Free volume properties of the zinc oxide nanoparticles/waterborne polyurethane coating system studied by a slow positron beam

Abstract

A variable, mono-energetic positron beam was employed to study the free volume characteristics of coating system composed of zinc oxide nanoparticles dispersed in waterborne polyurethane. Positron annihilation lifetime spectroscopy, in tandem with Doppler-broadening energy spectra and derived S parameter is used to quantitatively assess the free volume and interfacial interactions between zinc oxide nanoparticle surface and waterborne polyurethane. It was found that S parameter decreases with increasing ZnO load from 0 to 5%. The decrease of S parameter is explained as a result of strong interfacial interactions between the surface of the ZnO nanoparticles and WBPU chains. Furthermore, the half depth where the change in S parameter is reduced by half from the surface to bulk was found to increase with increasing ZnO concentration. These results are presented in the framework of free volume interactions in which the S parameter is quantitatively used to characterize the free volume in the inorganic/organic nanocomposite matrix.