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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.
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.