Polymer Science

Summary

Polymer science is a specialized field concerned with the structures, reactions, and applications of polymers. Polymer scientists generate basic knowledge that often leads to various industrial products such as plastics, synthetic fibers, elastomers, stabilizers, colorants, resins, adhesives, coatings, and many others. A mastery of this field is also essential for understanding the structures and functions of polymers found in living things, such as proteins and deoxyribonucleic acid (DNA).

Definition and Basic Principles

Polymers are very large and often complex molecules constructed by nature or humans through the repetitive yoking of smaller, simpler units. This results in linear chains in some cases and branched or interconnected chains in others. The polymer can be built up of repetitions of a single monomer (homopolymer) or different monomers (heteropolymer). The degree of polymerization is determined by the number of repeat units in a chain. No sharp boundary line exists between large molecules and the macromolecules characterized as polymers. Industrial polymers generally have molecular weights between ten thousand and one million, but some biopolymers extend into the billions.

Chemists usually synthesize polymers by condensation (or step-reaction polymerization) or addition (also called chain-reaction polymerization). A good example of chain polymerization is the free-radical mechanism in which free radicals are created (initiation), facilitating the addition of monomers (propagation), and ending when two free radicals react with each other (termination). A general example of step-reaction polymerization is the reaction of two or more polyfunctional molecules to produce a larger grouping, with the elimination of a small molecule, such as water, and the consequent repetition of the process until termination.

Besides free radicals, chemists have studied polymerizations utilizing charged atoms or groups of atoms (anions and cations). Physicists have been concerned with polymers’ thermal, electrical, and optical properties. Industrial scientists and engineers create polymers like plastics, elastomers, and synthetic fibers. These traditional applications have expanded to include advanced technologies such as biotechnology, photonics, polymeric drugs, and dental plastics. Other scientists have found uses for polymers in new fields like photochemistry and paleogenetics.

Background and History

Nature created the first polymers and, through chemical evolution, such complex and important macromolecules as proteins, DNA, and polysaccharides. These were pivotal in the development of increasingly multifaceted life forms, including Homo sapiens, who made better use of polymeric materials like pitch, woolen and linen fabrics, and leather as they evolved. Pre-Columbian American Indians used natural rubber, or cachucha, to waterproof fabrics, as did Scottish chemistCharles Macintosh in nineteenth-century Britain.

The Swedish chemist Jöns Jakob Berzelius coined the term “polymer” in 1833, though his meaning was far from a modern chemist's understanding. Some scholars argue that the French natural historian Henri Braconnot was the first polymer scientist since, in investigating resins and other plant products, he created polymeric derivatives not found in nature. In 1836, the Swiss chemist Christian Friedrich Schönbein reacted natural cellulose with nitric and sulfuric acid to generate semisynthetic polymers. In 1843 in the United States, hardware merchant Charles Goodyear accidentally discovered vulcanization by heating natural rubber and sulfur, forming a new product that retained its beneficial properties in cold and hot weather. Vulcanized rubber won prizes at the London and Paris Expositions in 1850, helping to launch the first commercially successful product of polymer scientific research.

In the early twentieth century, the Belgian-American chemist Leo Baekeland made the first entirely synthetic polymer when he reacted phenol and formaldehyde to create a plastic called Bakelite. The nature of this and other synthetic polymers was not understood until the 1920s and 1930s, when the German chemist Hermann Staudinger proved that these plastics (and other polymeric materials) were extremely long molecules built up from a sequential catenation of basic units, later called monomers. This enhanced understanding led the American chemist Wallace Hume Carothers to develop a synthetic rubber, neoprene, that had numerous applications, and nylon, a synthetic substitute for silk. Synthetic polymers were widely used in World War II. In the postwar period, the Austrian-American chemist Herman Francis Mark founded the Polymer Research Institute at Brooklyn Polytechnic, the first such facility in the United States. It helped foster explosive growth in polymer science and a flourishing commercial polymer industry in the second half of the twentieth century.

How It Works

After more than a century of development, scientists and engineers have discovered numerous techniques for making polymers, including a way to make them using ultrasound. Sometimes, these techniques depend on whether the polymer to be synthesized is inorganic or organic, fibrous or solid, plastic or elastomeric, crystalline or amorphous. How various polymers function depends on various propertiesmelting point, electrical conductivity, solubility, and interaction with light. Some polymers are fabricated as coatings, adhesives, fibers, or thermoplastics. Scientists have also created specialized polymers to function as ion-exchange resins, piezoelectrical devices, and anaerobic adhesives. Certain new fields have required the creation of specialized polymers like heat-resistant plastics for the aerospace industry.

Condensation Polymerization. Linking monomers into polymers requires the basic molecular building blocks to have reaction sites. Carothers recognized that most polymerizations fall into two broad categories—condensation and addition. In condensation, which many scientists prefer to call step, step-growth, or stepwise polymerization, the polymeric chain grows from monomers with two or more reactive groups that interact (or condense) intermolecularly, accompanied by the elimination of small molecules, often water. For example, the formation of a polyester begins with a bifunctional monomer containing a hydroxyl group (OH, oxygen bonded to hydrogen) and a carboxylic acid group (COOH, carbon bonded to an oxygen and an OH group). When a pair of such monomers reacts, water is eliminated, and a dimer forms. This dimer can now react with another monomer to form a trimer, and so on. The chain length increases steadily during the polymerization, necessitating long reaction times to get “high polymers” (those with large molecular weights).

Addition Polymerization. Many chemists prefer to call Carothers's addition polymerization chain, chain-growth, or chain-wise polymerization. In this process, the polymer is formed without the loss of molecules, and the chain grows by adding monomers repeatedly, one at a time. This means that monomer concentrations decline steadily throughout the polymerization, and high polymers appear quickly. Addition polymers are often derived from unsaturated monomers (those with a double bond), and in the polymerization process, the monomer's double bond is rearranged to form single bonds with other molecules. Many of these polymerizations require the use of catalysts and solvents, both of which have to be carefully chosen to maximize yields. Important examples of polymers produced by this mechanism are polyurethane and polyethylene.

Applications and Products

Since the start of the twentieth century, the discoveries of polymer scientists have led to the formation of hundreds of thousands of companies worldwide that manufacture thousands of products. In the United States, thousands of companies manufacture plastic and rubber products. These and other products exhibit phenomenal variety, from acrylics to zeolites. Chemists in academia, industry, and governmental agencies have discovered many applications for traditional and new polymers, particularly in such modern fields as aerospace, biomedicine, and computer science.

Elastomers and Plastics. From its simple beginnings manufacturing Bakelite and neoprene, the plastic and elastomeric industries have grown rapidly in the quantity and variety of the polymers their scientists and engineers synthesize and market. Some scholars believe that the modern elastomeric industry began with the commercial production of vulcanized rubber by Goodyear in the nineteenth century. Such synthetic rubber polymers as styrene-butadiene, neoprene, polystyrene, polybutadiene, and butyl rubber (a copolymer of butylene and isoprene) began to be made in the first half of the twentieth century, and they found extensive applications in the automotive and other industries in the second half.

Although an early synthetic plastic derived from cellulose was introduced in Europe in the nineteenth century, it was not until the twentieth century that the modern plastics industry was born, with the introduction of Bakelite, which found applications in the manufacture of telephones, phonograph records, and a variety of varnishes and enamels. Thermoplastics, such as polyethylene, polystyrene, and polyester, can be heated and molded, and billions of pounds of them are produced in the United States annually. Polyethylene, a low-weight, flexible material, has many applications, including packaging, electrical insulation, housewares, and toys. Polystyrene has found uses as an electrical insulator and, because of its clarity, in plastic optical components. Polyethylene terephthalate (PET) is an important polyester, with applications in fibers and plastic bottles. Polyvinyl chloride (PVC) is one of the most massively manufactured synthetic polymers. Its early applications were for raincoats, umbrellas, and shower curtains, but it later found uses in pipe fittings, automotive parts, and shoe soles.

Carothers synthesized a fiber that was stronger than silk, and it became known as nylon and led to a proliferation of other artificial textiles. Polyester fibers, such as PET, have become the world's principal man-made materials for fabrics. Polyesters and nylons have many applications in the garment industry because they exceed natural fibers, including cotton and wool, in such qualities as strength and wrinkle resistance. Less in demand are acrylic fibers, but because they are stronger than cotton, they have numerous applications for clothing, blankets, and carpet manufacturers.

Optoelectronic, Aerospace, Biomedical, and Computer Applications. As modern science and technology have expanded and diversified, so, too, have the applications of polymer science. For example, as researchers explored the electrical conductivity of various materials, they discovered polymers that have exhibited commercial potential as components in environmentally friendly battery systems. Transparent polymers have become essential to the fiber optics industry. Other polymers have helped improve solar-energy devices through products such as flexible polymeric film reflectors and photovoltaic encapsulants. Newly developed polymers have properties that make them suitable for optical information storage. The need for heat-resistant polymers led the U.S. Air Force to fund the research and development of several such plastics, and one of them, polybenzimidazole, has achieved commercial success not only in aerospace but also in other industries.

Following the discovery of the double-helix structure of DNA in 1953, multiple applications followed in biology, expanding into medicine and even to fields such as criminology. Nondegradable synthetic polymers have had multifarious medical applications, such as heart valves, catheters, prostheses, and contact lenses. Other polymeric materials show promise as blood-compatible linings for cardiovascular prostheses. Biodegradable synthetic polymers have found wide use in capsules that release drugs in carefully controlled ways. Dentists regularly use polymers for artificial teeth, composite restoratives, and various adhesives. Polymer scientists have also contributed to the acceleration of computer technology since the 1980s and 1990s by developing electrically conductive polymers, and, in turn, computer science and technology have enabled polymer scientists to optimize and control various polymerization reactions.

Careers and Course Work

Building on a base of advanced chemistry, mathematics, and chemical engineering courses, undergraduates generally take an introductory course in polymer science. Graduate students in polymer science usually take courses in line with their chosen career goals. For example, students aspiring to positions in the plastics industry would need to take advanced courses in macromolecular synthesis and the chemical engineering of polymer syntheses. Students interested in biotechnology or bioengineering would need to take graduate courses in molecular biology and biomolecular synthesis.

Many opportunities are available for those with undergraduate or graduate degrees in polymer science. The field is expanding, and research careers can be forged in government agencies, academic institutions, and various industries. Chemical, pharmaceutical, biomedical, cosmetics, plastics, and petroleum companies hire polymer scientists and engineers. Because of concerns raised by the modern environmental movement, many companies hire graduates with expertise in biodegradable polymers. The rapid development of the computer industry has led to a need for graduates with an understanding of electrically charged polymeric systems. In sum, traditional and new careers are accessible to polymer scientists and engineers in the United States and many foreign countries.

Social Context and Future Prospects

The expansion trend in polymer science and engineering was well-established in the twentieth century and continues in the twenty-first. As polymer scientists created new materials that contributed to twentieth-century advances in transportation, communications, clothing, and health, they were well-positioned to meet twenty-first-century challenges like energy, communications, human health, and the environment.

Polymer scientists increasingly create lightweight materials for use in energy-efficient automobiles. The role of polymer science in biotechnology continually expands, offering new options in wound care, tissue engineering, knee replacements, drug delivery, contact lenses, syringes, and more. The plastic waste produced by the medical industry is historically ample. Transitioning to environmentally friendly, medically safe options created by polymer scientists allows for a lower carbon footprint. As environmental scientists brought public awareness to the problematic nature of plastic manufacturing’s environmental impact, the demand for biodegradable polymers, called biopolymers, increased. Environmental concerns in the polymer industry persist in the twenty-first century as government regulations tighten and consumers become better informed.

Bibliography

Baker, Jaden. Essentials of Polymer Science and Engineering. Academic Press, 2022.

Carraher, Charles E., Jr. Introduction to Polymer Chemistry. 4th ed., CRC Press, 2017.

Collar, Emilia P., and Jesús-María García-Martínez. Polymers and the Environment. Multidisciplinary Digital Publishing Institute, 2023.

Gandini, Alessandro, and Talita M. Lacerda. Polymers from Plant Oils. 2nd ed., John Wiley & Sons, Inc., 2019.

"Chemical Engineers." U.S. Bureau of Labor Statistics, 17 Apr. 2024, www.bls.gov/ooh/architecture-and-engineering/chemical-engineers.htm. Accessed 20 May 2024.

Ferguson, Bethany. Polymer Science and Technology. Murphy & Moore Publishing, 2022.

Fried, Joel R. Polymer Science and Technology. 3rd ed., Prentice Hall, 2014.

Morawetz, Herbert. Polymers: The Origins and Growth of a Science. Dover, 2002.

Seymour, Raymond B., ed. Pioneers in Polymer Science: Chemists and Chemistry. Kluwer Academic Publishers, 1989.

Wypych, George. Handbook of Polymers. 3rd ed., ChemTec Publishing, 2022.