P. Jiang et al., Template-directed preparation of macroporous polymers with oriented and crystalline arrays of voids, J AM CHEM S, 121(50), 1999, pp. 11630-11637
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.