Tuesday, December 4, 2007 - 10:30 AM

Air Force Research Laboratory - Smart Surfaces Program

Joel Johnson, AFRL/RXBT

The AFRL “Smart Surfaces” basic research program involves three areas of technical focus: development of adaptable adhesive interfaces, molecular self-healing resins, and mechanisms of galvanic induced aluminum corrosion. The adaptable adhesion effort has focused on model gold surfaces with self-assembled monolayer (SAM) materials used as the interfacial moiety. A computational investigation of the conformation and dynamics of SAMs of a set of aromatic thiols arranged in the (√3×√3)-R30° packing ratio on an Au(111) surface was performed. It was found that the molecular conformations were better defined for the arylthiol with two phenyl groups as compared to the rest with a single phenyl group, and that the chemical structure of the head and the tail groups had a considerable influence on the system geometry. In line with the density functional theory (DFT) calculations of a single thiol molecule, we found for the SAMs that the FCC site on the Au(111) surface was the most preferred followed by the HCP site, while the Bridge position showed the characteristics of a local energy maximum. The dynamics of thiol head groups on these three Au sites was found to govern the overall dynamics of SAMs as measured by the mean square displacement. We also report that both the conformation and dynamics behaviors were driven by the SAM formation energy. The molecular self-healing resin effort has focused on synthesis of novel dihydroxy functional monomers based on imidizolium ionic liquid structures to provide enhanced intermolecular pseudo-crosslinking. The primary ionic liquid monomer of interest, N-(3-hydroxypropyl)-N’-methylimidazolium isethionic acid, [OH-PMIM SO3EOH], was synthesized in high yields with 85% purity by reacting N-(3-hydroxypropyl)-N’-methylimidazolium chloride, [OH-PMIM Cl] with isethionic acid sodium salt in acetonitrile. An anion exchange occurs substituting the chloride anion with isethionic acid. OH-PMIM SO3EOH is soluble in the acetonitrile while the by-product, sodium chloride, is not and is removed by filtration. The ionic liquid monomer is recovered after removal of the solvent. Polymerization of OH-PMIM SO3EOH with polyfunctional isocyanates is expected to yield novel polyurethane ionomers with covalently bound cations and anions. The investigation into mechanisms behind galvanic induced corrosion of aerospace aluminum alloys involved using the potentio-dynamic scan (PDS) technique to monitor corrosion of monolithic Ag and bare AA2024-T3 in the presence of four electrolytes: 3.5% NaCl, 5% NaCl, Dil.Harrison’s and Reg.Harrison’s solutions, and also with addition of 0.01%. 0.25% and 0.5% Na2Cr2O7 to quantify the effectiveness of Cr(VI) inhibitor in suppressing corrosion. Corrosion rates, Ecorr, Icorr were determined using Tafel analysis. In addition, the corrosion rate of AA2024-T3/Ag galvanic couples were investigated. A galvanic cell is formed when two dissimilar metals are connected electrically while both are immersed in a solution electrolyte. Galvanic cell tests were performed in dilute Harrison’s solution (0.35 wt% (NH4)2SO4 and 0.05 wt% NaCl) on a Gamry PC4 potentiostat functioning as Zero Resistance Ammeter (ZRA). The resultant corrosion currents as a function of cathode to anode ratio were determined to assist in identification of possible inhibitors to alleviate galvanic corrosion.