Self-assembly of hierarchical porous structure for stretchable superhydrophobic films by delicately controlling the surface energy

Herein, thermoplastic polyurethane (TPU) was not only used as the means of attaching modified silica (m-SiO2), but was also used as a flexible polymer substrate combining with poly(amide-imide) (PAI) for improving the robustness of stretchable superhydrophobic films by self-assembly. m-SiO2 floated up and TPU sank down in the superhydrophobic coating layer while TPU floated up and PAI sank down in the PAI-TPU stretchable substrate layer by delicately controlling the surface energies of the materials (gammaPAI > gammaTPU > gammam-SiO2). With this strategy, the two layers penetrated into each other, and the compatibility between the superhydrophobic coating and polymer substrate was improved due to the same component of TPU, which made m-SiO2 firmly attach to the stretchable substrate and uniformly disperse into the PAI-TPU substrate. In addition, during the up-down process, a hierarchical porous structure with robust microscale bumps was formed, which offered a stable Cassie state. As expected, the PAI-TPU/m-SiO2 superhydrophobic film was highly stretchable, and can bear 2000 cycles of stretching-releasing (0% -> 30% -> 0%) without sacrificing its superhydrophobicity. The tight adhesion between the decorated m-SiO2 and stretchable substrate rendered outstanding mechanical robustness with resistance to sandpaper abrasion, knife-scuffing, ultrasonic treatment, and hot-water jet impact. The PAI-TPU/m-SiO2 superhydrophobic surface also showed excellent durability when exposed to acid-base immersing, cooling or heating, and UV irradiation. Furthermore, the PAI-TPU/m-SiO2 superhydrophobic surface possessed excellent self-healing, and icephobic properties. For practical application, PAI-TPU/m-SiO2 stretchable superhydrophobic films were applied as water-proof covers for curved surfaces, or served as a self-cleaning coating. These versatile features demonstrated a simple and convenient method to fabricate stretchable superhydrophobic surfaces with multi-functionality.