(a) Preparation of porous polymeric membranes by the miniemulsion templating method [1] A miniemulsion templating method is one of sophisticated techniques to prepare porous polymeric membranes with a variety of pore sizes and morphologies. First, water nanodroplets were suspended in an oil phase by using a nonionic and polymeric surfactant to form a water-in-oil (W/O) miniemulsion. After the nanodroplets accumulated by centrifugation, a small amount of monomer was added as an oil phase to resuspend the water nanodroplets in a monomer phase. Then, photopolymerization of the monomer phase was conducted to generate pores in the polymeric matrix. The size of the nanodroplets was tuned by the surfactant concentration to control the pore size of the membranes. We could produce pore morphologies such as closed-cellular, open-cellular, and bicontinuous structures by tuning the volume fraction of the nanodroplets. (b) Observation of the Christiansen effect-based coloration of porous polymeric membranes [2] We fabricated a porous polymeric membrane with an open-cellular structure to investigate the transmitted and scattered lights in terms of the Christiansen filter effect. When the membrane pores were filled with an immersion liquid, we observed the transmission of light at a particular wavelength (λmax) where the refractive indices of the membrane and the liquid matched (Christiansen filter effect). At the same time, angle-independent color (structural color) was observed because multiple-scattering in all other wavelength occurs at the interface between the polymer and the liquid. In this miniemulsion templating method, porous membranes with the different refractive indices could be obtained by polymerization of different types of monomers. As a result, we could tune the λmax and the structural color by the polymer composition. Furthermore, we could estimate the refractive index of the polymeric phase using λmax values obtained by immersing it into several selected liquids with known refractive indices.

Creation of porous polymeric membrans by the "miniemulsion" templating method and control in coloration

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