Contact angle measurements as a tool to investigate the filler–matrix interactions in polyurethane–clay nanocomposites from blocked prepolymer

Optically transparent polyurethane-clay nanocomposite films were prepared by dispersing 5 wt% of various commercial organo-clays in a mixture of cycloaliphatic amines used as chain extender-cross-linker of a blocked prepolymer. For the first time, vibration-induced equilibrium contact angle measurements were successfully employed to rank the selected organo-clays accordingly to their hydrophobicity order. Polymer–clay intercalation degree in the nanocomposites, as assessed from X-ray diffraction, was strongly correlated to the water–clay equilibrium contact angle. Moreover, as the clay intercalation degree increased, a decrease of the cross-linking degree of the polyurethane matrix occured. Uniaxial tensile tests under quasi-static and impact conditions, and isothermal thermogravimetric analysis were performed on both unfilled polyurethane matrix and nanocomposites. Secant tensile modulus, tensile energy to break, and thermal lifetime showed a non monotonic trend with a maximum as a function of the intercalation degree. This behaviour is discussed considering that two concomitant and contrasting effects develop as the polymer–clay intercalation degree increases: a positive improvement of the filler matrix interactions, and a negative reduction of the matrix cross-linking degree.