gel layers swollen in ethanol as a good solvent. At the equilibrium swelling state, the relaxation function exhibits two decays in the time range between microseconds and seconds and the characteristic rates display a pure diffusive behavior. The fast cooperative diffusion increases with crosslinking density as a result of the decrease in the dynamic network mesh size. This increase is significantly stronger than the concentration dependence of the cooperative diffusion in uncrosslinked linear PNIPPAm solutions. Uniaxial swelling due to the surface attachment and structural inhomogeneities intrinsic to photo-crosslinked gels alter the dynamics of the surface anchored networks compared to the solutions. In contrast to the frozen inhomogeneities in conventional gels, the slow diffusion in the present anchored layers was found to be ergodic. It might relate to structural inhomogeneities but its nature is not clarified yet.