The swelling dynamics of the ultrathin polyacrylamide (PAM) spin-coated films in saturated vapor of D 2 O and H 2 O were studied using neutron and X-ray reflectivity. A uniform scattering length density (SLD) profile represents the dry PAM films, whereas the SLD profiles corresponding to the swelled films were characterized with a decreasing solvent concentration along the film thickness from top surface to the film/substrate interface. The diffusion mechanism of D 2 O into the films was found to be a non-Fickian process, as the D 2 O diffusion coefficient was observed to be decreasing as a function of film thickness. The thickness dependent structural changes in the dry polymer films were suggested from the increased density of thinner films. The diffusion coefficient of polymer chains in the solvent on the contrary was independent of film thickness. The different nature of D 2 O−PAM interaction (stronger) as compared to H 2 O−PAM interaction was found to play a crucial role on the diffusion of polymer, where the diffusion coefficient of the chains was an order of magnitude higher in D 2 O as compared to that in the H 2 O. A lower value of the excluded-volume parameter in the case of D 2 O also indicates stronger monomer−solvent interaction.
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Mukherjee et al. (2007) studied this question.
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