Self-Organized Fibrous Nanostructures on Poly[(aminopropyl)siloxane] Films Studied by Atomic Force Microscopy

We report the formation of highly organized fibrous networks on ∼50 nm thick oligomeric films of hydrolyzed γ-(aminopropyl)triethoxysilane (γ-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 (NH 2 ) functionality on the surface of the film reacts with atmospheric CO 2 and H 2 O 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 γ-APS oligomers capped with NHCOO - and NH 2 functional groups. The stabilization of the NHCOO - species by Na + ions leads to self-assembly via hydrogen bonding and electrostatic interactions.