Transparent Polymer Nanocomposites as Rayleigh’s Diffusers

Development of highly efficient and low cost LEDs makes possible to design lamps exploiting completely new concepts. In this perspective transparent polymer nanocomposites can play a fundamental role. In particular, a material that exhibits Rayleigh diffusion -i.e. having a scattering efficiency inversely proportional to the fourth power of the wavelength of light- is quite interesting for potential applications in the correction or introduction of color dispersion effects. Starting from Rayleigh’s equation, considering all the basic characteristics of the nanocomposite (volume fraction of the nanoparticles, optical path of the light inside the diffuser, refractive indices of the components) and choosing as polymer matrix the paradigm of transparent polymers, polymethylmethacrylate (PMMA), it is possible to predict that a PMMA nanocomposite containing nanoparticles of inorganic oxides, transparent in the visible range and endowed with high refractive index (like TiO2 or ZnO), shows a marked effect of the Rayleigh’s diffusion behavior when the dispersed nanoparticles present diameters comprised between 50 nm and 200 nm. Here we present the fabrication and characterization of such PMMA/TiO2 nanocomposites showing marked Rayleigh diffusion behavior. In a first step hydrophilic TiO2 rutile nanoparticles (116 nm average diameter) are converted to hydrophobic ones by modification of their surface by grafting of hexyl amine. The amount of attachment rate of surface-modifier on TiO2 nanoparticles is estimated by thermogravimetric analysis, while the structures of the modified titania surface are investigated by Solid-State NMR. Nanocomposites are fabricated as sheets (30 cm × 30 cm × 1 cm) by in situ polymerization of MMA containing the surface modified TiO2 nanoparticles. TEM measurements show homogeneous distribution of the nanoparticles in the PPMA matrix. Measurements of light transmittance in the visible range demonstrate that PMMA sheets containing between 50 ppm and 200 ppm of rutile nanoparticles behave indeed as Rayleigh’s diffusers. Typical effects of diffusion in the propagation of white light through the composite are also presented.