Refractometric discrimination of void-space filling and swelling during vapour sorption in polymer films

Thin polymeric films have been deposited as upper cladding layers on a new integrated optical interferometer fabricated from layers of silicon oxynitr ide on a silicon wafer. The evanescent field of the probing waveguide mode transduces refractive index changes in the polymer layer into the measured phase changes in the device. Real-time measurement of index change and its sign is obtained. Upon exposure to humid air, we record water sorption by f ilms of poly(vinyl pyrrolidone) by a rapid positive index change for void-s pace filling followed by a slow negative index change for swelling. Sorptio n of water vapor into a thin film of the viscous liquid polymer polyethylen imine shows only swelling mode behaviour and a simple constitutive model ca n be applied to give the fractional water occupied volume.