Examples of Advanced Materials Research at UNC

Three example areas of research emphasized in the Materials Science program are electronic and optical materials, polymer materials, and biomaterials. These areas are not discrete, however, as research projects in electronic polymers, nonlinear optics of polypeptides on surfaces, liquid crystals, and wear in polyethylene artificial joints demonstrate. Individual faculty members may have research interests in more than one of the primary areas, and may collaborate with others to address all three.

Materials Growth – Films and Surfaces

Silicon, used in computer applications, must be virtually defect-free and cannot contain more than part-per-billion levels of certain impurities. To control the purity of materials with this level of precision, each new layer of atoms must be added in the correct manner as the material grows. The new surface layer formed becomes buried by additional atoms as growth continues. In this sense, understanding materials growth boils down to understanding the behavior of atoms (or molecules) on surfaces. We are concerned with the growth of materials on pre-existing substrates. This technique, called thin-film growth, is used in many applications, the most important being the fabrication of integrated circuits for computer memory and logic chips.

Thin-film growth usually involves exposing a hot substrate to reactive gas phase atoms or molecules. The types of events that are important become obvious once you think about what might happen to these atoms. Atoms come raining down from a source of some kind and land on the surface. To find the right location for attachment to the existing surface these atoms must be able to move about. This is why the growth temperature is so important.

Carbon Nanotubes

Carbon nanotubes, UNC Physics Professor, Otto Zhou explains, materialize from a vapor after he uses a high-powered laser to blast a hole through a dime-sized pellet of graphite. The blasting is fiery and spectacular. The tubes are not. If you could actually see one, the tube would look something like a roll of chicken wire—a single layer of carbon atoms linked up in a fabric of hexagons.
But as modest as they seem, these little tubes have big possibilities, with attributes very much in demand for high-tech applications. They are as hard as diamond and tougher than graphite or steel. They can conduct (or semiconduct) electricity. But most important of all, they are tiny—much smaller than the carbon fibers used to reinforce some of today’s space-age materials.

Zhou and his students—including undergraduates—are putting the tubes through their paces, learning how to align them, imbed them in plastics, and assemble them into structures. So far, Zhou has been impressed with the nanotubes’ versatility and strength, though the tubes are costly and time-consuming to produce. (Zhou and others are investigating a different way of making the tubes, decomposing hydrocarbons to produce the tubes at lower temperatures and therefore lower energy costs.) Currently Professor Zhou is using tiny carbon nanotubes as a critical part of new types of X-ray tubes that are more efficient and safer than those now in use at airports and in doctor’s offices.

Polymer Theory and Computer Simulations

The unique properties of polymeric systems are due to the size, topology and interactions of the molecules they are made of. Our goal is to understand the properties of various polymeric systems and to design new systems with even more interesting and useful properties.

Our approach is based upon building and solving simple molecular models of different polymeric systems. The models we develop are simple enough to be solved either analytically or numerically, but contain the main features leading to unique properties of real polymers. Computer simulations of our models serve as an important bridge between analytical calculations and experiments.

The research projects that are profiled here are just a few of the many examples of the interdisciplinary research opportunities that exist at UNC in the Institute for Advanced Materials, Nanoscience and Technology (IAM) in connection with the Applied and Materials Science program.

Liquid Teflon in Microfluidics

Microfluidic devices developed in the early 1990s were fabricated from silicon and glass using photolithography and etching techniques. These processes were costly, required clean-room conditions, were labor intensive, and posed several disadvantages from a materials standpoint. For these reasons, soft materials have emerged as excellent alternatives for microfluidic device fabrication. Soft materials make possible the easy manufacture and actuation of devices containing valves, pumps, and mixers.1-5 This has allowed microfluidics to explode into a ubiquitous technology that has found application in genome mapping, rapid separations, sensors, nanoscale reactions, ink-jet printing, and drug screening.

Poly(dimethylsiloxane) (PDMS) has rapidly become the material of choice for many microfluidic device applications.2-6 PDMS offers numerous attractive properties in relation to microfluidics. Upon cross-linking, it becomes an elastomeric material with a low Young’s modulus of _750 kPa.3 This enables it to conform to surfaces and form reversible seals.

It has a low surface energy around 20 erg/cm2 which usually facilitates easy release from molds after patterning.2,4 Another important feature of PDMS is its outstanding gas permeability. This allows for gas bubbles within channels to permeate out of the device and is also useful in sustaining cells and microorganisms inside the features. Despite the advantages of PDMS for microfluidics technology, this material suffers from a serious drawback in that it swells in most organic solvents. Our approach to this problem has been to replace PDMS with photocurable perfluoropolyethers (PFPEs). PFPEs are a unique class of fluoropolymers that are liquids at room temperature, exhibit low surface energy, low modulus, high gas permeability, and low toxicity with the added feature of being extremely chemically resistant like Teflon.

The synthesis and photocuring of these materials is based on earlier work done by Bongiovanni et al. The reaction involves the methacrylate functionalization of a commercially available PFPE diol (Mn = 3800 g/mol) with isocyanatoethyl methacrylate (PFPE DMA). Subsequent photocuring of the material is accomplished through blending with one wt % of 2,2-dimethoxy-2-phenylacetophenone and exposure to UV radiation. Device fabrication was accomplished according to the procedure illustrated in Figure 1. This method utilizes partial curing techniques to adhere the two layers without compromising feature sizes. The PFPE DMA material is easily spin-coated and molded using procedures designed for standard PDMS materials. The DeSimone group has created a novel solvent-resistant microfluidic device fabricated from PFPE-based elastomers. Photocuring decreases fabrication from several hours to a matter of minutes. The device showed a remarkable resistance to organic solvents. This work has the potential to expand the field of microfluidics to many novel applications. Current efforts to use a PFPE-based device in a novel approach to DNA synthesis are underway.
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(1) Ouellette, J. The Industrial Physicist 2003, August/September, 14-17.
(2) Scherer, A.; Quake, S. R. Science 2000, 290, 1536-1539.
(3) Unger, M. A.; Chou, H. P.; Thorsen, T.; Scherer, A.; Quake, S. R. Science
2000, 288, 113-116.
(4) McDonald, J. C.; Whitesides, G. M. Acc. Chem. Res. 2002, 35, 491-
499.
(5) Thorsen, T.; Maerkl, S. J.; Quake, S. R. Science 2002, 298, 580-584.
(6) Liu, J.; Hansen, C.; Quake, S. R. Anal. Chem. 2003, 75, 4718-4723.

Samulski Biaxial Liquid Crystal

Liquid crystals form the basis of several flat-screen display technologies which are usually called LCDs. It is very challenging to design new liquid crystals for LCDs: typically the thermal energy in the melt phase overwhelms the delicate interplay between attractive forces and dynamic packing preferences needed for liquid crystallinity. Samulski’s group uses nuclear magnetic resonance (NMR) and computer simulation methods to characterize this interplay between intermolecular attraction and packing and try to extend our observations to less esoteric materials (polymer melts and networks). For the latter materials we have designed new experiments that enable us to visualize molecular phenomena while deforming fluid phases in the NMR spectrometer.

Samulski’s research program focuses on understanding structure-property relations, especially how rather modest structural changes at the molecular level are manifested in the supramolecular arrangements adopted by LC molecules and polymers. The alignment of the liquid crystal rod-like molecules, in what is known as the ‘nematic’ phase, can be achieved rapidly and with low voltage. According to theory, these materials should also exist in another nematic phase with potential for use in display applications.

In Physical Review Letters, Samulski’s group1 has reported firm evidence for this so-called biaxial nematic phase. The announcement has created considerable excitement, for it opens up new areas of both fundamental and applied research and maybe a type of “Holy Grail of liquid-crystal science” that has now been found. The hunt for this new liquid-crystal phase began more than 30 years ago, when it was recognized that the molecules forming liquid crystals deviate from their presumed cylindrical shape2. In fact, the molecules are more lozenge-like, and it is because of this lowering of the molecular symmetry that two nematic phases should be possible. In both phases, the molecular axes tend to align over large distances, although their centers of mass have no long-range order. This discovery may likely stimulate the search for other examples of biaxial nematics, especially those formed by V-shaped molecules. In fact, many such materials are already available, although produced for other reasons, and would certainly merit reexamination. The fundamental static and dynamic behavior of this new phase will be explored; perhaps creating a new area of research in the field of liquid-crystal science and technology.

1. Madsen, L. A., Dingemans, T. J., Nakata, M. & Samulski, E. T.
Phys. Rev. Lett. 92, 145505 (2004).
2. Freiser, M. J. Phys. Rev. Lett. 24, 1041–1043 (1970).