Cornell scientists have come up with a novel way to manipulate liquid crystal molecules so they self-assemble in a desired direction into a robust network, making them useful as a new material for a variety of applications in the computer, medical, automotive and aerospace industries.
The researchers have shown they can build a network of liquid crystal molecules that are linked together while aligned in an electric field. The field makes them lie parallel or perpendicular, depending on the AC frequency, so they orient on-demand.
"These are wonderful molecules. When you apply the electric field and crosslink them, you get controllable properties, so that it is possible to tailor these materials to possess specific optical, electronic and mechanical properties," said Christopher K. Ober, Cornell professor of materials science and engineering who led the work.
The research, reported in the journal Science (April 11, 1996), was done by Ober; Hilmar Koerner, a postdoctoral associate; Atsushi Shiota, a student now at Japan Synthetic Rubber; and Timothy J. Bunning, former postdoctoral researcher in Ober's lab now at Wright Laboratory in Dayton, Ohio. Their work was funded by the Electronic Packaging Alliance at Cornell (an industry-university partnership), the National Science Foundation and the Wright Laboratory for Laser Hardened Materials.
The team came up with not only new materials, but a new way of processing the materials. Cornell Research Foundation has applied for a patent on the technology.
Liquid crystal molecules -- like those in everyday watches or telephone displays, optical filters or data storage -- are self-assembling. Like tiny bricks, the molecules line up and assemble themselves into a wall. With an AC electric field from 10 hertz to 10,000 hertz applied, these "bricks" -- cigar-shaped, actually -- can be made to lie flat or stand up on demand, just by changing the frequency of the field. When heated, or cured, the molecules form bonds to create a network. The curing can take anywhere from five minutes to an hour, depending on the intended result.
Called liquid crystal thermosets, such materials could be used as an advanced adhesive, as barrier membranes in food or medicine, or as protective coatings -- a layer between materials, for instance.
"Our goal was to create a molecular system where one could not only align the components in external fields to form networks, but also selectively control and lock-in the direction of alignment by network formation," the researchers write in the Science paper. "Such materials would possess physical and chemical properties that are very different along each orientation, and one could conceive of using photochemistry, for example, to form films with order and orientation set in specified regions."
Another advantage of this technique is that the network is robust -- the network ensures that the molecules remain in the orientation and the bonds remain strong -- and can be used at temperatures above 100 degrees centigrade.
The scientists used X-ray diffraction from the Cornell High Energy Synchrotron Source to observe the orientation-on- demand thin films in real time. The high flux X-ray beam allowed them to monitor the curing process (network formation), alignment of the molecules and to simultaneously adjust the electrical field.
The researchers say the achievement is just a beginning step in a long process.
Said Koerner, "We have shown it is possible. Now we have to make it on a bigger scale to show it is practical. We will next produce large films with different orientations."