A new class of porous materials with an orderly crystal-like arrangement of ultra-small spherical spaces has been discovered by chemists at Penn State. A paper describing the "nanobubblepack" discovery will be published in the February 12 issue of the journal Science. The researchers report that they can produce the material in a range of pore sizes never before achieved, opening the door to a variety of potential uses in industry and research.
"It looks like we can easily make lots of this porous material out of anything, and we also can fill it up with anything," says Thomas Mallouk, professor of chemistry at Penn State and principal author of the paper describing the research. "It is intriguing to think about all the ways it might be used."
Mallouk says two of his graduate students, Stacy A. Johnson and Patricia J. Ollivier, initially proposed the idea for making the material. "Stacy and Patti were trying to do two things nobody had done before: make organic porous materials using uniform inorganic materials as a template, and make uniform pores in the size range between 10 and 100 nanometers," Mallouk says.
Other researchers had been able to make uniform pores using a fabrication process developed by Penn State materials scientists in the 1970s. They could make either pores larger than 100 nanometers, using polymer spheres as templates, or they could make pores smaller than 10 nanometers, using small molecules or groups of molecules as templates, but they could not make pores between 10 and 100 nanometers because there were no suitable templates in that size range--bigger than a molecule but smaller than a cell.
Johnson and Ollivier told Mallouk they had read a 1990 paper by Kwadwo A. Osseo-Asare, a professor of metallurgy at Penn State, in which he described how to make identical spheres of silica, an inorganic material, only 35 nanometers in diameter. The students wanted to try using Osseo-Asare's high-tech sand as a template for making an organic material with 35-nanometer pores--well within the size range not yet achieved by other researchers.
Under Mallouk's direction, the students first filled a pellet press with the 35-nanometer silica spheres, then pressed and heated them to form a colloidal-crystal pellet in which all the silica spheres were closely packed in an orderly arrangement. The researchers then used this pellet as a mold, saturating the spaces between the spheres with a liquid monomer--the chemical precursor of a polymer. They next processed the pellet to transform the liquid monomer into a solid polymer and chemically dissolved the silica spheres. "What you get is 75 percent empty space made up of identical spherical chambers surrounded by an organic polymer--the same shape and orderly arrangement as the silica spheres you started with," Mallouk says. "It's something like nanobubblepack--except that the chambers are connected by channels, which makes this material particularly porous."
Johnson and Ollivier next tried making the material with different mixtures of one monomer that has a tendency to shrink (EDMA) and another monomer that does not (DVB). "When you use EDMA alone, it shrinks by a factor of about 2.5 as soon as you etch the silica spheres away, so if you start with 35-nanometer spheres you know you're going to get 15-nanometer pores," Mallouk explains. "We can get a range of uniform pore sizes, anywhere between 15 and 35 nanometers, just by varying the mixture of EDMA and DVB--it's like having a reducing photocopy machine for porous materials," Mallouk says.
The researchers say they can dial in a specific pore size anywhere they want in the previously untouched size range. "Lots of people use the replication process to make materials, but it had not been used in this size range before and no one had ever made an organic replica of an inorganic material on anywhere near this length scale," Mallouk adds. "Unlike many other precision materials that are difficult to fabricate, you easily can make pounds of this very-well-behaved stuff," Mallouk says.
Other researchers in Mallouk's lab are now exploring methods for tailoring the chemistry of the pores for specific applications and are using the shrunken porous material to make miniature replicas of the initial silica spheres. "We are trying to use these materials to separate chiral molecules, which are chemically identical but come in left-handed and right-handed forms," Mallouk explains. The separation of chiral molecules is an important process in the pharmaceutical industry. "The goal is to tailor the chemistry of the pores so they will capture only the left or right form as a mixed stream of molecules flows through the material," he adds.
"There are all sorts of interesting potential uses, both in basic research and in industry, for ordered porous materials in this size range," Mallouk says.
One potential for basic research involves fundamental questions about how the physics of a material changes as it is cut into smaller and smaller pieces. "Magnetism, ferroelectricity in semiconductors, and certain optical properties disappear in a material if you break it into small enough pieces," Mallouk explains. Particles made in precise sizes in the pores of his new material could be useful to researchers trying to understand how the fundamental physics of such properties changes with decreasing size and to learn exactly when molecular-scale properties start to take over. Mallouk says intriguing uses also are possible in biological research because the smallest compartments he and his students made are about the right size for holding an individual enzyme molecule. "You might be able to make more stable and longer-lasting sensors or chemical reactors, like the glucose sensors used by diabetics, if you could use the pores as enzyme cages for controlling each enzyme's reaction environment," Mallouk speculates.
Other potential technological applications include using the pores as isolation chambers with the low dielectric conditions needed for preventing microelectronic circuit components from interfering with each other. "The lowest dielectric constant you can get is that of empty space, and these materials are 75 percent empty space," Mallouk comments.
"We also think that, because the length scales of these pores is well below the wavelength of light, any material we could make with them would be transparent--even a metal--because it would not be able to scatter light," Mallouk says. "It is exciting to have control over both the composition and the length scale of this new material because that gives us the tools for exploring lots of different possibilities."
Stacy A. Johnson and Patricia J. Ollivier have completed their graduate studies and are now employed by DuPont at its research facilities in Virginia and North Carolina. Mallouk has filed a patent application for the discovery. This research was supported by the National Institutes of Health.
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