Chapter 19 The Development of Nylon
Controversy on the Macromolecular Theory
In 2001, the writer co-worked with an experienced organic chemist. One day, the organic chemist synthesized a new compound with a double bond and said “Please synthesize a polymer for me.” A few days later, the writer returned him a small block of solid, saying “It is your polymer, polymerized by thermally free-radical bulk polymerization. Number-averaged relative molecular mass is about 52400 measured by gel permeation chromatography.” The organic chemist seemed to be shocked, “What? The relative molecular mass of my compound is 253.13. How did you get the number of 52400? You know, if there is only little difference in relative molecular mass, they are different substances. And how much is the melting point of the polymer?”“No melting point,” answered the writer. “Didn’t you do MS? NMR?”“No.”... “So, tell me what your polymer is!” the organic chemist was agitated.
“Rubber and other polymeric substances such as starch, cellulose and proteins are long chains of short repeating molecular units linked by covalent bonds. In other words, polymers are like chains of paper clips, made up of small constituent parts linked from end to end” proposed Staudinger in a landmark paper published in 1920. However, this proposition was very strange in the early 1920s, and was charged with controversy.
From a theoretical point of view, it was difficult to obtain key evidence for or against this view, mainly because measuring the relative molecular masses of that magnitude was an impossible mission from the technical point of view. It was also assumed that Staudinger’s hypothesis placed impossible demands on the strength of the chemical bonding forces, and the basic concept of a chemical compound might have to be revised. Macromolecular compounds are no longer completely identical, according to Staudinger. They consist of chain molecules with a characteristic average length but the length of the individual chains may rely on arbitrary circumstances.
This argument strikes us nowadays as completely obvious as that we are living in the age of polymers, but its historic truth and intensity as well as the real meaning of breakthrough in science are what we may actually feel in the head short story. Until 1930s, it was Wallace Hume Carothers (April 27, 1896-April 29, 1937) and his research group with Du Pont Experimental Station who placed a basic stone to hold up the Staudinger’s hypothesis.
Biography of Carothers
“In his research, Dr. Carothers showed even at this time the high degree of originality which marked his later work. He was never content to follow the beaten path or to accept the usual interpretations of organic reactions. His first thinking about polymerization and the structure of substances of high relative molecular mass began while he was at Harvard.” James B. Conant, the President of Harvard College in 1933, said of Carothers.
Carothers was a bookish-looking bespectacled man, born on April 27, 1896 in Burlington, Iowa. He was described as an egghead. As a youth, he was fascinated by tools and mechanical devices and wallowed in experiments. Forced by his father, the college vice-president, Carothers enrolled in a commercial college, majored in the accountancy and secretarial curriculum. Motivated by the interests in chemistry, after graduation, he entered Tarkio College and graduated with a bachelor’s degree of science in 1920. Then he went to the University of Illinois and was awarded the master’s degree of arts in 1921. With his master’s degree, Carothers got a position at the University of South Dakota, independently published a paper in Journal of the American Chemical Society. He went back to the University of Illinois in 1922 as a research assistant with the most prestigious award, Carr Fellowship, offered by the university, and received Ph. D. in 1924, majored in organic chemistry and minored in physical chemistry and mathematics.
Carothers worked as an instructor in organic chemistry at Harvard College for two years until 1928 when Du Pont Company unusually started a program to fund basic and pure research, which not deliberately targeted the development of money-making products. He was inquired of the possibility to lead the pure research of organic chemistry with an attractive offer that his beginning salary was $500 a month, much more than $267 at Harvard ($3200 per year). However, Carothers hesitated at first, partly worrying about his depression. He refused, explaining that “I suffer from neurotic spells of diminished capacity which might constitute a much more serious handicap there than here.”
Researches at Du Pont Experimental Station
On February 6, 1928, Carothers started his work at the Du Pont Experimental Station. At that time, as previously introduced, the concept of polymer was under hot arguments. Carothers stood for Staudinger’s hypothesis, thus his first goal was set to synthesize a polymer with a relative molecular mass of more than 4,200, which was the highest mass reported. However, he failed until Dr. Elmer K. Bolton became Carothers’ immediate boss. Bolton attended a lecture by chance at the University of Notre Dame and learned that divinyl acetylene was able to transform into an elastic compound similar to rubber when passed over sulfur dichloride. Du Pont purchased the patent rights. Therefore, he asked Carothers to examine the chemistry of an acetylene polymer with the goal of creating synthetic rubber. Carothers chose the monovinyl acetylene. In 1930, Carothers’s staff isolated chloroprene and polymerized it, obtaining a solid material resembling natural rubber. This was the first synthetic rubber and known today as the trade name, Neoprene (initially Duprene), or academically polychloroprene. It has routine applications in a variety of daily products such as electrical insulation, car fan belts, wetsuits, laptop sleeves, orthopedic braces (wrist, knee, etc.), and elastomeric membranes.
On belief of Staudinger’s hypothesis and his own knowledge of organic chemistry, Carothers started challenging reactions of dicarboxylic acids with diols and diamines, respectively. His new success was a synthetic polymer with a relative molecular mass of approximately 12,000, made in 1930. This success unambiguously proved the correctness of Staudinger’s hypothesis, and from then on, the macromolecular theory was universally accepted. Polyesters and polyamides are examples formed by step-growth polymerization. Carothers solved the theoretical challenges and established the relationship between the average degree of polymerization and the fractional conversion from monomer into polymer (“Carothers equation”).
Reaction to synthesize polyamides
Carothers had to suspend his research on polymers for a few years. Over 100 polymers were polymerized, but just shelved for academic studies. Comparatively, polyamides were easier to prepare, but difficult to treat due to their high melting points and lack of solvent to dissolve them. Polyesters demonstrated attractive properties such as good solubility in solvent and easy to work with in the laboratory. Hence, most of Carothers’ works were focused on polyesters. Dr. Julian Hill was one of the staff worked with polyester. During his experiment, he observed that a silky, fine fiber formed when drawing a stirring rod out of the beaker containing soft polyesters. He was curious to know how long a silky thread could be. One afternoon when Carothers was absent, Hill and his cohorts took a little ball on a stirring rod and ran down the hall and stretched them out into a string. However, the melting points of polyesters were too low to be used for making textile, thus they retrieved the polyamides from the shelf and repeated the experiment. They found that the strand of polyamide improved by stretching was a strong and excellent fiber. This accidental result inspired Du Pont, so they decided to choose a polymer for commercialization. In 1935, Carothers’ specimen number 66 was selected. After two years of efforts, the industrial production of the first synthetic fiber successfully realized.
Nylon 66 can be readily prepared by pouring the aqueous solution of hexamethylene diamine into the aqueous solution of 1,4-butylene dicarboxylic acid. The polymerization simultaneously occurs when the two solutions touch each other on their interface, thus called as interfacial polymerization.
Carothers invented a numbering system characterized by the number of atoms of carbon contained in the dicarboxylic acids and diamines. This numbering system was inherited into the new products. For example, polyamide made from hexamethylene diamine (C6) and 1,8-octanedicarboxylic acid (C10) was named nylon 610. However, Du Pont never patented the composition of nylon, rather they chose to patent the production process, cold-drawing, developed by unsupervised adults playing around in the lab. Nylon gained considerable strength by cold drawing under tension and by causing a reorientation of the crystals.
Nylons and Their Products
Nylon was first used commercially in a nylon-bristled toothbrush (1938), but famed by women’s stockings. In 1939, the nylon stockings hit the US market without equal in its impact before or since. In 1940, nylon stockings were brought into market throughout the US, and in New York City alone four million pairs were sold in a few hours. On the first day, 5 million pairs of the hosiery were sold out at $1.15_$1.35 a pair. “Nylon”, as it was called, freed the woman from the yoke of expensive, run-prone silk stockings. In the absence of Japanese silk during World War II , nylon was commandeered by the federal government to supply defense needs because it could be used to make parachutes, airplane-fire cord, combat clothing, netting and hammocks for the jungle, and life rafts. Du Pont now makes over $4 billion a year from nylon. This success created a new era that the pure academic study directly led to a commercial product. From then on, the enterprises began to attach importance to the academic researches.
Nowadays, nylon has become a part of our daily life as familiar to the world as wool, silk, wood, or steel. It is a thermoplastic silky material. Unlike the thermosetting resin of phenolic aldehyde where the polymer chains are tied in the crosslinked network, its movable linear molecules render the engineering processes applicable such as injection, extrusion, and cold-drawing. At meanwhile, Chapter 19 Wallace Carothers and the Development of Nylon contrast to the polyolefin such as polystyrene (PS), polyethylene (PE), polypropylene (PP), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), etc., the strong polar interaction between macromolecular chains in addition to the crystallization endows nylons excellent performance in the mechanical properties, accompanying with the most important property for modem society, that is, nylons are degradable by hydrolysis which is friendly to the environment. Numerous nylon products have been developed and their applications haven’t been just limited in the field of textiles, but expanded to various fields we can reach. In China, the first nylon product nylon 1010 was developed in 1958 and its large-scale production was launched in Shanghai Cellulose Plant in 1961. In addition, the name of nylon pronounced as “nilong” in Chinese pinyin is only referred to the polyamide plastic resins, but its fiber is usually called “Jinlun”,named after Jin-xi Chemical Plant with Liaoning Province where in China the first fiber of nylon 6 was born. By now, the annual production of two nylons alone, nylon 6 and nylon 66, is about 500 million tons over the world. Nylon 6 can be synthesized by ring-opening polymerization of caprolactam.
However, unfortunately, Wallace Carothers did not see the impact his invention would have on industry and everyday life over the world. “I find myself, even now, accepting incalculable benefits proffered out of sheer magnanimity and good will and failing to make even such trivial return as circumstances permit and human feeling and decency demand, out of obtuseness or fear or selfishness or mere indifference and complete lack of feeling,” wrote Carothers. Suffering from increasingly frequent attacks of depression, caused by the death of his sister and his conviction that he was a scientific failure, on April 29, 1937, two days after his 41st birthday, Carothers took his own life by drinking juice containing poison cyanide. He was survived by his widow and a daughter who was born after his death.
Carothers became a member of National Academy of Sciences in 1936. Today, nearly half of all the chemists around the world are employed in the industry sector related to the preparation, characterization, or applications of polymers.