Chemical properties of an oxide nanoskin on a polymeric surface

Vapor phase chemisorption of 1,3,5,7-tetramethylcyclotetrasiloxane onto polyimide (PI) substrates and subsequent photooxidation using 172 nm vacuum ultraviolet (VUV) light successfully produced a silicon dioxide (SiO2) layer of molecular-scale thickness, that is, an “oxide nanoskin” (ONS). This ONS was estimated to be around 1 nm thick by high-resolution transmission electron microscopy. Due to the ONS coating, the surface acidity of the PI substrate increased markedly and its isoelectric point shifted from around pH 3.5 to 2.0, which was almost equal to that of a native oxide-covered Si (SiO2/Si) substrate. PI surfaces both with and without the ONS were further modified using a vapor of fluoroalkylsilane (FAS). Based on contact angle analysis, the surface energy of the FAS/ONS/PI substrate was mostly consistent with that of a SiO2/Si substrate covered with a self-assembled monolayer (SAM) composed of FAS molecules. Our ONS was found to be effective in fabricating a FAS-SAM on a polymer substrate.