Mesoscale surface patterning of a laterally-grafted rod amphiphile: rings and fibers

An aromatic amphiphilic molecule based on branched oligo(ethylene oxide) was synthesized. Evaporation-driven ring formation and Langmuir-Blodgett films are investigated by utilizing this rigid-flexible block molecule. The size of the rings is strongly dependent on the solvent evaporation rate and the concentration of the molecule. In case of fast evaporation, volcano-like rings are formed by evaporating solution of high concentration. Perfectly symmetrical rings with diameters in the range of 2-6 microm are obtained by evaporating solution of low concentration. The formation mechanism of the ring is briefly discussed. The molecule at the air-water interface exhibits excellent amphiphilic properties. Upon transferring the monolayer onto solid substrates, AFM revealed the formation of fine and long, straight fibers. By combining the data obtained from the isotherms, AFM, water contact angle measurements, and UV/Vis and fluorescence spectra, the fibers are suggested to be formed by pi-pi stacking interaction of the aromatic rod segments as the oligo(ethylene oxide) branches are submerged in the water subphase upon compression. The fiber formation is associated with the transformation of the aromatic rod segments from the face-on conformation to the edge-on conformation.