Monolayer Formation of Hydrophobic Polysilane on Water through Hybridization with Liquid Crystal Molecules

Abstract

The monolayer formation of a hydrophobic polymer, poly(dihexylsilane) (PDHS), on the water surface was performed for the first time through cospreading with a liquid crystal molecule, 4‘-pentyl-4-cyanobiphenyl (5CB). The spread monolayer formation of PDHS was confirmed by surface pressure−area (π−A) isotherm measurements, Brewster angle microscopy (BAM), atomic force microscopy (AFM), and UV−visible absorption spectroscopy. The cospread film of PDHS and 5CB on water exhibited isotherms giving a new sharp pressure increase attributable to the hybridization of PDHS/5CB. The lift-off area corresponding to this pressure increase was in exact proportion to the mixing ratio (R = [Si unit of PDHS]/[5CB]). BAM revealed a highly homogeneous feature of the hybrid monolayers on water at the lift-off area of the PDHS/5CB hybrid film. The AFM images of these hybrid monolayers transferred onto mica indicated that the thickness of the spread PDHS film on the 5CB monolayer ranges from 1 to 1.5 nm. Most probably, the molecularly spread PDHS backbone is positioned on top of the 5CB monolayer. The hybrid films could be transferred onto a solid substrate. UV−visible absorption spectra showed that the Si main chain of PDHS adopts only a helical-gauche (disordered) conformation on both water and a quartz substrate, which shows a sharp contrast with that of the bulk film. On the solid substrate, a preferential orientation of the Si chain was observed in the lifting direction depending on the surface pressure applied for deposition.