Influence of nanoparticles on elastic and optical properties of a polymeric matrix: Hypersonic studies on ethylene-vinyl alcohol copolymer-titania nanocomposites

High resolution Brillouin spectroscopy (HRBS) backscattering elastic data in nanocomposites of ethylene-vinyl alcohol copolymer (EVOH) and TiO₂ nanoparticles present anomalous dependence with concentration, while Young’s modulus and microhardness data show the expected behaviour. When performing HRBS with the 90A scattering geometry to asses the effective elastic constant, the expected behaviour for low concentration of TiO₂ nanoparticles is again obtained. This unusual disagreement can be solved assuming that the inclusion of TiO₂ nanoparticles induces anomalous refractive index behaviour at the applied laser wavelength for the different EVOH-TiO₂ nanocomposites. Comparison with experimental elastic and optical data obtained in isotactic-polypropylene-TiO₂ nanocomposites proves that EVOH-TiO₂ nanocomposites show an unusual optical behaviour at the laser wavelength, possibly due to a singular bonding between the EVOH polymer and the TiO₂ nanoparticles.