Surface-initiated polymethylenation to grow superhydrophobic barrier films

Abstract

The modification of surface properties has become a vital focus in materials research and is fueled by the interesting ways of tailoring composition, modifying architecture, and optimizing surface characteristics to impact a target application. This dissertation focuses on new approaches for the preparation and characterization of superhydrophobic (SH) surfaces. Here, we show that ultrathin films of the world’s simplest and most common polymer, polymethylene (PM) (or the chemically equivalent polyethylene), exhibit dramatically large resistances against the penetration of aqueous ions if their topology is sufficiently rough on both micro- and nano-scales to merit superhydrophobic behavior. To achieve these rough, yet thin, PM films, we have reported a new surface-initiated polymerization strategy in which immobilized borane moieties serve as active centers for the reaction with diazomethane to grow PM chains one methylene group at a time from a variety of substrates. We have explored the effect of superhydrophobicity on the dielectric properties of the film based on impedance measurements and the rationalization of such measurements using the Helmholtz theory. We have established that SH films exhibit positive deviations from the inverse capacitance predicted by the Helmholtz theory, and we have modeled the effect of the entrapped air at the PM/solution interface of SH films relative to smooth and non-superhydrophobic (NSH) films using a composite factor. Experimental results have demonstrated the remarkable sensitivity of impedance-based methods to assess the superhydrophobicity in underwater conditions. To take advantage of this potential, we have developed a strategy to identify the Cassie and the Wenzel states for underwater surfaces using impedance measurements. We have established the principles and theoretical concepts of the technique and applied it to a situation in which we explore how SH surfaces recover their initial Cassie state after transitioning into the Wenzel state and drying the liquid present in the grooves. Finally, we have discussed the extension of the SIPM approach to virtually any substrate that has incorporated olefin groups; more specifically, we have explored a strategy to grow SH films from liquid polymer substrates to introduce the concept of a SH veneer atop a NSH surface.