Water Droplet Templating Technique to Design Three-Dimensionally Ordered Porous Structures of Polymer Film

We developed a unique water droplet templating method to fabricate polymer films with three-dimensionally ordered porous structures. This technique is based on a polymer/solvent/H₂O ternary system, and the key is to choose a volatile and hydrophobic solvent that is slightly miscible with H₂O. With the fast evaporation of the solvent, water droplets separate from the casting solution and condense from the air to act as pore templates inside the film and on the surface, respectively. According to this law, nitrocellulose (NC) films were produced from the NC/methyl acetate (MA)/H₂O system in which the solubility of H₂O in MA is 8.1 wt %. By modulating the solution concentration (density) from 3% to 9% NC, the distribution of separated water droplets (pores) in the solution can be flexibly controlled from sinking to floating. On the other hand, substantial ordered honeycomb pores, originated from condensed water droplets, distribute uniformly on the surface of NC films. This water droplet templating technique can be extensively applied in various polymer films, providing an effective pathway to designing polymer films with a desirable porous structure and diverse functionalities.