PHILADELPHIA – Researchers at the University of Pennsylvania have devised a new method for aligning isolated single wall carbon nanotubes and, in the process, have created a new kind of material with liquid crystal-like properties, which they call nematic nanotube gels. The gels could potentially serve as sensors in complex fluids, where changes in local chemical environment, such as acidity or solvent quality, can lead to visible changes in the gel's shape. The researchers describe their findings in the current issue of Physical Review Letters.
Single wall carbon nanotubes have astounded researchers with their remarkable strength and their ability to conduct heat and electricity. For many of their potential applications, however, these nanotubes work best when they are aligned parallel to one another, without forming aggregates or bundles. In solutions with low concentrations of single wall carbon nanotubes, the nanotubes are isotropic, or not oriented in a particular direction. If the concentration of the single wall carbon nanotubes is increased sufficiently, it becomes energetically favorable for the nanotubes to align. This is the nematic phase that many researchers have sought to create and utilize.
"Unfortunately, experience has shown that single wall carbon nanotubes tend to clump together or form three-dimensional networks in water at concentrations where theories otherwise predict they will form this nematic liquid crystal phase," said Arjun Yodh, senior author and a professor in Penn's Department of Physics and Astronomy. "Our gels effectively increase the concentration of isolated single wall carbon nanotubes without allowing them to bundle up or form networks."
Yodh and his colleagues embedded isolated nanotubes coated by surfactant into a cross-linked polymer matrix, a gel. The volume of the gel is highly temperature dependent, and the researchers were able to compress it to a fraction of its original size by changing its temperature. The gel network prevented the close contact between parallel nanotubes that produces bundling, and its compression produced concentrations of isolated nanotubes that favor nematic alignment. The condensed gel thus creates concentrations of isolated, aligned nanotubes that cannot be achieved when they are suspended in water.
Like liquid crystals, the resulting nanotube gels exhibit beautiful defect patterns revealed by polarized light transmission through the sample that correspond to the particular nanotube alignments. The topology of the defects are, in turn, coupled to the mechanical strains present in the gel. The researchers are now exploring applications for both the technique and the properties of the nematic nanotube gels.
"Certainly we expect the mechanical, electrical and perhaps thermal properties of the resulting composites to differ from their unaligned counterparts," said Mohammad Islam, a Penn postdoctoral fellow and co-author of the study. "It might be possible to use local influx of particular chemicals to cause mechanical deformations in the gel. Similarly, external fields could interact with the nanotubes, which in turn would interact and deform the background polymer network."
The research was funded by grants from the National Science Foundation and NASA. Penn has filed patent applications on this technology and the patent rights have been licensed to NanoSelect Inc. Commercial inquiries may be directed to NanoSelect.
Other Penn scientists involved in this study include Ahmed Alsayed, Zvonimir Dogic, Jian Zhang and Tom C. Lubensky.
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