Layer-by-layer assembly (LbL) which is the stepwise adsorption of polyelectrolytes in multilayers, and silanization, which is the assembly of silanes on hydroxylated surfaces such as silicon oxide, are two well-known methods of surface functionalization. The combination of these two complementary techniques offers interesting opportunities to fabricate complex functional films. However, exploiting fully the potential of this combination requires to understand in depth the phenomena occurring when silanizing LbL assemblies. Therefore, we focus here on improving our understanding of the gas phase silanization of typical polyelectrolyte multilayers based on poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA), and use this knowledge to fabricate sturdy superhydrophobic surfaces containing these polyelectrolytes and silica nanoparticles.<br/>The silanization process is first studied by Secondary Ion Mass Spectrometry, Atomic force Microscopy, contact angle and X-ray reflectometry, depending on reaction time and temperature, pH of multilayer assembly, and nature of the reacting silane group. Whereas monochlorosilanes only diffuse in the multilayer and graft in limited amount, trichloro- and triethoxy-silanes form rapidly a continuous gel layer on the surface of the multilayer, with a thickness of <i>ca. </i>10-20 nm. The reactivity is lower in the strongly-paired regime of the multilayers (neutral assembly conditions), but otherwise not affected by the pH of multilayer assembly.<br/>Then, robust thin superhydrophobic and self-cleaning films are prepared by assembling silica nanoparticles of various nature over a PAH/PAA cushion layer, followed by fluorosilanization, giving rise to systems of varying transparency and hydrophobicity. Characterization of the surface roughness by power spectral analysis confirms the Wenzel roughness to be a useful criterion to predict superhydrophobicity, whereas the components of the roughness of wavelength above a few hundred nanometers are responsible for the loss of transparency. Based on this analysis, an optimized design is proposed, leading to superhydrophobic transparent layers with contact angles as high as 155°, 0° roll-off angle and strong rebound of water droplets, demonstrating the potential for applications of the technology.

Silanization of layer-by-layer assemblies: Mechanisms and application for the fabrication of superhydrophobic surfaces

249th National Meeting (2015)

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