*1.3.5 Coupling agents*

KH-550 silane coupling agent chemically known as 3-(2,3-epoxypropoxy) propyl trimethoxy silane is selected for silica nanoparticle surface modification. It contains an organic functional group, a linker, a silicon, and a hydroxyl group. The organic functional group can bond to the organic aromatic polyimide ring. The general chemical formula for the silane group and the hydrolyzable functional group, typically alkoxy, amine, or chlorine, involved in the reaction with the inorganic silica substrate. The silanol groups of the nanosilica surface can react with the hydroxyl groups of the silane after hydroxylation through hydrogen bonding. The covalent bond improves interfacial adhesion between inorganic silica particles and organic monomer molecules.
