Fluorine-Free Superhydrophobic Film Based on Polybenzoxazine

Bioinspired superhydrophobic surfaces which possess the merit of a water contact angle (CA) larger than 150^o have numerous applications such as in self-cleaning, oil/water separation and liquid transportation. As is well known, chemical modification of a rough surface is rewarding to superhydrophobicity. Conventionally, the modification agent is a fluorine-containing low surface free energy material such as perfluorocarbon or fluoroalkylsilane. And roughening of a surface involves sophisticated processes such as etching and templating. Therefore, it has been desired to prepare superhydrophobic films by a cost-efficient and an easier process. As a new type of low surface free energy material, polybenzoxazine (PBZ) is promising for a superhydrophobic film due to its cross-linking structure, low water absorption and low cost. In our previous work, a fluorine-containing and fluorine-free silane-functionalized PBZ film with surface free energy of 15.5 mJ/m² and 14.91 mJ/m² has been synthesized respectively. (J. Liu, Z. Xin et al, Langmuir, 2013; L. Qu and Z. Xin, Langmuir, 2011)

In this paper, two important properties are combined: the low surface free energy property of polybenzoxazine (PBZ) and the photo-induced superhydrophilicity property of TiO₂. As a result, a multifunctional superhydrophobic PBZ/TiO₂ surface with a water contact angle of ~160 ^o and sliding angle less than 2 ^o is prepared by using spin coating method without any fluorine-containing surface modification agents. The as-prepared superhydrophobic film has good adhesion to substrate, and its superhydrophobicity is stable even after heat treatment at 300 ^oC and environmentally durable for more than half a year. When the content of TiO₂ is increased, the consequent nanocomposite film exhibits hydrophobicity-superhydrophilicity transitions with a variation of around 120^o in water CA upon ultraviolet (UV) exposure-heat treatment cycles.