Polyimide/Silica Nanocomposites with Enhanced Tensile Strength: Size Effects and Covalently Bonded Interface

In this work, the tensile strength of polyimide/silica composites with the covalently bonded interface (bonded PI/SiO₂) is investigated by molecular dynamic simulation. It is found that the nanofiller with smaller size can bring out a larger number of hydrogen bonds and interfacial non-bond energy in the composites, resulting in higher tensile strength. As the immobilization of the PI chains in the vicinity of SiO₂, the covalently bonded interface is found to offer a greater reinforcing effect than the unbonded interface that is confirmed by the self-diffusion coefficient. The tensile strength of 9 wt.% bonded PI/SiO₂ composites is 11.34% higher than that of the unbounded composites. The tensile strength of PI/SiO₂ composites is enhanced with the increase of SiO₂ concentration up to critical mass percent (X_c), beyond which it will be decreased. To quantitatively predict X_c of PI/SiO₂ composites, an empirical equation based on the non-bond energy of the composites is proposed. The empirical equation showed that the X_c of PI/SiO₂ composites ranged from 8.03 to 10.36 wt.%, which is consistent with experimental values. These results provided the understanding of size-dependent covalently bonded interface structure, which would be beneficial to the design of nanocomposites with excellent mechanical performances.