Effect of Solvent−Polymer Interaction in Swelling Dynamics of Ultrathin Polyacrylamide Films: A Neutron and X-ray Reflectivity Study

Abstract

The swelling dynamics of the ultrathin polyacrylamide (PAM) spin-coated films in saturated vapor of D2O and H2O 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 D2O into the films was found to be a non-Fickian process, as the D2O 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 D2O−PAM interaction (stronger) as compared to H2O−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 D2O as compared to that in the H2O. A lower value of the excluded-volume parameter in the case of D2O also indicates stronger monomer−solvent interaction.