Using neatly arranged silica spheres as templates, researchers at Pennsylvania State University, University Park, have prepared organic polymers with ordered arrays of nanometer-sized holes [Science,283, 963 (1999)]. The porous substances may be used to study cooperative materials properties like ferromagnetism, as separation media for large chiral molecules, and in other applications.

The study was conducted by graduate students Stacy A. Johnson and Patricia J. Ollivier and chemistry professor Thomas E. Mallouk.

"It's an exciting paper," remarks Andreas Stein, assistant professor of chemistry at the University of Minnesota, Minneapolis. "The work beautifully complements the newly emerging field of mesoporous/macroporous replica materials obtained from colloidal-crystal templates." Stein's group has made similar materials with inorganic shells.

Galen D. Stucky, a professor in the department of chemistry and the materials department at the University of California, Santa Barbara, describes the work as "an imaginative study," noting that the paper "fills in some of the open areas in the synthesis of patterned organic materials with pore sizes in the 10- to 1,000-nm regime."

In recent years, researchers have come up with a variety of procedures for controlling the size of voids in porous materials, Mallouk notes. Self-assembly and block-copolymer techniques, for example, have expanded the range of pores in zeolites and related materials from the bottom up (angstrom size and larger), while colloidal-crystal methods have been applied to the problem from the top down (micrometer size and smaller).

"But these methods left a gap--though not a complete one--for three-dimensional, ordered porous materials in the range of 10 to 100 nm," Mallouk says. And that's where the Penn State polymers come in. "Our procedure hits that gap with modifiable materials that are easy to synthesize on a preparative [multigram] scale." Because the new materials are made from organic polymers, they can be prepared as flexible or stiff samples and can contain chemical functionality. That aspect should be particularly useful for chemical separations, Mallouk adds.

Other applications may follow as well. For example, Ray H. Baughman, an aerospace fellow at AlliedSignal in Morristown, N.J., points out that generalizing this technique from insulating polymers to metals and semiconductors may result in nanostructured materials useful as high-performance thermoelectrics--devices like thermocouples that can interconvert thermal and electrical energy. %To make the materials, the Penn State group uses a laboratory press to coax 35-nm silica spheres to adopt an orderly arrangement of close-packed particles. Then the group subjects the pellets to high temperatures to sinter the samples--forming channels that interconnect the tiny dots.

Spheres assembled in close-packed arrangements occupy only about three-quarters of the volume, Mallouk notes. Voids between the spheres account for the remainder of the space. By permeating the empty space with monomeric solutions of divinylbenzene or ethylene glycol dimethacrylate, the chemists form a polymer frame that envelops the array of silica beads. The team treats the product with a hydrofluoric acid solution to dissolve the silica, leaving behind a polymer full of nicely aligned holes. By blending the monomers in suitable quantities, the team can finely control the pore size in the 15- to 35-nm range.

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