Fifty years of polymer research

Br. Polym. J. 1972,4, 465-489

Fifty Years of Polymer Research" Maurice L. Huggins

Arcudia Institute for Scientific Research, 135 Northridge Lane, Woodside, California, 94062, U.S.A.

(Paper received 6 September 1972 and accepted 17 November 1972)

The writer summarises his researches in the field of polymer science. He began with studies of the structures of space-network and sheet polymers in crystals. Currently he is working on the structures of macro-ions in silicate and borate crystals and glasses and on the development of a new theory of structures and thermodynamic properties of (polymer and other) solutions.

1. Introduction

I shall not attempt to cover the whole history of polymer research, but shall limit my discussion to a summary of my own relatively minor contributions to that history, beginning a little over 50 years ago. (Flory' has given an excellent summary of the early history of polymer science.)

2. Polymers in crystals

I think that I first thought about high polymers when, as an undergraduate student at the University of California in 1919, I learned about Emil Fischer's polypeptide syntheses and about the Braggs' determination2$ of the crystal structure of the diamond. In that substance (Figure 1) each carbon atom has four close neighbours tetrahedrally around it, suggesting chemical bonding. It follows that each crystal must be a single giant molecule. This structure was in agreement with G. N. Lewis' theory4$ of valence and molecular structure, which I was then applying to various theoretical problems. I studied other known crystal structures to see if Lewis' ideas, previously applied by him to small molecules and ions, could be used to account for the observed atomic arrangements. I found that the observed structures were indeed in agreement with the theory. Except for crystals in which the component units are obviously ions, the closest atoms are placed as they would be if held together by shared pairs of electrons. I concluded that crystals of many compounds, such as AISb,' Zn0,6 ZnS,6 AgL6 CuFeS2,R FeSZ6. * and the spinel^,^ are each, like the diamond, giant three-dimensional network macromolecules. (See Figure I .) From Lewis' theory,

"Based on a lecture presented before the Plastics and Polymer Group on 27 June 1972. Similar lectures were given at the Laboratory for Radiation Chemistry, Osaka, Japan, on 7 June 1972 and at The International Symposium on Macromolecular Chemistry, Helsinki, Finland, on 7 July 1972.

465 1

466 M. L. Huggh

0 *-

Figure 1. Arrangement of atoms and electronpair bonds in crystals of C (diamond), Sic, AISb, ZnS, and AgI. In the diamond, carbon atoms are in the positions represented by both filledandopencircles.

silicon dioxide (quartz) should have a structure in which each silicon has four oxygen neighbours (tetrahedrally) and each oxygen has two silicon neighbours (with the two 0-Si centrelines not colinear). Checking this prediction, I deduced1° a structure of this sort that fitted the Braggs’ X-ray datas, also later, more accurate, data. (See Figure 2.) Assuming SiOp molecular units, the Braggs had been unable to find any structure giving agreement.

Figure 2. Arrangement of atoms in the p-quartz (SOs) structure, projected onto a plane normal to the hexagonal axis. Filled circles represent oxygen atoms; open circles, silicon atoms. The a-quartz structures is very similar, but slightly less symmetrical.

Also in agreement with the published X-ray data and with my predictions from the Lewis theory, I showed that crystals of A S , ~ Sb,B Bi,O CdI,,” Mn(OH),,” and HgIala are composed of sheet polymer molecules (Figures 3 and 4) and that the “metallic” formsllp l9 of Se and Te are assemblages of helical linear polymers (Figure 5) . (The atomic arrangement in mercuric iodide was determined by X-ray diffraction analysis

Fifty years of polymer research 467

1 8

Figure 3. Arrangement of atoms and valence electronpairs (pairs of small filled circles) in a sheet polymer molecule in a crystal of As, Sb or Bi.

Figure 4. Arrangement of atomic centres and valence electronpairs in a sheet polymer molecule in a crystal of mercuric iodide, HgI,. Large open and filled circles represent Hg and I atoms, respectively.

Figure 5. Arrangement of atomic centres and chemical bonds in a portion of a crystal of metallic Se or Te. From Ref. 14.

by Magill and the writer.’* The atomic arrangements in the other substances listed were deduced and published by others.)

Applying my concept of hydrogen bonding to the structure of ice,ls I showedlB9 l7

that (if one defines a molecule in such a way as to include any assemblage in which all

468 M. L. Huggins

the atoms are held together by ordinary chemical bonds or hydrogen bonds) each ice crystal (or amorphous mass) consists of a single molecule. Similarly, an ammonium chloride crystal can be considered to be composed of two interpenetrating polymer rnole~ules~~. Cuprous oxidela and a form of p h o s p h o r u ~ ~ ~ also have structures in which two macromolecules interpenetrate each other. Certainly other examples exist or can be synthesised.

3. Conformation studies

At a meeting of the American Association for the Advancement of Science in 1922 I presented a paperao on “The Disposition of the Atoms in Molecules of Simple Ali- phatic Compounds”, in which I stated: “In spite of the so-called ‘principle of free rotation’ about a single bond, we might expect that the two parts of a molecule on opposite sides of a single linkage would tend to be oriented in some definite way with respect to each other.” Postulating that the hydrogen atoms in ethane repel each other weakly, I concluded that “the most stable orientation of the two halves of the ethane molecule would be that in which their hydrogen nuclei are farthest apart”-that is, the staggered conformation.

The same reasoning led to the conclusion that the larger normal alkane molecules and many of their derivatives would tend to assume a zigzag conformation. “Kinetic effects and the random distribution of the surrounding molecules should tend to produce other orientations within the molecules, in the gaseous and liquid states and in solution. The lower the temperature, however, the more effective should be the tendency” to exist in a zigzag arrangement, “and in the crystalline state . . . . we should expect the molecules of straight chain compounds” to have such an arrangement. I showed how this concept explained the known alternation of certain properties, such as melting point, molecular volume and solubilities.

The manuscript of this paper was never submitted for publication, because I learned of a paper by Pauly,21 with similar reasoning and conclusions, published the preceding year.

About 20 years later, applying the structural principleas that like atoms tend to be surrounded in like manner and making use of information from X-ray studies about the identity distances in the direction of the fibre axis, I concluded that, whereas in crystalline polyethylene the molecules are extended in a planar zigzag arrangement, in various other crystalline polymers (e.g. Sn,23 SenraS Ten,23 [CHzC(CHs)a1,,23 (CaH40)n,2s fibrous pr~teinsa~-~’) the chain atoms are arranged in helical fashion (Figure 6). The preferred structure is that which has the lowest Gibbs free energy, considering both intramolecular and intermolecular interactions. The polymer chains must each have a screw axis of symmetry or else two or more chains must be grouped around a screw axis. A fully extended zigzag polyethylene chain has a two-fold screw axis, but a fully extended zigzag chain of poly(oxymethy1ene) or polyisobutylene or stereo-regular polypeptide (other than polyglycine) has no such axis. Extended chains are less stable than helices for fibrous sulphur, metallic selenium and tellurium and poly(oxyethy1ene) because of interchain forces.

The chains in crystalline polyethylene may perhaps not be strictly planar extended


( 0 ) (b) (C)

Figure 6. Projections of certain helical structures. (a) Poly(~xymethylene),*~~ (CHIO)n (b) Metallic selenium or tellurium.a8. *@I ao (c) Fibrous sulphur.". (The number of a t o m per turn is probably only approximately 10/3, as

represented here.)

Figure 7. Projection of a portion of a polyethylene chain, assuming a fully extended planar zigzag structure. Each large circle represents a carbon atom; each small circle represents two hydrogen atoms.

zigzags but zigzag ribbons with a very slight twist. 32-34 The distance between hydrogen atoms in successive zigs of a planar zigzag is about 2.54 A (see Figure 7), which is less than the equilibrium distance for H . . . H interaction^,^^-^^ according to any of a variety of energy vs distance functions that have been proposed and used in recent years. See Figure 8 or a similar figure in Ref. 33. (One must of course except those functions that are based on the incorrect assumption that the equilibrium distance is twice the van der Waals radius, as deduced from interatomic distances in crystals. The H . . . H crystal distances are normally smaller than the true H . . . H equilibrium distances, because of the interactions-usually attractions-between atoms other than the two hydrogen atoms concerned.)

If the hydrogen atoms 2.54 A apart actually repel each other, a twist of the zigzag ribbon would reduce the energy and increase the stability. (Regularly twisted chains would give better interchain packing and so better stability than chains with random

470 M. L. Huggh

I 1 I I 2 5 3 0 3 5 4 .O

rH ,,(A)

Figure 8. Recently used energy-distance curves for two non-bonded hydrogen atoms. The letters indicate the scientists who have proposed the functions represented. For references and other details, see Refs. 34 and 35, from which this figure was copied.

directions of twist.) Twisting through a limited angle appears not to be in disagree- ment with the X-ray data. Twisting through a large total angle, however, would result in large interchain repulsions. As a result, chain folding or some other sudden break in the structure would be expected when the total angle of twist reaches some critical value (see Figure 9). I have suggested this as a possible explanation for the

\

(a) (b) ( C ) (d 1 (e)

Figure 9. Schematic representations of some hypothetical types of structural discontinuities. (From Ref. 33.)


chain folding that has been observed in polyethylene and other chain polymers. A spectroscopic argument has been presentedg6 against this idea. Although I do not believe the argument to be sound, I must admit that there is a serious flaw in the argument presented above : it is quite probable that the correct energy-distance relation varies with the orientation of the H . . . H vector relative to the C-H bond vectors. If so, the equilibrium distance for the hydrogen pairs in question may perhaps be less than the experimental distance of 2.54 A.

4. Stereoregularity

Atactic vinyl polymers, having an irregular distribution of steric orientations around their asymmetric carbon atoms, do not, in general, crystallise, even on stretching. If the sequence of orientations is regular, crystalline regions usually exist. In agreement with the principle that like groups tend to be surrounded in like manner, isotactic polymers, in which the steric orientation is the same for all mers, form helical structure^.^^ Many such structures have been deduced from X-ray diffraction measurements.

The difference in crystallisability between atactic and tactic polymers produces important differences in their properties-especially their physical properties. In 1944 I thus explained differences in properties of polystyrene prepared at different tempera- t u r e ~ ~ ' as resulting from differences in degree of stereoregularity.98 I proposed to my superiors in the company for which I then worked that research be initiated on the preparation and study of polymers differing in this respect, but my proposal was considered too theoretical and not worth following up. As all polymer scientists know, Natta and many others have done what I proposed, with very successful and commercially valuable results.

Dr A. L. Geddes and I collaborated in a study of the mechanism of Ziegler-Natta catalytic synthesis of stereoregular polymers. Based in part on evidence from Geddes' infrared studies, I proposed a structural mechanism that, for the special case of polymerisation at the surface of the TiC13 crystal with adsorbed R,AlCl, can be crudely represented by the two-dimensional equation

412 M. L. H&#

c1

CI-AIR, / I \

I CH, I

+CHR Cl

\I / +

/ I \ I Ti CH,-CHR-(CH,-CHR),-CH,-CHR

Cl-AIR,

+ Cl-AlRa--CH,--CH R--(CH,-CH R)m+ ,-CH,--CH R

/ I \ - c1

Ti AIR,

c1

\ I / \

/ I \ /

This theorySe has points in common with theories proposed by C o s ~ e e , ~ ~ Boor41 and others, but differs in some details. For example, in my theory the point of attach- ment of the growing polymer chain to the crystal surface shifts at each monomer addition.

It would be useful, in studying the problem experimentally, to be able to distinguish between chlorine (or carbon) atoms bridging between two metal atoms (e.g., Ti and Al) and such C1 (or C) atoms attached to only one metal atom. With this in mind, Sakurada, Anderson and I made a nuclear magnetic resonance studyq2 of solutions of a series of compounds

X X X

M M \ / \ /

/ \ / \ X X X

Firty years of polymer research 473

where M represents Al or Ti or both and X represents C1 or CH, or both.

solutions of such simple soluble compounds as For definitive studies of the polymerisation mechanism it should be helpful to use

R

R-Al I

CI

R

and

R R

if they can be prepared and if they will catalyse the polymerisation reaction. I hope someone will do this.

5. Proteins and polypeptides

Applying the structural principles already mentioned, plus the concept of strong N-H . . . 0 hydrogen bonding, to the problems of the structure of fibrous proteins, I concluded, in agreement with the experimental data by Astbury and others, that two nearly extended structures were theoretically possible for silk and P-keratin. Both are sheet structures. In 2 7 9 42 adjacent chains within each sheet run in opposite directions (Figure 10). In the 2 7 adjacent chains run in the same direction (Figure 11). Some distortions from the simple structures suggested by the bond diagrams in these figures are required (especially for the structure of Figure 11), in order to give stable bond lengths and interbond angles. The Figure 11 structure is essentially like the “pleated sheet” structure “discovered” by Pauling and Corey4, some eight years later.

414 M. L. Huggins

Figure 10. Bonding and hydrogen bonding in a sheet of extended polypeptide molecules, with adjacent chains running in opposite directions.*'

H-N H-N H-N .." \ ..." \ _..' \

Figure 11. Bonding and hydrogen bonding in a sheet of extended polypeptide molecules, with all chains running in the same direction."

For the less extended a-keratin, I concluded that the molecular structure should be helical, with NHO hydrogen bonds connecting successive turns of the

44* 45* 4o Several different helices seemed reasonable, but the meagre X-ray data then available were insufscient to enable one to decide which was correct. One helix, considered by mea4 and also by H. S. TaylorY4' is pictured in Figure 12. One difEculty was the problem of accounting for the strong 5.2 A reflection. I then thought it had to be accounted for by the structure of a single helix. Now I believe it to depend both on the helix structure and on the manner in which the helices are stacked. X-ray data from synthetic polypeptides were not then available. Many years later, when they did become available, Pauling and core^^^ showed that one of the class of helical structures that I had considered gave agreement with these data. They called this helix "the alpha helix". It is likely that this helix type occurs also in a-


Figure 12. A helical structure considered for a-keratin by Hugginsa4 and by Taylor". There are approximately three amino acid residues per turn of the spiral.

keratin, but otherwise the structures they for this substance are doubtless wrong and their explanation for the strong 5.2 A reflection is also incorrect51.

a-Keratin and other fibrous proteins give a number of relatively long X-ray spacings that are not attributable to the geometry of single chains, with all residues treated as alike. Some of the long spacings doubtless result from the sequences of residue types in the protein molecule. Others may result from the packing of adjacent chains to give the close-neighbour contacts that are energetically most stable.s1 The mode of chain packing pictured in Figure 13, based on this idea, accounts well for the five strongest long equatorial reflections. Although more recent experimental data seem to indicate that the chain packing pattern is not of this type, this figure illustrates some

Figure 13. A schematic representation of a method of packing helical polypeptide chains that will account for the main long equatorial spacings in a-keratin."

structural principles that I believe to be important. 6s now also appears to be

incorrect. I have begun a more detailed study of the theory of packing helical copoly- mer chains, hoping that it will contribute to the solving of keratin, collagen and other complex structures.

Another factor that may be responsible for many of the large meridional spacings in fibrous proteins is the f o l l o ~ i n g . ~ ~ ~ ~ Except fortuitously, the helical structure best satisfying the intramolecular structure is one in which the orientation (in a plane

A structure that I have suggested for collagena7*

476 M. L. Huggia9

normal to the chain axis) of residues of a given type gradually shifts relative to the directions to the axes of neighbouring helices (see Figure 14). As a result, an inter- molecularly unstable situation may eventually be reached. There should then be a

Figure 14. Illustrating changing intermolecular stability as the orientation of specific groups within each helical molecule changes. Left: Bulky groups that are most stably oriented toward “holes” at the centres of triplets (or larger groups). Right: Crosslinking groups that, for stability, must be oriented so as to permit bonding with similar groups in neighbouring helices. (From Ref. 34.)

break of some kind in the regular structure pattern. There might be chain folding, as discussed for polyethylene, a kink, a jog, a deviation from the otherwise regular sequence of residues, etc. (see Fig. 9). Although this is speculation, the logic on which it is based seems reasonable. New X-ray data from collagen have been interpreted to indicate a kinked structure for that substan~e.~*

Someone should check experimentally the structural principles just discussed, using appropriate synthetic polypeptides or other regular copolymers.

The deduction of nucleic acid structures came after I had left the study of the structures of proteins and related substances. It should be noted, however, that the nucleic acid structures conform to the structural principles (helical chains, hydrogen bonding, etc.) that I had used in my earlier theorising.

6. Hydrogen bonding within and between polymer chains

Hydrogen bonding between and within molecules, including polymer molecules, has had a special interest for me. The proteins I have just discussed are good examples. From my first thoughts about cellulose and starch I have been sure that hydrogen bonding in these hydroxyl-rich substances plays a very important role with regard to their structures and properties. I have proposed6s that in cellulose hydrogen bonding occurs between each ring oxygen and the nearest hydroxyl oxygen in the next ring and that this hydrogen bonding is largely responsible for the zigzag structure of the chain of rings in each molecule. This intramolecular hydrogen bonding has been confirmed.S6 There may also be a second hydrogen bond between each pair of adjacent rings; if so, there must be departures from the usually assumed 2-fold screw axis.

From theoretical considerations, reinforced by studies of known structures, I have concluded that O-H . . . 0 hydrogen bonds between hydroxyl groups are strongest if each oxygen atom is a bridgehead for two hydrogen bonds% 67

I I I

. . . O-H.. . O-H.. . O-H.. .


Figure 15. Suggested pattern of bonds and hydrogen bonds (dotted) in the V modification of amylose starch. Imagine this plane pattern to be bent around a cylinder to give a single continuous helical chain. (From Ref. 58.)

\ I \

O = i r y-H 0-5 1-H O=/ r y - H

o=l rH-/ y-H o=c o=l r y-H

H-Y C=O H-Y

C=O H-N \

H-N \

Figure 16. Hydrogen bonding in nylon 6 (polycaprolactam). From structure analysis by Holmes, Bunn and Smith.6g

478 M. L. Huggills

Figure 17. Hydrogen bonding in nylon 66. From structure analysis by Bunn and Gamer.Bo

Exemplifying this, I have 68 the distribution of ordinary bonds and hydrogen bonds shown in Figure 15 for the helical V form of amylose starch.

Intermolecular N-H . . . 0 hydrogen bonds occur in the nylons.m-Bo The bond and hydrogen bond patterns (Figures 16 and 17) are very similar to those in the extended zigzag forms of polypeptides (Figure 10 and 11). It would be interesting to see how the structures and properties would change if the

H H H H I I I I I I I I

I I I I I I I I

-c-c-c-c- . . . H H H H

H H H H chains were replaced by

4-c-c-c- . . . R R R R

or


H H H H I l l 1 -c-c-c-c- . . . I I I I H R H R

or other chains, with the same steric orientation at each asymmetric chain atom. One would expect the extended zigzags to be replaced by helices. I have neither read nor heard of any researches on such polymers.

7. Condensation polymers

In 1923 I read a papera1 reporting several condensations, of which that of benzidine

with terephthalaldehyde,

is typical. Chemical analysis of the products conformed to the assumption of rings, e.g.

Assuming such ring structures, the authors claimed that their results confirmed the Kaufler formula for diphenyl, according to which the planes of the two benzene rings are parallel.

The molecular weights of the products were not determined. These products were all insoluble in the ordinary organic solvents and melted, with decomposition, at unexpectedly high temperatures. It seemed to me that these properties indicated that the substances were polymers (but not necessarily linear polymers). When I suggested this to Roger Adams, the senior author, he merely said that it seemedsimplest to assume rings.

I then looked up, in Beilstein's Handbuch, a large number of syntheses of supposed ring compounds, produced by condensation reactions between bifunctional molecules.

480 M. L. Hugslns

With few exceptions, the products were insoluble in the solvents tried and decomposed on or before melting. Molecular weights were not determined. I decided that the usual products were polymers. (Linear polymers produced in this way I called “string molecules”.)

Because of pressure of other activities and because I kept hoping that I would have a chance to follow up these ideas experimentally, I did not report this conclusion until 1930. Then, after I presented a paper on the subject at a meeting of the American Chemical Society, someone informed me that Carothers had recently made products of this sort. (Carothers had been a student of Roger Adams. I never learned whether my discussion with Professor Adams was in any way connected with Carothers’ later interest in condensation polymers.) I think that I then first learned of Staudinger’s work in the polymer field.

8. Silicones and coordination polymers

In 1936, when I joined the research staff of the Eastman Kodak Company, it appeared at first as if I might start synthetic research in the polymer field. Because condensation polymers were being actively studied by Carothers and others at the duPont Company, I preferred to avoid them. Two new types of polymers appeared to me especially promising. In one type, the polymer molecules consist of chains having silicon and oxygen atoms alternating. These we now know as polymeric silicones. I did not learn until much later that Dr J. F. Hyde of Corning Glass Works was starting work in this field at about the same time.”*

The other type of special interest to me was that in which metal atoms are connected together through bifunctional organic ligands : coordination polymers. (I had been especially interested in coordination complexes since my early r e s e a r c h e ~ ~ ~ ~ 84 on extensions of the Lewis theory of atomic and molecular structure.) I synthesised a few rubbery coordination polymers and tried to get experimental help for continuing and expanding this work and that on silicone polymers. This was in the depression and I was unable to obtain such help. I repeatedly tried, unsuccessfully, to interest others at Kodak in this field, but everyone seemed more interested in following up his own ideas. As a result, I concentrated on theoretical problems.

Much later I returned, in a limited way, to the study of coordination polymers. I had concluded that certain types of resonating ions, such as the dihydroxyquinonate ion

0-

should, by double chelation, bond especially strongly to two appropriate complex- forming metal ions. If the metal ions have charges of + 2 units and coordination numbers of 4, rodlike coordination polymers should result. Oligomeric ring products would be impossible.

I suggested to a Japanese crystallographer friend, Professor Y. Saito, that the preparation and crystallographic study of polymeric compounds of this sort would be


interesting. One of his students, S. Kanda, made somee5 and, from X-ray and other evidence, concluded that they were indeed rodlike Later, Dr Eding and other colleagues of mine at Stanford Research Institute made others.s7 We were then more interested, however, in obtaining sheetlike dihydroxyquinonate polymers with a “chicken wire” structure-having hexagonal holes of uniform size-by using trivalent metal ions with coordination number six. In the time available to us on this project,eB we were unsuccessful in this aim. We did obtain amorphous, highly porous, semi- conducting, three-dimensional coordination polymer^.^^^ 70 Further study of the compositions, structures, properties, and uses of polymers of this class will, I predict, prove both interesting and rewarding.

9. Solution viscosities

Returning now to the late 1930’s, I admired Staudinger’s fine researches in the polymer field,71 but I could not accept his claim that the “Staudinger Law”-propor- tionality between intrinsic viscosity and molecular weight-could be explained on the assumption of rodlike polymer molecules. From my 1922 speculations20 about the conformations of simple chain molecules, I believed that most chain molecules in solution (and in the pure liquids and amorphous solids) are kinked in a very random way. This belief was strengthened by the work of Meyer, Mark, Kuhn and others in the early 1 9 3 0 ’ ~ . ~ * ~ ~

K ~ h n ~ ~ had dealt theoretically with the viscosity of solutions in which each solute molecule consists of a rectilinear array of spherical segments. In a solution in which there is a velocity gradient, nonspherical molecules will rotate. Assuming free draining (i.e. that the flow of solvent past each segment is unaffected by the presence of other segments in the vicinity), the frictional energy loss due to the relative motions of solvent molecules and solute segments, summed over all segments and over all solute orientations, gives the total energy loss. This is simply related to the viscosity. Replac- ing Kuhn’s rodlike molecule by a randomly kinked chain of segments, I Staudinger’s relation. This is equivalent to the empirical Mark-Houwink equation,s09

[q] = KMa (1)

with the exponent equal to unity. For rodlike solutes, Kuhn’s theory led to an exponent of 2. Einsteins2 had previously deduced an equation, equivalent to this equation with the exponent zero (hence no dependence of intrinsic viscosity on molecular weight) for large spherical solutes.

My theory also related the coefficient K to molecular properties and it furnished a basis for estimating departures from the simple relation at low molecular weights.

Especially for long chain solutes (high molecular weights), the actual lack of free draining should be taken into account. I have not dealt quantitatively with this effect, but others have done so.

Any factor that decreases the average extension of the solute molecules below that for purely random kinking decreases the magnitude of the exponent a, and any factor increasing the average extension increases a. Thus, in a polyelectrolyte solution the 2

482 M. L. Hugglns

intramolecular repulsions between like charges extend the solute molecules and so increase the viscosity over what it would be without the charges. If ionic charges of both sign are distributed along a polymer chain in equal numbers, as in a poly- ampholyte solution at the isoelectric point, the chain tends to coil more tightly than if it were uncharged, because of the attractions between oppositely charged groups ; this decreases a. I have used these concepts to explain the viscosity behaviour of gelatin solutions as dependent on pH.*',

The intrinsic viscosity is defined as the fractional increase of the viscosity of the solution over that of the solvent, divided by the concentration, extrapolated to infinite dilution :

[TI = (-) 77 - 770

77oc o = o

According to my theory,84

or

. . (3)

where k' is a constant (sometimes called "the Huggins constant") that depends on the geometry of the solvent and polymer molecules and their energies of mutual interaction. It is approximately independent of the degree of polymerisation, but varies with branching, temperature and other factors.

Figure 18 illustrates the agreement obtained with experimental data for typical polymer solutions.

28

2 6

2 4 - b0 " & 22 - 8 2 0

1 8

1 6

14

IOOC

Figure 18. Viscosity data (Philipp and Bjorks6) for cellulose acetate solutions in show agreement with the theoretical equation (4). (From Ref. 27.)

acetone, plotted to


In the 19303, M e ~ e r , ~ ~ Guth and Mark,86 Kuhne7 and others presented good evidence for the thesis that rubberlike elasticity is essentially an entropy phenomenon, related to the change in randomness of location of the rubber segments when the material is extended. On this basis they derived relationships between the initial elastic modulus, the degree of crosslinking and other molecular constants and the temperature. Many others have subsequently dealt with these relations.

It seemed to me, nevertheless, that it should be interesting and useful to relate the observed properties more closely to the appropriately summed energy, entropy and location changes of the shifting structural units in the material. Using a very simple hypothetical model, I derived a 8 B showing, for example, how the stress- strain curves are affected by the shape of the energy curve for the elementary re- arrangement process (see Figures 19 and 20). The treatment was very crude and I am sure that (if I could find the time) I could now improve on it and also extend it to make it applicable also to viscoelastic phenomena.

Figure 19. Energy curve for two orientations of a rearrangeable system. (From Ref. 88.)

10. Thermodynamic properties of solutions

The thermodynamic properties of solutions (their vapour pressures, osmotic pressures, freezing and boiling points, solubilities, etc.) are of course important. Until 1941, however, there was no theory of these properties that was even approximately applicable to solutions of polymers and other flexible long chain molecules. I believed that such a theory was possible and, using a simplified model, I derived one that has had a considerable amount of success-even though, as originally derived, it was strictly applicable only to very dilute solutions.

After I had published a notee0 about my theory and described it at a Colloid Sym- p o s i ~ r n , ~ ~ Flory reportede2, B 3 that he had independently arrived by a similar treatment at results that are equivalent at the infinite dilution limit. Since our derivations were practically contemporaneous, both of our names are customarily attached to this theory and to the fundamental equations derived therefrom.

the entropy of mixing solvent and solute molecules was calculated from the randomness of placing solvent molecules and polymer segments

In this theory27,

M. L. Huggins 484

I I I I I I 1 2

7

Figure 20. Theoretical stress-strain curves, with (dashed) an experimental curve for a typical rubber. c = AE/kT. (From Ref. 88.)

successively at the points of a hypothetical lattice. This randomness was assumed to vary with the number of segments per chain molecule and with concentration in the same way as the randomness of placement of the actual component molecules and segments in the solution.

The equation so obtained provides a useful relation between such experimental thermodynamic properties as osmotic pressure, vapour pressure and light scattering, extrapolated to infinite dilution and the average molecular weight of the polymeric solute (see Figures 21 and 22).

X

i 1

0 3

0 2

0.1

I 0.1 0 2 0 3 0 4 0 5 0.6 0.7 0 6 0.9 10

x2

Figure 21. Theoretical curves for solvent activity (a3 and solute activity (ae) as functions of mole fractions (xl, xz) for various molecular volume ratios (1,2,10,100, IOOO), for a polymer solution with X = 0. (From Ref. 98.)

Fifty years of polymer research

c (g/ccl

Figure 22. Dependence of osmotic pressure on concentration for solutions of gutta percha in carbon tetrachloride (0) and in toluene (0). [Data points by Staudinger and Fischer.O6 (From Refs. 27 and 97.)

The initial concentration dependence of these properties can be related to an interaction parametere8 (X), which is the sum of an enthalpy component (xh), related to the heat of mixing, and an entropy component (XJ, related (in my original theory) to the coordination number of the hypothetical lattice. Some researchers have tried to relate the lattice coordination number (and so X,) to molecular and segment sizes, neglecting various other factors and approximations in the theoretical development that also affect X,. To avoid such errors, I rederived the entropy of mixing, considering only molecular and segment volumes, without reference to a latti~e.~'9

Properties related to distribution of the components between two phases and processes, such as fractionation, depending on these properties are very sensitive to the magnitude of X and its concentration dependence. A theoretical treatmentloo of the equilibrium between two liquid phases, both polymer solutions, shows this to be the case. For the special case of constant X, this treatment yields quantitative equations for the dependence of fractionation efficiency on the overall concentration, the choice of solvent, the temperature, etc. For a given polymer-solvent system (with constant X) one can thus calculate the optimum conditions for most efficient fractionation.

e 7 9 lol aiming to deal better with the dependence of X on concentration and temperature and to relate it more closely to the composition and properties of the component molecules. The equations allowing for all of the factors that are sometimes important are quite complex and, chiefly because of the number of parameters involved, they are not easily applied to experimental data on actual solutions.

Modifying my solution theory to make it applicable to two dimensional systems, with solvent molecules replaced by vacant sites, I have derived and tested equations for the equilibrium properties of chain molecule monolayers on liquid surfaces.lo2

Partly because of the complexity of my earlier treatments of thermodynamic properties of nondilute polymer solutions and partly in the hope of being able to

I have revised my solution theory in various

486 M. L. Huggins

determine the necessary parameters for polymer solutions from measurements on simple mixtures of compounds of low molecular weight, I have recently been develop- ing a new, relatively simple, approach.log-llS It is applicable to, but not limited to, polymer solutions. The agreement with experiment so far obtained has been very encouraging, but much more remains to be done.

Consider, for example, systems (solutions) containing only two kinds of chemical units: “segments”. I assume lo4 the interaction energy to be the sum of the areas of intersegment contact of the three types of contact, each multiplied by the average contact energy per unit area for that type. I assume that the relative areas of contact for the three types are governed by an equilibrium constant. This minimises the total Gibbs free energy of interaction.

I assume107 the excess entropy of the mixture to be the sum of (i) a correction to the ideal solution entropy to allow for molecular size differences, computed from the Flory-Huggins equation; (ii) a correction for departures from perfect randomness of mixing, computed from the equilibrium constant (or constants) ; and (iii) a contribution due to the concentration dependence of the vibrational, rotational and orientational entropy (including, for polymers, the entropy related to chain flexibility).

The results obtained for polymer solutions are exemplified by Figure 23, for the polyisobutylene + benzene system. The details have been published elsewhere.l1° Here I shall only point out that the theory gives excellent agreement with the experimental X values over more than half of the concentration range. The minor disagree- ment at very high polymer concentrations can be attributed either to experimental

0 0.2 0.4 0.6 0.8 1

8 2 Figure 23. The interaction parameter, x, and its components, for polyisobutybne + benzene solutions. The curves are theoretical. (From Ref. 110.)


difficulties or (more likely) to inaccuracies of some of the simplifying assumptions of the theory.

11. Macroions in inorganic crystals and glasses

Mention should be made of my theoretical research in a quite different field. I have long been interested in the principles determining the structures and properties of crystals and glasses. I have concentrated especially on the types and relative numbers (of each type) of “structons” (close neighbour arrangements), as dependent on the overall composition. Recently I have been concerned especially with silicates and borates, most of which contain giant macro ion^."^-'^^ (In general, these are not linear macroions.) One result of these studies is that, for large ranges of overall composition of a silicate or borate glass, one can now predict the types of structon present and the number of each type. Since many bulk properties are closely additive functions of the structon properties, this result should be of considerable use. I am extending the application of the structon theory to other classes of crystals and glasses.

My research in this area is closely related to my new research on polymer solution theory. In both fields I concentrate on close-neighbour structures and relationships. Sometime in the not-distant future I hope to show explicitly how my theories in these two fields are related.

12. Conclusion

Although I have dealt here almost entirely with aspects of polymer science with which I personally have been concerned, I hope that this admittedly biased account is of interest to others. I also hope that this paper will contribute to a more general appreci- ation of the fact that the great development of polymer industry in the past 50 years is attributable, in large part, to fundamental theoretical work by polymer scientists. I predict that this dependence of industrial progress on theoretical progress will continue. There is nothing more practical than a good theory.

References 1. Flory, P. J. Principles of Polymer Chemistry. Cornell University Press, Ithaca, N.Y. 1953,

Chapter I. 2. Bragg, W. H.; Bragg, W. L. Proc. R. SOC. 1913, A89,277. 3. Bragg, W. H.; Bragg, W. L. X-Rays and Crystal Structure. Bell, London. 1916. 4. Lewis, G. N. J. Am. chem. SOC. 1916,38,762. 5 . Lewis, G. N. Valence and the Structure of Atoms and Molecules. Chemical Catalog Co., New

York. 1923. 6. Huggins, M. L. J. Am. chem. SOC. 1922,44,1841. 7. Huggins, M. L. Phys. Rev. 1926,27,286. 8. Huggins, M. L. Phys. Rev. 1922, 19,369. 9. Huggins, M. L. Phys. Rev. 1923,21, 509.

10. Huggins, M. L. Phys. Rev. 1922.19,363. 11. Huggins, M. L. J. chem. Educ. 1927,4,73. 12. Huggins, M. L.; Magill, P. L. J. Am. chem. SOC. 1927,49,2357. 13. Huggins, M. L. Phys. Rev. 1926,28,1086. 14. Ewald, P. P. ; Hermann, C. Strukturbericht, 1913-1928. Akademische Verlagsgesellschaft,

Leipzig. 1931.

488 M. L. Huggins

15. Bragg, W. H. Proc. phys. SOC. Lond. 1922,34,98. 16. Huggins, M. L. J. phys. Chem., Ithaca 1936,40,723. 17. Huggins, M. L. Angew. Chem. 1971,83, 163; Intern. Ed. 1971,10, 147. 18. Huggins, M. L. Chem. Rev. 1932,10,427. 19. Thurn, H.; Krebs, H. Acta crystallogr. 1969, B25, 125. 20. Huggins, M. L. Science, N. Y. 1923,57, 246. 21. Pauly, H. Z. anorg. allg. Chem. 1921,119,271. 22. Huggins, M. L. J. phys. Chem., Ithaca 1931,35,1270. 23. Huggins, M. L. J. chem. Phys. 1945,13, 37. 24. Huggins, M. L. Chem. Rev. 1943,32,195. 25. Huggins, M. L. J. Am. chem. SOC. 1952,74,3963. 26. Huggins, M. L. J. Polym. Sci. 1958.29, 5. 27. Huggins, M. L. Physical Chemistry of High Polymers. Wiley, New York. 1958. 28. Tadokoro, H.; Yasumoto, T.; Murahashi, S.; Nitta, I. J. Polym. Sci. 1960,44,266. 29. Bradley, A. J. Phil. Mag., 1924, 48, 77. 30. Slattery, M. K. Phys. Rev. 1925, 25,333. 31. Prins, J. A.; Tuinstra, F. Physica, 's Grav. 1963,29, 328. 32. Huggins, M. L. J. Polym. Sci. 1961, So, 65. 33. Huggins, M. L. Makromolek. Chem. 1966,92,260. 34. Huggins, M. L. Pure appl. Chem. 1967, 15,369. 35. Huggins, M. L. In Structural Chemistry and Molecular Biology. (A. Rich; N. Davidson eds.),

Freeman, San Francisco and London. 1968 p. 761. 36. Krimm, S.; Tasumi, M.; Opaskar, C. G. J. Polym. Sci. Part B, 1967.5, 105. 37. Alfrey, T.; Bartovia, A.; Mark, H. J. Am. chem. SOC. 1943, 65,2319. 38. Huggins, M. L. J. Am. chem. SOC. 1944,66,1991. 39. Huggins, M. L. J. Polym. Sci. 1960,43,473. 40. Cossee, P. J. Catal. 1964,3,80. 41. Boor, J. Ind. Engng. Chem. Prod. Res. Dev. 1970,9,437. 42. Huggins, M. L. J. org. Chem. 1936, 1,407. 43. Pauling, L.; Corey, R. B. Proc. natn. Acad. Sci. U.S.A. 1951,37, 251. 44. Huggins, M. L. A. Rev. Biochem. 1942, 11, 27. 45. Huggins, M. L. J. Polym. Sci. 1958,30, 5. 46. Huggins, M. L. J. Am. chem. Soc. 1952.74, 3963. 47. Taylor, H. S. Proc. Am. Phil. SOC. 1941,85,1. 48. Pauling, L.; Corey, R. B. J. Am. chem. SOC. 1950,72,5349; Proc. natn. Acad. Sci. 1951,37,241. 49. Pauling, L. ; Corey, R. B. Proc. natn. Acad. Sci. 1951,37, 261. 50. Pauling, L. ; Corey, R. B. Nature, Lond. 1953, 171, 59. 51. Huggins, M. L. Proc. natn. Acad. Sci. 1957,43, 204. 52. Huggins, M. L. Proc. natn. Acad. Sci. 1957,43,209. 53. Huggins, M L. In Collagen (N. Ramanathan, ed.), Interscience, New York. 1962, p. 79. 54. Nemetschek, T. ; Hosemann, R. Private communication. 55. Huggins, M. L. J. appl. Phys. 1939, 10, 700. 56. Tsnnesen, B. A.; Ellefsen, 0. In Cellulose and Cellulose Derivatives. (N. Bikales; L. S e d , eds.)

Vol. 5, Part 4. Wiley, New York. 1971. 57. Huggins, M. L. Nature, bnd . 1937,139,550. 58. Huggins, M. L. J. chem. Educ. 1957,34,480. 59. Holmes, D. R.; Bunn, C. W.; Smith, D. J. J. Polym. Sci., 1955, 17, 159. 60. Bunn, C. W.; Garner, E. V. Proc. R. SOC. 1947, A189, 39. 61. Adams, R. ; Bullock, J. E.; Wilson, W. C. J. Amer. chem. SOC. 1922, 44, 521. 62. Anon. Chem. Engng News, 1 March 1971, p. 20. 63. Huggins, M. L. Science, N. Y. 1922,55,459. 64. Huggins, M. L. J. phys. Chem., Zthaca 1922,26,601. 65. Kanda, S.; Saito, Y. Bull chem. SOC. Japan 1957, 30, 192. 66. Kanda, S. Kogyo KwagakuZasshi 1963,66,641. 67. Eding, H. J.; Huggins, M. L., et al. Unpublished research at Stanford Research Institute. 68. U.S. Office of Saline Water, Contract 14-01-0001-265. 69. Huggins, M. L. Pureappl. Chem. 1966,12,427. 70. Huggins. M. L. In Kinetics and Mechanism of Polyreactions, IUPAC, Vol. 6. Plenary and Main

Lectures (F. Tud6s ed.), Akadhia Kiad6, Budapest, 1971. p. 27. 71. Staudinger, H. Die hochmolekularen organischen Verbindungen. Springer, Berlin. 1932.


72. 73. 74. 75. 76. 77. 78. 19. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93 * 94. 95. 96. 97. 98. 99.

100.

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120.

Meyer, K. H.; von Susich, G.; Valk6, E. Kolloidzeitschri/t, 1932, 59, 208. Meyer, K. H.; Mark, H. Z. Elektrochem. 1934,40,449. Meyer, K. H.; Liihdemann, R. Helv. chem. Acta 1935, 18, 307. Kuhn, W. Kolloidzeitschri/f 1934,68,2. Kuhn, W. Z. phys. Chem. 1932, A161,421. Huggins, M. L. J. phys. Chem., Ithaca 1938,42,911. Huggins, M. L. J. phys. Chem., Ithaca 1939,43,439. Huggins, M. L. J. appl. Phys. 1939, 10,700. Mark, H. Der feste Korper. Hirzel, Leipzig. 1938 p. 103. Houwink, R. J. prukt. Chem. 1940,155,241. Einstein, A. Ann. Physik 1905, 17, 549; 1906, 19, 289. Abribat, M.; Pouradier, J.; Venet, A. M. J. Polym. Sci. 1949, 4, 523. Huggins, M. L. J. Am. chem. SOC. 1942,64,2716. Philipp, H. J.; Bjork, C. F. J. Polym. Sci. 1951,6, 549. Guth, E.; Mark, H. Monatsh 1934,65, 93. Kuhn, W. KoIloidzeitsch@t 1936, 75,258. Huggins, M. L. J. Polym. Res. (later called J. Polym. Sci.), 1946, 1, 1. Huggins, M. L. J. Chim. phys. 1947.44,99. Huggins, M. L. J. chem. Phys. 1941,9,440. Huggins, M. L. J. phys. Chem., Irhacu 1942,46, 151. Flory, P. J. J. chem. Phys. 1941, 9, 660. Flory, P. J. J. chem. Phys. 1942, 10, 51. Huggins, M. L. Ann. N. Y. Acud. Sci. 1942, 43, 1. Huggins, M. L. J. Am. chem. SOC. 1942,64,1712. Staudinger, H.; Fischer, K. J. prakt. Chem. 1940, 157, 19. Huggins, M. L. Ind. Engng. Chem. 1943,35216. Huggins, M. L. Ann. N. Y. Acad. Sci. 1943,44,431. Huggins, M. L. J. Polym. Sci. 1955,16,209. Huggins, M. L. In Polymer Fractionation (M. Cantow, ed.), Academic Press, New York. 1967, Chapter A. Huggins, M. L. J. Am. chem. SOC. 1964,86,3535. Huggins, M. L. Makromolek. Chem. 1965, 87, 119. Huggins, M. L. J. Paint Technol. 1969,41, 509. Huggins, M. L. J. phys. Chem. 1970, 74,371. Huggins, M. L. Polymer, 1971, 12, 357. Huggins, M. L. J. Polym. Sci. 1971, C33, 55. Huggins, M. L. J. phys. Chem. 1971,75,1255. Huggins, M. L. Macromolecules 1971, 4, 274. Huggins, M. L. J. Paint Technol. 1972,44, 55. Huggins, M. L. Pure Appl. Chem. 1972,31,245. Huggins, M. L. J. Polym. Sci. 1972, in press. Huggins, M. L. Polym. J. 1972, in press. Huggins, M. L. Polym. J. 1972, in press. Huggins, M. L. Macromolecules 1968, 1, 184. Huggins, M. L. Inorg. Chem. 1968, 7,2108. Huggins, M. L. Acta crystallogr. 1970, B26,219. Huggins, R. A.; Huggins, M. L. J. Solid State Chem. 1970, 2, 385. Huggins, M. L. Inorg. Chem. 1971, 10, 791. Huggins, M. L. In Amorphous Materials (R. W. Douglas and B. Ellis, eds.), Wiley, Chichester, England. 1972, p. 269. Huggins, M. L. J. Ceram. SOC. Japan 1972,80, 473.

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Fifty years of polymer research