Covalent molecular assembly in supercritical carbon dioxide: a comparative study between amine- and anhydride-derivatized surfaces

Layer-by-layer covalent assembly of an oligoimide on an anhydride- derivatized silicon dioxide surface is investigated using supercritical carbon dioxide (SCCO2) as the depositing medium. The deposited films were characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry (VASE), UV-visible spectroscopy, electrochemical impedance spectroscopy, nano indentation, and atomic force microscopy (AFM) and the properties of the films compared with those of an oligoimide deposited on amine-derivatized surfaces. Films formed on the anhydride surface are more uniform and stable possibly because the silane precursor for the anhydride is anchored to the surface through two (-Si-O-Si-) tripods. XPS results indicate that the interfacial reaction resulting in amide formation is almost complete in the case of the anhydride, but not in the case of the amine. We infer that the twin tripods linking the anhydride group to the surface may have improved the accessibility of the functional groups for immobilization of the next layer, thereby contributing to the better quality.