Molecular composites comprising TiO₂ and their optical properties

An in situ synthesis to form an organic-inorganic hybrid film with tunable optical properties is described. The nanocomposite comprises a titania nanophase that is formed from titanium alkoxide in the presence of a matrix polymer. XPS study indicates that titanium (Ti) in the nanophase is fully oxidized to Ti(IV), similar to that in TiO₂. When a random copolymer, of styrene and 3-methacryloxypropyltrimethoxysilane (MPTMS), comprising functional groups that form a covalent bond with the in situ synthesized TiO₂ is used, TEM and AFM show no phase contrast between the organic and inorganic phases at the nanometer level. This suggests a molecular level mixing of the polymer and formed inorganics and thus the formation of a molecular composite. This synthesis generates transparent composite films with any level of incorporation of TiO₂. The refractive indices of these molecular composite films are much higher than that predicted on the basis of effective medium theories; e.g., at a TiO₂ loading of 27 vol %, the resulting molecular composite film exhibits a measured refractive index of 1.76 compared to that of 1.66 predicted by effective medium theories.