When ORNL's new neutron sources go on-line in the next few years, researchers will have valuable tools for exploring the features of matter as small as a few billionths of a meter. That’s one reason why many researchers involved in nanoscience and nanotechnology are excited by two ongoing initiatives at ORNL. One initiative is the upgrade of the High Flux Isotope Reactor (HFIR), which will offer in 2002 the world’s highest thermal neutron intensities and in 2003 cold neutron fluxes comparable to the world's best. The other initiative is the construction of the Spallation Neutron Source (SNS), which by 2006 will provide 10 times the flux of any other pulsed (spallation) neutron source in the world.
"Reactor and spallation neutron sources are well matched to the nanoscale," states the 1999 Nanotechnology Initiative Report of the Department of Energy's Office of Basic Energy Sciences (BES). The report points out that neutrons, for example, "can be used not only to study nanoparticles themselves but also to examine protective coatings that are placed on the nanoparticles to prevent oxidation. Neutrons can also be used to study dispersants that disperse nanoparticles in a solvent or host medium."
Because nuclei of hydrogen and deuterium (heavy isotope of hydrogen) scatter neutrons differently, neutron scientists often selectively label organic material with deuterium or use deuterated solvents so they can highlight what they want to see and block out the rest. "Using this method," says George Wignall, a neutron scientist in ORNL's Solid State Division (SSD), "we can optimize the contrast between the nanoparticle and coating so we can study the structure and behavior of the organic material surrounding the nanoparticle."
The use of organic materials, self-assembled organic structures, and organic hybrid materials is emerging as a route to producing functional nanoscale structures. Wignall is a principal investigator in a study of nanoscale-structured materials formed by self-assembly of triblock copolymers. Participants in the study, which is supported by internal funding from the Laboratory Directed Research and Development Program at ORNL, include Tony Habenschuss of ORNL's Chemical and Analytical Sciences Division (CASD) and polymer chemist Frank Bates and his colleagues at the University of Minnesota. Triblock copolymers synthesized by Bates have been studied by Wignall and Habenschuss using small-angle neutron scattering (SANS), X-ray scattering, atomic force microscopy, and electron microscopy. SANS will be especially useful for studying these fascinating materials at ORNL when the HFIR upgrade is complete.
In 2003, the cold neutron flux at HFIR will be comparable to that of the Institut Laue Langevin in Grenoble, France, which is the best research reactor in the world for neutron scattering research. "The upgrade will boost HFIR's cold neutron flux and detection efficiency," says Wignall. "Because our neutron detector will be two-and-a-half times larger than what we have now, we will be able to make simultaneous measurements over a much wider range of angles and perform time-resolved experiments. For example, we will be able to make real-time studies of materials as they are processed and observe the self-assembly of triblock copolymers."
Triblock copolymers are made by joining three chemically distinct polymer blocks (large molecules), each a linear series of identical monomers (small molecules).
"Because the three blocks linked together in a linear arrangement may be thermodynamically incompatible, they will try to separate from each other," Habenschuss says. "But since they are joined together, they can only form separate domains on a scale of the individual block sizesthat is, on a nanometer scale for typical block lengths. These separate domains self-assemble into complex ordered nanostructures."
Examples of these structures are rods or spheres of one material regularly arranged in the matrix of another. A particularly interesting morphology studied at ORNL is the continuous core-shell gyroid structure in which three-dimensionally continuous channels wind through a matrix.
"Previously we had used SANS to study diblock copolymers made of polystyrene and polybutadiene," Wignall says. "In the LDRD study we investigated triblock copolymers of polyisoprene, polystyrene, and polydimethylsiloxane."
Self-assembled triblock copolymers could create hundreds of new morphologies. Such materials could result in improved power-producing fuel cells and gas-separating membranes.
"A large fraction of potential users of the upgraded HFIR and the SNS have expressed interest in studying soft matter, such as proteins and polymers, using SANS, reflectometry, and other instruments," Wignall says. As noted in the BES Nanotechnology Initiative Report, "the potential impact of neutron sources for the elucidation of these nanostructures is tremendous."
According to Wignall, "We can expect studies of very small features in soft materials to get a big boost when ORNL becomes the world's leading center for neutron scattering research in the middle of this decade."
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