In Situ Dynamic Measurements of Sol−Gel Processed Thin Chemically Selective PDMDAAC−Silica Films by Spectroscopic Ellipsometry

Spectroscopic ellipsometry studies of thin sol−gel processed polyelectrolyte−silica composite films used in chemical sensing are presented. Dynamic measurements were used to characterize PDMDAAC−SiO2 film behavior when equilibrated in 0.1 M KNO3 solution (supporting electrolyte). Optical modeling of ellipsometric data revealed three transformation phases during the equilibration: (1) a slow initial phase when the film refractive index approached that of the solution while film thickness changed little; (2) the disintegration of the silica xerogel matrix during which the film incorporated a large amount of water expanding to a hydrogel of thickness many times it's original size; (3) the slow dissolution of the swollen hydrogel film. Simultaneous electrochemical studies of the incorporation of Fe(CN)6 3- into films coated on indium tin oxide glass substrates identified the cause of previously observed film failures in sensing applications after long equilibration in aqueous environments.