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 itʹs original size; (3) the slow dissolution of the swollen hydrogel film. Simultaneous electrochemical studies of the incorporation of Fe(CN)63- 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.