Self-Assembly Assisted by Poly[(aminopropyl)siloxane] Films:

nanoribbons and micron diameter tubes

J. M. White

Texas Materials Institute

University of Texas at Austin

Highly organized fibrous networks form on ~50 nm thick oligomeric films of hydrolyzed g-aminopropyltriethoxysilane (g-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 atomic force microscopy (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. While the molecular level structure remains unresolved, we propose that the diffusing Na⁺ ions participate in acid-base interactions within the film leading to the formation of small g-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.

Homogeneous mixtures, ~ 50 mm thick, of KOH, H₂O, CO₂ and hydrolyzed g-aminopropyltriethoxysilane placed on a clean, oxidized surface of Si(100) evolves to form fibers of crystalline KHCO₃. The fibers are fluid filled, narrow (1-4 mm), long (aspect ratios up to at least 300), and both straight and curved. The fibers emerge from sites at interfaces between microclusters of KHCO₃ (mc-KHCO₃) and poly[(aminopropyl)siloxane] (poly-APS). Beginning with the homogeneous film, we propose a multi-step physical model for the nucleation and growth of tubes.