Figure 6 shows the typical shape of water droplet on the surfaces of neat P(MMA-co-BA) and nano-ZnO/P(MMAco- BA) nanocomposite. In Fig. 6(a), the water droplet could spread to wet the surface of neat P(MMA-co-BA) with the static contact angle of 71°, indicating the hydrophilicity of P(MMA-co-BA) as schematically shown in Fig. 7(a). The static contact angle of the nano-ZnO/P(MMA-co-BA) nanocomposite slightly increased from that of the neat P(MMA-co-BA) to 78° as shown in Fig. 6(b). This result indicated that the adding of 1.0 wt%nano-ZnO insignificantly effected on the wettability of nanocomposite. Nevertheless, the hydrothermal growth of ZnO nanostructure on the surfaces of the nano-ZnO/P(MMA-co-BA) considerably resulted in an increase of static contact angle, so the shape of water droplet gradually changed to be spherical droplet as shown in Fig. 6(c)–(f). The higher pH of hydrothermal treatment, the higher contact angle was obtained, implying the greater hydrophobicity of the nanocomposite surfaces. These results were considered to be because the hydrothermal grown ZnO consisted of a large number of multidirectional aligned nanorods and many groove between individual rods, generating a roughness of nanocomposite surface. The rough ZnO surface entrapped the large fraction of air within the interstices, preventing the penetration of water droplet into the grooves and then creating the hydrophobicity of the nanocomposite surface.When using the higher pH of hydrothermal treatment, the higher amount and the more complicated shape of hydrothermal grown ZnO were obtained, resulting in the more surface roughness. Although the flower-like ZnOclusters formed at pH=9 created more surface roughness than the nanorods formed at pH=8, the contact angle at pH=9 did not significantly increase because the formed flower-like ZnO was discrete clusters so the water droplet was in contact with the solid asperities in the Wenzel state model as shown in Fig. 7(b) [26]. The static contact angle drastically increased to the highest value of 121° when treated the nano-ZnO/P(MMA-co-BA) at pH=10. This result was because lots of ZnO clusters and ZnO nanofibers with nanospine formed on the composite surface, so the water droplet rested on the tops of the solid asperities as Cassie–Baxter model as schematically shown in Fig. 7(c) [27]. These hydrophobic surfaces allowed to improve the self-cleaning property of nano-ZnO/P(MMA-co-BA) nanocomposites.