Template-directed preparation of macroporous polymers with oriented and crystalline arrays of voids

The fabrication of polymeric materials with ordered submicron-sized void st ructures is potentially valuable for many separation technologies as well a s for emerging optical applications. This paper reports the preparation of macroporous polymer membranes with regular voids and the characterization o f their diffractive optical properties. These materials are made using a co lloidal crystal template of silica microspheres: the air between the sphere s can be replaced by monomers that can be subsequently polymerized. The use of silica microspheres as templates makes it possible to employ chemical r ather than thermal methods for template removal. For this reason, polymers as diverse as polyurethane and polystyrene can be used to create free-stand ing macroporous films, with thickness ranging from 0.5 to 50 mu m. Scanning electron microscopy of these samples indicates a well-formed porous struct ure consisting of voids ranging in diameter from 200 to 400 nm. These large cavities are not isolated, but rather interconnected by a network of monod isperse smaller pores (d = 50-130 nm) whose size can be controlled by varyi ng the polymerization temperature. These membranes exhibit striking optical properties due to the periodic arrangement of air spheres in the polymer m edium. Normal-incidence transmission measurements of these samples are comp ared to a theoretical model based on a scalar wave approximation. This mode l assumes an ordered structure of close-packed, three-dimensional air spher es. The good agreement between theory and experiment provides additional ev idence of the long-range order of these samples.