We report ultrathin polymer coatings with unprecedented control of hydrophobicity ultimately reaching a contact angle (CA) &gt;120° when deposited on flat surfaces by aqueous layer-by-layer (LbL) deposition. Reminiscent of rose petals, these versatile coatings reach a CA &gt;150° when deposited on structured surfaces while maintaining an adhesivity that enables precise droplet manipulation. Through LbL assembly of fluorinated, carboxylated polyphosphazenes (PPzs) with polyamines, we show that control over both morphology and surface-oriented chemical groups is achieved by varying the degree of PPz fluorination (up to 86%), polyamine structure, and deposition pH. Importantly, morphology, obtained by AFM, revealed a regular formation of self-assembled microphase domains likely due to hydrophobic interactions. The deposition pH influenced the size of the microdomains, resulting in controlled, pH-dependent roughness. In addition to the morphology, the carboxyl ionization and film growth curves, monitored by ATR-FTIR and spectroscopic ellipsometry, correlate with the CA and CA hysteresis. Depositing as few as five bilayers (~25 nm) on a microstructured silicon substrate enhanced the originally superhydrophilic surfaces (CA ~0°) to superhydrophobic (CA &gt;150°) with CA hystereses dependent on both the deposition conditions and structural dimensions. Lastly, we explore the pH response of surface carboxyl groups to demonstrate pH-dependent droplet adhesion. Controlling CA hysteresis through the chemical composition of PPzs, type of polycation partner, pH, and the substrate microstructure enabled programmed adhesion and transfer of water droplets between surfaces for applications in droplet-based microfluidics and water collection/harvesting.

All-aqueous layer-by-layer coatings of fluorinated polyelectrolytes for controllable hydrophobicity and droplet adhesion

ACS Spring Meeting 2021

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