Self-Patterning of Polyelectrolyte Multilayer Films: The Roles of PSS Molecular Weight, the Top Layer, and Post-Preparation Treatment

Abstract

The self-patterning of thin films is relevant for both fundamental research and applications. We investigate polyelectrolyte multilayer films made from poly(diallyldimethylammonium) chloride and poly(styrene sulfonate) sodium salt (PDADMA/PSS). Various PSS with low molecular weight were used. First, the film thickness increases exponentially with the number of deposited PDADMA/PSS bilayers. The separation and height of the pillar-like domains increase significantly with each deposited PDADMA/PSS bilayer, as AFM images show. After the exponential growth regime, either a parabolic (and then a linear) or a linear growth regime follows, depending on the PSS molecular weight. The domain separation changes less and correlates with the vertical growth regimes. The domain separation varies between 70 and 750 nm and always exceeds the domain height. PSS-terminated films show the same domain distance in water and air. However, when PDADMA-terminated films are dried, the domain distance in air increases while the domain height decreases, causing a reduction in total area. In the air, the surface energy is greater than in water and a highly textured surface costs a lot of energy. We propose that the changed surface pattern is attributable to energy minimization. Furthermore, the domains are stable when exposed to 1 M NaCl solution but shrink enormously in 2 M NaCl, while their separation increases slightly. Under certain conditions, 50 nm broad filaments consisting of PDADMA/PSS complexes are observed. We suggest that these complexes diffuse between the domains and adjust their separation.