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

Abstract

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 °C the film was cloudy, while above 25 °C it was transparent. The temperature region from 20–25 °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 °C⁻¹ per cycle while in the transparent temperature region it was as high as 15–16 nm °C⁻¹. At 50 °C, the film thickness increment of a single deposition step reached 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 °C, the Fabry-Pérot fringes gradually disappeared and a morphology full of pits formed, like the film that was prepared by layer-by-layer assembly at 50 °C.