Self-organized fibrous nanostructures on poly[(aminopropyl)siloxane] films studied by atomic force microscopy

We report the formation of highly organized fibrous networks on similar to 50 nm thick oligomeric films of hydrolyzed gamma-(aminopropyl)triethoxysilane (gamma -APS) adsorbed on Na+-containing substrates. The growth of these nanostructures is dependent on the surface segregation of Na+ ions and exposure to ambient conditions. Using an atomic force microscope (AFM), we have followed the growth characteristics of the fibrous nanostructures in a time-resolved manner. The fibers, which grow 2-dimensionally, have a uniform height of approximately 3 nm and widths varying from a few nanometers to hundreds of nanometers. The AFM further shows that each individual fiber consists of a well-ordered parallel assembly of "nanostrands" with widths of approximately 8 nm and a peak-to-valley corrugation of approximately 0.4 nm. Changes in the chemical nature of the APS films as a function of film age were characterized by X-ray photoelectron spectroscopy (XPS) and transmission infrared spectroscopy (IR). Results indicate that the amine (NH2) functionality on the surface of the film reacts with atmospheric CO2 and H2O to form n-propyl carbamate species (NHCOO-) TO explain the fiber growth, we postulate that the diffusing Na+ ions participate in acid-base interactions within the film leading to the formation of small gamma -APS oligomers capped with NHCOO- and NH2 functional groups. The stabilization of the NHCOO- species by Na+ ions leads to self-assembly via hydrogen bonding and electrostatic interactions.