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Chemistry: Challenges and Solutions

Modern Materials and the Solid State: Crystals, Polymers, and Alloys

n this unit, the focus shifts from fluids and their solutions to solids, whose atoms and molecules are fixed in definite arrangements. Examples of molecular configurations range from crystals in a mineral collection, to metal alloys, to long polymer chains. One promising area of polymer research may allow the delivery of essential drugs such as insulin to be taken orally and delivered directly into the bloodstream. Bioplastic-enclosed nanoparticles encasing the drug have the ability to withstand the body’s corrosive digestive system. Extremely high-temperature resistant alloys are another important application of the chemistry of the solid state.

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While chemical reactions in gases and liquids are essential to the understanding of chemistry, the chemistry of solid-state materials characterizes most of the interactions we have with matter on a daily basis. Chemists take advantage of the complexity of solids to engineer new materials, including nanoparticles, polymers, and advanced metal alloys. These new materials have many potential applications in sensors, advanced drug delivery systems, and space exploration. Today, modern materials are following a heritage—one that can be traced back to earlier civilizations—in which the properties of solids are manipulated to advance human needs.
Hosted by Ainissa Ramirez.

Unit Overview

Understanding the inter-atomic forces that give structure and properties to different types of solids is essential for the creation of new alloys, the development of useful polymers, and the creation of many other kinds of human-made materials. In this closing chapter, we see chemistry not only as an excellent entry point to predicting how a new material behaves, but also as a continuous process of innovation and discovery.

Section 1: Introduction

In the first human chemistry experiments, people worked with solid materials—food, pottery, and metals—for tools and weapons. When chemistry became a laboratory science in the 18th and 19th centuries, many breakthroughs involved work with gases and liquids. Now we’ve come full circle to a new era of discoveries about the chemistry of solid materials that started in the 20th century and continues today.

Solid materials come in many forms with a dizzying variety of properties. As one example, orthopedists have developed a new way to mend bones broken so badly that they need surgery. They repair the bones with plates made of a shape memory alloy. Otherwise known as a “memory metal,” this alloy possesses the ability to change shape in response to alterations in temperature. In the cold operating room, the surgeon stretches the alloy plate and links it to both ends of the broken bone. Once the repair is sewn up, the patient’s body heat causes the metal to flip back to its original unstretched shape. As it does so, the metal pulls the bone ends together to begin the healing process. And once the ends have joined, the metal continues to hold them in place, strengthening the bone during rehabilitation.

Figure 13-1. ENIAC, the First Electronic General-Purpose Computer (1945), and a Modern Smart Phone User
The ENIAC computer weighed more than 30 tons, took up 18 square feet, and consumed 170 kilowatts of power. Today’s smart phones, powered by microchips made from high-purity silicon, consume a tiny fraction of as much energy and space, and perform operations in seconds that would have taken days a generation earlier.
© Left: Wikimedia Commons, Public Domain. Author: U.S. Army. Right: Wikimedia Commons, Creative Commons License. Author, PictureYouth.

At Cambridge University in England, chemists have used another new type of solid—electrically conducting plastic—to create flexible solar cells. Clustered together to form curtains and placed over a window, the cells could provide electric power for homes that are not connected to power grids.

Memory metals and electrically conducting polymers are just two examples of important advances in the science and technology of solids. Like the original chemists of the Bronze Age and Iron Age, solid-state chemists are making discoveries that are transforming modern life. Solid materials are central to today’s societies at all scales, from the steel beams that hold up bridges and skyscrapers to silicon microchips that have revolutionized the global information industry. (Figure 13-1)

Today, solid-state chemistry is one of the most innovative areas of chemical research, producing advanced materials for many uses. As historian Trevor H. Levere (born 1944) observes, “Chemistry is the only science that now builds or creates much of what it goes on to study, from artificial elements to the latest plastics and the most powerful pharmaceutical chemicals, from fertilizers to microchips.”1 Solid-state chemistry intersects with many other scientific fields, including geology, energy storage, and materials science—the study of a material’s structure, properties, and performance under various conditions.

Figure 13-2. German Steel Rolling Mill (1983)
Manufacturing steel requires an understanding of phase changes that take place as the steel is heated, cast, and cooled.
© Wikimedia Commons, Creative Commons License. Author: Deutsche Fotothek‎.

Increasingly, modern societies are demanding materials that not only have unique properties, but also are environmentally benign. To develop new materials with finely tuned properties, scientists constantly seek further understanding of how solids and materials are structured, and how small changes can drastically alter their characteristics.

This unit reviews the features that distinguish solids from liquids and gases, and the types of bonds that hold solids together. We will examine four major types of solids—ionic, covalent-network, molecular, and metallic—and how the forces that hold each group’s particles together shape those materials’ physical characteristics and performance. Then we will use two well-known processes, blacksmithing and steel manufacturing, to consider how solids can exist in multiple phases. (Figure 13-2) Finally, we will look at two broad categories of modern materials that demonstrate the diversity of solids: metal alloys and both natural and synthetic polymers.

1Trevor H. Levere, Transforming Matter: A History of Chemistry from Alchemy to the Buckyball(Johns Hopkins University Press, 2001), p. 182.

Section 2: What Is a Solid?

As we first learned in Unit 2, solids are distinguished from liquids and gases by the strength of the forces that hold them together. The intermolecular attractive forces in solids are stronger than those in liquids or gases. They hold molecules close together, with very little space between them. Solid particles have low kinetic energy, so they cannot move freely, although they vibrate in place. Because their particles are closely packed, solids are hard to compress. And when scientists add a material to a solid to change its properties, the additive diffuses (is transported) through the solid very slowly. One common process that involves diffusion is galvanizing steel—applying a protective zinc coating to steel objects to keep them from rusting.

In most solids, the atoms or molecules that make up the substance are arranged in a regular order that repeats throughout the solid. These substances are called crystalline solids. The building-block unit of a crystalline solid that repeats throughout the material (like a single brick in a brick wall) is called the unit cell, and is usually a six-sided, three-dimensional shape whose faces have four straight sides. Each unit cell is packed with atoms, molecules, or ions of the substance. Familiar examples of crystalline solids include ice, table salt, and diamonds. Figure 13-3 shows the structure of unit cells for several salts.

Figure 13-3. Unit Cells for a Variety of Salts
Unit cells are the simplest repeating units in crystalline substances.
© Purdue, Bodner Research Group.

Crystalline solids have well-defined melting points because the same type of bonds hold particles together throughout the solid. So when the melting point is reached, the particles all respond in identical ways. Adding that maximum heat energy will uniformly disrupt the placement of the atoms, which then can move around. However, when some energy is added very quickly—for example, hitting a crystal with a hammer—this change isn’t enough to overcome all of the bonds, so only some are broken. When crystals crack, they break apart along flat planes where their atomic bonding forces are the weakest.

Since the 1920s, scientists have used a technique called X-ray crystallography to analyze the structure of crystalline solids. This method was developed by British physicist William Henry Bragg (1862–1942) and his son William Lawrence Bragg (1890–1971), who recognized that when a beam of X-rays was aimed at a crystal, planes of atoms within the crystal would scatter the rays in patterns that could be used to map the crystal’s internal structure. The Braggs shared a Nobel Prize in physics in 1915 for their work. Over the next several decades, other scientists applied diffraction to increasingly complicated substances. Dorothy Crowfoot Hodgkin (1910–1994), a British chemist, became a leading expert in the technique, which she used to determine the structures of insulin, penicillin, and vitamin B-12, an important nutrient. In 1964, Hodgkin became the third woman to receive the Nobel Prize in chemistry, following Marie Curie (1867–1934) in 1903, and again in 1911, and Irène Joliot-Curie (1897–1956) in 1935.

Figure 13-4. Silicon Dioxide in Crystalline and Amorphous Forms
When silicon dioxide is heated and then cooled rapidly, its ordered crystalline structure is unable to reform and it becomes glass, an amorphous solid.
© NDT Resource Center, http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/solidstate.htm, Center for NDE, Iowa State University.

Solids whose particles are not arranged in an orderly structure are called [/tooltip]amorphous solids. In these materials, particles are arranged randomly, so they do not have the well-defined shapes that characterize crystalline structures. Amorphous solids may be compounds of molecules that do not combine neatly together or may be composed of a single substance with large, complex molecules. Common examples include rubber and some types of plastic.

Some substances can occur in both crystalline and amorphous forms. A familiar example is silicon dioxide (SiO2), which occurs naturally as quartz sand with a crystalline structure. When it is melted and quickly cooled, however, it forms glass, which is amorphous. (Figure 13-4) Glass is also rigid, but, unlike quartz, it breaks into random fragments because its structure does not contain well-defined planes. Many products are made today with safety glass, which has either been heat-treated so that it will break into small, granular chunks or layered with plastic so that pieces will stick together if the glass breaks. Common uses for safety glass include car windows, skylights, and shower doors.

As this example shows, the properties of solid materials are shaped by the bonds that hold them together. In the next several sections, we will see how bond types affect the characteristics of four major categories of solids: ionic, covalent-network, molecular, and metallic.

Section 3: Ionic and Covalent-Network Solids

By contrasting two types of solids, ionic and covalent-network, we can easily see how centrally chemical bonds determine the characteristics of solid materials. Ionic solids are made up of ions held together by ionic bonds. All salts and most minerals are ionic solids. As we learned in Unit 5, ionic bonds form when electrons jump from the valence shell of one atom to another, creating negatively charged and positively charged ions. The ions attract each other and are held together by Coulombic forces.

Figure 13-5. Conceptual Design for a Molten Salt Reactor, Using Salt as Both Fuel and Coolant
Molten salt reactors take advantage of salts’ high melting points, substituting salts for pressurized water as a coolant and sometimes as fuel.
© Wikimedia Commons, Public Domain. Author: U.S. Department of Energy, 2009.

Ionic bonds are strong, so it takes a lot of energy to break them. This property gives ionic solids high melting and boiling points. For example, sodium chloride melts at 800°C. The sodium and chloride ions that make up sodium chloride have charges of 1+ and 1, respectively. The ions in magnesium oxide (MgO) have charges of 2+ and 2, respectively, so their bonds are even stronger: Magnesium oxide’s melting point is 2852°C. Ions cannot conduct electricity in their solid state, but can conduct electricity if they are dissolved in water or melted, thereby breaking their ionic bonds.

Because ionic solids have high melting points, nuclear engineers are studying them as a potential cooling material for nuclear reactors. Without a coolant, the heat generated by chain reactions inside nuclear fuel rods would melt the fuel, releasing radiation. Commercial reactors today use water as a coolant; but since water boils at 100°C, most reactors are pressurized to prevent the coolant from boiling (remember the role of pressure in phase changes from Unit 2). Reactors using molten salt as a coolant can operate at much higher temperatures and at near-atmospheric pressure, which reduces stress on the equipment.2 Figure 13-5 shows a conceptual design for a molten salt reactor.

Covalent-network solids are made up of many atoms held together in large, regular lattices by covalent bonds. These substances are extremely strong and have very high melting points because they contain so many covalent bonds, which are stronger than intermolecular forces. A covalent-network crystal is like one big molecule of the substance, since its units are held together in a continuous network of covalent bonds. Breaking the material is difficult because it involves breaking many chemical bonds. Examples include diamonds, silica, and graphite.

Figure 13-6. Bonding in Diamonds and Graphite
Diamonds and graphite are forms of carbon with very different properties due to their contrasting chemical structure.
© Science Media Group.

Many elements can be found as allotropes: different forms made up of the same element arranged in different structures. Diamonds and graphite are allotropes of carbon. (Figure 13-6) Each atom of a diamond is bonded to four other carbon atoms in a repeating three-dimensional structure, which makes diamonds so hard and durable that they are widely used in industry to cut other materials. In contrast, graphite’s atoms are arranged in layers of flat six-sided rings, which are stacked and can slide freely past each other. This structure makes graphite a good lubricant. Cores of conventional writing pencils, widely referred to as “leads,” are actually made of graphite mixed with a clay binder.

2For more information on molten salt reactors and other advanced designs, see M. Mitchell Waldrop, “Nuclear Energy: Radical Reactors,” Nature, December 5, 2012. http://www.nature.com/news/nuclear-energy-radical-reactors-1.11957.

Section 4: Molecular Solids

Figure 13-7. Liquid and Solid Water Molecules
Ice is less dense than water because frozen water molecules are locked together in a fairly open network by rigid hydrogen bonds.
© Science Media Group.

Molecular solids are materials whose molecules (or atoms) are held together by intermolecular forces, such as London dispersion forces, dipole-dipole forces, and hydrogen bonds. As we saw in Unit 5, these forces are much weaker than ionic or covalent bonds. As a result, many molecular substances are gases or liquids at normal temperature and pressure.

Molecular solids are soft and low density, and have low melting points, usually lower than 300°C. Examples include sugar, iodine, and dry ice. Dry ice has such weak bonds that it sublimates directly from a solid to a gas at -78.5°C. Iodine will sublimate at room temperature from a black solid to a purple gas. The familiar form of iodine that is widely used for disinfecting cuts is actually a small amount of iodine dissolved in a water and alcohol base.

Figure 13-8. Morphology of Snow Crystals
Snow crystals form simple shapes at lower humidity levels and more complex shapes at higher humidity. However, all have a six-sided structure because of the way hydrogen bonding occurs between ice molecules.
© Science Media Group, adapted from image by Caltech.

As we learned in the discussion of phase diagrams in Unit 2, ice (a molecular solid) has an unusual property: It is less dense in its solid form than as a liquid. This occurs because weak hydrogen bonds between liquid water molecules constantly form and break, so the molecules are randomly distributed; when water freezes, however, the bonds form rigid lattices with each water molecule bonded to four others. These lattices contain relatively large gaps, which would be filled with atoms if the water were liquid. (Figure 13-7)

Note in Figure 13-7 that the water molecules in solid form create a six-sided structure. This happens because, at temperatures and pressures that are typical on Earth’s surface, water crystallizes into six-sided rings when it freezes. Snowflakes are six-sided because they form in clouds when water vapor turns into ice. As more water molecules in the cloud strike against the central snow crystal, some of them stick to its corners and freeze, growing into branches. A snowflake may be a single hexagonal ice crystal, a few crystals stuck together, or a large clump of crystals. For reasons that are not yet completely understood, snowflakes grow in different shapes (always six-sided) under different temperature and humidity conditions. (Figure 13-8)

Section 5: Metallic Solids

Metallic solids are made up entirely of metal atoms arranged in crystal lattices. They are joined together by metallic bonds, which are similar to covalent bonds in that they involve shared electrons. However, valence electrons in metals are shared among all the atoms in a crystal, not just between two individual atoms. The atoms in the crystal become positive ions surrounded by a sea of electrons, and the interaction between these ions and the valence electrons binds the entire crystal together. Figure 13-9 shows this arrangement for atoms in a block of aluminum, which has three valence electrons: The valence electrons merge into an electron cloud around Al 3+ cations.

Figure 13-9. Metallic Bonding in Aluminum
Aluminum atoms in a crystal lattice of positively charged ions and negative electrons.
© NDT Resource Center, Center for NDE, Iowa State University.

Metals generally are solids at room temperature, except for mercury, but their melting points vary widely. Generally, the hardest metals are the ones at the center of the periodic table, as metals with more valence electrons for binding are harder and have higher melting points. The softest are the alkali metals, with only one valence electron: Sodium, potassium, rubidium, and cesium all have melting points below 100°C and are easy to cut with a knife. At the other extreme, tungsten melts at 3422°C and is 70 percent denser than lead. But metals with more electrons, such as silver and zinc, start to become softer again because they only have one or two electrons in their valence shell’s sorbital.

Metals have other properties that make them extremely useful materials. They are less brittle than ionic or covalent-network solids: Their atoms can slide past each other without breaking their metallic bonds because they are surrounded by a sea of electrons. Most metals are ductile (easily stretched into wires) and malleable (easily shaped and formed by hammering or pressure). They also conduct electricity well, because free electrons flow readily between metal atoms. Copper is widely used in electrical wiring because it excels in many of these qualities. It is highly ductile, so it can be drawn into thin wires; has high tensile strength, so considerable force is required to break it; and has high electrical conductivity, so it transmits electricity with little resistance. (Resistance converts a fraction of the current passing through the wire into waste heat that does not reach the destination point.)

Well before chemistry became a science, it was widely known that metals could be made even more useful by combining them in an alloy. An alloy contains more than one element, including at least one metal, and has metallic properties. Modern solid-state chemistry is producing many highly sophisticated alloys, which are discussed in Section 9 of this unit.

Section 6: Phase Changes in Solids

Unit 2 of this course presented some simple phase diagrams for water and carbon dioxide, showing the combinations of pressure and temperature at which these substances change between solid, liquid, gas, and supercritical phases. Phase changes can also occur within solids: As they undergo heat or pressure changes, the molecules’ structure is altered, changing the material’s properties. These shifts are known as “solid-solid phase changes.”

Figure 13-10. Iron-Carbon Equilibrium Diagram
As iron and steel are heated, they undergo solid phase changes, with crystals within the metal changing form.
© Science Media Group, adapted from original image by U.S. Department of Transportation, Federal Highway Administration.

The historic practice of blacksmithing requires an extensive understanding of phase changes in solid metals, even though most blacksmiths never studied chemistry. Humans began working with iron more than 3,000 years ago, and iron was a centrally important material for many tools and applications until it was replaced by mass-produced steel in the latter half of the 19th century.

The first step in ironmaking is smelting iron from iron ore (iron oxide mixed with other trace elements), which was done for centuries by heating the ore with charcoal. Oxygen in the iron oxide combines with carbon from the charcoal to form carbon dioxide, leaving iron metal behind. As the iron metal is heated further, it absorbs carbon from the charcoal and forms different crystal shapes. Steel is an alloy of iron with less than 2 percent carbon. If the metal absorbs more carbon, it becomes cast iron, which contains about 2 to 4.5 percent carbon and is hard and brittle. Figure 13-10 shows some of the crystal structures that form in iron at different temperatures and carbon contents. Ferrite consists of iron molecules with no carbon attached, cementite is an iron-carbon compound (Fe3C), and pearlite is a mixture of ferrite and cementite.

Early blacksmiths did not have thermometers that could measure these high temperatures, but knew from practice that metal glowed in different colors as it heated, first turning red and then orange, yellow, and white. After smiths forged metal into a specific shape, such as a horseshoe, they often would let the metal cool to a certain point, then quench the object by plunging it into water or another liquid to cool it quickly. This process converted some atoms in the metal into hard, brittle crystals, giving the object strength. Today, advanced alloys may be quenched with oil, brine, or forced air.

Blacksmiths also developed many other techniques for manipulating the properties of alloys, which are widely used today in industry. Typically, they involve bringing the material to a certain temperature and holding it there for a specific length of time to form or dissolve certain crystals in the material. Examples include:

  • Annealing: heating metal to a high temperature, then cooling it very slowly. During this process, small grains in the metal re-crystallize into large grains, which makes the metal softer and easier to form.
  • Tempering: heating iron alloys to a lower temperature to reduce their hardness.

The Chemistry of Steelmaking
Steel is an alloy of iron with about 0.2 to 1.5 percent carbon. It is stronger, more flexible, and more resistant to corrosion than iron, and is the most widely used metal in the world today. But making steel in large quantities was difficult until the mid-19th century because there was no good method for controlling the amount of carbon in the alloy.

In 1856, British metallurgist Henry Bessemer (1813–1898) invented a new system: blowing air through the molten pig iron that had just been smelted from iron ore and typically contained 3 to 4 percent carbon. Carbon from the iron combined with the oxygen, leaving molten steel behind. Bessemer invented a pear-shaped furnace to contain this process, which came to be known as a “Bessemer converter.” Iron was loaded into the top of the vessel and heated from the bottom. Once the iron was melted, air was injected through it, oxidizing the unwanted materials. The entire container could be tipped to pour out the molten steel for shaping and further processing.

Bessemer Converter on Display Outside the Kelham Island Museum, Sheffield, England. © Wikimedia Commons, Public Domain.

Other inventors found ways to improve the Bessemer process. Adding a mixture of iron, carbon, and manganese, known as spiegeleisen or spiegel, drove off excess oxygen from the steel and ensured that it retained the right amount of carbon. And adding limestone removed unwanted phosphorus, which reacted with the limestone to form “slag,” a glassy byproduct that was separated from the molten steel before it was cast.

In the 20th century, Bessemer converters were replaced by other technologies: first the open-hearth furnace, then the basic oxygen furnace, and finally electric arc furnaces. Each of these systems provided better control over the heating and refining process. Producers also developed increasingly sophisticated methods for analyzing the chemical composition of steel—a critical step in manufacturing highly specialized alloys. Today, many steps in steel production are computer-controlled, from operating furnaces to testing metallurgical content.

Recycled scrap iron and steel is the single largest source of raw material for modern steel production. Scrap metal contains many different types of alloys; but by analyzing its chemical composition, manufacturers can blend metals from various sources to make new steel products.

Recycling saves money for steel producers because it is cheaper to buy scrap metal than to mine fresh supplies of iron ore, and less energy is required to melt scrap metal than to convert new iron into steel. In North America, steel manufacturers recycle steel each year that is equivalent to about 80 percent of total annual production, mainly from cans, automobiles, construction materials, and appliances.3

3Steel Recycling Institute, “Steel Recycling Rates,” http://www.recycle-steel.org/Recycling%20Resources/Steel%20Recycling%20Rates.aspx.

Section 7: Polymers—Materials Adapted from Nature

Polymers are a deceptively simple concept—large molecules made up of many small, uniform molecules linked together in long chains. Over the past century, polymer chemistry has produced an enormous number of products, including synthetic fabrics, spray-on insulation, waterproof films and coatings, carpet fibers, and all kinds of plastic goods. To understand why polymer chemistry is such a diverse and important field, it is helpful to know about polymers’ basic properties and to see how many useful polymers occur in nature.

Figure 13-11. Polyethylene Chain
Polyethylene is produced by breaking the double bonds in ethylene molecules, freeing two electrons to form new single bonds with other ethylene molecules. Note that only a fragment of the polyethylene chain is shown; it continues in both directions.
© Science Media Group.

Polymers are molecular solids, held together by covalent bonds. They can have amorphous or crystalline structures. Polymers consist of long chains of molecules, often containing hundreds of thousands of atoms. Figure 13-11 shows the structure of an ethylene molecule and a polyethylene chain made up of ethylene monomers.

This structure makes many polymers very flexible, although some types that have cross-links between individual strands are rigid. Adding materials with lower molecular weights to a polymer alters its properties. It can make the base material harder by creating cross-links between polymer chains, or can make it softer by interfering with intermolecular forces and preventing crystals from forming. Additives that make a material softer or more fluid are called “plasticizers,” alluding to the word “plastic” as an adjective—something that can be shaped or formed.

Figure 13-12. Vulcanization of Rubber with Sulfur
Vulcanizing rubber by heating it with sulfur locks together soft rubber molecules, making them harder and more durable.
© Science Media Group.

Many important natural materials are polymers, including wool, silk, and cellulose (plant fiber). Humans were working with and manipulating polymers long before they understood their chemical structure. For example, natural rubber is made by tapping rubber trees to extract latex, a milky fluid polymer, and letting it dry. Explorers brought it back from the Americas to Europe in the 18th century. English chemist Joseph Priestley, whom we met in Unit 1, found that this so-called “Indian gum” could erase lead pencil marks, and dubbed it “rubber.”

Natural rubber was soft, and early rubber goods melted on hot days until the 1830s, when American merchant Charles Goodyear (1800–1860) found that treating rubber with sulfur and then heating it made it weatherproof. English scientist Thomas Hancock (1786–1865) adapted Goodyear’s idea and patented it as vulcanized rubber in 1844. Vulcanization creates cross-links between polymer chains in rubber, making it more durable and water-repellent. (Figure 13-12)

Figure 13-13. The Structure of Silk (Yellow Squares Are Beta-Sheet Crystals)
Silk’s unusual structure, which makes it very lightweight and strong, shows that hydrogen bonds can produce strong materials if they are arranged in the right ways.
© Science Media Group, adapted from an image by M. Buehler from the paper: S. Keten, Z. Xu, B. Ihle, M.J. Buehler, “Nanoconfinement controls stiffness, strength, and mechanical toughness of beta-sheet crystals in silk,” Nature Materials, Vol. 9. pp. 359–367, 2010.

Silk is another natural polymer that is widely used worldwide, mainly for textiles. Silk threads are extremely light, but also very strong relative to their weight, as anyone knows who has seen beetles snared in a spider’s web. It is also extremely ductile, so it can bend and stretch without breaking. Silk is made up of proteins, including some that form thin, flat structures called “beta sheets.” Beta sheets can stack together in a crystalline formation, joined by hydrogen bonds. Hydrogen bonds are relatively weak; but when the bonds in silk fail, they quickly re-form. And silk beta-sheets are stacked in arrangements that allow hydrogen bonds to reinforce adjacent chains, making silk very flexible and strong. (Figure 13-13)

Section 8: Modern Materials—Synthetic Polymers

As Section 7 illustrates, polymers such as rubber and silk are extremely useful materials with many applications. Before chemists even understood atomic structure, early pioneers started developing synthetic polymer materials—new materials that were not based on modifying a naturally occurring substance. Inventors quickly recognized that these new substances could be useful for applications such as fillers and coatings. In the 20th century, synthetic polymer chemistry grew rapidly, driven by two related subfields: petroleum refining, which broke down oil into many different fuel products, and plastics manufacturing. (All plastics are polymers, but not all polymers are plastics.)

Figure 13-14. Bakelite Buttons
Bakelite was used for jewelry, telephones, toys, kitchenware, billiard balls, and many other products.
© Chemical Heritage Society, photograph by Gregory Tobias.

The first synthetic plastics were created as substitutes for natural materials. John Wesley Hyatt (1837–1920), an American printer, patented a process for making celluloid (a shiny coating similar to shellac, which was derived from beetle shells) in 1870. Hyatt mixed ground camphor, a compound from the wood of the camphor tree, with nitrocellulose (cotton treated with nitric acid and sulfuric acid), drained off the water, and compressed the mixture at a high temperature. The product was light, strong, and colorless, and could be dyed and molded in various shapes. Next came Bakelite, a synthetic plastic invented by Belgian chemist Leo Baekeland (1863–1964) in 1907. Baekeland mixed phenol and formaldehyde, then heated the mixture under pressure. The end product had a high degree of cross-linking; so once molded, it hardened and could not be reshaped. Mixed with fillers, it made a hard, moldable plastic that could be dyed in different colors. Many products were made from Bakelite in the early 20th century. (Figure 13-14)

Between World War I and World War II, chemists worked to understand how polymeric molecules were structured. Important discoveries in the 1930s included neoprene, the first synthetic rubber, and nylon, the first synthetic fiber. After World War II, as oil became the primary U.S. energy source, oil companies started looking for ways to use chemicals like propylene and ethylene, which were byproducts from petroleum refining. Two researchers at Philips Petroleum, J. Paul Hogan (1919–2012) and Robert L. Banks (1921–1989), discovered that these chemicals could be reacted with chromium catalysts to produce crystalline polypropylene and high-density polyethylene (HDPE)—two new types of plastic resin that soon were manufactured into numerous products, from auto parts to food packaging and medical instruments.6

Polyethylene and polypropylene are thermoplastics: Their polymer chains are only weakly bonded together. So when the material is heated, the intermolecular forces that hold the chains together are overcome, and the materials can be molded into new shapes. About 80 percent of plastics manufactured today are thermoplastics. The rest are thermosets, which harden into a fixed shape after they are heated and cooled, and cannot be melted and reformed. Since their polymer chains are strongly cross-linked together, thermosets are used in many products that are exposed to heat, such as electronic circuit boards and cooking utensils.

Figure 13-15. Structure of Kevlar Fiber
Kevlar® polymer chains are aligned in a crystalline form that makes the fiber extremely strong.
© Science Media Group, adapted from an image by the American Chemical Society, Chemical & Engineering News.

Modern polymer chemistry has produced many materials with unique properties that make them suitable for a wide range of uses. Polymers can be crystalline or amorphous solids, and their structures determine how they will behave. One well-known example is Kevlar®, an extremely strong fiber that is used to make bulletproof vests, as well as ropes, cables, sports equipment, and many other items. A single Kevlar fiber has up to one million segments bonded together in a highly ordered crystalline structure. Normally, it is very difficult to align polymer fibers in one direction to make them highly ordered and crystalline, but the monomers used to make Kevlar help this process by aligning into a liquid crystal before the polymerization takes place. This means that the fibers are polymerized in place and already highly ordered with no post-processing necessary. Sheets of Kevlar fibers, held together by hydrogen bonds, are stacked in a radial form around the axis of each fiber. (Figure 13-15)

In the 1980s, scientists developed polymers that could conduct electricity. This was a radical innovation: Up to that point, carbon-based polymers had always been used as insulation around the metal wires in electric cables. Researchers found that a polymer could be made conductive if it was constructed with alternating single and double bonds between carbon atoms, and was “doped” by removing electrons through oxidation or adding them through reduction. This process allowed electrons to move along the molecule by jumping from one vacant hole to another.

Three scientists—Alan J. Heeger (born 1936) of the University of California at Santa Barbara, Alan G. MacDiarmid (1927–2007) of the University of Pennsylvania, and Hideki Shirakawa (born 1936) of Tsukuba University—won the Nobel Prize in Chemistry in 2000 for pioneering the field of conducting polymers. Conducting polymers have been used in many applications, including batteries, light-emitting diodes, and microwave-resistant coatings.

Recycling Plastics
Many consumers worldwide are familiar with the “chasing arrows” symbol, a public-domain logo that manufacturers use to indicate that materials can be recycled. But among the materials that households typically recycle, one category often causes confusion: plastics. Consumer items made of plastic contain different types of resins that cannot be mixed during recycling because their chemical structures are different. If different types of plastic are melted together, the materials will tend to separate, like oil and water.

To identify the types of resin in plastic goods, the chasing arrows symbol on recyclable plastic items contains a number from 1 through 7. These resin identification codes, developed by the industry in the late 1980s, indicate from which type of plastic the item is made. Resin codes represent broad categories, each of which includes thousands of types of plastics.

Resin Identification Codes for Recyclable Plastics © Symbols: Public Domain, Chart: Science Media Group.

Among these resin types, PET and HDPE plastics have the best-developed markets for recycled goods. These categories (#1 and #2) are the types collected most widely by recycling programs. The majority of recycled plastics are processed mechanically: Materials are crushed or shredded, washed, dried, and pelletized. The pellets can then be heat extruded into new forms. Mechanical recycling changes the physical form of the plastic but not its chemical structure. Plastics can only be recycled this way a few times before their polymers start to break down and their quality degrades. And thermoset (rigid) plastics cannot be recycled mechanically because they cannot be melted and remolded.

Chemical recycling is a less common method in which plastics are chemically broken down into their monomeric building blocks using heat or a catalyst, a process called “depolymerization.” This approach produces monomers that can be used to make new, virgin-quality resins. One demonstrated use is recovering nylon from scrap carpet.4

Another chemical process under development is using pyrolysis (decomposing a chemical compound using intense heat) to convert non-recyclable scrap plastic into petroleum products. This approach faces potential barriers—for example, questions about whether it can be considered recycling or environmentally friendly—but advocates call it a ready way to treat unwashed, hard-to-recycle plastic materials instead of sending them to landfills.5

44R Sustainability, Inc., “Conversion Technology: A Complement to Plastic Recycling,” report for the American Chemistry Council, April 2011, p. 4. http://plastics.americanchemistry.com/Plastics-to-Oil.

5Ibid., Daniel Robison, “Startup Converts Plastic to Oil, and Finds a Niche,” National Public Radio, March 19, 2012. http://www.npr.org/2012/03/19/147506525/startup-converts-plastic-to-oil-and-finds-a-niche.

6American Chemical Society, “National Historic Chemical Landmark: Polypropylene and High-Density Polyethylene,” http://portal.acs.org/portal/acs/corg/content?_nfpb=true&_pageLabel=PP_SUPERARTICLE&node_id=715&use_sec=false&sec_url_var=region1&__uuid=f999a40d-92c5-48aa-ae0f-c66f55948117.

Section 9: Modern Materials—Alloys

As we learned in Section 5 of this unit, alloys are materials that contain more than one element and have the characteristic properties of metals: They are ductile, malleable, and good electrical conductors. Alloys are widely used in all kinds of modern applications, from jewelry to medical implants and aircraft parts. Many have been developed to combine strength with other attributes. For example, titanium alloys are widely used to make artificial hips, knees, and other body implants because they are strong yet lightweight, and do not cause reactions in human tissue.

Figure 13-16. Atomic Arrangements in Alloys
When a metal is mixed with another material, forming an alloy, atoms of the added component may either take the place of host metal atoms or fit between them.
© Wikimedia Commons, Public Domain.

Alloys that are homogeneous mixtures in which the components are distributed uniformly are known as “solution alloys.” The components can blend in two ways. Atoms of the added component(s) can take up spaces that are normally occupied by atoms of the host metal, forming a substitutional alloy; or they can occupy spaces in between atoms of the host metal, forming an interstitial alloy. (Figure 13-16) Substitutional alloys are much more common than interstitial alloys.

Alloys can also be heterogeneous mixtures in which the component materials are not uniformly distributed. However the alloy is structured, combining a metal with another substance changes its properties. The first known manmade alloy to be produced was bronze (a substitutional alloy), which came into use more than 3,000 years BCE, giving the Bronze Age its name. Bronze is roughly 70 to 90 percent copper mixed with tin; the resulting alloy is harder and more durable than copper, and easier to melt and cast. It is also harder than iron and much more resistant to corrosion.

Other well-known alloys include brass (copper and zinc), pewter (tin, copper, and bismuth), solder (tin and lead), steel (iron, carbon, and other metals), and stainless steel (steel and chromium). Brass and pewter are substitutional alloys. Steel is interstitial: Small carbon atoms fill spaces between larger iron atoms. Stainless steel is both substitutional and interstitial: Carbon atoms fit between the iron atoms, but nickel and chromium atoms replace some iron atoms.

Figure 13-17. Common Steel Alloying Agents
Many metals can be alloyed with steel to alter its characteristics for different products and applications.
© Science Media Group.

To form a substitutional alloy, two components must have atomic radii of similar sizes (no more than about 15 percent difference) and similar chemical bonding characteristics. Metals from the d block of the periodic table have similar radii and form a wide range of alloys with each other. Common examples include gold-silver alloys, widely used for making jewelry, and brass, which is an alloy of copper and zinc.

As we learned in The Chemistry of Steelmaking Sidebar in Section 6, mass production of steel (an alloy of iron mixed with small amounts of interstitial elements, mainly carbon) developed in the 19th century. In the 20th century, manufacturers learned that alloying steel with other metals could further improve its properties. Nickel, chromium, molybdenum, vanadium, and copper are among the metals that modern steelmakers use to alter properties such as the hardness and corrosion resistance of their products. (Figure 13-17) For example, stainless steel is a steel alloy that contains about 10 percent chromium by mass. Stainless steel is not completely immune to corrosion, however. As one example, the Gateway Arch in St. Louis, which has a skin made of stainless steel, shows visible stains and corrosion, which National Park Service managers say are cosmetic.

To understand why alloys are usually stronger than pure metals, remember that metals are crystalline solids. When a metal is mixed with another material to make a substitutional alloy, the less abundant atoms of the solute element distort the crystal lattices of the more abundant element. This makes it more difficult for planes of the material to slide past each other, which makes the alloy hard and strong. In interstitial alloys, the nonmetal atoms fit between the metal atoms and provide extra bonding to neighboring atoms, which makes the alloy harder, stronger, and less ductile.

However, a few alloys are designed to be weaker than their constituent metals. The best-known example is solder, which is essentially metallic glue used to join metal parts together. Solder is a blend of lead and tin, sometimes with other metals added to make the alloy harder or more elastic. Another fusible alloy (one that melts at low temperatures) is Wood’s metal, a blend of bismuth, lead, tin, and cadmium that melts at 70°C. Wood’s metal is used as the triggering element in fire sprinkler systems: Heat from nearby flames melts a plug of Wood’s metal, releasing water from sprinklers.

Section 10: Conclusion

Solid state chemistry is a rapidly growing field that draws on knowledge from many other disciplines, including physics, materials science, and engineering. It is directly relevant to many contemporary challenges that industrial societies face today.

For example, a key hurdle to making solar power an affordable energy source is increasing the efficiency of photovoltaic cells that convert solar radiation to electricity—in other words, making solid materials in solar cells as conductive as possible. And solid chemistry is central to oral health. For more than a century, dentists have used silver amalgam (a blend of silver and mercury) to fill cavities. Silver amalgam is very durable, but mercury is highly toxic when it is released into the environment. Now fillings made of other solid materials, such as composite resin (a mixture of plastic and glass) or ceramic (a hard, heat-resistant ionic solid), are becoming viable alternatives for many dental patients.7

Figure 13-18. Quasicrystals
Israeli scientist Daniel Shechtman (born 1941) received the 2011 Nobel Prize in Chemistry for discovering quasicrystals, a type of crystal structure that other scientists initially believed did not exist. Similar patterns can be found in Islamic tile designs.
© Wikimedia Commons, Public Domain. Author: Ames Laboratory, U.S. Department of Energy.

Chemists are also making new finds whose applications are not yet known. In 2011, Israeli scientist Daniel Shechtman (born 1941) received the Nobel Prize in Chemistry for discovering quasicrystals—units in a rapidly cooled alloy of aluminum and manganese, packed together in orderly patterns. Unlike known crystal shapes, such as squares, triangles, or the hexagons that we saw in ice crystals in Section 4 of this unit, the crystals he saw in the alloy had five-sided symmetry, and formed unique patterns that did not repeat. (Figure 13-18) Shechtman’s finding was so controversial that he was asked to leave his research group, but it was validated by further research.

Quasicrystals are very strong and non-reactive, but the real significance of Shechtman’s discovery was in changing how chemists viewed solid matter. The five-sided units “break all the rules of being a crystal at all,” said David Phillips (born 1939), former president of the Royal Society of Chemistry, when Shechtman received the Nobel Prize, which Phillips called “a celebration of fundamental research.”8

The discovery of quasicrystals shows that chemistry is still a rapidly evolving field, and that our understanding of the basic properties of matter is far from complete. At the same time, however, chemistry is producing countless, highly useful materials and products, from pharmaceuticals to solar cells. As the evolution of dental fillings suggests, many of these new products are more effective and have fewer harmful impacts on the environment than the items they are replacing. From basic needs like safe food and clean drinking water to advanced industrial materials, humans continue to use chemistry to understand how our world works and to improve our daily lives.

7For a comparison, see “Dental Health and Tooth Fillings,” WebMD, http://www.webmd.com/oral-health/guide/dental-health-fillings.

8Ian Sample, “Nobel Prize in Chemistry for Dogged Work on ‘Impossible’ Quasicrystals,” The Guardian, October 5, 2011. http://www.guardian.co.uk/science/2011/oct/05/nobel-prize-chemistry-work-quasicrystals.

Section 11: Further Reading

Boyd, Jane E. “Celluloid: The Eternal Substitute.” Chemical Heritage Magazine,November–December 2011. http://www.chemheritage.org/discover/media/magazine/articles/29-3-celluloid-the-eternal-substitute.aspx.

Kunzig, Robert. “The Chemistry of . . . Plastics.” Discover Magazine, December 1, 2000. http://discovermagazine.com/2000/dec/featchemistry#.Ud70blN5HKJ.

Riordan, Michael, and Lillian Hoddeson. “Birth of an Era.” Scientific American: The Solid-State Century (special issue), June 2, 1997–December 2, 1997. http://solidstate.um.ac.ir/parameters/solidstate/filemanager/Scientific.American.Special.Edition.1997-12_Solid_State_Century.pdf.

Stubbles, John. “The Basic Oxygen Steelmaking (BOS) Process.” American Iron and Steel Institute. http://www.steel.org/en/Making Steel/How Its Made/Processes/Processes Info/The Basic Oxygen Steelmaking Process.aspx.

Glossary

Allotropes
Any two or more forms of the same element with different arrangements of atoms in their structures. For example, dioxygen (O2) and ozone (O3) are allotropes.
Alloy
A substance that contains more than one (usually metallic) element and has metallic properties, such as strength, conductivity, ductility, and malleability.
Amorphous solid
A solid whose atoms or molecules are not arranged in regular, repeating patterns.
Crystalline solid
A solid whose atoms or molecules are arranged in regular, repeating patterns in any direction throughout the solid.
Monomer
A small molecule that reacts to form covalent bonds with other monomer molecules over and over again to make a polymer.
Polymer
A large molecule formed by the joining together (through covalent bonds) of a large number of individual molecules with low molecular mass (known as monomers).
Unit cell
The smallest repeating unit of atoms in a crystalline solid.
X-ray crystallography
A technique for determining the structure of a crystal by passing X-ray beams through it and analyzing how the repeating units of atoms diffracts (spreads) the X-rays.

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