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CHAPTER SIX Functional Organic and Hybrid Materials CHAPTER SUMMARY The Panel on Functional Organic and Hybrid Materials addressed what its members expect to be defining general concepts that will emerge in the next two decades to fundamentally change the science and engineering of organic and hybrid materials. Many of these changes will be truly revolutionary. The panel predicts that organic materials of high and low molar mass will continue to increase their penetration of military materials applications for the foreseeable future because of the clear advantages they have in terms of functional flexibility, low weight, and facile processibility—all leading to economic gain over the life cycle. This prediction is based on an extrapolation of materials developments over the last 50 years. The panel has identified a number of research opportunities, among them: Promotion of the convergence and integration of organic and Si electronics and other semiconductor and photonics into hybrid architectures; New synthetic strategies to produce high yields of selected polymers with completely defined chemical structures and with enhanced homogeneity and purity; Computer modeling and simulation, accessible to experimentalists, to optimize chemical and structure selection for specific functionalities (organic materials, especially macromolecular, that display high photovoltaic and thermoelectric figures of merit are particularly valuable for military applications);
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Use of organic materials to provide robust defenses against laser threats to personnel and equipment; and Novel catalyst systems to provide in situ defenses by neutralizing chemical and biological attack. DoD investments in such areas will maximize the development of important novel organic materials with specific military applications. If these opportunities are pursued, the panel expects that: Modeling will become a routine first step in organic materials development. Synthesis and processing of organic materials will tend to converge. Polymers of high purity with totally controlled microstructure will become available, with important applicabilities. Aggregates of organic materials on the nanometer scale will yield new opportunities in material functionality. Combinations of low- and high-molar-mass organic molecules with inorganic materials will become widespread, offering unique functional advantages. INTRODUCTION The Panel on Functional and Organic Hybrid Materials believes that organic materials of low or high molar mass are destined to play a vastly increased role throughout the entire spectrum of military applications for the foreseeable future. By virtue of their functional flexibility, facile processibility, and intrinsic low weight—all of which contribute to an economic advantage over the application life cycle—their penetration into regimes hitherto held by metallic and other inorganic materials will continue at an unabated, and perhaps accelerated, pace. In this chapter, the panel discusses what it believes to be the defining general concepts that will emerge in the next two decades to fundamentally change the science and engineering of organic and hybrid materials. Advances based on these materials are foreseen in electronic and photonic devices, eye protection against laser weapons, lightweight full-color displays, photovoltaic energy collectors, protection against chemical and biological agents, and many other areas. Many of these changes will be
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truly revolutionary. The chapter concludes with a discussion of R&D priorities that can help to bring about these revolutionary changes in the areas of greatest interest to the U.S. military. DOD NEEDS FOR FUNCTIONAL ORGANIC AND HYBRID MATERIALS Electronic Devices The military will always have a need for low-cost expendable and long-term durable electronic devices. Though current silicon technology is viable for today’s devices, there will be a need for molecular electronic devices in the future. Single electron-conducting molecules, including small clusters of metal atoms (hybrid systems), may be the basic technology for advanced electronic circuits and components in future small electronic devices (Lewin, 2001). Gimzewski (2000) projects that Si-based complementary metal oxide semiconductors (CMOS) will reach their limit in 10-20 years. Molecular electronics is expected to surpass this technology, providing that new fabrication methods and probes will allow individual or very small numbers of molecules to be connected to create actual devices. The current capabilities of information technology (IT) are primarily associated with information processing and transmission. In the future it is expected that acquiring and acting on information will be a critical need, and current semiconductor technology is not projected to meet these challenges. Polymer electronics (organic electronic materials) may be the enabling technology for future IT needs. Microelectronics today is already on the road to nanoelectronics, but there are exponentially increasing costs and diminishing returns associated with building new integrated circuit (IC) fabrication capabilities. This high infrastructure cost is limiting competition and innovation from smaller companies, while the technical miniaturization challenges (wiring, power dissipation, etc.) are increasing rapidly. Alternative technologies are required to advance IT needs in the next 20 years (Xu, 2000). Current electronic systems have excellent information-processing speeds, but the hardware is difficult to reconfigure or rapidly evolve into more powerful systems. Molecular computers based on combinatorial syntheses of complex families of materials may be used to create new reconfiguration and evolution concepts. This could ultimately connect
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electronic processing speed with molecular design, structure, and flexibility (McCaskill and Wagler, 2000). Current state-of-the-art molecular electronics technologies will have many challenges over the next 10-20 years, but they have the potential to deliver 1,000 times better performance in IT application areas than can be achieved with existing materials and systems (Wada et al., 2000). Quantum effects associated with nanometer size dimensions are already considered in the design of microelectronic devices. Thus, it is possible that all organic molecular electronics will find either a significant niche apart from conventional CMOS systems or that hybrid products will be created using both technologies (Seabaugh and Mazumder, 1999). Polymers, including composites and hybrid systems, are easy to process/fabricate, low-cost, lightweight, and flexible; they can have unique structural features and be made very durable. Thus, polymer electronic devices may become competitive with semiconductor and metal devices (Rughooputh and Rughooputh, 1999). Miniaturization in electronic circuits, and ultimately devices, will reach the scale of atoms or small numbers of molecules (molecular electronics). There already have been many instances where single molecules have been embedded between electrodes and demonstrated basic digital electronic functions (Ellenbogen and Love, 2000). Among the challenges still remaining are to theoretically design new materials using computational chemistry, synthesize these materials, and assemble and connect the molecular circuits and components to create practical devices (Joachim et al., 2000). Photonics In 2020 the military will need to effectively control the human-to-human, human-to-machine (weapon or network), and machine-to-machine interfaces. This means being able to rapidly sense or obtain large volumes (terabytes) of information, quickly analyze the information, and react accurately in time frames (nano- to picoseconds) that may not be possible today with current electronic and photonic devices. For these reasons it is projected that a number of advanced developments must be made in the area of photonics in order to design and produce devices and equipment that will process the massive amount of information that the military needs to be effective in future operations, both in peacetime and in conflict.
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Optical Limiting Materials Progress in compact laser systems made laser weapons possible, but it is a challenging task to protect sensors and eyes from laser light damage because there is a wide range of laser systems with different temporal and spectral characteristics. Two strategies can be employed to achieve sensor protection: all-optical switches and optical limiters. Both devices use nonlinear optical effects. The challenge will be to synthesize materials that exhibit large enough optical nonlinearity and chemical and photochemical stability. If such optical limiting materials can be synthesized, their impact on soldier protection will be enormous. Organic Light-Emitting Materials Low-cost, addressable, lightweight, full-color displays on flexible organic substrates with efficiencies in the 50-100 lumen/watt range or higher have substantial implications for information dispersion at the soldier level. Their development will require integration of several technologies. From a fabrication point of view, the printing technology that can in principle apply to polymer materials (less obviously to low-molecular-weight organics and probably not at all to inorganics) will give integrated all-macromolecule systems a competitive advantage. However, probable advances in other display methodologies make the equation less predictable. Other areas of military application are in solid state organic light-emitting diode (OLED) and polymer light-emitting diode (PLED) lighting, again taking advantage of simpler fabrication. Polymeric materials also form the basis for polymer lasers. At present there have been many displays of stimulated emission in chromophores using optical pumping. Electrically pumped systems have been demonstrated from low-molecular-weight organics and are certainly imminent from macromolecules also. Many technical issues remain, including the architecture of cavity designs, but it is highly probable that electrically pumped polymer lasers will become available soon. The materials issues to be resolved deal with obtaining sufficiently high excitation densities by avoiding defect structures and morphologies. Again, host-guest polymer systems represent an attractive focus for research over the next 2 decades. The implications of having low-cost, efficient polymer lasers in information storage and retrieval systems are profound (McGhee and Heeger, 2000; Friend et al., 1999).
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Molecular Magnetic Materials It is anticipated that through 2020 the field of molecular magnetic materials will produce advances in lightweight motors and electric generators that incorporate organic- and polymer-based magnets for substantial weight savings. The field will continue to expand in terms of materials options, synthesis and processing choices, and new phenomena unique to the organic-, molecule-, and polymer-based architecture. It is expected that the magnetic ordering temperature for some examples of this class will exceed 600 K together with approximate thermal stability. It also is expected that the organic-, molecule-, and polymer-based magnets will be combined with organic- and polymer-based conducting, semiconducting, and photonic materials in integrated multifunctional smart materials. An example of such integration would be “spintronics” devices that are all-organic. New processing options, such as self-assembly of structures, will be commonplace. Photorefractive Materials The photorefractive effect has long been recognized to possess great potential for military applications (Günter and Huignard, 1998; Solymar et al., 1996), including high-capacity optical memories, dynamic hologram formations, massive interconnections, high-speed tunable filters, phase conjugation, real-time handling of large quantities of information, and real-time relay lines for phase-array antenna processing. Numerous device concepts using inorganic materials have been explored. However, only a handful evolved into military devices to date. Inorganic materials are difficult to prepare with defined composition (impurity levels) and are expensive. The successful demonstration of new devices based on organic photorefractive materials depends heavily on the emergence of new materials and processes. Photovoltaics Soldiers are equipped with sophisticated electronics and communications gear. Portable power for soldiers is the most immediate application for large-area flexible photovoltaics. Field stations and mobile armor units would also benefit from access to such devices. While the efficiencies of
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inorganic semiconductor-based photovoltaic devices are being improved, there is great appeal for an organic counterpart. Organic materials, either as low-molar-mass compounds on a flexible support or as polymers, are attractive due to the typical prospects of facile large-area fabrication, mechanical flexibility, potential very low cost, and the ability to tune optical properties to match the absorption characteristics of the solar spectrum. Membranes Three major military needs for membrane materials have been identified in the next 20 years: (1) soldier protection from chemical and biological agents, which will be the driver of development of “smart” membrane technologies. Such membranes ideally will protect the soldier while detecting and reporting the nature of the chem/bio agent, followed by decontamination and reactivation of the sensing elements; (2) membranes with high throughput and selectivity for water purification; and (3) membranes for power (especially portable) sources. Metal Organic Catalysts The principal areas important to metal organic catalysts for future materials for defense are smart materials, fuel conversion, and self-healing structures. In the case of smart materials, embedded catalysts are expected to act through a feedback loop as both sensors and actuators; an example is a metal organic that senses a biohazard and actuates a catalytic antidote. For fuel conversion, the catalysts may be for fuel reforming, along with a fuel cell, or for producing nutritional substances on the battlefield. One pressing need is for electrocatalysts for direct methanol oxidation. For fuel cells that run on hydrogen, the difficulty is hydrogen storage. One solution would be to generate hydrogen on demand by direct oxidation of methanol, a fuel that is easier to store and transport. These electrocatalysts, which typically contain Pt and Ru, require a delicate balance in the metal-on-hydrous oxide structure (Long et al., 2000). More rugged materials that protect this structure are needed. The third area is self-healing structures. Here the need is for catalysts embedded in textiles that can initiate polymerizations to repair tears and punctures (White et al., 2001).
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SPECIFIC AREAS OF OPPORTUNITY This section describes the challenges and opportunities presented if functional organic and hybrid materials are to be inserted into militarily important applications. In addition, a few crosscutting and high-risk opportunities for use of these materials are described. Electronic Devices The basic components needed to create electronic devices on a molecular scale are wires, switches, rectifiers, and transistors. Molecular Wires An organic solid [tetrathiafulvalene (TTF)–tetracyanoquinodimethane (TCNQ)] that exhibited metal conductivity below 59 K was reported some time ago (Heeger and Garito, 1972; Cowan and Wiygul, 1986). Later, Shirakawa et al. (1977) reported a doped polyacetylene material that had a room-temperature conductivity of 500 (ohm-cm)−1. Work has continued in this area with primary attention to polythiophenes, polypyrroles, and other highly conjugated organic, heterorganic, and organometallic systems (Skotheim, 1986; Joachim et al., 2000). Certain modified polythiophene structures (Figure 6-1) have shown superconductivity properties at 2.5 K (Schön et al., 2001; Skotheim, 1986). FIGURE 6-1 Superconducting organic polymer.
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Another approach to creating potential molecular wires is using σ bonds that are associated with polyorganosilane materials (Figure 6-2). These polymers exhibit semiconducting properties like photoconductivity, high hole drift mobility, and electroluminescence. They can be designed and fabricated into either rigid or flexible polymer chains that can be precisely connected to silicon surfaces (Fujiki, 1996). These types of polyorganosilane hybrid materials could find applications in bridging conventional silicon-based circuits with molecular organic-base building blocks or components (Fujiki, 1996). Carbon nanotubes represent a special class of wires in that they are inherently conductive but their conductivity is propagated via tube-to-tube contact points or through substrate-tube-substrate connection geometries. Actually, the electronic structure of nanotubes can be metal-like or semiconducting, depending on the diameter of the tube and on the geometrical arrangement of the carbon atoms. This ability to have or create conducting or semiconducting properties has allowed several researchers to design and build nanotube diodes, T and Y junctions, and field-effect transistors (FETs) (Collins and Avouris, 2000; Lefebvre et al., 2000; Meyyappan and Srivastava, 2000; Dekker, 1999). Switches A simple molecular switch allows transport of electrons through a molecule while at the same time being able to disrupt the transport process in the molecule via conformational changes or other reversible reactions. For example, photochromic molecules can undergo a photo-induced intramolecular change in their molecular orbital structure that can favor or reduce electron transfer, depending upon the final molecular configuration FIGURE 6-2 Potential molecular wire material that takes advantage of σ bonds in polyorganosilane materials. R1 and R2 are alkyl groups (branched and linear).
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(Fraysse et al., 2000). Also, supermolecular structures, rotaxanes, are reported to have mechanical switching capabilities (Bissell et al., 1994). Organic molecules that can be electronically switched on or off for extended periods might be used as the basic components of memory and logic devices. The molecules of choice are highly conjugated donor/ acceptor functionalized phenylene ethynylene oligomers that are very sensitive to their local environments. When these molecules are constrained in a well-ordered monolayer stack of dodecanethiolate, their switching ability between conductive (on) and nonconductive (off) states under an electronic field is severely reduced. When their environment is changed to a less ordered structure, they can switch ability much more rapidly (Jacoby, 2001). It has also been shown that when several thousands of these types of molecules are configured between gold electrodes, they can be switched between conductive and nonconductive states that allow data to be written, read, and erased just as in magnetic storage media (Reed, 1999a,b; Reed et al., 1997, 1998). Rectifiers Molecules that allow electrical conductivity in one direction through the molecule but not the other can be classified as rectifiers (Aviram and Ratner, 1994). An example of this type of molecule is γ-(n-hexadecyl) quinolinum tricyanoquinodimethane (C16H33Q-3CNQ) (Figure 6-3). This particular molecule has a high dipole moment (43 debyes) zwitterionic ground state (donor + –π bridge – acceptor–) and a first excited state with lower polarity (donor ° – π bridge – acceptor°) and corresponding lower dipole moment (3 to 9 debyes). The ability to affect the flow of current in FIGURE 6-3 Molecular rectifier.
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one direction and then in the opposite direction is a function not only of the difference in dipole moments of the molecule but also of how the molecule is configured between the electrodes of a test cell (Aviram and Ratner, 1974). A Langmuir-Blodgett monolayer film of C16H33Q-3CNQ has been shown to rectify by intramolecular tunneling, while monolayers and multilayers tend to rectify as macroscopic film structures. These systems also show rectification between 105 K and 370 K, but there are concerns about voltage recycle capabilities and alignment stability of the film structures (Metzger et al., 1997). Polymer Transistors Transistors, as the major building blocks of any electronic circuit, should be the focus of interdisciplinary research teams in the future. Integrated circuits have been created using polymers in such conventional fabrication techniques as inkjet printing and microcontact printing (Garnier et al., 1994; Gelinck et al., 2000). Figure 6-4 is a representation of a polymer field-effect transistor. The electrodes (gate, sources, and drain) were deposited by inkjet technology and the semiconducting or dielectric layers were created by conventional spincoating techniques. The polyimide channels were fabricated using photolithography and oxygen plasma processes on glass substrates, but polymer film materials, flexible or rigid, could be used as well. This particular all-organic polymer transistor had a high mobility of 0.02 cm2/V-sec and its on-off current switching ratio was 105 (Sirringhaus et al., 2000). Other recent developments in non-inorganic transistor technologies include the use of an organic semiconductor (pentacene) as the thin-film active layer transistor constructed on a glass substrate (Klauk et al., 1999). In the most recent molecular scale transistor (field-effect transistor), monolayers of 4,4’-biphenyldithiol were self-assembled on a gold substrate and then sandwiched under another gold top electrode. Two of these transistors (approximately 1,000 molecules) were used to create a “0” to “1”/ “1” to “0” input switch (Schön et al., 2001). Electronic Circuits It has now been established that most, if not all, of the individual basic molecular-scale building blocks that are required to create electronic circuits have been demonstrated. The next step is to create economically viable and highly reliable electronic circuits that are equivalent or superior
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Advances in synthesis have improved methodologies for designing metal organic catalysts. Work on polyoxometalates has led to stronger metal-ligand bonds (Schroden et al., 2001), and the capability of forming clusters. Sol-gel processes have been used to create better supports and high-surface-area materials (Avnir et al., 1998). Sol-gel processing also has been used to encapsulate catalysts to protect them from degradation. New methods of complexation have given better ways to anchor catalysts. Silsesquioxanes are a class of materials that expand the types of support structures available (Zheng et al., 2001). With silsesquioxanes, mesoporous materials, and zeolites, other support structures are needed to prevent clustering and loss of activity. A new development in sol-gel encapsulation is the successful incorporation of an acid (molybdic acid) and a base (N-2-aminoethylamino-propylated silica) in the same silica matrix. This concept allows sequential acid- and base-catalyzed reactions to take place. Further extension of the concept is needed. The perceived benefits of these hybrid organic-inorganic materials in the context of metal organic catalysts are that an inorganic matrix (1) improves thermal stability, (2) enhances chemical stability, (3) reduces air sensitivity, and (4) may increase selectivity through chromatographic behavior of the matrix porosity of the metal organic catalyst. Clearly, further R&D is required to optimize methods. Transparent Electrodes/Organic Interfaces A crosscutting issue in hybrid materials technology is transparent electrodes. In OLEDs, liquid crystal displays, photodetectors, solar cells, optical filters, electrical heating, anti-fogging devices and touch-screen sensors, the prevailing material is ITO. In passively driven displays, the anode and cathode are ITO on rigid substrates. In actively driven displays, the ITO is used in conjunction with thin-film transistor (TFT) arrays. The ITO layer is typically 100-500 nm thick, with >80 percent transmission in the visible light range. The ITO layer must be electrochemically compatible with metals like Al and maintain a low resistance (5-10 ohm/square). In addition, the ITO layer has to survive all processing of other layers in the device, including cleaning, patterning, UV/ozone exposure, and heating. For all devices that rely on ITO, the present needs are for lower cost, fewer processing steps, reduced sensitivity to atmosphere to eliminate need
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for vacuum equipment, and better adhesion to flexible substrates to permit roll-to-roll processing. This list of improvements presents challenges that may or may not be met by ITO. Other transparent conductors, such as antimony-doped tin oxide (ATO), fluorine-doped tin oxide (FTO), and possibly ZnO, are being pursued. Parallel efforts need to be carried forward on both the processing and the chemistry sides to meet the requirements of planarization and adhesion. Higher-Risk Developments Examples of material developments that are as yet on the far horizon include the following: Organic Thermoelectrics These materials (especially the macromolecular ones) could provide extremely versatile power generation or cooling for a very wide range of military applications. Present (metallic) materials are expensive and difficult to fabricate and typically use heavy elements. Figures of merit (300 K) over Z = 1 have been demonstrated, but higher Zs are a material challenge (see Appendix D). Organic Room-Temperature Superconductors Oligomeric and polymeric regioregular thiophenes have demonstrated superconductivity in special circumstances at <5 K. As with ceramic/ metallic superconductivity, what is needed is a material that will not only substantially increase Tc and current carrying capacity but also be easy to process. Such materials may become available by 2020; if so, they will have profound implications for myriad device and data-processing applications. RESEARCH AND DEVELOPMENT PRIORITIES In this chapter, the panel outlines its assessment of the opportunities that functional and organic hybrid materials offer for revolutionary new military capabilities by 2020. From this analysis, the panel has extracted five broad R&D priorities, discussed below, that are considered critical to the realization of these opportunities.
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Convergence and Integration of Organic and Si (and Other Semiconductor) Electronics and Photonics in Hybrid Architectures Opportunities will continue to arise for the use of polymers as components in hybrid materials systems, along with metals, ceramics, and electronic materials. Familiar composite materials (e.g., graphite-reinforced epoxy) will find additional applications, but significant potential is seen for new materials combination, such as organic electroactive materials and silicon for hybrid electronic and optical devices. Major issues will need to be addressed, such as how to match sometimes disparate properties like thermal expansion and optical absorption. New Synthetic Strategies to Produce High Yields of Selected Polymers with Completely Defined Chemical Structures, Enhanced Homogeneity, and Purity Synthesis and processing today are typically separate operations. This will change over the next two decades as opportunities emerge to carry out simultaneous synthesis and processing. This idea is not new (e.g., reaction injection molding or chemical vapor deposition), but it will become more widespread. Of particular interest will be polymers and small molecules that self-assemble into ordered molecular structures (e.g., liquid crystals) or morphological structures (e.g., block copolymers). Combinatorial synthesis of polymers will become more routine. The panel anticipates the emergence of combinatorial processing approaches to rapidly identify conditions for fabricating polymers to achieve maximum properties. Polymerization techniques have limited opportunities to control the sequence of adding two or more monomers. Block copolymers are possible with successive addition of monomer charges to active chain ends, and alternating copolymers can be obtained under special circumstances. However, there currently is no viable means to prepare, for example, vinyltype copolymers with sequence control (e.g., poly[(monomerA)1– (monomerB)2]n). This is in stark contrast to peptide synthesis on ribosomes within biological cells, which employ a template to code for specific amino acids that are enzymatically linked. A major opportunity and challenge is thus sequence-controlled polymerization of a wide variety of monomers using systems that mimic the functions of ribosomes. Electrochemical polymer synthesis is an attractive option because properly
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patterned electrodes may simultaneously serve as solid templates for synthesis and as energy sources. The panel notes that other lessons from biology, such as self-assembly and development of hierarchical structures, will continue to be borrowed and built on. Polymers are by nature complex materials, typically having a distribution of chain lengths, isomer content, degrees of orientation, and fractional crystallinity (if any). Thus, structure in polymeric materials can be hard to define compared to small molecules. This difference will gradually disappear over the next two decades with synthesis of long-chain molecules with greater compositional and structural precision. The implications will be significant: Very precise structure/property relationships will be possible, and through the integration of modeling, synthesis and processing, properties can be maximized. Many common examples of nanoscale (one dimension of <100 nm) organic materials exist, including block copolymer films and collagen fibers that function as scaffolds for tissues and organs. However, there will be an increasing push to exploit the properties of individual molecules, or very small aggregates of molecules, for the next generation of electronic and optical devices. For example, carbon nanotubes and collections of only a few organic molecules are being studied as components of diodes, transistors, and memory elements. The ability to create well-defined organic molecular structures and manipulate them to form complex and functional arrangements will drive a revolution in information storage and processing, sensing, and communications. Computer Modeling and Simulation, Accessible to Experimentalists, to Optimize Chemical and Structure Selection for Specific Functionalities The creation of new organic materials will begin with broad evaluation of properties using high-level modeling and simulation to determine critical parameters (isomeric structure, molecular weight, degree of chain orientation) that influence a property of interest. In particular, modeling will be used to predict complex organization of functional low-molar-mass molecules and polymers, as is beginning to be done for the difficult problem of predicting protein-folding motifs. Modeling will also extend to synthesis routes to define the best approach, as well as to processing. Much guiding information will be in hand before any wet chemistry is done.
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Organic Materials to Provide Robust Defenses Against Laser Threats to Personnel and Equipment The challenge will be to synthesize materials that exhibit enough optical nonlinearity and chemical and photochemical stability. If such optical limiting materials can be synthesized, their impact on soldier protection will be enormous. Catalyst Systems to Provide in Situ Defenses by Neutralizing Chemical and Biological Attack Metal organic catalysts have a role to play in making materials multifunctional in the true sense of “smart” materials. In case of chemical or biological attack, embedded catalysts are expected to act as both sensors and actuators through a feedback loop. An example is a metal organic that senses a biohazard and actuates a catalytic antidote. However, new production methods, lower-cost catalysts, and new support structures will be necessary before these materials can realize their potential. REFERENCES Abdeldayem, H., D.O. Frazier, M.S. Paley, and W.K. Witherow. 2000. Recent advances in photonic devices for optical computing. Available online at <http://science.nasa.gov/headlines/images/nanosecond/thepaper.pdf>. Accessed February 21, 2002. Allemand, P.M., K.C. Khemani, A. Koch, F. Wudlf, K. Holczer, S. Donovan, G. Gruner, and J.D. Thompson. 1991. Organic molecular soft ferromagnetism in a fullerene-C60. Science 253(5017):301-303. Aviram, A., and M. Ratner. 1974. Molecular rectifiers. Chem. Phys. Lett. 29:277-283. Avnir, D., L.C. Klein, D. Levy, U. Schubert, and A.B. Wojcik. 1998. Organo-silica sol-gel materials. Pp. 2317-2362 in The Chemistry of Organic Silicon Compounds, Vol. 2. Z. Rappoport and Y. Apeloig, eds. New York: Wiley & Sons. Bane, P.W., and S.P. Bradley. 1999. The light at the end of the pipe. Scientific American 281(4):110-115. Bissell, R.A., E. Cordova, A.E. Kaifer, and J.F. Stoddart. 1994. Chemically and electrochemically switchable molecular shuttle. Nature 369:133-137 [see also Borman, S. 1994. Reversible molecular switch synthesized. Chem. Eng. News, May 16, pp. 8-9, and Borman, S. 1999. Key step made toward molecular computing. Chem. Eng. News 77(29):11-12]. Black, C.T., C.B. Murray, R.L. Sadstrom, and S.H. Sun. 2000. Spin-dependent tunneling in self-assembled cobalt-nanocrystal superlattices. Science 290(5494):1131-1134.
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