Control of density gradient in polystyrene porous films

Silica colloidal crystals and multilayer heterostructures were fabricated by using monodisperse silica microspheres. Non-closely-packed colloidal crystals, whose volume ratio of voids was larger than 26% in face-centered closely-packed structure, were obtained after calcining and etching silica colloidal crystals sequentially. The volume ratio of voids increases as the etching time is prolonged. It was found that the smaller the spheres forming colloidal crystals were, the bigger the ratio of voids was, after colloidal crystals were etched under the same condition. Graded silica colloidal films were obtained by calcining and etching silica multilayer heterostructures, which were used as templates to prepare polystyrene porous films with graded density. The different layers in porous films displayed a smooth transition where there were no obvious interlayer defects. Moreover, there were no large channels between pores that existed in conventional three-dimensionally ordered porous structures.