Thermal imprint techniques for preparation of superhydrophobic polymer coatings

Abstract

The self-cleaning property of superhydrophobic surfaces has raised considerable interest among scientists. Many preparation methods require complex and expensive fabrication techniques that cannot be used to directly modify substrates. We designed an imprint process using a micro/nano-structured mold to emboss several polymer coatings in order to introduce superhydrophobicity and still maintain their transparent/translucent properties. Polydimethylsiloxane (PDMS) elastic molds with various micro and nano features were fabricated using different grades of sandpaper as master molds, increasing the water contact angles (WCAs) on the PDMS surfaces from 120° (plain) to more than 170°. The PDMS molds were then applied to replicate some commercial polymer coatings at an elevated temperature of about 140°C to transfer the mold pattern to the coatings. The hydrophobic property of those polymer coatings was then enhanced to a WCA of greater than 150°. After fluoroalkylsilane deposition on the surfaces, the surfaces displayed an improved lipophobic property (hexadecane contact angle (HCA) went from 90° to 110°). The arbitrarily-rough surfaces displayed ‘sticky superhydrophobicity’ due to the water drops being adsorbed and not rolling off from the surfaces. Atomic force microscopy (AFM) was employed to measure the morphologies of imprinted coatings and also estimate the roughness, occupying polymer ratios, and critical CA to evaluate the wetting of sample surfaces. From the evaluation of surface properties, all the samples using sandpaper as templates exhibited stable Cassie or metastable wetting. The rougher surfaces provided some large protrusions to contact with and be immersed into water droplets, leading to high water penetration depth and thus stickiness. Additionally, the rougher the mold was, the greater the WCA was, but the transparency of the coating was reduced. However, the imprinted polymer surfaces could still maintain sufficient transparency for use in applications requiring preservation of the appearance of the underlying surface.