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/m2) and high
mixture viscosity (>65 mPa·s).