Highly Refractive Poly(2-ethyl-2-oxazoline) Coatings Incorporating TiO2 Nanoparticles

In recent years a few examples of nanocomposites with optimized optical properties have been reported. The research has been focused mainly on the following goals: luminescence, change (increase) of the refractive index (RI), UV absorption. In particular high RI polymer nanocomposites can be obtained by incorporation of inorganic building blocks with high RI (such as InP, PbS, TiO2, ZrO2, or ZnS) into organic matrices on the nanoscale. Concerning polymers, which are the embedding matrix for the inorganic nanoparticles, poly(meth)acrylates represent the most studied class since their feature high intrinsic optical clarity -especially in the case of polymethylmethacrylate- and the possibility to modulate the physico-chemical properties by simply varying the nature of the side chains. Quite rarely water soluble polymers are used, although the hydrophilicity of the macromolecular chains can, in principle, avoid aggregation of the nanoparticles, especially in the case of oxides that present hydrophilic surfaces. Here we report for the first time the preparation and optical characterization of nanocomposite films made of TiO2 dispersed in the water soluble polymer poly(2-ethyl-2-oxazoline). This polymer was chosen because of its solubility in water, lack of toxicity and stability. It features several recognized applications as an adhesive and as a coating. Nanocomposite films characterized by three different concentrations of TiO2 nanoparticles (20 nm average radius) in the polymeric matrix (16%, 28% and 44% w/w) were obtained. Optical characterization using UV-visible spectroscopy has shown that the films feature significant UV absorption below visible wavelengths and good transparency for visible wavelengths. Moreover, the highly TiO2 loaded films feature a high-refractive index of up to 1.67. The nanocomposites surface was studied by AFM, showing good coverage of extruding nanoparticles by the soft polymer. The roughness of the surface related to the high fraction of nanoparticles is not having a negative impact on the optical quality of the films which remains excellent, as clearly assessed by specific optical measurements of specular gloss and distinctness-of-image (DOI).