Hole formation induced by ionic strength increase in exponentially growing multilayer films

Polyelectrolyte multilayer (PEM) films consist of polyanion/polycation super-structures that are sensitive to various stresses like ionic strength changes. We investigate the swelling process of the exponentially growing poly(L-lysine)/hyaluronic acid (PLL/HA) films induced by changes of the ionic strength of the contact solution. We show that above a first critical ionic strength the swelling is accompanied by a release of both polyelectrolytes constituting the film, leading to its subsequent dissolution. At a second critical ionic strength, the swelling of the multilayer is so important that, in addition to this polyelectrolyte release, formation of spherical holes is observed inside the film. The presence of dissolved PLL and HA chains in these holes leads to an increase of the concentration of their counterions inside of them, and thus induces an extra osmotic pressure. This in turn favors the size increase of the holes before they coalesce. The release of both polyelectrolytes from the film into the supernatant ultimately allows a decrease of the osmotic pressure inside the PLL/HA film, which finally leads to the disappearance of the holes and concomitantly to a complete dissolution of the film. When the release of polyelectrolytes into the solution is prevented by a poly(diallyldimethyl ammonium chloride)/poly(styrene sulfonate) (PDADMAC/PSS) capping film, the holes appear at a smaller critical ionic strength compared to uncapped films. Here too the formation of the holes is attributed to an increase of the osmotic pressure inside the film. As soon as the capping barrier ruptures because of the swelling of the film, both PLL and HA chains can diffuse out of the film and the holes decrease in size and disappear, as does the film.