Fabrication of an Organic–Inorganic Hybrid Hard Coat with a Gradient Structure Controlled by Photoirradiation

[Image: see text] Organic–inorganic materials have attracted attention because of the advantages of both organic and inorganic resins. Among their disadvantages, hard coating films made of organic–inorganic mixtures of resins have opacity and interface peeling problems because of organic–inorganic phase separation and surface segregation of inorganic resins. Although an organic–inorganic gradient-structured material comprising an inorganic-rich domain at the air interface and an organic-rich domain at the organic substrate has the potential to solve these problems, the fabrication of a gradient-structured material has not yet been achieved. Here, we describe the fabrication of an organic–inorganic gradient film by impeding the movement of organic and inorganic resins through radical photopolymerization of organic and inorganic oligomers. Moreover, we successfully enhanced gouge hardness by cross-linking with photobase-catalyzed sol–gel reactions of inorganic resins at the air interface. As a result, the organic–inorganic gradient coating contributed excellent gouge hardness (pencil hardness >9H), adhesion to an organic substrate such as polycarbonate, and transparency (visible light transmittance >99%T). In addition, we demonstrated that the formation of organic–inorganic gradient structures is dominated by the surface free energy and viscosity of each resin. Achieving a gradient structure required a significant difference in surface free energy (>20 mJ/m(2)) and high mixture viscosity (>65 mPa·s).