Water uptake behavior of hydrogen-bonded PVPON-PAA LBL film

Hydrogen-bonded films of poly(vinylpyrrolidone) (PVPON) and poly(acrylic acid) (PAA) were built on a silicon and quartz surface using the layer-by-layer (LBL) assembly technique. Upon incubation, PVPON and PAA chains in the film can attain high-level miscibility and the film becomes homogeneous and transparent. TGA and elemental analysis indicate that the water content of the incubated PVPON-PAA film is only ∼4%. FT-IR proved that at high temperature these water molecules can be easily driven out of the film. When the incubated film was characterized with a spectrometer, it exhibited Fabry-Pérot fringes in the UV-visible-NIR spectrum. Such Fabry-Pérot fringes, sensitive to the change of water content in the film, were applied to study drying, drying-rewetting cycle, and humidity-responsive behavior of the film. The film exhibited reversible swelling-deswelling behavior during the incubation-heating cycles. The film thickness was found to decrease ∼10% upon drying. When the dry film was exposed to different humidity environments, it was found that the optical thickness of the film has a linear relationship with ambient humidity.