Probing differential hydration of poly(vinylpyrrolidone) thin films using tracer mobility: an insight from...

Dynamics of small probe molecules have been routinely used to unravel the intrinsic details of charged ion transport in polymer brushes and polyelectrolyte multilayer (PEM) thin films. However, corresponding morphological properties affected with absorption of moisture have been hardly dealt with despite numerous applications of isotropic thin films in material chemistry and medical purposes. We have explored the overall structural changes associated with plasticization of PVP thin films by probing dynamics of small reporter (rhodamine 6G, Rh6G) molecules using fluorescence correlation spectroscopy (FCS). It was observed that under lesser amounts of absorbed moisture, the rigidity of the film matrix was high enough to inhibit appreciable molecular mobility. Nonetheless, with gradual increase in the moisture level within the film, molecular movement became extremely facile, so much so that it almost attained close to a solution like state. Molecular mobility was found to be dependent on both the method of preparation and the thickness of the thin films. The diffusivities mostly followed anomalous subdiffusive behaviors, reminiscent of dynamics of tracers in crowded cellular environments. The mobility was found to be independent of any electrostatic interaction between probe and polymer thin film. Hence, the tracer dynamics was attributed most likely to the viscoelasticity of the thin film matrix.