Lowering of surface free energy of Polythiophenes through entropy control in side chains without Perfluoroalkyl...

A hydrophobic surface with low surface free energy is crucial in various advanced functional materials. The incorporation of fluorine-based functional groups onto the surface enhances surface hydrophobicity and oleophobicity. However, the use of per- and polyfluoroalkyl substances (PFAS) poses a significant issue due to their environmental and biological accumulation. Therefore, the development of fluorine-free alternatives to PFAS has garnered much attention for low surface energy surface. Polythiophenes with the low side chain densities and the rigid thiophene backbone should provide large entropy and low surface free energy, and achieve omniphobic, namely hydrophobic and oleophobic, surfaces.

We synthesized various polythiophenes with linear alkyl, branched alkyl, and siloxane side chains and evaluated their surface structure and properties. From dynamic contact angle measurements at 20 °C, the effects of the branched alkyl and siloxane side chains on surface free energies were compared. The correlations between the surface free energies and the branched structure and densities were evaluated. In addition, the surface free energies were measured at 20 °C, 30 °C and 40 °C, and their entropic effects were evaluated.

The low densities of the polythiophenes with highly branched alkyl chains and large siloxane side chains led to low surface free energies of their thin films. The surface free energies were lower and the omniphobicity were larger relative to polytetrafluoroethylene (PTFE). Moreover, polythiophenes with branched alkyl and siloxane side chains possessed larger entropies. In conclusion, we successfully accessed to an entropy-driven low surface free energy without any fluorine groups.