Effects of Nanoparticles’ Size and Shape on the Thermodynamic Properties of PI/SiO2 Nanocomposites

In this paper, the effects of SiO 2 size (ranging from 5.7 $$ Å to 7.50 $$ Å) and shape (spherical, cylindrical and rod) on the interface structure and the thermodynamic performance of polyimide (PI)/SiO 2 composites were investigated by molecular dynamics simulation. Also, whether the nanofiller with isocyanate propyl triethoxysilane (ICTOS) can highly enhance interaction in PI/SiO 2 (bonded PI/SiO 2 ) ${}_2)$ was investigated by experiment. To enhance the interfacial intensity efficiency, the coupling agent ICTOS was used to modify the surface of the silica nanoparticle. The results showed that the size and shape of SiO 2 not only affect the interfacial number of hydrogen bonds and the interfacial area, but also impact on the glass transition temperature of the composites. As the radius of the embedded nanosilica increased, the bonded energy and T g $T_g$ gradually decreased, showing a prominent filler size effect. At the same time, the T g $T_g$ of spherical-type system was significantly greater than the nanosystems of other shapes. This is because as the number of surface atoms and superficial area of the spherical filler are higher than the nanofillers of other shapes, the combination between spherical-type silica and the PI chains is more compact than those between cylindrical and rod-like silica and the PI chains. The contributions of different nanofillers to the filler size dependency of T g $T_g$ of the nanocomposites can be elucidated by comparing the interfacial interaction and net structure between the PI matrix and silica which exhibit prominent filler shape and size dependency. Finally, the results were confirmed by the nonbonded interaction energy and reduced radial distribution function (RRDF), and the highly interaction interface is found to bring about a greater reinforcing effect.