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Advancing Materials Research The Role of Chemistry in Materials Science GEORGE M.WHITESIDES, MARK S.WRIGHTON, and GEORGE PARSHALL Chemistry has always been an indispensable contributor to materials science. Materials are synthesized from chemical precursors and assembled using processes requiring chemical transformations. Of the six major classes of products produced by the chemical industry—fuels, commodity chemicals, polymers, agricultural chemicals, pharmaceuticals, and specialty chemicals—two contribute directly to materials science.1 Synthetic organic polymers constitute a major class of materials, and specialty chemicals (e.g., paints, corrosion inhibitors, lubricants, adhesives and adhesion promoters, electronics chemicals) are essential components of many assembled materials systems. Until recently, chemistry has been relatively inactive in other major areas of materials: metallurgy has attracted little activity from chemists, and only in the last few years has chemistry again become involved in ceramics. The importance of chemistry to materials science has been masked by at least two factors. First, the most obvious contribution of chemistry to materials—large-volume organic polymers—is now a mature technology. These substances are perceived (incorrectly) as being scientifically unexciting. The very scale of these large-volume materials and the evolutionary nature of changes in the technology for their production and use have masked the recent emergence and application of exciting new classes of polymeric materials.2,3 Both large-volume commodity polymers and small-volume specialty polymers are subsumed under the general term “polymers,” and the interest and potential of the latter is overshadowed by the volume and importance of the former. Second, the many small components required for the operations of materials systems—e.g., adhesives, lubricants, corrosion inhibitors, mold release agents, surfactants, imaging systems—are indispensable but almost
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Advancing Materials Research invisible in the final systems. Overall, chemistry is perceived as playing a supporting role in materials science, and a relatively unexciting one at that. This situation will change. Materials science is facing problems for which existing systems do not offer satisfactory solutions. The great strength of materials science has rested in its ability to process, combine, and fabricate final systems from existing materials (e.g., metals, polymers, ceramics). The potential for major innovation through improved processing of existing materials is not decreasing, but many important problems cannot be solved simply by processing. The strength of chemistry is its ability to design, synthesize, and produce new materials. The current requirement in materials science for new classes of materials capable of meeting high-performance specifications has stimulated great interest in chemistry. Both the intellectual and practical commercial aspects of materials design and production are becoming major foci of research and development in chemistry.4 This chapter outlines representative recent contributions of chemistry to materials science, suggests areas of materials science to which chemical research might directly contribute in the immediate future, and speculates about the long-range impact of chemical research in producing new types of materials systems and new concepts for materials science. It is not a comprehensive survey of the relations between chemistry and materials science. Rather, it offers informed opinions concerning areas in which new chemistry might lead to new materials and ultimately to new, technologically significant materials systems. THE CURRENT ROLE OF CHEMISTRY IN MATERIALS SCIENCE The production of a final assembled object can be broken down into three major processes (Figure 1). The components of the system—polymers, metals, ceramics, and functional agents—are prepared or synthesized from raw materials. These components are then processed into materials having desired properties and shapes, and, finally, are assembled into the final product. Chemistry contributes to all these stages but plays its largest role in the first—the synthesis of individual components. The importance of the adhesives, protective coatings, lubricants, and other products used in later stages of the process is often overlooked. One major change expected in the future is the more active involvement of chemistry in the later stages of the transformation from raw materials to final product. Ceramics processing is an example of a field of opportunity for chemists. In ceramics processing, a ceramic powder is typically suspended in an aqueous or nonaqueous medium with the help of wetting agents and dispersants. A polymeric binder is added to convert the dispersion to a thick paste. Once the “green” ceramic paste has been cast or molded, the polymeric binder is extracted or pyrolyzed to leave a hard, brittle ceramic residue in the final
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Advancing Materials Research FIGURE 1 The materials production process. shape. High-temperature sintering and densification yield the finished ceramic product, somewhat smaller than the “green” object. Both the binder pyrolysis and the sintering process are subject to catalysis by chemical processes that are totally empirical in character. Major opportunities for improvements such as reduction in pyrolysis and sintering temperatures and control in crystallite structure in the final object await better understanding of the chemistry. In fact, there are three distinct ways in which chemistry may contribute to materials science: first, in the design of chemical additives (coupling agents, lubricants, corrosion inhibitors, barrier films) to facilitate processing, assembly, and performance of the final products; second, in the synthesis of major chemical components (high-performance matrix resins, nonlinear optical materials, ceramic precursors) that result in desirable properties and promote processing in the final pieces; third, in providing new chemical starting materials and reactions for use in processing. It is useful to consider the contributions of several traditional disciplines to materials science (Figure 2) and to anticipate new contributions from each. The strength of chemistry is its ability to manipulate matter at the molecular level: it synthesizes starting components in materials science, provides techniques for analyzing molecular and atomic structures, and generates the understanding required to manipulate properties by changing structure at the molecular level. Physics is particularly successful in rationalizing the delocalized and cooperative properties of materials, and it provides new analytical and processing techniques. Engineering contributes techniques for design, processing, and assembly of fabricated structures, and an understanding (or,
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Advancing Materials Research FIGURE 2 Contributions of various disciplines to materials science. in some cases, empirical modeling) of important macroscopic materials properties. The general character of the contributions from chemistry to materials science is not likely to change. As chemistry turns increasingly to investigations of the solid state, however, and becomes more active in connecting the molecular structure of starting components with the macroscopic properties of final assemblies, the capability of chemistry to contribute throughout materials science will increase. The major classes of substances derived explicitly by chemical synthesis and processing, and used in materials science, have been organic polymers. Polymers are low-density, processable materials having desirable properties such as high tensile strength or elasticity. These are among the properties one can expect from new organic materials. The great power of organic chemistry is its versatility: many organic compounds with different properties can be synthesized. It is this versatility— the ability to manipulate molecular-level structure and produce materials with desired properties—that offers hope for the generation of fundamentally new types of materials. The disadvantages associated with organic materials have been low stability at high temperatures, high sensitivity to oxidation, and low electrical and thermal conductivity. Although it is unlikely that the temperature stability and oxidation resistance of even new organic materials will ever reach those of ceramics and stable metals, strategies for improving these properties are well understood in theory and are being actively explored experimentally.5 Perhaps as important as the potential development of new organic components for materials systems is the opportunity afforded by other classes of chemicals—inorganics, organometallics, and biologicals. Their potential utility
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Advancing Materials Research to materials science has only begun to be explored. Moreover, their combinations with one another and with more traditional organic materials offer opportunities for entirely new types of systems. Two examples suggest the current role of chemistry in materials science. The first concerns the production of carbon-fiber-reinforced composite structures; the second illustrates the importance of organosilicon chemistry in materials science. Carbon-Fiber-Reinforced Composites Carbon-fiber-reinforced composite materials are increasingly substituted for metals in situations where the ratio of weight to strength is a critical issue. The largest current use of these materials is in military aircraft, but their use is being extended to commercial aircraft and to other structural applications as well. The contributions of chemistry to this technology include the production of both the carbon fiber and the matrix resin as shown in Figure 3 with polyether ether ketone (PEEK). The production of carbon fiber involves many chemical steps: conversion of propylene to acrylonitrile (a FIGURE 3 The chemistry of fiber-reinforced composites, as shown with polyether ether ketone (PEEK).
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Advancing Materials Research commodity chemical), conversion of acrylonitrile to a special grade of polyacrylonitrile (PAN), spinning of this polymer into fiber form, and careful oxidation and heat treatment of the fiber under tension to produce carbonized fiber. The corresponding pathway from crude oil to PEEK is too complex to trace in detail, but is shown generally in Figure 3. This type of polymer— a high-performance engineering polymer—represents a major accomplishment in molecular design and chemical synthesis and processing. The combination of PEEK with carbon fiber to form the prepregnated (prepreg) tape from which complex aeronautical shapes are commonly constructed also depends strongly on chemistry. A major problem in this process is to achieve intimate contact between the fiber and the matrix polymer; the resin is viscous and the fibers are fragile. The electrochemical surface treatment applied to the carbon fiber before wetting it with the resin is essential to the success of the final composite. Chemistry has been little involved in the later stages of the process—lay-up of the prepreg tape and cure of the resulting multilayer system to the final object. However, it is precisely in such areas that chemistry may be able to contribute most effectively in the immediate future. The major mode of stress-or impact-induced failure of fiber-reinforced composites is fracture occurring at the interface between the layers of tape and at the interface between the fiber and the matrix polymer.6 Examination of these interfaces and development of chemical treatments and additives to strengthen them is just beginning. Such studies offer probably the best avenue to improvement of the performance of composites generated by this highly evolved materials technology. It is superfluous to point out the complexity and the sophistication of the chemical technology already used to produce matrix polymers and reinforcing fibers for composites. More to the point is the fact that much greater sophistication can and will be applied to certain aspects of each. Further, the chemical aspects of the later stages in this process—especially the molecular tailoring of the many interfaces between the components of these systems—offer extraordinary opportunity both for the development of new interfacial science and for the design, synthesis, and production of chemical reagents to control the properties of these interfaces in useful ways. Organosilicon Chemistry A second representative example of the role of chemistry in present-day materials science is provided by the chemistry of derivatives of silicon (Figure 4). The central column in the figure outlines the current process used to prepare the highly purified silicon that provides the basic materials for the production of semiconductor devices. This process depends upon the conversion of the raw material (silicon dioxide) to a volatile form (silicon tetrachloride) that can be purified by distillation. An alternative version of this
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Advancing Materials Research FIGURE 4 Chemistry of derivatives of silicon. Center column shows the process for preparing the highly purified silicon used in semiconductor devices. At left is the conversion of SiCl4 to triethoxypropylaminosilane, used in the production of glass fiber-reinforced composites. At right is the route to development of silicon carbide fibers. SiCl4 is converted to dimethyldichlorosilane and then to the polymer Si(CH3)2. The resulting structure, SiC, remains unclear. scheme involving silane (SiH4) as the penultimate volatile silicon compound is being developed. An alternative technology, critical for materials science, that depends upon the production of silicon derivatives is based on the conversion of SiCl4 in several steps to triethoxypropylaminosilane [(CH3CH2O)3SiCH2CH2CH2NH2] (shown in left-hand part of Figure 4). This compound reacts with hydroxyl groups present on the surface of glass, silicon, and related materials and attaches the amine-terminated silane covalently to these surfaces. This surface modification is a component in the production of, among other things, glass-fiber-reinforced composites. Without the silane surface modification, adhesion between the glass fiber and the matrix is poor, and the resulting composites have unsatisfactory mechanical properties.7 The right-hand part of Figure 4 sketches a new technology in development. Silicon tetrachloride can be converted to dimethyldichlorosilane and then to a polymer having elemental composition Si(CH3)2 by using the techniques of organometallic chemistry. Thermal rearrangement of this polymer, spin-
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Advancing Materials Research ning into a fiber, and controlled oxidation and heat treatment provide the most thoroughly developed route to materials loosely called “silicon carbide fibers.” The detailed structure of this “SiC” remains unclear. It appears to be a ceramic glass having approximately this composition, and the mechanical properties of these fibers are not yet those of silicon carbide whiskers. Nonetheless, this sophisticated combination of organometallic chemistry, polymer chemistry, and fiber technology typifies a major activity at the border between chemistry and materials science—that is, the generation of useful ceramic forms from organometallic precursors. PROBLEMS IN MATERIALS SCIENCE AND POTENTIAL SOLUTIONS THROUGH CHEMISTRY The preceding examples illustrate the role of chemistry in materials science. Chemistry is critical at the front end of the materials chain in providing components and reagents; its involvement (although not necessarily its importance) declines in the processing and assembly steps. Does existing chemical science have the potential to solve important problems in existing materials processes? Are there classes of problems for which one might expect useful solutions in the short term (less than five years)? Any near-term solution to problems with a commercial process must involve modification of existing technology rather than development of fundamentally new ones. Opportunities to develop specific chemical components and reagents that add value and performance to existing materials systems abound, and many of these opportunities seem well within the reach of existing chemical technology. However, the most important issue in these types of problems is often as much one of management as of science or technology. The market for an additive—for example, a coupling agent or adhesion promoter—that adds value to a materials system may be small, even if the value added is large. Thus it may not be worth the effort of a commodity-oriented chemical company to develop such an additive for sale to others, and it is often outside the competence of a typical specialty chemical company to evaluate performance of additives in complex materials systems. The users most competent to judge the value of such new additives and to take advantage of the value they add—the materials processors themselves— may not be willing or competent to carry out the chemistry required. University chemistry laboratories are typically not familiar with problems in materials science, and again, are unable either to test the performance of compounds they might make or to arrange for commercialization of successful materials. Thus, although the exploration of problems depending on the design, synthesis, and production of specialty additives for materials systems seems practical in technical terms, it is unclear how or where this type of research should be conducted. Government laboratories and university-
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Advancing Materials Research industry collaborative groups seem possibilities. A few specific problems and the possible role of chemistry in their solution will illustrate some of the issues. Multilayer Substrates for Electronics The fabrication of high-performance microelectronic devices depends increasingly on a sophisticated technology in which individual components are located on complex substrates containing the necessary power distribution lines and interconnects. These substrates comprise multiple layers of ceramic (alumina), metal, and thin-film organic insulators. The fabrication of these substrates is made difficult by a number of problems, prominent among which is that of ensuring adhesion between the different components/These components commonly exhibit widely divergent coefficients of thermal expansion and fundamentally incompatible surface chemistries. Current adhesion promoters and coupling agents used in these devices are not entirely satisfactory, and the mechanisms of interfacial failure are being explored. This area represents an important opportunity for chemistry—the development of new series of coupling agents designed and optimized to improve adhesion between the ceramic, organic, and metallic components of these multilayer substrates. The packaging of microelectronic components, which are commonly assembled on fiberglass-reinforced epoxy circuit boards, presents many other significant problems. As the density of circuit interconnects on these boards increases along with requirements for flexibility and durability, the intrinsic limitations of such boards become more evident. For instance, delamination resulting from adhesive failure at the epoxy-fiberglass interface during drilling of interconnect holes can cause short-circuiting between leads and consequently is a serious problem in large, high-density circuit boards. In the near term, development of improved, water-resistant coupling agents to improve adhesion between fiberglass and epoxy will improve this technology. In the long term, development of homogeneous substrates or composite substrates that have fewer problems with delamination will be important. The production of ceramics for microelectronics presents additional problems. The most commonly used ceramic in microelectronics is alumina. It has a high dielectric constant which reduces the speed of electrical signal transmission. Ceramics with a lower dielectric constant would be preferable, but none of the present alternatives is as easily processed as alumina. The development of new, processable, low-dielectric-constant ceramics using organometallic ceramic precursors and sol-gel methods is attractive. The microelectronic packaging requirements for low-temperature sintering, low dielectric constant, and controlled thermal expansion cannot be met by conventional ceramic materials such as alumina. New multiphase ceramic
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Advancing Materials Research composites offer many of the needed properties, but the relationships between structure and property are highly empirical. Chemistry can make an enormous contribution by providing new materials and by making possible rational design of composite materials with preselected properties. Even when conventional materials such as alumina possess adequate electronic properties, they often have unacceptable particle characteristics and levels of impurities. The electronic ceramics industry is moving toward chemically synthesized ceramic powders in which sol-gel8 and other innovative techniques yield materials that disperse, form, and sinter more readily and yield more acceptable physical properties. High-Performance Composite Structures Tailoring the properties of the interface between reinforcing component and matrix is a major application of chemistry to improving the performance of composites. Failure in multilayer fiber-reinforced composite structures often occurs either at the fiber-matrix interface or at the matrix-matrix interface. The surface treatments now used to modify the surface properties of reinforcing fibers in composites are largely empirical. Coupling agents and adhesion promoters operate by principles that are poorly understood and fail by mechanisms that are only partially understood. The practicality of developing fundamentally different types of adhesion promoters is high if the principles of adhesion promotion in these systems is better understood. An example of the need for improved performance of a fiber-composite interface can be seen in efforts to develop ceramic-metal composites for use in automobile engines.9 Because of the poor adhesion between the dissimilar materials in these composites, the designer of an aluminum connecting rod reinforced with alumina fibers must deal with the fact that aluminum metal does not wet aluminum oxide. This problem has been partially solved in an empirical way, but a better understanding of surface chemistry would be a major step toward a more satisfactory solution. The payoff would be broad commercial use of lightweight aluminum connecting rods in automobile engines to achieve greater fuel economy and improved acceleration. New Electronic Materials Chemistry already plays an important role in the fabrication of microelectronic devices. For example, photoresist technology and etch processes are crucial in achieving state-of-the-art integrated circuits. Understanding of the basic chemistry underlying the achievement of very-large-scale integration (VLSI) lags somewhat behind the technological achievements so far.
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Advancing Materials Research However, it is clear that emphasis on the chemistry of electronic materials will yield important advances in the next generations of devices.10–14 Part of the basis for an increasingly important role for molecular chemistry is that the size of individual devices now approaches the size of large molecules that chemists are accustomed to characterizing. An even more significant consideration is that new electronic materials and devices will be needed in certain technological applications. Most notably, the fabrication of semiconductor microelectronic devices from molecular precursors may emerge as the method of choice for such devices as infrared detectors and semiconductor lasers. The use of organometallic substances as precursors to semiconductor materials of device quality is obviously an area where chemists’ knowledge can be profitably applied. In particular, the purity of the materials and their handling and storage pose unprecedented problems because of the performance demanded of microelectronic devices. There is a clear need for chemists to become involved in the science of semiconductor devices, to appreciate the materials properties required. For instance, remarkable progress has been made in the fabrication of certain gallium-arsenide-based devices through application of chemistry, but a more thorough understanding of semiconductor growth is required to exploit metal-organics in chemical vapor deposition growth processes. Molecular chemistry and associated particle-assisted (photon, electron, and ion) deposition processes represent another area where molecular chemists can contribute directly to the fabrication of electronic devices. The fundamentals of laser chemistry of small molecules can be addressed by chemists and may well lead to new ways of fabricating complex devices. By taking advantage of various nonlinear phenomena, it is possible to produce structures that are smaller than the wavelength of the light used to cause deposition of a solid material, and there is the prospect of nanometer-sized structures from focused ion or electron beams. Although hybrid electronic circuitry depends heavily on ceramics—ceramic resistors, capacitors, dielectrics, and metal-glass composite conductors—the potential exists for a new, parallel, polymer-based technology in which binders, dielectrics, and other components are made from polymers rather than ceramics. In the low-technology end of consumer electronics, such polymer-based systems are well developed. Conductors are fabricated from silver-polymer pastes and resistors from carbon-polymer composites. Although this technology will probably never reach the high end of the electronics market (aerospace, for example), it has enormous potential for modestly sophisticated electronic circuitry. Despite needs for improved stability and reliability of polymer-based systems, the ease and simplicity of fabrication offer opportunity for more varied and less expensive circuit designs.
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Advancing Materials Research Polymeric Precursors to Ceramics Recently, great interest has emerged in synthesizing polymers that can be processed (into fiber, for example) at low temperature and subsequently converted to high-performance ceramic objects.9,15–17 One notable system has already been mentioned, namely, the formation of high-performance SiC fibers. Emphasis has been placed on forming Si3N4 from polymer precursors. Interestingly, the molecular materials requirements (and problems) are similar to those in the fabrication of semiconductor devices from molecular precursors in that high purity is required in the final materials. In the case of the Si3N4 fibers, it is important to prepare oxygen-free materials in order to realize the maximum materials performance. Like many of the precursors to important electronic materials, the precursors to ceramics are often sensitive to oxygen and water. Therefore, unusual synthesis, storage, and transfer techniques are required. Materials in Energy Conversion Chemistry is central to present energy systems. The diversity of energy systems ranges from the lead-acid batteries in automobiles to nuclear reactors to combustion of fossil fuels. Chemistry has contributed heavily to the science and technology of energy production and storage: electrochemical principles, separations chemistry for enrichment of uranium, catalytic chemistry involved in refining petroleum products, and combustion chemistry.18–25 For the foreseeable future, the production of energy will involve many materials problems which chemistry should be able to solve. In electrical energy storage, for example, recent discoveries of polymeric electronic conductors21 may lead to the development of new kinds of energy storage systems having high energy density coupled with the advantages of high discharge rate and the ability to undergo many complete charge-discharge cycles. New electrical energy storage systems are needed for the development of electric vehicles, but more near-term applications of polymer batteries are likely to emerge from advantages associated with rechargeability and the high discharge rate. Development of large-scale solar energy systems (based on the excitation of electrons) for the direct generation of electricity or production of chemical fuels is a possibility, but many materials problems must be solved before a large-scale technology can be realized.22 In the last decade, chemistry has contributed significantly to the development of semiconductor-based photoelectrochemical cells that can convert sunlight to electrical energy or to chemical energy in the form of H2 from the reduction of H2O with an efficiency exceeding 10 percent. Advances in solar photovoltaics and solar-driven chemical fuel formation depend on the ability to synthesize photo-
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Advancing Materials Research sensitive materials that cover many square miles of surface. Approaches to semiconductor fabrication involving molecular precursors may be most appropriate for such large-scale synthesis. Catalytic chemistry is important in the production of gasoline. In the future it is likely that catalytic chemistry will continue to play a crucial role, independent of whether inexpensive fusion or solar electricity generation systems become available. There will remain a need for chemical fuels. Catalysts will be important at interfaces between solids and liquids to accelerate the interconversion of electrical and chemical energy. Devices such as fuel cells, for example, require the use of catalysts to achieve practical rates of conversion from chemical to electrical energy. If inexpensive electricity becomes available, from any source, there will be the need to develop redox catalysts for electrolyzers that will be used to produce chemical fuels.23 Chemistry can contribute significantly to the solution of problems in heterogeneous redox catalysis, through synthesis, characterization, and theoretical modeling. NEW MATERIALS, PROCESSES, AND STRATEGIES FROM MOLECULAR SCIENCE The chemistry of materials is now an active field in molecular science. Major activity in this field can be expected in both industrial and university communities. The motivations of these two communities are different, but both bring important skills. The chemical industry is motivated by declining profitability of the existing commodity and agricultural chemical markets, and by the high value of materials produced for high-technology applications in electronics and defense. The profitability of these areas for a traditional chemical company is unclear, but probably irrelevant in the short term to the amount of research that will be devoted to generating new products for these areas. The chemical industry brings to materials science several important capabilities in developing economical processes for the production of chemical substances, especially polymers. For example, catalysis has been applied with great success to the development of commodity chemicals but has not been widely applied to processes leading directly to materials. The industry has organized complex, multidisciplinary development groups needed to span the range from chemical synthesis to materials testing. And, it has the financial resources required for commercial development. University research is motivated by a growing interest in the solid state, in surface and interfacial phenomena,26,27 and in cooperative effects occurring in solids and at surfaces. University science has generated much information in organometallic chemistry, electrochemistry, and inorganic chemistry, little of which has been applied to the development of new materials. It has available new theoretical techniques for relating molecular structure and
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Advancing Materials Research materials properties. It provides the connections between chemistry, materials science, and apparently unrelated areas such as molecular biology. What new molecular science will chemistry provide for use in materials science in the future? Among the topics of high current activity in chemistry are the following: New Polymers A wide variety of polymers are now being developed for use in an impressive range of engineering plastics. The commercialization of many of these materials is held up in part by difficulties in their processing, and in part by uncertainty about markets. New inorganic and organometallic polymers are being explored, and increasing activity is being focused on functional polymers (that is, polymers showing properties such as piezoelectric and ferroelectric response). Interface-Modifying Agents The application of chemistry to the development of agents that modify interfaces has been mentioned in connection with microelectronics and composites. Related principles of molecular design and synthesis are directly applicable to the generation of new barrier films, corrosion inhibitors, and agents to control wetting, surface reflectivity, and many other phenomena. Functional Systems Chemistry is increasingly used to generate new types of membranes for gas separation and electrochemistry. New technologies are being developed for the preparation of highly monodispersed particles in wide ranges of sizes. Optically Responsive Systems Organic materials show more promise in applications such as information storage and optical communications, which require nonlinear optical effects, than many of the traditional inorganic materials. Perhaps this promise arises because damage induced by high radiation intensities is able to heal spontaneously.28 Organic and organometallic materials also have the great advantage that their optical properties can be easily tailored. Their disadvantages are thermal instability and sensitivity to oxidation. Electroactive Systems A range of organic conductors has been developed, and organic superconductors have recently been identified. Although none of these materials is likely to compete with conventional metallic conductors and superconductors in the near future, the information derived from their synthesis and study should be useful in the synthesis of weakly conducting materials for other applications, such as static charge dissipation and electromagnetic shielding. Biocompatible Materials Practical economic considerations stemming from rising costs of health care for an aging population, coupled with the great increases in fundamental understanding of biological systems, now make it practical to design materials for applications requiring biocompatibility. Such applications include joint, tooth, tendon, and ligament re-
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Advancing Materials Research placements, vascular grafts, contact lenses, artificial skin, implantable insulin pumps and monitors, extracorporeal shunts, and related systems. High-Purity Chemicals A key consideration in the fabrication of microelectronic devices is the purity in the chemical components used. The development of volatile organometallics promises to provide new routes to high-purity sources of elements for use in chemical vapor deposition and related materials processing techniques. Solvents and inert gases are important components of current and future processes. Chemistry also has important contributions to make in materials processing. Probably the most important single area for the immediate future is a scientifically unglamorous but economically and politically critical one—the development of environmentally acceptable processes. Chemical industries— both commodity and pharmaceutical—are facing this problem first. Many techniques learned in these industries for safe, low-pollution processing and for safety testing and cost-benefit analysis should be transferable to manufacturing in materials science. Other prospective new materials processes include the following: Economical Syntheses Many materials (for example, rigid-rod polymers such as polybenzthiazole) are known that have highly attractive properties as components in materials systems but are not currently produced in significant quantities partly because no economical routes for their synthesis now exist. As the chemical industry turns increasingly to materials science for new markets, the techniques of economical catalytic synthesis should be applied to make available these types of materials. Molecular Control of Processes Several processes (especially chemical vapor deposition) are widely used in materials processing but are poorly understood at the molecular level. Chemists are now studying the detailed kinetics and molecular mechanisms of these processes, with the objective of better control. Practical benefits from these studies will be the development of processes that will operate at higher pressure, lower temperature, with faster growth rates, and that will yield more uniform products. Self-Assembling Systems The importance of composite structures in materials science is beyond question. An activity of increasing significance, both intellectually and practically, in chemistry is the development of techniques for generating systems that spontaneously self-assemble into microscopically heterogeneous domains—that is, to molecular-level composites. The most highly developed of these systems are phase-separated block copolymer systems. They combine desirable properties characteristic of the individual polymeric blocks but can be processed as homogeneous materials. Self-assembling or supramolecular chemistry is being actively studied in areas of chemistry from surface science (Langmuir-Blodgett-like self-assembling monolayer films) to micelles and liposomes.
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Advancing Materials Research FIGURE 5 Representative examples of molecular spontaneous self-assembly. A long-term objective of this type of work is summarized schematically in Figure 5. Polymer blends or block copolymers can be handled and formed as homogeneous blends or melts. After standing, they would phase-separate into well-defined domains with spatial arrangements appropriate for a composite structure. Appropriate curing would convert one component of the two-phase system to the equivalent of the stiff fiber reinforcement in conventional composites; the second component would remain the equivalent of the matrix resin; the interface between these two phases would transmit stress between them. The result would be the formation of an in situ composite in which, in principle, it would be possible to achieve geometries for the “reinforcing phase” that would be impossible using existing composite technology, and in which the perfection of the structure, the ease of its assembly, and the control of its fiber-matrix interface would be greater than could be achieved using existing technology. RELEVANCE OF BASIC CHEMICAL RESEARCH TO MATERIALS SCIENCE One contribution that chemistry makes to materials science is the ability to control composition and structure at the atomic and molecular levels. This ability is highly developed for molecules with molecular weight less than
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Advancing Materials Research 2,000 (pharmaceuticals, complex natural products), moderately developed in several highly evolved routes to certain classes of macromolecules (synthetic polymers produced by condensation and addition polymerizations), and relatively undeveloped in preparations of other systems (most types of solids, thin films, and surfaces). The development of rational techniques for controlling the extended, molecular-level structure of the solid state (in both two and three dimensions) is one of the most interesting challenges now facing chemistry. The intellectual relation between molecular chemistry and materials science can be summarized in two questions: What are the relations between atomic- and molecular-level structure (elemental composition, bond connectivity) and macroscopic properties (e.g., tensile modulus, thermal and electrical conductivity, second-order optical response) of materials? What new compounds or compositions of matter will provide new properties or facilitate processing and assembly of materials? How can these new compounds be synthesized? As these questions are answered in specific materials systems, those systems and their properties will fall under rational synthetic control and eventually under economic synthetic control. The areas of intellectual excitement in chemistry today are the areas in which new synthetic materials will probably first emerge. A selected list includes Solids whose properties can be rationally controlled by varying molecular and electronic structure (organic conductors, optically responsive materials, materials with nonlinear optical effects, ferromagnetics); Surfaces and thin films, especially those relevant to catalysis by metals and metal oxides, and those that are active in electron transport; Solids having large internal surface or volume, such as zeolites, layer structures, pillared clays, and intercalates; Self-assembling structures, such as ordered monolayer films, liposomes, liquid crystals, micelles, and ordered two-phase systems. Several other areas of chemistry that are, in principle, important to materials science are either inactive or in a state in which development is steady but few new ideas are being introduced. These fields are less active than those mentioned previously, either because of the perceived difficulty of working in them or because they are not currently fashionable. Examples include the synthesis of hard, tough, or thermally conducting solids; the rational synthetic control of the crystal structure of organic, inorganic, and organometallic solids; and the study of adhesion and tribology. As the chemistry community’s interest in materials science grows, these fields will develop.
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Advancing Materials Research BIOLOGY: A STIMULUS TO CHEMISTRY AND MATERIALS SCIENCE As a final note, it is worthwhile briefly to consider the role another molecular science—biology—might play in the future of materials science. Biology and biochemistry and their several subfields, especially molecular genetics, immunology, endocrinology, neurobiology, and enzymology, have developed explosively. Although much of this development is arguably irrelevant to materials science, some threads of biology do have major long-term relevance as sources of new materials and processes (Figure 6) and as stimulating intellectual concepts (Figure 7). One of the true scientific revolutions of this century has been the development of molecular genetics. Among the practical applications of this knowledge is the ability to produce a virtually limitless number of proteins, both natural and unnatural, in large quantities. Thus, for the first time, proteins can be considered as components of materials systems, although it is not known which proteins and structures are appropriate for what purposes. None of these questions can be answered at present. Proteins are, however, a class of macromolecular species with potentially great versatility; the development of protein structure-property relationships could be of great practical importance in materials science. At the same time, more mundane but possibly more immediately useful advances in classical microbiology have made available new polysaccharides FIGURE 6 Biologically derived materials and processes relevant to materials science.
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Advancing Materials Research FIGURE 7 Examples of biological structures representing concepts not now exploited in materials science. and certain other classes of biologically derived polymers. Some of these materials are useful in biomaterials applications (for example, polylactic acid has been developed for biodegradable sutures, and hyaluronic acid is a valuable lubricant for use in vivo); others provide engineering plastics (polyhydroxybutyrate/propionate) whose impact may ultimately be greater in Third World nations lacking a petrochemical base than in developed countries. Finally, one should not forget that it is not yet possible to duplicate the properties of many useful and interesting biomaterials, for example, barnacle cement and spider silk. The interaction between conventional materials and living systems involves a different type of activity. For example, microbiological action contributes significantly to corrosion of metals in marine and underground environments, and marine colonization and fouling limit the use of all materials in marine environments. By understanding the physiological, metabolic, and molecular mechanisms responsible for such processes, it may be possible to limit their impact on materials systems. Biology also offers hints of important processes and concepts currently alien to practical chemistry and materials science (Figure 7). These biologically derived concepts may, in fact, never be used directly in materials science, but their existence provides an intellectual stimulus and an opportunity to reexamine familiar hypotheses. The nervous system is one obvious example. Control and computation in mammalian systems is conducted by processes relying on information transmission using a bizarre hybrid system
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Advancing Materials Research containing ionic conductivity and chemical diffusion as major components; the logic of the brain is based on still mysterious processing, storage, and retrieval algorithms. All of these processes are unarguably slower, more delicate, and more limited than semiconductor-based methods, but the biological systems are nonetheless able to deal with certain tasks such as pattern recognition, training, and associative recall with a facility that is difficult to duplicate with existing silicon-based devices. Will new types of materials be required to model these processes and to accomplish similar results? Can existing materials be assembled in new configurations to duplicate them? A less familiar example is provided by vectorial chemistry. Energy is stored in biological systems in two forms: reactive molecules and in concentration gradients across cell membranes. There are no purely chemical processes that duplicate the biological processes that convert the energy generated by dissipation of a concentration gradient into useful work. The concepts on which these conversions rest involve systems containing areas of active membrane, and promise to stimulate efforts to develop similar active membrane systems for other purposes. These systems may eventually appear as problems and opportunities in materials science. CONCLUSIONS Chemistry and materials science have been considered largely separate disciplines, to the disadvantage of both. In the past decades researchers in both fields have been profitably occupied in separate spheres: chemists with the products of the classical chemical economy (fuels, commodity chemicals, polymers, agricultural chemicals, pharmaceuticals, specialty chemicals) and materials scientists with the core of classical materials (metals, glasses, and ceramics). Many of these areas are now scientifically and technologically mature. New problems require new solutions, and new science yields new technology. The push toward high-strength, lightweight, durable materials has stimulated interest in composite structures. The importance of microelectronics, and the specialized requirements of the U.S. Department of Defense, have focused attention on new classes of materials. Environmental constraints and competition from other nations have rendered many existing processes obsolete or unacceptable. In chemistry, a range of types of information—in catalysis, polymers, organometallics, synthetic methods—awaits sophisticated application to the solid state. The boundary between chemistry and materials science is beginning to blur, but the pace of evolutionary change is too slow. Materials scientists should learn what chemistry has to offer; chemists must actively seek to understand and solve the problems in materials science.
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Advancing Materials Research ACKNOWLEDGMENT This work was supported in part by the National Science Foundation, grants CHE-82–05142, DMR-83–16979, and CHE-83–20096. NOTES 1. H.A.Wittcoff and B.G.Reuben, Industrial Organic Chemicals in Perspective, Parts 1 & 2 (Interscience, New York, 1980). 2. H.R.Allcock and F.W.Lampe, Contemporary Polymer Chemistry (Prentice-Hall, Englewood Cliffs, N.J., 1981). 3. F.A.Bovey and F.H.Winslow, editors, Macromolecules: An Introduction to Polymer Science (Academic Press, New York, 1979). 4. Committee to Survey Opportunities in the Chemical Sciences, National Research Council, Opportunities in Chemistry (National Academy Press, Washington, D.C., 1985). 5. P.M.Hergenrother, ChemTech 14, 496 (1984). 6. J.L.Kardos, J.Adhesion 5, 119 (1973); J.L.Kardos, ChemTech 14, 430 (1984); J.L. Kardos, Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem. 24, 185 (1983). 7. E.P.Plueddemann, Silane Coupling Agents (Plenum, New York, 1982). 8. J.L.Woodhead and D.L.Segal, Chem. Br. 20, 310 (1984). 9. H.J.Sanders, Chem. Eng. News 62 (28), 26–40 (1984). 10. R.D.Dupuis, Science 226, 623 (1984). 11. R.Solanki, C.A.Moore, and G.J.Collins, Solid State Technol. 28, 220 (1985). 12. K.A.Jones, Solid State Technol. 28, 151 (1985). 13. R.M.Osgood and T.F.Deutsch, Science 227, 709 (1985). 14. S.M.Sze, editor, VLSI Technology (McGraw-Hill, New York, 1983). 15. R.F.Davis, H.Palmour III, and R.L.Porter, editors, Emergent Process Methods for High Technology Ceramics (Plenum, New York, 1984). 16. R.R.Ulrich and L.L.Hench, editors, Ultrastructure Processing of Ceramics, Glasses, and Composites (Wiley, New York, 1984). 17. R.W.Rice, Ceram. Bull. 62, 889 (1983). 18. R.H.Baughman, J.L.Bredas, R.R.Chance, R.L.Elsenbaumer, and L.W.Shacklette, Chem. Rev. 82, 209 (1982). 19. D.F.Shriver and G.C.Farrington, Chem. Eng. News 63 (20), 42–57 (1985). 20. H.R.Allcock, Chem. Eng. News 63 (11), 22–36 (1985). 21. R.L.Greene and G.B.Street, Science 226, 651 (1984); M.R.Bryce and L.C.Murphy, Nature 309, 119 (1984). 22. S.M.Sze, Physics of Semiconductor Devices, 2nd ed. (Wiley, New York, 1981). 23. M.H.Chisholm, editor, Inorganic Chemistry: Toward the Twenty-First Century, ACS Symposium Series, No. 211 (American Chemical Society, Washington, D.C., 1983). 24. A.Heller, Science 223, 1141 (1984). 25. M.S.Wrighton, J. Vac. Sci. Technol. A 2, 795 (1984). 26. R.D.Vold and M.J.Vold, Colloid and Interface Science (Addison-Wesley, Reading, Mass., 1983). 27. A.W.Adamson, Physical Chemistry of Surfaces, 3rd ed. (Wiley, New York, 1976). 28. D.J.Williams, editor, Nonlinear Optical Properties of Organic and Polymeric Materials, ACS Symposium Series, No. 233 (American Chemical Society, Washington, D.C., 1983).
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