Waterborne polyurethane/silica nanocomposites based on electrostatic interaction: Interfacial interactions and properties

To study the effect of electrostatic interaction on the interface and mechanical properties of nano-composites, two kinds of silica nanoparticles with positive and negative charges were prepared and introduced into waterborne polyurethane (WPU) emulsion with negative charges to prepare composite emulsion and film. Molecular dynamics simulation and instrumental characterization were used to investigate the properties of nanoparticles and hybrid films. Molecular dynamics simulation results showed that WPU/SiO₂₍₊₎ nanocomposite demonstrated higher interfacial free energy, lower free volume fraction and diffusion coefficient than those of WPU/SiO₂₍₋₎ nanocomposite. The mechanical properties of the composite films indicated that there is no positive effect on the mechanical properties as the charges of SiO₂ and WPU keep the same. Therefore, the mechanical properties of the composite films can be effectively enhanced by introducing positively charged SiO₂, and the optimized introduction amount is 1 %. SEM images of the cross-section for the film show that SiO₂₍₊₎ exhibits higher interfacial interaction strength with the WPU matrix than SiO₂₍₋₎. Accordingly, molecular dynamics simulation and instrumental analysis afford similar results.