Effect of temperature on the build-up and post hydrothermal processing of hydrogen-bonded PVPON/PAA film

Hydrogen-bonded films were prepared by layer-by-layer (LBL) assembly of poly(vinylpyrrolidone) (PVPON) and poly(acrylic acid) (PAA). The effect of temperature on the build-up and post hydrothermal process was studied in detail. It was found that the film's cloudiness was sensitive to the fabrication temperature. Below 20 degrees C the film was cloudy, while above 25 degrees C it was transparent. The temperature region from 20-25 degrees C was a mixed zone, transiting from cloudy to transparent. The thickness growth exhibited different temperature dependencies in the cloudy and transparent temperature regions. In the cloudy temperature region it was 5-6 nm degrees C-1 per cycle while in the transparent temperature region it was as high as 15-16 nm degrees C-1. At 50 degrees C, the film thickness increment of a single deposition step reached to similar to 600 nm, which was considered as one of the fastest growth rates in LBL assembly. The post hydrothermal processing of the film was performed in acidic water (pH = 2.0) at temperatures higher than the build-up temperature. As the hydrothermal processing temperature elevated to 90 degrees C, the Fabry-Perot fringes gradually disappeared and a morphology full of pits formed, like the film that was prepared by layer-by-layer assembly at 50 degrees C.