Not too many years ago, certainly no more than two decades, no one would have dreamed that an aircraft could circumnavigate the earth without landing or refueling. Yet in 1986 the novel aircraft Voyager did just that. The secret of Voyager's long flight lies in advanced materials that did not exist twenty years ago. Much of the airframe was constructed from strong, lightweight polymer-fiber composite sections assembled with durable, high-strength adhesives; the engine was lubricated with a synthetic multicomponent liquid designed to maintain lubricity for a long time under continuous operation. These special materials typify the advances being made by scientists and engineers to meet the demands of modern society. The flight of Voyager is significant enough in the history of flight that the aircraft is displayed prominently in the Smithsonian's Air and Space Museum in Washington, DC. The future of industries such as transportation, communications, electronics, and energy conversion hinges on new and improved materials and the processing technologies required to produce them. Recent years have seen rapid advances in our understanding of how to combine substances into materials with special, high-performance properties and how to best use these materials in sophisticated designs. Chemical engineers have long been involved in materials science and engineering and will become increasingly important in the future. Their contributions will fall in two categories. For commodity materials, which are nonproprietary formulations with well-established chemical compositions and property standards, chemical engineers are helping to maintain U.S. competitiveness by creating and improving processes to make these chemicals as pure as possible and in high yields at the lowest possible investment and operating costs. For advanced materials, which are generally multicomponent, often proprietary, compositions designed to have very specific performance properties in specific uses, the competitive edge will come from chemical engineers who excel in controlling molecular conformation, microscopic and macroscopic structure, and methods of combining the components in a way that will maximize product performance. In the last topic we discussed chemical engineering challenges presented by materials and chemically processed devices for information storage and handling. In this section additional classes of materials are covered: polymers, polymer composites, advanced ceramics, and ceramic composites. The revolution in materials science and engineering presents both opportunities and challenges to chemical engineers. With their basic background in chemistry, physics, and mathematics and their understanding of transport phenomena (i.e., how fluids flow, heat is tranported, and chemicals move in a mixture), thermodynamics, reaction engineering, and process design, chemical engineers are bringing innovative solutions to the problems of modem materials technologies. Part of this approach to advanced materials is a necessary departure from the "think big" philosophy of the profession; to participate effectively in modern materials science and engineering it is essential to "think small. " The crucial phenomena in making modern advanced materials occur at the molecular and microscale levels, and chemical engineers must understand and learn to control such phenomena if they are to engineer the new products and processes for making them.

The modern era of polymer science belongs to the chemical engineer. Over the years, polymer chemists have invented a wealth of novel macromolecules and polymers. Yet understanding how these molecules can be synthesized and processed to exhibit their maximum theoretical properties is still a frontier for research. Only recently has modern instrumentation been developed to help us understand the fundamental interactions of macromolecules with themselves, with particulate solids, with organic and inorganic fibers, and with other surfaces. Chemical engineers are using these tools to probe the microscale dynamics of macromolecules. Using the insight gained from these techniques, they are manipulating macromolecular interactions both to develop improved processes and to create new materials. The power of chemical processing for controlling materials structure on the microscale is illustrated by the current generation of high-strength polymer fibers, some of which have strength-to-weight ratios an order of magnitude greater than steel ("Order of magnitude" is synonymous with "a factor of 10." Each additional order of magnitude corresponds to an additional factor of 10, so, for example, 3 orders of magnitude is 10x10x10 = 1000 times). One of the best known example of these fibers, Kevlar, is prepared by spinning an aramid polymer—part of the family of nylons—from an anisotropic phase (a liquid phase in which molecules are spontaneously oriented with one another over microscopic dimensions). This spontaneous orientation is the result of both the processing conditions chosen and the highly rigid linear molecular structure of the aramid polymer. During spinning, the oriented regions in the liquid phase align with the fiber axis to give the resulting fiber high strength and rigidity. The concept of spinning fibers from anisotropic phases has been extended to both solutions and melts of newer polymers, such as polybenzothiazole, as well as traditional polymers such as polyethylene. Ultrahigh-strength fibers of polyethylene have been prepared by gel spinning. The same concept, controlling the molecular orientation of polymers to produce high strength, is also being achieved through other processes, such as fiber-stretching carried out under precise conditions. What is a polymer? The word comes from the greek polumeres, which means "having many parts." Polymers are large molecules consisting of repeated chemical units ('mers') joined together, usually in a line, like beads on a string. Each 'mer' is typically made up of more than 5 and less than 500 atoms; the word "polymer" is applied when you have more than about 50 'mers' stuck together. Most of the plastics that make up the pieces of junk that fill our lives are made of polymers. Historically, polymers have mostly been used to make solid plastics where the chains virtually don't move. But nowadays people dream of new applications of polymer liquids where microscopic random fluctuations (Brownian motion) and interactions (the sticking together or association of different types of molecules) can play a more important role. Many of the most important research problems involve polymers free to fluctuate about in a small-molecule solvent. Naturally, the most important solvent is also the hardest one to understand: water. An important area of research is the modification of the properties of surfaces using thin polymer coatings. The sky is the limit for these wet technologies: living organisms are mainly composed of polymerized amino acids (proteins) nucleic acids (RNA and DNA), and other biopolymers. One example of a polymer is Kevlar, a polyamide material, that consists of long chains of the chemical group above repeated n times, where n is usually a large number. Each hexagonal ring is a benzene group. Synthetic polymers are produced by chemical reactions, termed "polymerizations." Polymerizations occur in a variety of ways, but the common thread is reactions consisting of the repetitive chemical bonding of individual molecules, or monomers. Assorted combinations of heat, pressure, and catalysis alter the chemical bonds that hold monomers together, causing them to bond with one another. Most often, they do so in a linear fashion, creating chains of monomers called polymers. Some polymerizations join entire monomers together, whereas others join only portions of monomers and create "leftover" materials, or by-products. Co-polymers can be formed using two or more different monomers. And two or more polymers can be combined to produce an alloy, or blend, that displays characteristics of each component. In addition to processes that result in materials with specific high-performance properties, chemical engineers continue to design new processes for the low-cost manufacture of polymers. The UNIPOL process for the manufacture of polyethylene is a good example of the contributions of engineering research to polymer processing. Polyethylene is probably the quintessential commodity polymer. It has been manufactured worldwide for decades, and current U.S. production exceeds 20 billion pounds per year. Linear polyethylene consists of an large number of CH2 units linked together. Considering the global capital investment in existing plants for making polyethylene, it could be argued that inventing a new process for its manufacture is a waste of time and money. Not so. Chemical engineers at Union Carbide designed a proprietary catalyst that allowed polyethylene to be made in a fluidized-bed, gas-phase reactor operating at low temperature and pressure (below 100 ºC and 21 atmospheres pressure). The resulting process produces a polymer with exceptional uniformity and can precisely control the molecular weight and density of the product. The advantages of the process (including a low safety hazard from the mild operating conditions and minimal environmental impact since there are no liquid effluents and unreacted gases are recirculated) are such that, in 1986, UNIPOL process licensees had a combined capacity sufficient to supply 25 percent of the world's demand for polyethylene. This is remarkable market penetration for a new process technology for a mature commodity, particularly in light of the tremendous existing (and fully amortized) worldwide capacity for polyethylene. In 1985, Union Carbide and Shell Chemical successfully extended the UNIPOL process to the manufacture of polypropylene, another major polymer commodity. Interestingly, the first two licensees for the new polypropylene process were a Japanese chemical company and a Korean petroleum company.

Polymer composites consist of high-strength or high-modulus fibers embedded in and bonded to a continuous polymer matrix, such as that shown in the figure below. These fibers may be short, long, or continuous. They may be randomly oriented so that they impart greater strength or stiffness in all directions to the composite (isotropic composites), or they may be oriented in a specific direction so that the high-performance characteristics of the composite are exhibited preferentially along one axis of the material (anisotropic composites). These latter fiber composites are based on the principle of one-dimensional microstructural reinforcement by disconnected, tension-bearing "cables" or "rods." To achieve a material with improved properties (e.g., strength, stiffness, or toughness) in more than one dimension, composite laminates can be formed by bonding individual sheets of anisotropic composite in alternating orientations. Alternatively, two-dimensional reinforcement can be achieved in a single sheet by using fabrics of high-performance fibers that have been woven with enough bonding in the cross-overs that the reinforcing structure acts as a connected net or trusswork. One can imagine that an interdisciplinary collaboration between chemical engineers and textile engineers might lead to ways of selecting the warp, woof, and weave in fabrics of high-strength fibers to end up with trussworks for composites with highly tailored dimensional distributions of properties.

Fibers that are very strong or very stiff can be used to reinforce polymers and ceramics. The resulting materials, known as composites, usually have one of the structures depicted in this figure.

First-generation polymer composites (e.g., fiberglass) used thermosetting epoxy polymers reinforced with randomly oriented short glass fibers. The filled epoxy resin could be cured into a permanent shape in a mold to give lightweight, moderately strong shapes. The current generation of composites is being made by hand laying woven glass fabric onto a mold or preform, impregnating it with resin, and curing to shape. Use of these composites was pioneered for certain types of military aircraft because the lighter airframes provided greater cruising range. Today, major components for aircraft and spacecraft are manufactured in this manner, as are an increasing number of automobile components. The current generation of composites are being used in automotive and truck parts such as body panels, hoods, trunk lids, ducts, drive shafts, and fuel tanks. In such applications, they exhibit a better strength-to-weight ratio than metals, as well as improved corrosion resistance. For example, a polymer composite automobile hood is slightly lighter than one of aluminum and more than twice as light as one of steel. The level of energy required to manufacture this hood is slightly lower than that required for steel and about 20% of that for aluminum; molding and tooling costs are lower and permit more rapid model changeover to accommodate new designs. Polymer composite hoods, trunk lids, and side panels are now standard on a number of automobiles, because the early problems of higher manufacturing cost and of achieving adequate production have been largely overcome. The mechanical strength exhibited by these composites is essentially that of the reinforcing glass fibers, although this is often compromised by structural defects. Engineering studies are yielding important information about how the properties of these structures are influenced by the nature of the glass-resin interface and by structural voids and similar defects and how microdefects can propagate into structural failure. These composites and the information gained from studying them have set the stage for the next generation of polymer composites, based on high-strength fibers such as the aramids.

For most people, the word "ceramics" conjures up the notion of things like china, pottery, tiles, and bricks. Advanced ceramics differ from these conventional ceramics by their composition, processing, and microstructure. For example: Conventional ceramics are made from natural raw materials such as clay or silica; advanced ceramics require extremely pure manmade starting materials such as silicon carbide, silicon nitride, zirconium oxide, or aluminum oxide and may also incorporate sophisticated additives to produce specific microstructures. Conventional ceramics initially take shape on a potter's wheel or by slip casting and are fired (sintered) in kilns; advanced ceramics are formed and sintered in more complex processes such as hot isostatic pressing. The microstructure of conventional ceramics contains flaws readily visible under optical microscopes; the microstructure of advanced cerwnics is far more uniform and typically is examined for defects under electron microscopes capable of magnifications of 50,000 times or more. Advanced ceramics have a wide range of applications. Some examples of these are shown in the table below. In many cases, they do not constitute a final product in themselves, but are assembled into components critical to the successful performance of some other complex system. Commercial applications of advanced ceramics can be seen in cutting tools, engine nozzles, components of turbines and turbochargers, tiles for space vehicles, cylinders to store atomic and chemical waste, gas and oil drilling valves, motor plates and shields, and electrodes for corrosive liquids. Because advanced ceramics provide key components to other technologies for major improvements in performance, their impact on the U.S. economy is much greater than is indicated by their sales figures. Ceramic components used in turbines permit the construction of engines that operate at much higher temperatures than metallic engines, thus greatly increasing their thermodynamic efficiency and compactness. Ceramic liners and other ceramic components in diesel engines provide added benefits, such as the elimination of the need for water cooling and the prompter ignition of the fuel. An investment in wear-resistant ceramic cutting tools can be more than repaid by the decrease in downtime for sharpening or replacing a dulled or worn metallic tool. Function Properties Applications Thermal Thermal conductivity insulation refractoriness heat sink in electronics electrode material furnace liner jet engine rotors Mechanical High strength wear resistance low thermal expansion lubrication tools and jigs abrasives turbine blades solid lubricants precision instrument parts Biological/ chemical Biological compatibility catalysis adsorption corrosion resistance artificial bone and tooth heat exchanger geothermal and offshore equipment catalyst support Electric/ magnetic Electrical insulation electrical conductivity semiconductivity piezoelectric; dielectric piezoelectric filter computer memory resistance heating element integrated circuit substrate Optical Optical conductivity translucence fluorescence optical condensing fiber optic cable translucent porcelain light-emitting diode laser diode Nuclear Radiation resistance refractoriness high-temperature strength nuclear fuel fuel cladding control material moderating material Given these advantages, it is not surprising that market forecasts for advanced ceramics (including ceramic composites) are optimistic; in fact, sales in the year 2000 are expected to be $20 billion. The market for advanced ceramics in heat engines has grown by almost 40% per year, to a total of $1 billion in 2000. The use of advanced ceramics is predicted to grow 20% per year over the next 5 years, and sales for automotive applications increased from $53 million per year in 1986 to an anticipated $6 billion in 2000. Uniform microstructure is crucial to the superior performance of advanced ceramics. In a ceramic material, atoms are held in place by strong chemical bonds that are impervious to attack by corrosive materials or heat. At the same time, these bonds are not capable of much "give." When a ceramic material is subjected to mechanical stresses, these stresses concentrate at minute imperfections in the microstructure, initiating a crack, as shown in the figure below. The stresses at the top of the crack exceed the threshold for breaking the adjacent atomic bonds, and the crack propagates throughout the material causing a catastrophic brittle failure of the ceramic body. The reliability of a ceramic component is directly related to the number and type of imperfections in its microstructure.

Zirconia (ZrO2) ceramics can be made stronger and less brittle using chemical additives to stabilize a more compact structure that does not naturally occur at room temperature. In the absence of these additives, cracks such as the one shown here readily form.

As the requirements for greater homogeneity in ceramics become more stringent, and the scale at which imperfections occur becomes smaller, the need for chemical processing of ceramics becomes more compelling. Traditional approaches to controlling ceramic microstructure, such as the grinding of powders, are reaching the limits of their utility for microstructural control. Chemical engineers have used opportunities in this area to contribute their expertise in reaction engineering to problems that required new analytical, synthetic, and processing tools. These include sol-gel processing and the use of chemical additives in ceramic processing. What is an advanced ceramic? Ceramics are traditionally described as inorganic, nonmetallic solids that are prepared from powdered materials, are fabricated into products through the application of heat, and display such characteristic properties as hardness, strength, low electrical conductivity, and brittleness. Advanced ceramics represent an "advancement" over this traditional definition. Through the application of a modern materials science approach, new materials or new combinations of existing materials have been designed that exhibit surprising variations on the properties traditionally ascribed to ceramics. As a result, there are now ceramic products that are as tough and electrically conductive as some metals. With the development of advanced ceramics, a more detailed, "advanced" definition of the material is required. Since 1993 an advanced ceramic is described as "an inorganic, nonmetallic (ceramic), basically crystalline material of rigorously controlled composition and manufactured with detailed regulation from highly refined and/or characterized raw materials giving precisely specified attributes." A number of distinguishing features of advanced ceramics are pointed out in this definition. First, they tend to lack a glassy component; i.e., they are "basically crystalline." Second, microstructures are usually highly engineered, meaning that grain sizes, grain shapes, porosity, and phase distributions (for instance, the arrangements of second phases such as whiskers and fibres) are carefully planned and controlled. Such planning and control require "detailed regulation" of composition and processing, with "clean-room" processing being the norm and pure synthetic compounds rather than naturally occurring raw materials being used as precursors in manufacturing. Finally, advanced ceramics tend to exhibit unique or superior functional attributes that can be "precisely specified" by careful processing and quality control. Examples include unique electrical properties such as superconductivity or superior mechanical properties such as enhanced toughness or high-temperature strength. Because of the attention to microstructural design and processing control, advanced ceramics often are high value-added products.   Sol-gel processing The use of sol-gel techniques to prepare ceramic powders has attracted must interest over the past twenty years, both in industry and academia. Sol-gel techniques involve dissolving a ceramic precursor (e.g., tetramethyl orthosilicate) in a solvent and subjecting it to a carefully controlled chemical reaction involving water, known as hydrolysis. When the hydrolysis products first appear as a separate phase, they are fine colloids consisting of small particles, some with radii as small as a few nanometers. This colloidal suspension (the sol) further reacts and polymerizes to form a porous high-molecular-weight solid (the gel) that contains the solvent as a highly dispersed fluid component in its internal network structure. Removal of the solvent leaves behind solids with a wide variety of macrostructures depend- ing on the solvent and the way in which it was removed. These macrostructures can be sintered to convert them to dense ceramics. Sol-gel techniques are of interest because they can be used to prepare powders with a narrow distribution of particle size. These small particles undergo sintering to high density at temperatures lower by several hundred degrees centigrade than those used in conventional ceramic processing. Sol-gel processes may also be used to prepare novel glasses and ceramics such as ceramics with novel microstructures and distributions of phases, amorphous powders and dried gels tht can be processed without cryswlization to fully dense amorphous materials whose synthesis, might not otherwise be possible, materials with controlled degrees of porosity and possibly tailored surfaces within pores, and ceramics with surfaces modified to alter their response to mechanical forces or to promote their adhesion to other materials. Sol-gel processes also allow the manufacture of preforms that, upon sintering, collapse to a final product with the proper shape. There are many unresolved problems in sol gel processing, many of which revolve around the poorly characterized chemistry of the process. Understanding and controlling the polymerization reactions that produce the gel are key challenges, as are characterizing and optimizing both the removal of fluid from the gel and the subsequent sintering of the porous solid to a fully dense ceramic body. Solving these problems will make sol-gel processing the process of choice for the synthesis of a wide variety of ceramics, glasses, and coatings. What is a colloid? A colloid is any substance consisting of particles substantially larger than atoms or ordinary molecules but much too small to be visible to the unaided eye. Such particles range in size from about 0.0000001 to 0.001 centimeters and are linked or bonded together in various ways. Colloidal systems may exist as dispersions of one substance in another, as, for example, smoke particles in air, or as single substances, such as rubber. Colloids are generally classified into two systems, reversible and irreversible. In a reversible system the products of a physical or chemical reaction may be induced to interact so as to reproduce the original components. In a system of this kind, the colloidal material may have a high molecular weight, with single molecules of colloidal size, as in polymers, polyelectrolytes, and proteins, or substances with small molecular weights may associate spontaneously to form particles (called micelles) of colloidal size, as in soaps, detergents, and some dyes. An irreversible system is one in which the products of a reaction are so stable or are removed so effectively from the system that its original components cannot be reproduced. Examples of irreversible systems include sols (dilute suspensions), pastes (concentrated suspensions), emulsions, foams, and certain varieties of gels. The size of the particles of these colloids is greatly dependent on the method of preparation employed.   Chemical additives Another area which chemical engineers have made a significant contribution is the use of chemical additives to improve the properties of ceramic materials. For example, as shown in the earlier figure, a crack can readily form in zirconium oxide, also known as zirconia (ZrO2), when the material is exposed to small stresses. However, the addition of small amounts of chemical impurities improve its microstructural stability. A crack can still form due to the application of a load or stress, but the morphology of the grains allow the crack to "heal" before it can reach a length where the material fails. Similarly, aluminum oxide, or alumina (Al2O3) is usually optically opaque in its pure, ceramic form. This is shown in the micrograph on the left, below. However, when very small amount of magnesium (Mg) are added to the system during processing, the not-quite-pure alumina film becomes optically translucent, shown in the right figure. It's not perfectly transparent, but a thin film of this doped alumina film applied to the inside of a light bulb strengthens the glass. The addition of the magnesium changes the phase of the solid material, orienting the grains so that they are more efficient at transmitting light. The phase of a material is determined at the atomic level—it depends on the way the atoms are bonded together, and the angles and lengths of the bonds. Carbon is a well-known system with different solid phases. One phase of pure carbon is graphite and another is diamond. Although both of these phases are pure carbon, their properties are remarkably different.

Pure aluminum oxide (alumina) is optically opaque when pure, and cannot readily transmit light.

When alumina is doped with small amounts of magnesium its morphology is completely different, and as a result becomes optically translucent.

Like polymer composites, ceramic composites consist of high-strength or high-modulus fibers embedded in a continuous matrix. Fibers may be in the form of "whiskers" of substances such as silicon carbide (SiC) or aluminum oxide (Al2O3) that are grown as single crystals and that therefore have virtually no defects. Fibers in a ceramic composite serve to block crack propagation; a growing crack may be deflected to a fiber or might pull the fiber from the matrix. Both processes absorb energy, slowing the propagation of the crack. The strength, stiffness, and toughness of a ceramic composite is prin- cipally a function of the reinforcing fibers, but the matrix makes its own contribution to these properties. The ability of the composite material to conduct heat and current is strongly influenced by the conductivity of the matrix. The interaction between the fiber and the matrix is also important to the mechanical properties of the composite material and is mediated by the chemical compatibility between fiber and matrix at the fiber surface. A prerequisite for adhesion between these two materials is that the matrix, in its fluid form, be capable of wetting the fibers. Chemical bonding between the two components can then take place. Ceramic matrix composites are produced by one of several methods. Short fibers and whiskers can be mixed with a ceramic powder before the body is sintered. Long fibers and yarns can be impregnated with a slurry of ceramic particles and, after drying, be sintered. Metals (e.g., aluminum, magnesium, and titanium) are frequently used as matrixes for ceramic composites as well. Ceramic metal-matrix composites are fabricated by infiltrating arrays of fibers with molten metal so that a chemical reaction between the fiber and the metal can take place in a thin layer surrounding the fiber. As with advanced ceramics, chemical reactions play a crucial role in the fabrication of ceramic composites. Both defect-free ceramic fibers and optimal chemical bonds between fiber and matrix are required for these composites to exhibit the desired mechanical properties in use. Engineering these chemical reactions in reliable manufacturing processes requires the expertise of chemical engineers.

A fractured (broken) sample of a ceramic composite: alumina with 30% silicon carbide whiskers. The lighter regions of circular or cylindrical shape are randomly oriented whiskers protruding from the surface.

A final important class of composite materials is the composite liquids. Composite liquids are highly structured fluids based either on particles or droplets in suspension, surfactants, liquid crystalline phases, or other macromolecules. A number of composite liquids are essential to the needs of modem industry and society because they exhibit properties important to special end uses. Examples include lubricants, hydraulic traction fluids, cutting fluids, and oil-drilling muds. Paints, coatings, and adhesives may also be composite liquids. Indeed, composite liquids are valuable in any case where a well-designed liquid state is absolutely essential for proper delivery and action. All composite liquids are produced by the chemical processing industries, and chemical engineers face continuing challenges in tailoring their end-use properties. Some of these challenges are illustrated in the following examples: Motor lubricants are complex liquid composites in which components provide different performance characteristics. The basic component is a hydrocarbon oil with a fixed boiling range. It must have sufficient viscosity at engine operating temperatures to prevent the friction and wear of moving surfaces, but must be fluid enough below freezing temperatures for winter start-up. Viscosity modifiers are high-molecular-weight polymers that reduce the temperature coefficient of viscosity (viscosity index). Suspended colloidal particles of calcium or magnesium carbonate are added to neutralize engine acids and are stabilized by adsorbed polymers and surfactants to prevent coalescence. Solids dispersants are low-motecular-weight polymers with functional groups that pick up carbon particles generated in combustion and maintain them in suspension. At low temperatures, the waxes (straight-chain paraffin hydrocarbons) in the lubricant form long crystals to set up a solid gel. To prevent this, low-molecular-weight polymers, called pour point depressants, are added to co-crystallize with the wax; the resulting smaller crystals do not gel. Finally, there are antiwear additives and antioxidants to reduce engine wear and deposits. Lubricants with outstanding viscosity indexes enable an engine to start when the lubricant temperature is as low as -40 ºC and yet operate well when the lubricant temperature is as high as 200 ºC. Other additives allow broadening the temperature range further by providing increased thermal and oxidative stability. The use of synthetic base oils allows still broader ranges of operating temperatures, up to 500 ºC. Advanced adhesives are composite liquids that can be used, for example, to join aircraft parts, thus avoiding the use of some 30,000 rivets that are heavy, are labor-intensive to install, and pose quality-control problems. Adhesives research has not involved many chemical engineers, but the generic problems include surface science, polymer rheology (how the liquid polymer responds to forces or stresses) and thermodynamics, and molecular modeling of materials near or at interfaces. The scientific and engineering skills needed are very similar to those needed for polymer composites and multicomponent polymer blends. The time-tested mechanical methods developed for joining metals are not satisfactory for composite and other advanced materials, and chemical engineers skilled in interfacial science are well qualified to contribute to this area. Another class of liquid composites is that of coating compositions used to deposit thin films on a substrate or other films; these composites have evolved from typical paints and varnishes into multilayer films in which each layer contributes specific properties to the ensemble. Such films may be paints used for sealing and decorative purposes, films used for printing or packaging purposes, or multilayer products used in recording tapes and photographic products. All are based on generic scientific principles that include many common elements from thermodynamics, polymer science, rheology, and fluid mechanics. Liquid composites seldom behave as Newtonian fluids. For a fluid to be called Newtonian it must behave according to a known set of rules when exposed to stresses or forces. The simplest way to describe a Newtonian fluid is to say that its viscosity must remain constant, independent of how hard it's being pushed, pulled, or squeezed, or how fast it's flowing. Water is a good example of a Newtonian fluid. Most liquid polymers, colloid suspensions, and liquid composites are known as non-Newtonian. These complex mixtures usually contain macromolecules, suspended particles, and surfactants. They are frequently multiphase (solid+liquid or multiple immisible liquids), and changes in phase composition or formation of new liquid phases may occur over the range of operating conditions. Phase composition may be shifted by chemical reaction, by shear forces, or simply by changes in temperature or pressure. Liquid-liquid and liquid-solid equilibria are crucial. Detailed molecular understanding of the interactions among such components as surfactants, polymers, and particles is essential for the rational design of liquid composites. Much of this design is now accomplished by informed empiricism—observation plus experience—which is useful for the incremental improvement of current products but inadequate for major changes and innovation.

A wide variety of chemical engineering challenges involve advanced materials and are now coming into the mainstream of the discipline. The study of materials has traditionally centered on the influence of molecular composition and microstructure on mechanical, electrical, optical, and chemical properties. At the molecular level there are a variety of research frontiers that draw chemical engineers into close collaboration with physical chemists, applied physicists, and materials scientists. These areas can be grouped into several categories. Molecular design of composite materials One of the toughest challenges in chemical engineering is the development of wholly new designs for composite solids that possess previously unattainable physical properties. Such materials are typified by composites reinforced by three-dimensional networks and truss-works—microstructures that are multiply connected and that interpenetrate the multiplyc connected matrix in which they are embedded. In such materials, both reinforcements and matrix are continuous in all three dimensions. Geometric properties of such structures are found in certain liquid crystals and colloidal gels and in bones and shells. The challenge is to break away from today's technology, to go beyond today's research in two-dimensional fabric-like reinforcement, and to determine how to create truly three-dimensional microstructures and design, construct, and process them so as to control the properties of the composite. A potentially promising area is "molecular composites," in which the fiber and its surrounding matrix have the same composition and differ only in molecular structure or morphology. This might involve forming the composite from very stiff, linear polymer molecules, some of which are aligned during the forming step as reinforcing crystallites in the amorphous regions—the matrix. An analogous ceramic composite may be envisioned. There are difficult engineering problems to be solved in learning how to control the orientation of the crystalline regions and the ratio between crystalline and amorphous regions in the material. Interfaces and materials chemistry Two general problems relate to the role of interfaces in advanced materials. The first is simply that we do not have the theory or the computational or experimental ability to understand the interatomic and microscopic interactions at the interfaces between components of an advanced material, on which its properties are critically dependent. There is a general need for research on processes at interfaces and on the structure-property-performance relationships of interfaces.

What is an amorphous material? There are two main classes of solids: crystalline and amorphous. What distinguishes them from one another is the nature of their atomic-scale structure. The essential differences are displayed in the figure below. The salient features of the atomic arrangements in amorphous solids (also called glasses), as opposed to crystals, are illustrated in the figure for two-dimensional structures; the key points carry over to the actual three-dimensional structures of real materials. For the sketches representing crystal and glass structures, the solid dots denote the fixed points about which the atoms oscillate. Atomic positions in a crystal exhibit a property called long-range order or translational periodicity; positions repeat in space in a regular array, as in the left figure. In an amorphous solid, translational periodicity is absent. As indicated in the right figure, there is no long-range order. In the glass example illustrated in the figure, each atom has three nearest-neighbour atoms at the same distance (called the chemical bond length) from it, just as in the corresponding crystal. All solids, both crystalline and amorphous, exhibit short-range (atomic-scale) order. (Thus, the term amorphous, literally "without form or structure," is actually a misnomer in the context of the standard expression amorphous solid.) The well-defined short-range order is a consequence of the chemical bonding between atoms, which is responsible for holding the solid together.

The second problem relates to the role that interfaces play in mediating chemical reactions in the synthesis of composite materials. This problem has three parts, which are illustrated here for polymeric composites. First, in composites with high fiber concentrations, there is little matrix in the system that is not near a fiber surface. Inasmuch as polymerization processes are influenced by the diffusion of free radicals from initiators and from reactive sites, and because free radicals can be deactivated when they are intercepted at solid boundaries, the high interfacial area of a prepolymerized composite represents a radically different environment from a conventional bulk polymerization reactor, where solid boundaries are few and very distant from the regions in which most of the polymerization takes place. The polymer molecular weight distribution and cross-link density produced under such diffusion-controlled conditions will differ appreciably from those in bulk polymerizations. Second, the molecular orientation of the fiber and the prepolymer matrix is important. The rate of crystal nucleation at the fiber-matrix interface depends on the orientation of matrix molecules just prior to their change of phase from liquid to solid. Thus, surface-nucleated morphologies are likely to dominate the matrix structure. Third, the ultimate mechanical properties of a composite will be strongly influenced by the degree to which the matrix wets the fiber surface and by the degree of adhesion between the two after curing. Both phenomena depend on intimate details of the surface science of the two phases, about which little is known. Molecular modeling techniques, augmented by careful measurements of the structure of the interfacial regions, hold promise for elucidating details of these three aspects of interfacial control of matrix polymerization. Molecular behavior of complex liquids Basic understanding of the liquid state of matter still lags behind that of the solid and gaseous states. Our knowledge of interactions in multicomponent liquids containing macromolecules and suspended solids is extremely limited. Thus, the study of complex liquids, including polymer solutions, sols, gels, and composite liquids, is a significant challenge for chemical engineers. The ability to predict liquid-liquid and liquid-solid equilibria in complex systems is still rather undeveloped, in part because of the lack of systematic and molecularly interpreted experimental information. Considerable research has been conducted on the behavior of liquids near their critical points, on lower critical solution phenomena, on spinodal decomposition, and on related dynamics such as the growth and morphology of new phases, but generalized correlations and connections of theory to practice are few. Molecularly motivated empiricisms have been valuable in dealing with mixtures of weakly interacting small molecules where surface forces are small. However, they are completely inadequate for mixtures that involve macromolecules, associating entities like surfactants, rod-like or plate-like species that can form ordered phases. New theories and models are needed to describe and understand these tems. This is an active research area where advances could lead to better understanding of the dynamics of polymers and colloids in solution, the rheological and mechanical properties of these solutions, and, more generally fluid mechanics of non-Newtonian liquids. Chemical dynamics and molecular processes Chemical dynamics and modeling are known to be extremely important research frontiers in chemical engineering, and their impact can be felt in biomaterials, electronics, advanced materials, and biomedical engineering. At the molecular scale, important areas of investigation include studies of statistical mechanics, molecular and particle dynamics, dependence of molecular motion on intermolecular and interfacial forces, and kinetics of chemical processes and phase changes. Mechanistic studies are particularly needed for the hydrolysis and polymerization reactions that occur in sol-gel processing. Currently, little is known about these reactions, even in simple systems. A short list of needs includes such rudimentary data as the kinetics of hydrolysis and polymerization of single alkoxide sot-gel systems and identification of the species present at various stages of gel polymerization. A study of the kinetics of hydrolysis and polymerization of double alkoxide gol-gel systems might lead to the production of more homogeneous ceramics by sol-gel routes. Another major area for exploration is the chemistry of sol-gel systems that might lead to nonoxide ceramics (i.e., ceramics that do not contain oxygen atoms in their makeup).

Materials synthesis and materials processing have traditionally been thought of as separate activities, and in the days of simple, homogeneous materials, they were. But today's complex materials are bringing these two areas closer together in research and in practice. Four outstanding intellectual challenges demonstrating this connection are described here. Complex liquids processing Complex liquids are ubiquitous in materials manufacture. In some cases, they are formed and must be handled at intermediate steps in the manufacture of materials (e.g., sols and gels in the making of ceramics, mixtures of monomer and polymer in reactive processing of polymers). In other cases (e.g., composite liquids), they are the actual products. Understanding the properties of complex fluids and the implications of fluid properties for the design of materials processes or end uses presents a formidable intellectual challenge. Complex liquids seldom behave as classical Newtonian fluids; thus, analysis of their behavior requires a thorough understanding of non-Newtonian rheology. The importance of this knowledge is illustrated by the following two examples: The problem of processing complex liquids while they are undergoing rapid polymerization is an important challenge in reactive polymer processing (e.g., reactive injection molding and reactive extrusion). In these processes, the viscosity of a reaction mixture, as it proceeds from a feed of monomers to a polymer mell product, may change by 7 decades or more in magnitude (i.e., by up to 107). Fluid mixtures flowing into a mold of complicated geometry may exhibit large temperature gradients from the highly exothermic (high heat release) chemical reactions taking place and significant spatial variations in viscosity and molecular weight distribution. Rheology is especially important to the understanding of composite liquids in their many applications as products (e.g., lubricants, surface coating agents, and additives for enhanced oil recovery and drag reduction). Such liquids usually contain polymers, and their behavior is frequently viscoelastic under use conditions. While data on linear response and relatively mild shear flows are available for nonassociating polymer solutions in the relevant ranges of molecular size and concentration, far fewer data are available on liquid systems that contain particles or micelles, particularly those in which there are strong interparticle interactions. Knowledge of the fluid mechanics of ordered liquids is similarly sparse. Information on the response to rapid shear flows and extensional flows, even in simple polymer solutions, is very limited. Thus, we are far from having dependable equations from which models of such fluids could be developed and farther still from a generalized molecular understanding of the structure-property relations of these fluids and from extrapolations of the flow patterns and stress distributions in such fluids in geometries close to those in which they are used. For example, there is significant divergence between theoretical prediction and empirical observation of the flow of lubricants in journal bearings. Even if satisfactory equations of state and constitutive equations can be developed for complex fluids, large-scale computation will still be required to predict flow fields and stress distributions in complex fluids in vessels with complicated geometries. A major obstacle is that even simple equations of state that have been proposed for fluids do not always converge to a solution. That is, the solution to the proposed equations cannot always be found. It is not known whether this difficulty stems from the oversimplified nature of the equations, from problems with numerical mathematics, or from the absence of a laminar steady-state solution to the equations. Since the equations themselves are not exact it is very difficult to pinpoint the cause of the problem. Powder processing One route to better ceramic powders, sol-gel processing, has already been described in this section. There are, however, many other possible routes to improved ceramic powders. These routes include refinements of older processes, such as precipitation and thermal decomposition, as well as newer processes, such as plasma processing and chemical vapor deposition. The nucleation and particle-growth processes in such systems need to be described quantitatively to enable better process development and scale-up. Chemical engineering frontiers include the development of new chemical processes for producing ceramic raw materials, such as submicron, spherical, uniform powders, and high- strength fibers and whiskers. Chemical engineers also work to devise processes to improve the flow characteristics of powders after they are formed. Such research has helped to control agglomeration of particles in subsequent processing steps as well as facilitate the production of compacted ceramic preforms. For example, gas-solid chemical reac- tions might be used to tailor the chemical composition of powders. As another example, better methods of compounding powders with binders might be achieved by processes that mix powders with suitable binders in a liquid and then spray dry the resulting suspension. Powder processing is also one element in the engineering of grain boundaries in large, complex parts. Such engineering would allow sintering ceramics to full density without degrading oxidation resistance and long-term strength. Polymer processing Other important research challenges confront chemical engineers in the area of polymer processing. One concerns the interactions of polymers with their environment. For example, contacting a glassy polymer with a solvent or swelling agent may lead to unusual diffusion characteristics in the polymer, stress formation, crazing, or cracking. Such phenomena are poorly understood because glassy polymers may exhibit complex viscoelastic behavior in the presence of a liquid or during their second-order (glass) transitions. The study of diffusion in glassy polymers is a virgin research area for chemical engineers. A better understanding of polymer-solvent interactions could have important payoffs in the development of positive resists for microcircuit manufacture because the dissolution characteristics of polymeric resists are crucial to their application and removal during microlithography. (Lithography was discussed in the previous topic.) The focus of research on engineering thermoplastics with enhanced mechanical, thermal, electrical, and chemical properties has shifted away from synthesizing novel polymers toward combining existing polymers. Multicomponent polymer blends pose interesting and challenging new problems for chemical engineers. Many multicornponent polymeric melts are homogeneous at processing temperatures but separate during cooling. Judicious choice of stress levels during cooling and of the cooling rate can effect changes in the structure and morphology of the end product and hence in its properties. The fluid prepolymer in which the load-bearing fibers of a polymer composite are placed undergoes further polymerization and cross- linking during the thermal curing of the composite. The chemical reactions that occur during curing are exothermic and are difficult to control. Some regions in the composite material react adiabatically (they're thermally insulating) while others lose heat by conduction to their surroundings. The resulting point-to-point variations in polymer matrix molecular weight and cross-link density result in changes in the composite's properties and quality. We need to better understand and control these variations in well-characterized processes and to deduce how to change the geometry of the finished object or the distribution of fibers within it to compensate for the variations in polymer structure that might inherently arise during processing. Process control and design Because processing conditions and history have such an important influence on the conformation and properties of materials, there is a need to develop models and systems for the measurement and control of materials manufacturing processes so that processes can be better designed, more precisely controlled, and automated. Opportunities for chemical engineers in process design and control, including advanced mathematical modeling of polymer processing have never been greater. It is particularly important to study process phenomena under dynamic (rather than static) conditions. Most current analytical techniques are designed to determine the initial and final states of a material or process. Instruments must be designed for the analysis of materials processing in real time, so that the crucial chemical reactions in materials synthesis and processing can be monitored as they occur. Recent advances in nuclear magnetic resonance and laser probes indicate valuable lines of development for new techniques and comparable instrumentation for the study of interfaces, complex liquids, microstructures, and hierarchical assemblies of materials.