Disruption of self-assembly and altered mechanical behavior in polyurethane/zinc oxide nanocomposites

The addition of less than 1 vol%, 33 nm zinc oxide nanoparticles into a polyurethane matrix resulted in approximately 40% decrease in the Young's modulus, 80% decrease in strain at fracture, and 50% decrease in the storage modulus, but at the same time resulted in an ∼11 °C increase in the glass transition temperature of the polymer. These results appear to contradict the general principle observed for many polymeric systems, where higher glass transition temperature generally means higher elastic modulus. Detailed experiments with FTIR, DMTA, FE-SEM, and AFM indicated that the addition of ZnO nanoparticles disrupts the phase separation in the polymer, resulting in weaker mechanical properties. The special interaction between the particles and polymer possibly constrains the mobility of polymer chains, which increases the glass transition temperature. The most likely reason for the disruption and the nature of the interaction is the reaction between the surface hydroxyl groups of the zinc oxide nanoparticles and the isocyanate groups of the polyurethane pre-polymer.