Hydrophobic ultrathin films formed by fluorofunctional cage silsesquioxanes

Abstract

The usefulness of fluorofunctional cage silsesquioxanes (POSS) as coating materials effectively changing the surface properties of model substrates was analyzed. Five fully condensed silsesquioxanes (containing two types of organic groups attached to the Si-O cage: octafluoropentyloxypropyl (OFP) and trimethoxysilylethyl (TMS) at different ratios, one monofunctional derivative with OFP groups only and one open cage POSS derivative with three OFP and seven isobutyl groups as organic substituents were investigated. All POSS derivatives were applied to enhance hydrophobicity of quartz or glass plates by deposition of thin LB film transferred from the water subphase. Then, by comparison of water contact angles (WCA) on modified surfaces the effect of silsesquioxane structure (i.e. the TMS and fluoroalkyl group contents as well as the structure of silicon-oxygen cage) on hydrophobic properties was determined. Moreover, the texture of LB films formed by POSS considered was analyzed using the AFM technique. It has been established that the hydrophobizing properties of silsesquioxanes are considerably influenced not only by the number of OFP groups, but also by the silsesquioxane structure. The most effective appeared to be the open cage OFP-POSS derivative which produced coatings with water contact angles (WCA) equal to about 100 deg. Additionally the POSS derivatives studied were also used for dip-coating and spin-coating surface modification. The study has shown that fluorofunctional POSS derivatives can be effective hydrophobizing agents. Additionally applying the LB technique for surface modification allows obtaining substrate of enhanced hydrophobicity by using vanishingly small amount of the modifying substance in comparison to the dip-coating and spin-coating method, which is especially important from the economic viewpoint.