“Petal Effect” Superhydrophobic Surface Based On Polybenzoxazine

"Petal effect" superhydrophobic surface based on polybenzoxazine

Juan Liu, Zhong Xin*, Xin Lu, Changlu Zhou

State key laboratory of chemistry engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China

Roses are not only the symbol of romantic love, but also have great contribution to the scientific researches. Rose petals show super water repellency with a CA of about 152.4^o; however, these droplets become pinned to the surface and cannot roll off, even when the petal is turned upside down. This phenomenon is the "pinned effect" or "petal effect" .¹  Such surface has the potential to transfer a droplet from a hydrophobic to a hydrophilic one².  And that has attracted great interest for both basic research and practical applications in recent years³.  Jiang and co-workers⁴ noted that there are hierarchical micropapillae and nanofolds existed on the petals' surfaces of red roses. Also, the "petal effect " surface has been prepared by adding TiO₂,⁵  SiO₂,⁶ and other nanoparticles into some low surface free energy materials. Polybenzoxazine^{7, 8} is one type of the low surface energy materials. In this work, SiO₂ nanoparticles were added into the polybenzoxazine by one-step thermal curing method to fabricate the "petal effect " surface.

In our previous study⁹, we discovered a series of non-fluorine low surface free energy polybenzoxazine: P-tmos, MP-tmos and PTBP-tmos. The polybenzoxazine possesses a surface free energy (¦Ã_s=14.91mJ/m²) that is even lower than that of pure poly(tetrafluoroethylene) (PTFE) (¦Ã_s=21mJ/m²). Although these polybenzoxazines and fluoropolymers both possess low surface free energies, the former is cheaper to prepare, easier to process and higher adhesive force to the substrates. Herein, the"petal effect" superhydrophobic surface with high water-adhesion was fabricated through adding nano-silica particles into poly(3-(trimethoxysilyl)-n-propyl-3,4-dihydro-6-methyl-2H-1,3-benzoxazine) (poly(MP-tmos) ) with one-step thermal curing method. The surface properties of the polybenzoxazine films were proved through contact angle measurement. The results show that this surface is superhydrophobic with a CA of 161.5^o, and the droplets become pinned to the surface and cannot roll off, even when it is turned upside down. The morphological characterization of the samples was examined by using scanning electronic microscope(SEM). It was clearly seen that the surface had micro- and nano- structures.

Meanwhile, the surface possessed good resistance to ultraviolet and excellent thermal stability. After irradiation by 254nm UV-light for several minutes, the surface still exhibited "petal effect". The water contact angle was almost unchanged, and the water droplet still cannot roll off.  The thermogravimetric analysis (TGA) results indicated that the polybenzoxazine added nano SiO₂ particles, with a high char yield of 83.10%, possessed excellent thermal stability. On the other hand , there was a strong adhesive force adhesion between the surface and the substrate which was proved by the cross-cut method.

Keywords: superhydrophobic; polybenzoxazine; petal effect

Juan Liu, E-mail: juan_liu86@163.com; Correspondence Zhong Xin, Professor, Tel.: +86 21 64240862; Fax: +86 21 64251772, E-mail: xzh@ecust.edu.cn

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