Impregnation of weakly charged anionic microhydrogels with cationic polyelectrolytes and their swelling properties monitored by a high resolution interferometric technique. Transformation from a polyelectrolyte to polyampholyte hydrogel

Deposition of polycations on weakly charged anionic hydrogels and the resulting swelling properties were studied to determine the influence of molecular parameters on the properties of the resulting composite soft materials. The deposition process and swelling properties at various ionic strengths were followed in situ by an interferometric technique allowing continuous monitoring of changes in optical length within the hydrogels with 2 nm resolution. The distributions of the polycations labeled with fluorescent dye Alexa Fluor 488 were additionally monitored employing confocal microscopy. Polyallylamine hydrochloride (PAH) of three different molecular weights and a high molecular weight chitosan were employed as polycations. The high resolution interferometric data revealed the classical ionic hydrogel increase in swelling with decreasing ionic strength that increases with the charge density of the anionic hydrogel. Impregnating the anionic hydrogels with PAH polycation resulted in composite materials where PAH where distributed throughout the original polyanionic hydrogel, and where the changes in swelling at various ionic strengths resembled a polyampholyte hydrogel. Exposure to a chitosan yielded a polycation distribution with a distinct chitosan enriched shell at the surface of the hydrogel, and less penetration within the core of the anionic hydrogels. These materials displayed a typically ionic hydrogel swelling signature on changes in ionic strength. The results here show that tuning of molecular parameters can be used to control transport properties of polycations into polyanionic hydrogels and one can also prepare composite soft hydrogels with polyampholyte properties using such a route.