(365b) Covalent Molecular Assembly in Supercritical Carbon Dioxide: a Comparative Study between Derivatised Surfaces

Molecular films have aroused widespread interest because the ability to tailor the functionality of the constituent molecules makes them ideal for the investigation of intermolecular forces, molecule-substrate and molecule-solvent interactions that affect interfacial properties such as wettability, biocompatibility, and corrosion resistance of the surfaces of a wide range of materials.

In this work, we have investigated the effect of the nature of the reactive substrate surface (anhydride-terminated versus amine-terminated) on the molecular assembly process and the properties of the resulting film when supercritical carbon dioxide is deployed as the solvent vehicle. The molecular assembly was carried out on silicon dioxide and quartz surfaces derivatised by chemisorption of p-amino phenyl trimethoxysilane (APhS) to yield an amine surface or 3-cyanopropyl trichlorosilane (CPS) ? that was subsequently hydrolysed and converted) for an anhydride surface. In order to compare the immobilization capacities of different starting substrates, an oligoimide was fabricated on the amine and anhydride terminated substrate through alternate layer-by-layer assembly (LbL) of pyromellitic dianhydride (PMDA) and diaminodiphenylether (DDE) in SCCO2. 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 oligoimide deposited on amine derivatised surfaces. Films formed on the anhydride surface were 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.

We have also studied the behavior of mixed reactive surfaces comprising APhS and CPS deposited in various ratios. Optimization of the mixing ratios of the organosilanes were carried out to obtain a homogeneous and well-organized reactive surface. PAMAM dendrimers were deposited selectively on the reactive ends of the anhydrides using SCCO2. Individual dendrimer molecules could be clearly observed in the AFM images, which showed that these dendritic molecules appear to be monodispersed, dome-shaped, and randomly distributed on the reactive surfaces in conformation to the different ratios of APhS: CPS that were deposited on the surface. AFM results also shows that the aggregation of dendrimer molecules can be reduced by optimizing the ratio of APhS: CPS.