Molecular dynamics simulation study on the structure and mechanical properties of polyimide/KTa0.5Nb0.5O3 nanoparticle composites

The polyimide/potassium tantalite niobate (PI/KTa0.5Nb0.5O3) nanoparticle composite models were established by multi-scale modeling method, the effect of KTa0.5Nb0.5O3 nanoparticles with different sizes (5.5?, 8.0 ?, 9.4 ?, 10.5 ?, 11.5 ?) on the structure, elastic modulus and interaction energy of the polyimide-based nanocomposites were investigated using the molecular dynamics (MD) simulation. The cell parameters, cohesive energy density, solubility parameter, Young's modulus and Poisson's ratio were calculated respectively; and the bond energy and the number of atoms per unit surface area of the nanoparticles were analyzed to explore the internal mechanisms of mechanical properties improvement. The results demonstrate that the density of PI matrix is 1.24—1.35 g/cm3, the cohesive energy density of PI matrix is 2.025×108J/m3, and the solubility parameter of PI matrix is 1.422×104(J/m3)1/2, which are consist with the actual PI parameters. Meanwhile, the Young's modulus of the PI and PI/KTa0.5Nb0.5O3 composites are 2.914 GPa and 3.169 GPa, and the Poisson's ratio are 0.370 and 0.353, which illustrate that the mechanical properties of the PI could be significantly improved by the introduction of the KTa0.5Nb0.5O3 nanoparticles. At the same pressure, the increases of Young's modulus are basically the same after the KTa0.5Nb0.5O3 nanoparticles are doped into the PI matrix; and when the temperatures are different, the standard deviation of elastic modulus of the PI matrix and PI/KTa0.5Nb0.5O3 composite are almost the same. No matter what the pressures and the temperature are, the Young's modulus of PI/KTa0.5Nb0.5O3 composite is always larger than PI matrix. These all indicate that the effect of KTa0.5Nb0.5O3 nanoparticle on the elastic modulus has a similar variation rule under the selected pressure and temperature conditions. In addition, the bond energy of particle surface atoms are 8.62—54.37 kJ/mol, which shows the binding force between particles and the matrix are mainly van der Waals force, and hydrogen bonds exist at the same time. When the doping concentration is fixed, the proportion of nanoparticles surface atoms increase significantly as the size decreases, the interaction between particles with the matrix becomes stronger, the Young's modulus increases obviously and the size effect is more significant. Therefore, it is confirmed that doping small size KTa0.5Nb0.5O3 nanoparticles into the polyimide matrix is the effective way to improve the mechanical properties of the composite.