## 3 Nanocomposite Properties
### High refractive index nanocomposites  
Recently, great effort has been devoted by the scientific community to extend the applicative potential of hybrids formed of inorganic NP modified polymers, for the production of high refractive index materials, for highly reflective and antireflection coatings and advanced optoelectronic systems [116]. In particular, optically transparent TiO\({}_{2}\) based nanocomposites with high refractive index are very appealing materials for UV filters and high refractive coatings. Due to their high refractive index and UV absorption, TiO\({}_{2}\) NPs have been incorporated in polymers to improve linear and non linear optical properties and to prepare photonic materials. Nevertheless, in order to avoid Rayleigh scattering and intensity loss of the transmitting light, an uniform dispersion and a small size of NCs in the polymer is required. Variation in the titania NP concentration, size, morphology and surface functionalization, as well as a change in the dielectric constant of the matrix result in the possibility of tuning the optical properties of the materials for practical applications [117].  
In a recent work, sol-gel synthesized TiO\({}_{2}\) NPs have been embedded in hyperbranched polymers and cured to prepare hard coating or films. The optimization of the preparative conditions has leaded to the improvement of both thermal and mechanical properties of the nanocomposite material [118].  
ZnS NPs have been used to prepare polymer nanocomposites with tunable refractive index by varying the nanostructures content in the matrix. [PERSON] and co-authors have investigated optical, dynamic and thermomechanical properties of nanocomposites formed of 2-mercaptoethanol(ME)-capped ZnS NPs in polymer matrix obtained by free radical initiated polymerization [119]. The nanocomposites exhibited optical transparency in the visible range, thermal stability and good mechanical properties, although at the increasing of the nanofiller content, the plasticizing effect of ME-capped ZnS NPs has resulted in a decrease of _T_g values. Thanks to the optical transparency of the nanocomposites and the dependence of the refractive indices on NP loading, the prepared nanocomposites can be potentially applied for fabricating optical devices.  
Fluorescent and highly transparent nanocomposite thin films with high refractive index have been recently prepared by blending o-phenylene diamine (o-PDA) functionalized ZnS NC solutions with poly(vinylpyrrolidone) (PVP) [120]. Atomic force microscopy (AFM) and X-ray diffraction (XRD) characterizations have revealed the preservation of the NP size distribution (3.0 \(\pm\) 0.30 nm) in the polymeric matrix. Compared to the bare polymer, the ZnS NC polymeric films have exhibited enhanced refractive index, high optical transparency and preservation of the NC band-edge emission. Nanocomposites with highly dispersed ZnS nanophase contents have been carried out by [PERSON] and colleagues [121]. Nanomaterial characterizations have shown that ZnS NPs and the polymer matrix are linked by covalent-bonds, thus resulting in the enhancement of thermal and mechanical properties. The small size and the homogeneous dispersion of ZnS NPs contribute to the transparency and the linear behavior of the refractive index with the volume fraction of the polymer matrix.