Fabrication of Water-Resistant Materials using Nanofiber Membranes Inspired by Penguin Feathers

When diving, penguins do not get wet due to the feathers that cover their skin, and emperor penguins dive to a maximum depth of 564 meters. This is probably due to the penguin's feather density, which is the largest among birds, the hierarchical microstructure of its feathers with gaps of up to 17 μm, and surface-modifying fat secreted by the tail blubber gland. Even with the feathers without the fat removed by organic solvents, contact angles were as high as about 130°. When a water pressure resistance was measured of surface modified feathers and wire meshes with various pore sizes by self-assembled monolayers, the smaller the pore size and the smaller the surface free energy, the better the water resistance. This result suggests that water repellency and water resistance are synergistic effects of microstructure and surface modification.<br/>This study aimed to fabricate water-resistant materials using nanofiber membranes by electrospinning, inspired by the mechanism of penguin feathers. Polyacrylonitrile (PAN) nanofiber membranes fabricated in previous studies had problems with water absorption. A high surface free energy of PAN causes capillary action within the pores of the nanofiber membrane. Therefore, to improve the water absorption phenomenon, we investigated methods of reducing the pore size by heat pressing and changing to a polymer with low surface free energy. The experimental conditions for the electrospinning were a drum collector rotation speed of 15 rpm, a needle diameter of 0.41 mm, a feed rate of 0.10 ml/min, an applied voltage of 20 kV, and a distance between a collector and needle of 10 cm. Samples of heat pressing were yarned from a 12.5 wt% N,N-dimethylformamide (DMF) solution of PAN, and were pressed under three conditions: room temperature (25°C), glass transition temperature (Tg) of PAN (104°C), and 150°C. A hydrophobic polymer nanofibers were spun from DMF solution of Poly(vinylidene fluoride) (PVDF) (15, 20, 25, and 30 wt%). Static contact angles, water absorption , and wetting behaviors of the nanofiber membranes were discussed.<br/>As a result, water absorption was observed in the nanofiber membrane pressed at room temperature, but no water absorption was observed in the heated press at or above the glass transition temperature. The fibers were not adhered each other pressed at room temperature, resulting in capillary action. On the other hand, the fibers pressed over Tg were adhered each other. Then pores of the nanofiber membranes became smaller and capillary action did not take place. The contact angles of the pressed nanofiber membrane at over Tg were about 70°. It is considered that the difference in pressing temperature over Tg has no effect on the surface structure. In the case of PVDF, only beads were formed at 15 and 20 wt%, which resembled "electrospray", and 25 and 30 wt% resulted in fibers, although some beads were formed. The beads were probably caused by aggregation on the collector side because the solvent was not fully volatilized. The PVDF nanofiber membrane did not show water absorption, and the contact angle was about 140°. The contact angle of smooth PVDF was about 85°, suggesting that the microstructure of PVDF improves hydrophobicity. These results indicate that the combination of microstructure and low surface free energy has potential for superhydrophobic fabrics and will contribute to the development of new water resistance mechanisms.