On the other hand, after the conversion to ZrP, the 30 nm substrate showed the highest pore-filling ratio, while the ratio of ZrP decreased in the substrates with smaller pore diameters. It might be because the reaction condition is suitable for conversion to ZrP in the 30 nm substrate. Since PE is hydrophobic, the hydrophobic surface area could increase as the pore size of the substrate decreases, which is expected to increase the hydrophobicity of the substrate surface. This effect could make it difficult for hydrophilic compounds such as phosphoric acid, a reactant of ZrP, to penetrate the substrate. As a result, the chemical reaction between AcAc-Zr and phosphoric acid did not occur sufficiently. It suggests that it could be possible to successfully convert 20 nm/16 nm AcAc-Zr membranes to the ZrP membranes retaining the high pore-filling ratio by increasing nonpolar solvent in phosphoric acid solution.