Effect of chemical structure of fluorinated polyhedral oligomeric silsesquioxanes on formation of Langmuir monolayers and Langmuir-Blodgett films

Abstract

Fluorinated polyhedral oligomeric silsesquioxanes (F-POSS) due to their nanoscale hybrid structure coupled with hydrophobic substituted chains have drawn significant attention as highly hydrophobic and oleophobic materials. That is why the surface properties of such compounds at the molecular level should be analyzed in details. Therefore we studied four different fluorinated closed cage polyhedral oligomeric silsesquioxanes (POSS) using Langmuir and Langmuir-Blodgett (LB) techniques. The monolayer behavior was investigated by measurements of the surface pressure-area (π-A) isotherm and morphology of the films was visualized using Brewster angle microscopy (BAM). The rheological behavior was investigated using interfacial shear rheometer (ISR). The unusual properties of these silsesquioxanes result from their unsymmetrical structure. The hydrophobic character of their fluorinated substituents, combined with the hydrophilicity of the trimethoxysilane moieties lead to specific monolayer properties. The compounds were found to form stable monolayers at the air/water (A/W) interface however the surface activity of POSS derivatives decreases with increasing content of fluorinated moieties in the POSS structure. The monolayers of the studied POSS compounds were also successfully transferred onto the solid substrates as Y-type Langmuir-Blodgett films. The wettability and the structure of the LB film were investigated by measuring the static water contact angle on the glass surface. The presence of fluorinated moieties in the structure of POSS molecules governs the enhanced hydrophobic properties of solid substrate modified by the LB film.