Phase separation of composite materials through simultaneous polymerization and crystallization

Composite materials have attracted much interest because of the emergent properties originating from the components. A variety of methods have been studied to control the morphology of composites based on noncrystalline polymers and crystalline materials. However, it is not easy to control complex morphologies, such as segregated sea-island structures, on the submicrometer scale. Polymerization induces crystallization, because supersaturation, which is required for crystallization, is achieved by the consumption of the monomer. Here we report a phase-separation approach based on simultaneous polymerization and crystallization as a new method for the morphological control of composite materials. Segregated polymer and organic crystal domains are obtained by polymerization of an organic monomer solution accompanied by simultaneous crystallization. The phase separation induced the generation of composite materials consisting of a redox-active quinone crystal and conductive polymer with a segregated structure on the submicrometer scale. The segregated composite of 2,3-dichloro-1,4-naphthoquinone and polypyrrole showed enhanced charge-storage properties based on the smooth redox reaction. The present phase-separation approach can be applied to a variety of functional segregated composite materials consisting of crystalline and polymer materials. A method that induces a pair of organic precursors to simultaneously polymerize and crystallize holds promise for low-cost energy storage. Conductive polymers are tricky to combine with other ingredients into all-in-one composites because of their rigid, insoluble frameworks. Yuya Oaki and colleagues at Keio University in Japan developed a simple way to quash this problem using a mortar and pestle. The team crushed a redox-active quinone compound with a pyrrole polymer precursor and a pinch of oxidative copper chloride powder. After a few minutes, a spontaneous composite emerged thanks to phase separation: as the pyrrole units link together, they cause tiny quinone regions to supersaturate and crystallize inside the polymer. Electron microscopy revealed the composite had a sub-microscale sea-island structure that is ideal for smooth transfer of charges to the conductive matrix. Phase separation based on simultaneous polymerization and crystallization generated the segregated composite of organic crystals and conductive polymers in submicrometer scale. The phase-segregated structure of redox-active conductive polymer and quinone crystal showed the enhanced charge-storage properties based on the smooth redox reaction.