Manipulating the molecular conformation of a nanometer-thick environmentally friendly coating to control the surface energy
Long-chain fluorocarbons are the state-of-the-art materials as nanometer-thick coatings in growing nanotechnology industries. The low surface energy, resulting from the molecular nature of C-F bonds, is the key feature of the perfluoro-materials that no other material can provide. However, the research in the past decades showed that long-chain fluorocarbons pose serious toxicological and environmental concerns because their degradation products, which has at least six fluorocarbons, are bioaccumulative, toxic and have high global warming potential. One possible solution for the dilemma is to develop hydrocarbons with short fluorocarbon side chains (HC-SFSC), which has been demonstrated to be environmentally much more friendly. However, there has been few experimental study of the nanometer-thick HC-SFSC on the solid substrate. Moreover, it is very challenging for HC-SFSC to provide the low surface energy as fluorocarbon does. In the current paper, a nanometer-thick HC-SFSC has been deposited on the silica substrate by dip-coating. Ellipsometry and X-ray photoelectron spectroscopy (XPS) results indicated that as-coated HC-SFSC molecules take a conformation between “random coil” and “flat chain”. Contact angle testing results showed that the as-coated HC-SFSC/silica has relatively high surface energy and, interestingly, a simple thermal annealing reduces the surface energy to the value close to that of Teflon. XPS studies suggested that the thermodynamic equilibrium conformation of HC-SFSCs on a solid substrate is actually in favor of low surface energy and the low mobility of HC-SFSCs induced by the solid confinement traps the molecule in a non-equilibrium conformation with higher surface energy. The finding here potentially provides a viable approach to make the surface energy of HC-SFSCs as low as the conventional perfluoro-materials.