Adsorption isotherm and kinetics of diffusion of water accumulated between polypropylene thin film and Si substrate; Neutron reflectivity investigation

Miyazaki, Tsukasa*; Miyata, Noboru*; Arima-Osonoi, Hiroshi*; Shimokita, Keisuke*; Yamamoto, Katsuhiro*; Takenaka, Mikihito*; Nakanishi, Yohei*; Shibata, Motoki*; Aoki, Hiroyuki  ; Yamada, Norifumi*; Yamada, Masako*; Seto, Hideki*

Isotactic polypropylene (PP) thin films deposited on Si substrates were subjected to neutron reflectivity (NR) measurements under various humidity conditions to evaluate the isothermal adsorption of water accumulated between the PP layer and Si substrate, with focus on the temperature-dependence of the adsorption isotherm. The adsorption of accumulated water followed a single type II isotherm according to the Brunauer-Emmett-Teller (BET) classification, regardless of temperature. This can be attributed to the weak interactions between the PP layer and Si substrate, and between the PP layer and water molecules. Hydrophobic PP does not significantly interact with water molecules and the hydrophilic Si surface. Therefore, the interfacial water molecules are simply adsorbed on the native oxide layer on Si via simple interaction between the water molecules and the silicon oxide surface without being affected by the PP layer. Consequently, the adsorption isotherm of the accumulated water follows the single type II adsorption isotherm regardless of temperature, similar to the adsorption isotherm of water simply adsorbed on exposed silica surfaces. These weak interactions of the PP layer with the Si surface and water molecules also lead to fast diffusion kinetics for the accumulated water along the interface.