The Nobel Prize in Chemistry 1974

Paul J. Flory

EARLY LIFE, EDUCATION, CAREER, AND FAMILY

Paul Flory was a warm and loyal friend to those people who, like he, had high standards of integrity and were honestly modest about their own accomplishments and potential. These friends in turn greatly admired Paul. On the other hand, Paul was not everyone's friend. Indeed, he was not reluctant to show his distain for those whose behavior suggested that they had exalted opinions of themselves, particularly if they were in dominating positions (e.g., administration) where they could influence the lives of others. Paul was a strong and vociferous champion of the oppressed in such situations and a fierce adversary of the offender.

Flory's puritanical principles could well have been derived from his background. The Flory family traces its roots back to Alsace, then England, later to Pennsylvania, and then to Ohio. Paul appeared to be especially proud of his Huguenot origin. His father, Ezra Flory, was a minister in the Church of the Brethren, a sect somewhat like the Quakers. The family moved frequently as he was appointed to different parishes. Ezra married Emma Brumbaugh, by whom he had two daughters, Margaret and Miriam. After Emma died in childbirth, Ezra married her cousin, Martha Brumbaugh, and they had two boys, James and Paul. The farmland outside of Dayton was given to the Florys by a Presidential grant and is still in the family.

Paul was rather frail as a child but was very precocious. He was always especially attached to his half-sister, Margaret, who was also his sixth-grade teacher. She recognized his potential, and was eager to have him further his education. As he matured Paul worked diligently on developing his physique through activities such as ditch digging, vigorous swimming, and mountain hiking. He became a strong man with great vitality, which he enjoyed for the better part of his life. He was always adamantly opposed to having regular physical checkups even when he began to be bothered by tiring while swimming not very long before he died of a massive heart attack.

Although it was during the Great Depression, Paul managed to attend Manchester College in Indiana, graduating in three years and supporting himself by various jobs. It was at Manchester that his interest in science, particularly chemistry, was inspired by Professor Carl W. Holl, who encouraged Paul to enter graduate school at Ohio State University in 1931. During the early period at Ohio State he helped to support himself by digging ditches and working in the Kelvinator factory, and he first pursued a master's program in organic chemistry under Professor Cecil E. Boord. In his second year, having decided to opt for physical chemistry, he became laboratory assistant to his dissertation adviser, Professor Herrick L. Johnston, whom Paul described as "having boundless zeal for scientific research which made a lasting impression on his students." On the other hand, a fellow graduate student of that time has recalled that Johnston and Flory "did not see eye to eye."

Paul was a restless person and hardly ever was satisfied with the status quo. He was always looking for better places or conditions where his scientific interests and those of his colleagues could flourish. After graduate school he joined DuPont in 1934 and four years later, in 1938, he left to join the Basic Research Laboratory at the University of Cincinnati. The urgency of the development of synthetic rubber provoked by World War II brought him back to industrial research at the Esso Laboratories of the Standard Oil Development Company (1940-43) and then in the Research Laboratory of the Goodyear Tire Company (1943-48). In 1948 he accepted a professorship at Cornell University, where he was fairly content for nine years. Then in 1957 he was lured to the Mellon Institute in Pittsburgh to establish a broad program of basic research. Under his direction this enterprise thrived for several years until top management began to lose interest in the project. In 1961 he accepted a professorship at Stanford University, where he remained until his death in 1985.

Paul enjoyed a rich family life. In 1936 he married Emily Catherine Tabor, who was strongly supportive of all of her husband's activities. They had three children: Susan, who is now the wife of George S. Springer, a professor in the Department of Aeronautics and Astronautics at Stanford University; Melinda, whose husband, Donald E. Groom, is professor of physics at the University of Utah; and Dr. Paul John Flory, Jr., research associate in the Department of Human Genetics at the Yale University School of Medicine. There are five grandchildren in the family: Elizabeth Springer, Mary Springer, Susanna Groom, Jeremy Groom, and Charles Groom.

SCIENTIFIC WORK

Commencing in 1934 Flory dealt with most of the major problems in the physical chemistry of polymeric substances, among them the kinetics and mechanism of polymerization, molar mass distribution, solution thermodynamics and hydrodynamics, melt viscosity, glass formation, crystallization, chain conformation, rubberlike elasticity, and liquid crystals. The restricted bibliography presented at the end of this memoir necessarily cannot convey fully the content of his more than 300 publications.

The special characteristics of Flory's work were well stated by his longtime friend and collaborator Thomas G. Fox. The secret of his success is unparalleled intuition for grasping the physical essentials of a problem, for visualizing a phenomenon in terms of simple models amenable to straightforward treatment and productive of results that are valid to the degree required by the original statement of the problem. Consequently, Flory's concepts and results are presented in a way that is instructive, understandable, and directly useful to the reader. This is equally true for those working in basic polymer science and those interested in industrial applications.

ACADEME I: CINCINNATI (1938-1940) While continuing to accumulate experimental results on linear systems, Flory turned his attention to polyesters containing an ingredient bearing three or more functional groups, so-called "three-dimensional" polymers, containing branched structures. One example of this type was already a well-known commercial product, glyptal (made from glycerol and phthalic anhydride), and it was well known that such systems attain a state of zero fluidity (the gel point) at a stage well short of complete reaction. Carothers had correctly concluded that this state indicated infinite molecular weight, with the chains forming a giant network; but he calculated from simple stoichiometry the number average molecular weight as the appropriate signal. In fact, the gel point is found to occur much earlier, when the number average molecular weight is still modest. Here Flory recognized that the branched polymers would have a size distribution much broader than that of linear polymers, and that the gel point corresponds to a diverging weight average molecular weight. In a series of three papers, characterized by mathematical sophistication far in advance of his previous work, he developed the quantitative theory of the gel point and of the entire molar mass distribution.

ESSO LABORATORIES (1940-1943) The onset of World War II greatly increased the urgency of development of synthetic rubber and convinced Flory to return to industry. He nevertheless managed to produce some very fundamental results in macromolecular physical chemistry. With John Rehner, Jr., he developed a useful model of rubber networks and its application to the swelling phenomenon. In polyisobutylene solutions he personally measured viscosities over a very wide range of molecular weights, far greater than any earlier examples, and showed their strict adherence to the Mark-Houwink-Sakurada law with a fractional exponent of 0.64. Doubtless his outstanding achievement of those years was the development of the famous Flory-Huggins, or "volume fraction," formula for the entropy of mixing of polymer solutions. (This result was obtained essentially simultaneously by Maurice L. Huggins in the United States and by A. J. Staverman in Nazi-occupied Holland.) This now classic formula plays a role analogous to that of the van der Waals equation of state for real gases, because although approximate, it conveys the essential physics and leads to reliable qualitative predictions. It remains the norm to which real behavior is customarily compared. He later extended his treatment to polymer solutions of arbitrary complexity.

GOODYEAR RESEARCH LABORATORY (1943-1948) In these years Flory's concerns with applied polymer science were at their height. He studied the tensile strength of elastomers in relation to network structural defects, and measured viscosities and glass temperatures of polymer melts. He also began work on the thermodynamics of polymer crystallization, a field that previously was not well defined. His theories predicted the dependence of the degree of crystallinity on temperature, molar mass, chain stiffness, chemical uniformity of the polymer, and elongation under a tensile force. From his equations one can determine the heat and entropy of fusion of the polymer and the thermodynamic interaction parameters with added diluent. In the spring of 1948 Flory was invited to Cornell University to deliver the George Fisher Baker Non-Resident Lectures, and he found the atmosphere in Ithaca so congenial that he readily accepted an offer to join the faculty there.

ACADEME II: CORNELL (1948-1957) During the Baker lectureship Flory had started to work on a major project that was finished only in 1953: the composition of his massive Principles of Polymer Chemistry (672 pages), which after almost half a century is still a greatly used text. No other single book has had such a great influence in an ever expanding field. Also first conceived during the Baker year, one of his greatest achievements was speedily completed: a viable theory of the so-called excluded volume effect, accounting for the fact that real chain molecules have effective lateral dimensions and therefore cannot intersect themselves, and that furthermore their atoms experience van der Waals interactions with their close neighbors whether these belong to the same chain or to surrounding molecules. Proceeding beyond earlier incomplete discussions by Werner Kuhn, by Huggins, and by Robert Simha, Flory's "mean field" theory is still in extensive use today. Except in special circumstances (see below) the net effect of the volume exclusion and other interactions does not vanish. In a good solvent, chain molecules experience a net perturbation that increases without limit as the chain is lengthened, and the numerical relation between molecular weight and effective radius (the root-mean-square radius of gyration measurable by light scattering) deviates from the square-root law that must hold for flexible chains if all the interactions could be ignored. Flory's theory leads to a limiting exponent of 3/5 relating radius to molecular weight, which is not very far from the value 0.5887 yielded by the best modern theories. Flory's result was not welcomed at the time by Debye and many other workers, for an "unperturbed" chain following the square-root law would precisely obey the laws of random flights already well understood in the theory of Brownian motion. However, he showed that very often there was a special temperature (called the "theta" temperature by Flory, but the "Flory temperature" by most others) at which the attractive and repulsive interactions would just cancel. This special state could be recognized (as in the analogous case of the Boyle temperature of an imperfect gas) by the vanishing of the osmotic second virial coefficient, also the subject of intensive study by Flory and Krigbaum. Flory next turned to an interpretation of polymer solution viscosity. Recognizing that the incomplete hydrodynamic shielding featured in the earlier theories of Kirkwood and of Debye could be neglected, he and Fox showed that the increase in viscosity produced by each chain molecule is proportional to the cube of its effective radius, as given by the excluded volume theory, and that the proportionality constant is essentially universal for all flexible chains in all solvents. There was thus made available an especially simple method for extracting from a vast body of existing data and information about chain conformations, which became one of Flory's major preoccupations for the rest of his career. Soon after the viscosity breakthrough Flory with coworkers Mandelkern and Scheraga produced a similar treatment of sedimentation velocity in the ultracentrifuge and showed that from both measurements taken together one could extract the molecular weight of the polymer. For some years this method was much used by biochemists, as it required less sample than the other methods available at that time. Another pioneering effort of the Cornell years was the production, during a sabbatical term in Manchester, United Kingdom, of a theory for the thermodynamic properties of stiff chains, which Flory put to further use many years later in his work on liquid crystals. Also, his Goodyear work on polymer crystallization was applied to the phase behavior of fibrous proteins.

MELLON INSTITUTE (1957-1961) Flory, having served on the Mellon Board of Trustees for several years, strongly urged the board to modify its long-standing program of industrial fellowships and to move heavily into basic research. The board's response was that Flory was just the man to lead this effort, and so he felt obliged to take up the offer, on condition that the institute's considerable financial resources would be firmly dedicated to this goal. After several years, however, the board had failed to follow through, and Flory decided to return to academic life.

ACADEME III: STANFORD (1961-1985)

The circumstances of Paul's move to Stanford are related by William S. Johnson in the next section. Continuing work started before the move and, with the special help of R. L. Jernigan and later Do Yoon, he developed powerful matrix methods for describing the conformations of chain molecules. He not only mastered the works of M. V. Volkenshtein (Soviet Union), K. Nagai (Japan), and S. Lifson (Israel) but also actually surpassed them and produced significant new results. These are embodied in his second book (1969), Statistical Mechanics of Chain Molecules, and applied to a great variety of polymers, including polypeptides and polynucleotides. Some examples are described in his 1974 Nobel lecture.

Flory also returned to one of his favorite topics: the thermodynamics of polymer solutions. The Flory-Huggins entropy was not abandoned, but much effort was expended on improving the details of the enthalpy of mixing. Compressibility and free-volume effects were introduced, called by Flory the "equation of state" terms. The treatment was also applied with considerable success to non-polymeric liquid mixtures.

Two other areas of earlier interest were also resumed. The theory of anistropic solutions, begun in his 1956 paper, was developed to deal also with mixtures of rigid and flexible chains. The theory of rubber networks, begun in 1943, has been greatly refined. An important source of information on the energetics of chain conformation is the temperature dependence of the elastic force in rubbery polymers, provided that the excluded volume effect can be neglected. Flory regarded this neglect as justified. In his own words: "Although a chain molecule in the bulk state interferes with itself, it has nothing to gain by expanding, for the decrease in interaction with itself would be compensated by increased interference with its neighbors." Many years after he made this statement, neutron-scattering studies at Grenoble and Julich confirmed it. By taking advantage of the big difference in neutron-scattering cross-sections between deuterium and hydrogen, it was directly shown that the mean dimensions of a number of different polymers in undiluted amorphous samples are identical to their "unperturbed" dimensions in dilute solution.

Questions concerning the morphology of semi-crystalline polymers have given rise to an extensive and thorny literature, and the principal matter at issue was not resolved during Flory's lifetime. When polymers crystallize from dilute solution in thin plates, single crystals can be observed, and it is found that the direction of the elongated chains is perpendicular to the lamellar plane. The chain length typically exceeds the lamellar thickness by a factor of 10 or more, so the chains must therefore fold back and forth many times. When polymers crystallize in the bulk, lamellar crystals also frequently form, and the question is whether the chains usually fold sharply at the crystal surface and reenter the lattice in an adjacent position, or whether they make larger loops in an amorphous region before finding reentry some distance away. This latter "telephone switchboard" model was strongly favored by Flory and Yoon, but the adjacent reentry model also had many strong and able supporters. It has turned out that an intermediate situation is needed to reconcile all the facts, with a figure of roughly 50 percent to 70 percent adjacent reentry taking place.

I was born on 19 June, 1910, in Sterling, Illinois, of Huguenot-German parentage, mine being the sixth generation native to America. My father was Ezra Flory, a clergyman-educator; my mother, nee Martha Brumbaugh, had been a schoolteacher. Both were descended from generations of farmers in the New World. They were the first of their families of record to have attended college.

My interest in science, and in chemistry in particular, was kindled by a remarkable teacher, Carl W. Holl, Professor of Chemistry at Manchester College, a liberal arts college in Indiana, where I graduated in 1931. With his encouragement, I entered the Graduate School of The Ohio State University where my interests turned to physical chemistry. Research for my dissertation was in the field of photochemistry and spectroscopy. It was carried out under the guidance of the late Professor Herrick L. Johnston whose boundless zeal for scientific research made a lasting impression on his students.

Upon completion of my Ph.D. in 1934, I joined the Central Research Department of the DuPont Company. There it was my good fortune to be assigned to the small group headed by Dr. Wallace H. Carothers, inventor of nylon and neoprene, and a scientist of extraordinary breadth and originality. It was through the association with him that I first became interested in exploration of the fundamentals of polymerization and polymeric substances. His conviction that polymers are valid objects of scientific inquiry proved contagious. The time was propitious, for the hypothesis that polymers are in fact covalently linked macromolecules had been established by the works of Staudinger and of Carothers only a few years earlier.

A year after the untimely death of Carothers, in 1937, I joined the Basic Science Research Laboratory of the University of Cincinnati for a period of two years. With the outbreak of World War II and the urgency of research and development on synthetic rubber, supply of which was imperiled, I returned to industry, first at the Esso (now Exxon) Laboratories of the Standard Oil Development Company (1940-43) and later at the Research Laboratory of the Goodyear Tire and Rubber Company (1943-48). Provision of opportunities for continuation of basic research by these two industrial laboratories to the limit that the severe pressures of the times would allow, and their liberal policies on publication, permitted continuation of the beginnings of a scientific career which might otherwise have been stifled by the exigencies of those difficult years.

In the Spring of 1948 it was my privilege to hold the George Fisher Baker Non-Resident Lectureship in Chemistry at Cornell University. The invitation on behalf of the Department of Chemistry had been tendered by the late Professor Peter J.W. Debye, then Chairman of that Department. The experience of this lectureship and the stimulating asociations with the Cornell faculty led me to accept, without hesitation, their offer of a professorship commencing in the Autumn of 1948. There followed a most productive and satisfying period of research and teaching "Principles of Polymer Chemistry," published by the Cornell University Press in 1953, was an outgrowth of the Baker Lectures.

It was during the Baker Lectureship that I perceived a way to treat the effect of excluded volume on the configuration of polymer chains. I had long suspected that the effect would be non-asymptotic with the length of the chain; that is, that the perturbation of the configuration by the exclusion of one segment of the chain from the space occupied by another would increase without limit as the chain is lengthened. The treatment of the effect by resort to a relatively simple "smoothed density" model confirmed this expectation and provided an expression relating the perturbation of the configuration to the chain length and the effective volume of a chain segment. It became apparent that the physical properties of dilute solutions of macromolecules could not be properly treated and comprehended without taking account of the perturbation of the macromolecule by these intramolecular interactions. The hydrodynamic theories of dilute polymer solutions developed a year or two earlier by Kirkwood and by Debye were therefore reinterpreted in light of the excluded volume effect. Agreement with a broad range of experimental information on viscosities, diffusion coefficients and sedimentation velocities was demonstrated soon thereafter.

Out of these developments came the formulation of the hydrodynamic constant called theta, and the recognition of the Theta point at which excluded volume interactions are neutralized. Criteria for experimental identification of the Theta point are easily applied. Ideal behavior of polymers, natural and synthetic, under Theta conditions has subsequently received abundant confirmation in many laboratories. These findings are most gratifying. More importantly, they provide the essential basis for rational interpretation of physical measurements on dilute polymer solutions, and hence for the quantitative characterization of macromolecules.

In 1957 my family and I moved to Pittsburgh where I undertook to establish a broad program of basic research at the Mellon Institute. The opportunity to achieve this objective having been subsequently withdrawn, I accepted a professorship in the Department of Chemistry at Stanford University in 1961. In 1966, I was appointed to the J.G. Jackson - C.J. Wood Professorship in Chemistry at Stanford.

The change in situation upon moving to Stanford afforded the opportunity to recast my research efforts in new directions. Two areas have dominated the interests of my co-workers and myself since 1961. The one concerns the spatial configuration of chain molecules and the treatment of their configuration-dependent properties by rigorous mathematical methods; the other constitutes a new approach to an old subject, namely, the thermodynamics of solutions.

Our investigations in the former area have proceeded from foundations laid by Professor M.V. Volkenstein and his collaborators in the Soviet Union, and were supplemented by major contributions of the late Professor Kazuo Nagai in Japan. Theory and methods in their present state of development permit realistic, quantitative correlations of the properties of chain molecules with their chemical constitution and structure. They have been applied to a wide variety of macromolecules, both natural and synthetic, including polypeptides and polynucleotides in the former category. The success of these efforts has been due in no small measure to the outstanding students and research fellows who have collaborated with me at Stanford during the past thirteen years. A book entitled "Statistical Mechanics of Chain Molecules", published in 1969, summarizes the development of the theory and its applications up to that date.

Mrs. Flory, the former Emily Catherine Tabor, and I were married in 1936. We have three children: Susan, wife of Professor George S. Springer of the Department of Mechanical Engineering at the University of Michigan; Melinda, wife of Professor Donald E. Groom of the Department of Physics at the University of Utah; and Dr. Paul John Flory, Jr., currently a post-doctoral Research Associate at the Medical Nobel Institute in Stockholm. We have four grandchildren: Elizabeth Springer, Mary Springer, Susanna Groom and Jeremy Groom.

Banquet Speech Paul J. Flory's speech at the Nobel Banquet, December 10, 1974 Your Majesty, Your Royal Highnesses, Ladies and Gentlemen, Acknowledgment of the privilege of receiving the Nobel Prize in words commensurate with the distinction it conveys overtaxes the resources of language. It must suffice to say that I am profoundly grateful to the Royal Swedish Academy of Sciences for their gracious decision in my favor. I take genuine pleasure in being afforded the opportunity to express my highest thanks to them and to the Nobel Foundation for this, the ultimate prize in science. Perhaps I may be permitted to reflect briefly on Alfred Nobel the man vis-?-vis the prizes that bear his name. Lest it seems presumptuous of me to comment on that great but little appreciated man, may I remind you that I too am a chemist. In fact, my researches have touched upon one of the principal ingredients of his epochal discoveries and inventions. I refer to nitrocellulose. To be sure, our interests in this substance differed: his of a scope leading to developments warranting world-wide fame, mine obscure by comparison. Be this as it may, nitrocellulose is a duly respected member of the family of macromolecules, and I take pride in laying claim to scientific kinship to Alfred Nobel through an interest in this substance, however tenuous the connection may be. The Nobel Prizes have gained universal recognition as pre-eminent symbols of the importance and significance of intellectual achievement. They are much better known than the man who founded them. Yet, that wise but modest man, whose extraordinary vision and perception were obscured by a self-effacing manner, would not be offended, I believe, by the contrast between his own fame in the world of 1974 and that of his prizes. He founded them from the purest of motives, not as a means of memorializing himself. His will does not suggest, much less require, that the prizes bear his name; this was a decision of his executors, a well reasoned one to be sure. Alfred Nobel appears to have been motivated by the conviction that science and learning should be encouraged and more widely appreciated. And so, on this splendid occasion, I am persuaded to pay tribute to Alfred Nobel, inventive genius, humanitarian and scholar, who had both the foresight and the magnanimity to commit his fortune for the encouragement of future generations to devote themselves to the cause of Peace, to the cultivation of science and to the enrichment of literature, endeavors which the burdens of his other responsibilities allowed him far too little time to pursue and enjoy. To this I should like to add a word in tribute to the executors of his estate and to the Nobel Foundation for implementing Alfred Nobel's intentions and desires with such remarkable success.

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