Swelling dynamics and swelling induced structural changes of polyelectrolyte ultrathin films

Swelling dynamics and swelling induced structural evolution of spin coated poly(sodium-4-styrenesulfonate) (NaPSS) ultrathin films in presence of water vapor have been studied using X-ray scattering and AFM techniques. Swelling dynamics of the films has been modelled in terms of the swelling of gyration ellipsoid of polyelectrolyte chains. In presence of water vapor swelling of these gyration ellipsoids is found to be independent of the film thickness. In presence of water vapor not only the thickness of the film increases but their in-depth and in-plane structures undergo continuous modification; that being different for thinner and thicker films. In–plane structural studies of the films indicate that undulations or faceting are present on film surface and contribution from each facet shows an oscillating behaviour with swelling time for thinner films. While for thicker films in-plane modification was unidirectional. The structural changes of the films in presence of water vapor are found to be permanent in nature for both thinner and thicker films and overall film quality diminishes on prolonged swelling. Pre and post annealing XRR data shows that rather than stress relaxation, solvent–molecule interaction is mainly responsible for the structural evolution of the polyelectrolyte films.