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Flory, Paul John

FLORY, PAUL JOHN

(b. Sterling, Illinois, 19 June 1910; d. Big Sur, California, 8 September 1985),

chemistry, physical chemistry of polymers, kinetics, thermodynamics.

Once polymer chemistry had been established in the 1920s and early 1930s by Hermann Staudinger, Hermann Mark, and Wallace Hume Carothers, there was a pressing need to establish the basic underlying physiochemical principles of this new field, especially the kinetics of polymerization and the thermodynamics of polymer solutions. A start had been made by Mark and the Swiss chemist Werner Kuhn, but the physical chemistry of polymers was still in its infancy in the mid-1930s. Paul Flory met this need almost single-handedly. He had an extraordinary knack for creating simple models that could be applied to existing data with remarkable effects. This was a result of his strong intuition for how polymers behaved. He continued to apply these models over many years to new problems. He also had a great talent for explaining the physical chemistry of polymers to colleagues and other chemists who often had a nonmathematical organic chemistry background. Moving constantly between academia and industry in the first half of his career, he also assisted the transfer of knowledge and especially concepts between the two sectors. This process was assisted by Flory’s association with the government-backed synthetic rubber research program in the 1940s and early 1950s, which promoted the free flow of information between different universities and firms. The net effect was the rapid development of the physical chemistry of polymers and its immediate application to industrial problems, thereby accelerating the development of plastics and synthetic rubbers, especially in the United States.

Childhood and Education . Paul John Flory was born on 19 June 1910 in Sterling, Illinois, the son of Ezra Flory, a minister in the Church of the Brethren, and Martha Flory (née Brumburgh), a former schoolteacher. The Florys were Alsatian Huguenots who had come to America from England. Sterling was a medium-sized town on the Rock River, between Chicago and the Mississippi River border with Iowa. Flory thus shared a rural midwestern background with several other American polymer chemists of his generation, including Wallace Carothers and Carl Marvel, both of whom were born within 100 miles of Sterling. Like the other members of this group, Flory took his first degree in 1931 at a small liberal arts college. In Flory’s case this was Manchester College in North Manchester, Indiana, run by the Church of the Brethren, where his interest in chemistry was aroused by Professor Carl Holl. During this time, Flory shared a dorm with Roy Plunkett, another midwestern polymer chemist (from rural Ohio), who later discovered Teflon at du Pont. Again in keeping with the other Midwesterners, Flory took his master’s degree in 1932 at a leading university, in his case Ohio State University in Columbus, Ohio. He then took a PhD under Professor Herrick Johnston with a dissertation on the photochemical dissociation of nitric oxide. When Flory completed his doctorate in 1934, during the Great Depression, he was fortunate to be offered a position at du Pont’s Experimental Station by the director of the organic chemistry section, Arthur Tanberg.

Polymerization Kinetics: First Work . When Flory joined Carothers’s group in July 1934, he had no background in polymer science, but his expertise in chemical kinetics was very valuable to Carothers, whose background was in organic chemistry. Carothers gave Flory the task of working out the relationship between the length of a polymer chain and the number of polymer molecules of that length for a condensation polymer such as a polyester. At this time it was taken for granted that the reactivity of a molecule decreased as it grew in size. Flory quickly showed that beyond a low threshold size (a few carbon atoms), the reactivity of the functional groups in a condensation polymer remained the same, regardless of the length of the chain, as long as the external conditions remained unchanged. Armed with this knowledge, he was then able to show that the number of chains of a given length decreased exponentially with increasing length.

The kinetics of addition polymerization had presented similar problems, and hitherto there had been little agreement between theory and the practical results. As with condensation polymerization, it had been assumed that the polymerization stopped when the molecule became too big to react. Flory showed that the polymerization stopped when the free radical at the end of a growing chain took an atom (usually hydrogen) from a monomer or solvent molecule (or even another chain), creating a new free radical that could start a new chain. The active site is thus transferred to another molecule, a process called “chain transfer.” Using the concept of chain transfer, Flory was able to achieve good agreement between theory and practice.

While he was working at du Pont, Flory met Emily Catherine Tabor, a member of a well-established Philadelphia family; they were married in late 1936. Their marriage was very successful, and Emily was a major force behind Flory’s burgeoning career. She enabled Flory to shake off his rural background and become a sophisticated international scientist. They had three children: Susan, Melinda, and Paul John Flory Jr. (known as John), who became a research scientist in genetics at the Yale University School of Medicine.

Elastic Polymers . In the space of a few years, the young chemist had revolutionized the study of polymerization kinetics at du Pont, but Carothers had in the meantime committed suicide in April 1937. Without Carothers to protect him from routine work, Flory decided to reenter the academic world, and he went to the basic science research laboratory at the University of Cincinnati in Ohio in 1938. Although the conditions there were rather rudimentary, Flory now had the freedom to develop his own ideas. He had become interested in the theory of gelation, the formation of an insoluble elastic material from a viscous polymer fluid, which occurs well before all the starting material has been used up. This process is particularly common with polymers, such as rubber or the glycerol polyesters (gyptals) used in paint, which undergo crosslinking between different chains. Flory developed a mathematical “network” theory which showed that gelation occurred at a specific “gel point.” He thus showed that gelation is mathematically similar to the kinetics of explosions or nuclear chain reactions. Carothers had been interested in the kinetics of gelation but had lacked the mathematical background to explore the process properly.

With war looming, Flory returned to industry in 1940 and joined the Standard Oil Development Company (part of Standard Oil of New Jersey, now Exxon Mobil) at Linden, New Jersey. Standard Oil, as the former partner of I.G. Farben in the polymer field, was playing a leading role in the development of synthetic rubber. Flory took part in this research and applied his network theory to the swelling of rubber. He developed a statistical-mechanical model of polymer solutions that explained their thermodynamic properties, and in particular, the entropy of mixing of polymer solutions. He was disconcerted to hear Maurice Huggins, a chemist at Eastman Kodak, expound a similar theory at a conference in the summer of 1941, but when Flory offered to collaborate, Huggins encouraged the younger chemist to publish his work independently.

The rubber firm Goodyear set up a fundamental research group at its Akron, Ohio, headquarters in 1943 and invited Flory to lead it. With Norman Rabjohn, he now applied his network theory to the physical properties of rubber, relating, for instance, the degree of crosslinking to the tensile strength. Flory also began his study of the crystallization of polymers and the factors that influenced this process, a line of research that was to lead him a decade later to the study of liquid crystals.

Principles of Polymer Chemistry . The American-Dutch chemist Peter Debye—who shared a mutual interest in the gelation of rubber—invited Flory to give the Baker Lectures at Cornell University in Ithaca, New York, in 1948. As a result of this series of lectures, Flory was invited to join the chemistry faculty at Cornell. The Baker Lectures were eventually published in 1953 as Principles of Polymer Chemistry, which for the first time—in the United States at least—established the idea that polymer science could be studied in terms of theory rather than as a body of empirical information. This monograph was read by leading polymer chemists and students alike, and it soon became the standard work in this field, above all in industrial laboratories.

While Flory was preparing his Baker Lectures, he took up the concept of “excluded volume” in polymer solutions, which had been originally introduced by the Swiss chemist Werner Kuhn in 1934. Flory and William Krigbaum applied this concept to a statistical mechanical theory of dilute polymer solutions to show that there was a temperature at which a given polymer behaved in an “ideal” manner, acting according to theory so that its behavior could be easily analyzed. This temperature became known as the theta point or Flory temperature. Flory then used the Flory-Krigbaum theory to study the viscosity of polymer solutions and showed that several supposed complications with such solutions were not in fact relevant. With Thomas Fox, who had followed him from Goodyear, Flory showed that the increase in viscosity produced by each chain molecule was proportional to the cube of its radius. As the ratio of the intrinsic viscosity to the hydrodynamic volume was found to be constant for a wide range of polymers and solvents, Flory was able to mine existing data to study the configuration of many polymers, a task that occupied him for much of the rest of his career. With his colleagues Leo Mandelkern and Harold Scheraga, he also applied this model to the sedimentation of polymers in the ultracentrifuge. Mandelkern also collaborated with Flory on the subject of crystalline polymers, and Scheraga shared his interest in chain conformation.

In 1953 the rubber chemist Geoffrey Gee, who had just been appointed to a chair at Manchester University, invited Flory to take his sabbatical at Manchester so they could work together. Gee had previously worked at the

British Rubber Producers’ Research Association (now the Malaysian Rubber Producers’ Research Association), outside Hertford, England, and the two chemists shared an interest in the physical chemistry of natural rubber and the thermodynamics of polymer solutions. They were good friends and, rather exceptionally, Flory took Gee’s comments on his research seriously. During this sabbatical, as a result of a personal discussion with Gee and Geoffrey Allen—then an assistant lecturer at Manchester—Flory became interested in the thermodynamics of stiff polymer chains. He later used this as a starting point for his work on the theory of liquid crystalline polymers, such as the aramid polymer Kevlar. Liquid crystals subsequently became important materials for electronic displays, but the materials used are not polymers.

The Mellon Institute and Stanford . In the mid-1950s the Mellon Institute in Pittsburgh—which had hitherto acted mainly as a research service for industry—decided to concentrate on basic and fundamental research. Paul Mellon and his board invited Flory (who was already a member of the board of trustees) to become its executive director in 1957. Flory’s collaborator, Thomas Fox, became manager of polymer research and stayed on after Flory departed for Stanford University four years later. During his period at the Mellon Institute, Flory published a seminal paper, “Theory of Light Scattering by Polymer Solutions” (1958), with Arthur Bueche, carried out considerable work on elasticity with Cor Hoeve and Alberto Ciferri, and continued his research on polyelectrolytes with Thomas Orofino. However, the reform program did not go according to the agreed plan, and Flory did not see eye to eye with Paul Mellon.

Flory was already looking for an academic position in 1961 when he was offered a chair at Stanford University in California. When Fred Terman, an electrical engineer, became provost of Stanford in 1955, he was determined to turn the hitherto intellectually undistinguished but wealthy college into the equal of Harvard or Yale. He hired William S. Johnson from the University of Wisconsin as professor of chemistry in 1960. When Johnson heard that Flory was planning to leave the Mellon Institute, he invited him to visit California. Flory accepted Johnson’s offer of a chair, and as a result other leading chemists, including Henry Taube and Eugene van Tame-len, also moved to Stanford. Flory subsequently became the Jackson-Wood Professor of Chemistry in 1966. The effect on the Chemistry Department was dramatic, and Stanford chemistry rose from fifteenth position in 1957 to fifth in 1964. As head of the growing department between 1969 and 1972, Flory was concerned about its lack of good laboratory facilities. A new chemistry building was eventually sanctioned by the university in 1974.

At Stanford, Flory continued his work on the configuration of polymer molecules and with Robert Jernigan developed matrix methods of describing these configurations. Flory summarized his research in this field in his classic work, Statistical Mechanics of Chain Molecules, in 1969. He also continued to improve the Flory-Huggins theory of mixing and his network theory of rubbers. In particular, he argued that the excluded volume effect could be ignored in the study of temperature dependence of elasticity. This simplification was controversial at the time, but it was eventually shown to be correct by the new technique of neutron scattering. This technique revealed that the size of the chains in bulk increased linearly with the chain length, consistent with a lack of excluded volume.

Later Years . Flory was awarded the Nobel Prize in 1974 for “his fundamental achievements, both theoretical and experimental, in the physical chemistry of macromolecules.” While this accolade was well deserved, the award of the prize to Flory alone could also be considered controversial, as he was only one of a number of chemists who had developed the physical chemistry of polymers. By 1974 Werner Kuhn and Peter Debye had died, but Hermann Mark and Maurice Huggins were still alive, and there were other chemists of Flory’s generation— including Eugene Guth and Walter Stockmayer—who had made major contributions to this field. With hindsight, it must surely be regretted that this prize for the physical chemistry of polymers was not awarded jointly to Mark.

After he retired in 1975, Flory remained an active researcher at Stanford and also became a consultant to IBM. He continued to work on liquid crystals using his model of rigid and flexible chains, and he was also interested in the structure of semicrystalline polymers. Polymers can crystallize from dilute solutions to form lamellar crystals, in which the polymer chains must be folded up. How the chains fold and reenter the crystal was a matter of controversy. Flory and Do Yoon argued that the chain formed a loop and reentered some distance away, but other polymer scientists—particularly the Anglo-Hungarian chemist Andrew Keller—insisted that the chains were folded sharply in a regular pattern with adjacent reentry. In the early twenty-first century it appears that the actual situation lies between these two extremes. Flory was still hard at work in these related fields when he died unexpectedly of a massive heart attack at his weekend home at Big Sur, California, on 8 September 1985.

Aiding Soviet Scientists . Flory was a hard-working and driven individual who was described by Johnson as “a human dynamo.” He expected similar high standards from those who worked with him but also filled his students with a passion for doing science. His rather puritanical personality was balanced by his sense of humor and his loyalty to his friends. Flory had a sympathy for the underdog, and after he won the Nobel Prize, Flory and his wife put much effort into supporting scientists persecuted by the authorities in the Soviet Union—notably Andrei Sakharov, Yuri Orlov, and Anatoly (Natan) Shcharansky (the so-called SOS dissidents). He was particularly critical of the neutral stance on this issue of the American Chemical Society and the National Academy of Sciences (United States). He persuaded some nine thousand scientists to boycott scientific cooperation with the Soviet Union. At a personal level, Flory traded his agreement to give lectures in Soviet bloc countries for exit visas for persecuted scientists, including Jan Bares and Witold Brostow. He also made an unsuccessful offer to become a temporary hostage to allow Sakharov’s wife, Yelena Bonner, to travel to the West for medical treatment.

BIBLIOGRAPHY

Selected Works of Paul J. Flory (cited below) includes a bibliography of Flory’s publications.

WORKS BY FLORY

Principles of Polymer Chemistry. Ithaca, NY: Cornell University Press, 1953.

With Arthur M. Bueche. “Theory of Light Scattering by Polymer Solutions.” Journal of Polymer Science 27 (1958): 219–229.

“Autobiography.” 1974. Available from http://nobelprize.org/nobel_prizes/chemistry/laureates/1974/flory-autobio.html.

Selected Works of Paul J. Flory. Edited by Leo Mandelkern, James E. Mark, Ulrich W. Suter, et al. 3 vols. Stanford, CA: Stanford University Press, 1985. This includes an authorized biography.

Statistical Mechanics of Chain Molecules. New York: Wiley-Interscience, 1969. Reprinted with corrections, 1989.

OTHER SOURCES

Chayut, Michael. “New Sites for Scientific Change: Paul Flory’s Initiation into Polymer Chemistry.” Historical Studies in the Physical and Biological Sciences 23 (1993): 193–218.

Johnson, William S., Walter H. Stockmayer, and Henry Taube. “Paul John Flory, June 19, 1910–September 8, 1985.” Biographical Memoirs of the National Academy of Sciences 82 (2003): 114–141.

Morris, Peter J. T. Polymer Pioneers. Philadelphia: Center for the History of Chemistry, 1986.

Seymour, Raymond B. “Paul J. Flory—Nobel Laureate and Polymer Scientist.” In Pioneers in Polymer Science, edited by Raymond B. Seymour. Dordrecht, Netherlands, and Boston: Kluwer, 1989.

Peter J. T. Morris

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