Herman Mark

Marvin:

This is an interview with Professor Herman Mark at his office in the Polytechnic Institute of New York, March 19, 1979, conducted by R. S. Marvin in connection with the preparation of a history of the Society of Rheology.

Marvin:

Dr. Mark, I saw a very nice general history of your life that just appeared in the Journal of Chemical Education. So, if you don’t mind, I’d like to concentrate on just the aspects of your work and career connected with rheology — the science of rheology and the Society of Rheology — instead of going into the much more extensive work you have done in other branches of polymer science.

Mark:

Fine and I’ll be, of course, very pleased to tell you a little bit about how I, myself, and the group that worked with me became interested in rheology. I think the earliest contact with problems of rheology and with their practical utilization occurred when I worked for I.G. Farben in Ludwigshaven between 1926 and 1932. At that time, of course, we worked on phenomena of polymerization and on the utilization of polymers. Our main target in those days was polystyrene. Very soon it turned out that it was a material which was very much liked by our customers The customers in the plastics industry at that time were injection molders and extruders And they used cellulose acetate Cellulose acetate is a beautiful material, but it is a little bit too sensitive against moisture. So we extruded sheets, films, and other types, combs and what not. They would get “hazy.” Moisture, alkali, acidic materials. So when we presented them first with a bag of polystyrene beads, they injected it with this material, brush handles and I don’t know what else. And they really liked, first, the ease of injection molding, fluidity, as today we would say excellent rheological behavior in an extruder, in a molding machine, and then complete resistance against moisture.

Marvin:

Yes. When did you hear the term “rheology”? I believe it was just invented about 1929.

Mark:

Yes.

Marvin:

Did you know about that?

Mark:

Oh yes, sure. Now, when we polymerized styrene, of course, under all kinds of conditions, and depending on the conditions, we got a different molecular weight.

Marvin:

Yes.

Mark:

And even though the softening area, the softening range, was the same, the viscosity, the melt viscosity, was very different. So we were, therefore, interested in two phenomena. First, how could one possibly use the solution viscosity to determine the molecular weight, and then how could one predict from the molecular weight the melt viscosity? Now, at that time, earlier work by Duclaux and by Ostwald, and by Sacoor, had already given a kind of empirical correlation between the molecular weight, or the degree of polymerization, and the solution viscosity. But there was no tested formula — equation. So Dr. Fikentscher and I set out to develop such an equation, empirically, by a large number of tests, because we knew the molecular weight from osmotic measurements. We had an idea, I don’t want to say we knew, but we had an idea about the molecular weight from osmotic measurements which were also carried out in the group there. And so he, Fikentscher, then developed the famous Fikentscher equation. Really, the idea was that one would like to have a 1-constant equation — not an equation with 2 constants, but an equation with 1 constant, which would cover a practical range of polymeric systems which was between, let us say, a molecular weight of 20,000 and 100,000 Below 20,000 the material was brittle, above 100,000 it was so viscous that it was not easy to handle. So it was a range of property-proper processibility combination — a compromise. And there he developed the famous K value. So the K value that was the only constant in his equation, the K value would indicate the molecular weight, and then the dimensions were chosen in such a manner that the K value multiplied with 1,000 was about the molecular weight. So a K value of 0 would mean a molecular weight of around 30,000. Eighty would mean a molecular weight of around 80,000. This K value of Fikentscher, developed in probably 1928 or 1929, is still used in industry as an empirical means to determine — to specify — and in fact, even now, polymers are still sold on the basis of the K value. It’s one specification, one practical specification. If a customer has a certain machine and he wants to make combs, then he knows that the viscosity shouldn’t be too high because the comb is a difficult thing to make by an injection molding machine, to fill out all the tiny bits of the part of the comb. So he will have a K value between 30 and 40. But, if he wanted to make a brush handle, something which had to be stronger, and where the injection molding problems were not so complicated, he would say 1 want a K value between 70 and 80. And it’s done here today. So really that was the first. And at that time we also established an empirical relationship, of course, again, between the melt viscosity and the molecular weight. And at that time we arrived at the melt viscosity, the log of the melt viscosity was log of molecular weight times 3 to 4. So that the melt viscosity was a constant times the molecular weight to the power between 3 and 4. Then, later, all these things were developed very much more in detail and today we know that it’s about 3.4.

Marvin:

Yes, 3.4. I suppose at that time the distinction between number average and weight average was not so clear.

Mark:

It didn’t exist.

Marvin:

You just talked about molecular weight.

Mark:

Well, that was in the days of Ludwigshaven.

Marvin:

Let’s see now, that’s just about the time the U.S. Society of Rheology was formed, in 1929. Were you aware of the formation of that group?

Mark:

Yes. In those years, every year there was an annual meeting of the Colloid Gesellschaft in Germany and, of course, the most prominent members — or personalities — were Ostwald, Staudinger, Rideal, Perrin, and Duclaux.

Marvin:

Of course, Ostwald was one of the founders.

Mark:

He was one of the founders. And this Society met every year. It’s all in the literature. Of course, usually someone from Ludwigshaven gave a lecture. Fikentscher, for instance, gave lectures, I gave lectures, Voludel gave lectures; we were very close to it. I don’t know — I think in 1927 or ‘28, one of those meetings, it was proposed, instead of using viscosity, the viscosity, to use the opposite, the reciprocal of it, namely rheology. Now, I don’t think that .Bingham ever was at one of those meetings. He didn’t like to leave the U. S. He may have been, I don’t know.

Marvin:

I have not seen any mention of his going to Europe.

Mark:

I don’t think he was. I mentioned earlier the names of Berl, of Biltz, and of Duclaux, and of Ostwald. But Rideal was the British representative there.

Marvin:

Yes.

Mark:

I really don’t remember exactly when the word rheology was officially propounded.

Marvin:

It was proposed in 1929, by Bingham. Some… 1928, 1929. A professor of classics suggested this term to him.

Mark:

Panta rei.

Marvin:

Panta rei. And the... it was in 1929... the decision to form The Society was in 1928. A group got together and drew up the proposed constitution and organization plan and plans for a journal, and then it was started in December of 1929. Interestingly enough, the first issue of the Journal of Rheology came out before the first meeting of the Society at which it was officially established. Bingham was very ambitious; apparently, he got the Journal going right away.

Mark:

Well, that’s right. Then, in 1932, I came to the University of Vienna; I left the I.G. Farben and I came to the University of Vienna.

Marvin:

Excuse me. Did you join the Society while you were still in Europe?

Mark:

Yes, sure.

Marvin:

Do you remember about what time, just for the record?

Mark:

Now, I would guess it was some time in ‘32, maybe a little bit later.

Marvin:

Unfortunately, our records from those days are lost.

Mark:

I’ll tell you… of course when I came to Vienna, all of the interest which I had in high polymers and macromolecules and their behavior was shifted more to the fundamental end. Because now I was a University Professor, and I was supposed to do fundamental research, I was supposed to educate people, to produce chemists and maybe also rheologists. So we immediately switched over to fundamentals, and at that time I had a number of collaborators, Guth, Gold, Eirich, Simha, Margareta, Bunzl, and others. And this whole group started to work on viscosity. Now, not so much on the practical aspects, but on the fundamental aspects. Which means that we all went back to the Einstein equation of 1910, and started to work from then on. Now Guth and Gold were physicists who started to work on the extension of the Einstein Equation to the next, to a quadratic term. You know, 2. 5 x phi or whatever + what phi squared.

Marvin:

Yes.

Mark:

And they worked it out, that it was 14.1 or something like that. Guth and Gold published a paper, and Guth and Simha published a paper, in the Kolloid Zeitschrift, between 1932 and 1936. And then one of our main concerns was to verify the equation, experimentally, because it wasn’t.

Marvin:

Yes.

Mark:

Well, that meant that you would have to have spheres or almost spherical particles suspended in a liquid at low concentration and decreasing concentration because we also wanted to experimentally test the second term and then we would have to make viscosity measurements. So, Eirich, actually together with two or three of his collaborators, carried out this work. It wasn’t easy to find a material which we microscopically could establish in terms of spherical character and diameter and to have a liquid which had the same density. Because that was the problem — that they wouldn’t settle.

Marvin:

Yes.

Mark:

And in the paper it is described, they did it in several ways. First, we used hollow glass spheres, then we increased the specific gravity of the solvent by a molecular solute, for instance we used toluene in a mixture of ethyl iodide. See, ethyl iodide has a high — methane tetra bromide — they have high specific gravities. So, we adjusted the specific gravity of the solvent to the specific gravity of our solute and the solute was either very small beads of polystyrene or poly (methyl methacrylate), but glass was the best, because the other materials were usually attacked by the solvent — this is all in the literature. And sometime in 1936, yes 1936, Eirich and I published an article, a long article in Fortschritte, of which part of it was an experimental verification of the Einstein equation. Also with the second term.

Marvin:

Yes.

Mark:

Then, of course, we also got again interested in the problem of viscosity of dilute polymer solutions.

Marvin:

What kind of measurements did you use for your viscosity? Capillary —

Mark:

Not all of them — we made a lot in the Couette. The capillary and the Couette were the main ones. We had a very good Couette machine which was made in our workshop. As far as the viscosity of dilute polymer systems goes, at the same time Kuhn, in Basel, worked on the same problem, and Shultz in Freiburg worked on the same problem, and we exchanged our results. Rideal worked on the same problem, Melville, for instance, published. There were a number of laboratories interested in this approach at that time. Actually, in 1934, Guth and I, and at the same time, independently, Kuhn, published a paper; he published one and we published one, which would indicate that because of the random conformation of a macromolecule in solution, the viscosity should not be proportional to the first power of the molecular weight as Staudinger had indicated, empirically, but to the square root. Now then, of course, we started to, particularly Simha and I carry out a number of measurements on cellulose acetate, polystyrene, poly (methyl methacrylate), cellulose nitrate, in various solvents. And we knew the molecular weight from osmotic measurements — osmotic measurements at that time weren’t very good — hot as good as they are now. But still, they were in the right ballpark. And Houwink did the same thing in Holland at the same time; you know, he was the pre-eminent Dutch rheologists — he published the famous Proceedings on Viscosity of the Dutch Academy of Sciences. I have them here. They’re called “First and Second Report on Viscosity,” Amsterdam, 1935 and 1938.

Marvin:

I’ve seen those. Burgers was also involved in that.

Mark:

That’s correct, Burgers was also involved.

Marvin:

Did you know those people?

Mark:

Yes. And Houwink, while we did this kind of work in Vienna, he did it in Holland.

Marvin:

Yes.

Mark:

Sometime in 1938, I don’t quite know when it was, he published an equation where the exponent on the molecular weight was left open and I published the same equation where I also said, neither is it one nor is it one-half, but it may be something in between. And that is the so-called Mark-Houwink Equaton. The intrinsic viscosity is equal to a constant K times the molecular weight to another constant A, and both constants depend on the temperature and on the solvent And then, of course, Huggins took this equation and then interpreted it in all details.

Marvin:

That was later, though, wasn’t it?

Mark:

That was later, yes. In 1934... a little bit after that, 1935 or 1936, Houwink and I independently published this equation which, of course, having two constants, is more adaptable and more general. And then later, of course, Huggins took it up ‘and Flory and Debye, and now today we know what those two constants mean.

Marvin:

Let’s see, weren’t you also, or your group, involved to some extent in the chain kinetics and kinetic theory elasticity and...

Mark:

Sure. We started it, Guth and I.

Marvin:

I know. Wasn’t that in the ‘30’s?

Mark:

‘34.

Marvin:

‘34.

Mark:

You see, Guth and I, and Kuhn, independently in Basel, at the other end of the Alps, we said a long chain consists of a large number of beads, a large number, thousands Therefore, it would not be possible to follow the actual movement of every bead, it would be necessary to use statistics. That was the beginning of the statistical treatment of macromolecular chains.

Marvin:

Right.

Mark:

Now, already — maybe a year earlier, in Ludwigshaven, when I was still in Ludwigshaven — ’32 — Valko and Meyer and I said that, but qualitatively. We said it probably would be necessary ‘to use statistics. And, of course, that’s all been published. But neither Meyer, nor Valko, nor I were enough mathematicians to do that. But when I came to Vienna, Guth — he was a pupil of Han’s Thirring, a professor of theoretical physics in Vienna. And Guth was one of his students. When I saw Thirring and told him, look here, I think we have a problem — to treat macromolecules statistically—he said, “Well, why don’t you talk with Guth about it?” He had already his Ph.D. — he was a kind of a postdoc. Then Guth came to our laboratory and I told him, “Look here, I think there is an interesting prob1em” And then he did it — he did all of the mathematics in this article, and we published it in 1934, and then he kept on doing that. Then, of course, later, there were many improvements. You know, Huggins improved it, and Treloar in England improved it, then Flory did a lot of very important work Guth and James, Huggins, Flory, and Treloar, I think, were some of the ultimate refiners of this theory.

Marvin:

Guth and Flory still argue a little bit, but it’s —

Mark:

They will argue. You know how those things are.

Marvin:

Yes.

Mark:

I remember there was a Gordon Conference in 1940 or ’41 — they were still at Gibson Island on the Chesapeake Bay. And there was Treloar, Flory, Guth, and each of them had an entirely different approach. So the differences were that. (Hands far apart) Now the differences are that. (Hands close together)

Marvin:

During that period, did you keep in touch with the U.S. Society of Rheology — did you get the publications and notices and so on?

Mark:

Yes. Well, we got them in those days, and actually earlier. We got, of course, the Colloid Symposium Monograph. You see, every year it was published in this country — the Colloid Symposium Monograph. It was a volume about that (indicating thickness). I don’t know when the first volume appeared — in '25 or '26 — the Colloid Symposium Monograph.

Marvin:

Well, I think they were started as early as 1924.

Mark:

Anyway, we got them at the library.

Marvin:

Yes.

Mark:

Because you know one of them, I think in ‘26 or ’27 — there was this article by Sponsler and Done — which had nothing to do with rheology, but was on the structure of cellulose. There was a mistake in it — but it had nothing to do with rheology. Well, that started the other end, namely, the fundamental end of viscosity. Of course I did keep in touch with the empirical end, the practical end of it, because during those years — 1932 to 1938 — I still was a consultant for the I.G. Farben in Ludwigshaven, so I visited them twice a year, and Fikentscher was still there, and Valko and Susich — all these people were still there. Hauser-Ernst Hauser — was an important man in rheology in the ‘20’s and in the early ‘30’s. He was with the Metallgesellschaft in Frankfurt and he had the Rubber Department and worked on lattices. And of course, viscosity of latex was a very important problem, because the lattices were eventually sold with viscosity. And for cellulose viscosity was a very important factor, because pulp was sold according to its alpha content and according to its viscosity. Those were the two-millions of tons of wood pulp were sold with two specifications. Content of alpha cellulose which was that insoluble in a certain sodium hydroxide solution and viscosity.

Marvin:

Yes. Did you ever happen to meet Marcus Reiner? Did he come to Vienna sometimes?

Mark:

Yes. Of course, Marcus Reiner visited us some time in the ‘20’s in Ludwigshaven.

Marvin:

Did he?

Mark:

I think he even visited the Kaiser Wilhelm Institut in Berlin early in the ‘20’s.

Marvin:

Yes.

Mark:

But then he was in Ludwigshaven. And then he was in Vienna as a visiting lecturer to present talks. He was one of the fundamentalists of rheology from the —

Marvin:

Perhaps the first mathematician—mathematical rheologist, I suppose —

Mark:

Stress and strain, flow —

Marvin:

Yes. He was also quite instrumental in the Society’s start, when he visited with Bingham in 1928.

Mark:

Of course, at that time he worked at — Haifa?

Marvin:

Yes — no, I think he was at —

Mark:

Was it Tel Aviv?

Marvin:

I can’t remember exactly. When he — he was in Israel certainly — when the British mandate was there, he was an engineer in Palestine, and I don’t remember the exact dates, but anyhow. What about Scott-Blair — did you know him?

Mark:

Sure. Scott Blair practically always attended the meetings of the Colloid Society. He and Rideal and Melville and Hugh Scott Taylor came over from England to these meetings. Then, in the development of the Fikentscher Equation, we were anxious to use low concentration in order to escape the non-Newtonian behavior. And, of course, later in the development of a rational relationship between intrinsic viscosity and molecular weight, intrinsic viscosity means extrapolation to zero concentration so that was really the lowest concentration. But, at the same time, there was a group which were interested in the deviations — in the non-Newtonian behavior. Mainly the people — the injection molding people — you see. Because they worked with the stuff itself. They worked with a melt — of course, all those melts where highly non-Newtonian. That, of course, was where Bingham started. Everything is quite different when you go into the non-Newtonian field.

Marvin:

I’m not sure that Bingham himself worked with plastics, or polymers so much, did he? Wasn’t his interest more in oils and greases?

Mark:

I think he was interested in oils and greases, and I think he was also interested in suspensions. Those suspensions are also non-Newtonian.

Marvin:

He really was a colloid chemist, I think.

Mark:

Yes, sure he was.

Marvin:

Most rheologists were colloid chemists.

Mark:

Really, there was a complete overlapping of rheology and colloid chemistry, and of macro molecular chemistry.

Marvin:

Yes.

Mark:

All of the polymer people went to the meetings of the Colloid Society.

Marvin:

But my impression is that rheology really got its start in the ‘20’s, primarily from the colloid chemists, rather than from the polymer chemists. The polymer chemists, by and large, came in a little bit later — was it the ‘30’s?

Mark:

Yes: We had to use it. Now, Weissenberg at that time — even as far back as 1928 — he started to develop his rheogoniometer.

Marvin:

Was it that early?

Mark:

Rabinowitz, Reiner, and Weissenberg — Rabinowitz was 29, Reiner was 31, and Weissenberg was 28 — particularly had interested themselves in the anomalous behavior of viscous liquids. Then .Bingham suggested a substitute for the Newtonian equation which was adopted by Freundlich who was another colloid chemist who was on the Board of Editors of the Journal of Rheology. So it was Freundlich, Segwari, Hauser, Rabinowitz, and Weissenberg, and, of course, Reiner — they were extending the classical theory.

Marvin:

Back in those times, there seemed to have been a good deal of uncertainty about the connection between viscosity and plasticity and I don’t think people were entirely clear about what they really meant when they used the word “plasticity.”

Mark:

Yes. It was an empirical term — something is plastic if you can deform it at a certain temperature, and if you cool it down it will maintain its shape. That was essentially what the plastics did, what the thermoplastics did. Then, out of this conglomerate of ideas and of contributors, emerged one branch which deliberately only used very dilute solutions or suspensions for the purpose to determine molecular weight. Then there was a branch which was not interested in the molecular weight, not primarily at least, but in the behavior — in what can you do with a plastic system — was it a concentrated solution or a melt?

Marvin:

Let’s see then, it was about 1938 when you went to Canada briefly?

Mark:

Yes — then in 1938 I went to Canada. Eirich went to England. I took Simha with me to Canada and there, of course, I was asked to come to the Canadian International Paper Company by its Technical Director, Dr. Thorne, to modernize their research laboratory. They had a rather well-equipped research laboratory with some six or seven academic people, but it was entirely empirical. They would determine the alpha, the beta, the gamma cellulose, but they didn’t know what it meant. They would determine all the properties of a polymer — like plasticity — they didn’t know what it meant they didn’t have the equipment. And he knew, Thorne, who was a Norwegian by birth, he visited Europe each year, and he knew there were a lot of things going on in Europe. So he wanted me to do that. So I went to Hawkesbury and did just that. In other words, we established that alpha meant molecular weight and, that alpha and the viscosity were correlated with each other. Beta meant a smaller molecular weight, gamma meant a very small molecular weight, and the viscosity had to be carried out at the lower concentration in order to be meaningful, and so on.

Marvin:

There was really no academic work on this continent at that time?

Mark:

No.

Marvin:

In any branch of polymers.

Mark:

I went there in 1938 and I left in 1940, so it was three years that I was there. And I told Dr. Thorne, I think I have done what you wanted me to do. You have a lot of good people here, make one of them Director of that ‘laboratory. I would like to go back to the scientific world. And he said, “Fine, and I’m sorry. I would like you to stay. And after all, didn’t we treat you very nicely?” And I said, “My God, I’ve never been treated so nicely in my life.” But he understood. Then I went to Poly. When I came here, of course, a lot, of work on all these things was done in industry. DuPont, Dow — empirical work. But nothing was done at any university. So the word “polymer” was unknown in the academic world. Because the only man who had really done fundamental works on polymers was Carothers. He was in the DuPont Laboratory and there was not very much contact. C. S. Marvel at Illinois was the only man in an academic position who had already done a great deal of work in the polymer field, but he was interested in synthesizing... He was an organic chemist. So he was interested in making — but he wasn’t too interested in how they behaved.

Marvin:

That’s interesting, because by about 1946 or 1948 there were quite a number of centers in this country.

Mark:

Yes, that was ‘48. But this is now ‘40.

Marvin:

But even so, it seems a little surprising that it could grow so much during the war years.

 

Mark:

Well, I’ll tell you. When I came down here, the first thing was to edit the papers of Carothers. When the Interscience Publishing Company and I started seriously on high polymers, the first volume was to collect the papers of Carothers. Then the next one was Physics and Chemistry of High Polymers. I wrote that. Well, of course, there was Bingham and his group, but they were not polymer chemists. Then came the war, and during the war polymers became very popular because of the Rubber Project. See, there was the great emergency of rubber and the synthetic rubber program — it was a crash program to adopt as fast as possible the German technology for making synthetic rubber. And since I had worked in Germany in a synthetic rubber plant, I was very closely in contact with this project. Now, there was a big committee with Debye, and Marvel, and many, many other people where we tried to help the industry develop this technology with the aid of all the fundamental information available, Molecular weight determinations and viscosity determinations and all these things. Now, then we had a large group in those days — there was Tobolsky, Doty, Zimm, Simha — Eirich was there. No, he came later. Overberger. Mesrobian. Between 1940 and ‘46 they were all here at Polytech. Then they dispersed, and each of them founded a new small group on polymer chemistry. And that was the reason why by 1948, see Zimm was in California, and he had a small group working on polymer chemistry.

Marvin:

Not that early.

Mark:

Yes.

Marvin:

I though Zimm went to G.E. for a while —

Mark:

Yes — you’re right.

Marvin:

I think he went to California much later. Anyhow...

Mark:

I think he went to California first, and from California to G.E.

Marvin:

Perhaps he did. [Ed. Note: Mark is correct here.]

Mark:

And Doty went first to Notre Dame and then to Harvard.

Marvin:

And then, of course, Flory switched back and forth between industry and academia.

Mark:

Yes, many times. He got his Ph.D. at the University of Cincinnati sometime in 1927. Then he went to DuPont and worked with Carothers. And then he went to Esso, Goodyear, and Cornell for a while, then he was in Pittsburgh at the Mellon Institute, then he went to Stanford. While he was at Cornell he published the famous book on Principles of Polymer Chemistry, for which he won an award. In 1953 he published his book.

Marvin:

And at that time you apparently became very active in the Society of Rheology. As soon as you came to Brooklyn, and very shortly after that, you became President in 1942.

Mark:

When I came to Brooklyn, in 1940, one of the earliest things I did was to visit Bingham. I went to Pennsylvania and visited with Bingham. He was a great man.

Marvin:

Yes. Everyone who met him comments about what a fine person he was.

Mark:

Yes. A real gentlemen, and I told him what I intended to do here and, of course, he said that’s wonderful. And he was, at that time, I think, President of the Society.

Marvin:

No, he was never President. He was Editor, and perhaps Vice President one year, but he was never President.

Mark:

Who was?

Marvin:

Let me tell you — I have a list of the officers here — in 1940-41, A. Stuart Hunter was President, and Bingham had to retire as Editor sometime in 1939, he was quite ill for a while, although he did recover. Bingham was Second Vice President in 1938 and ‘39, and Melvin Mooney was President. So you probably met Mooney also at that time.

Mark:

Yes. Today he is a test.

Marvin:

Unfortunately, Mooney died a few years back. And Jack Dillon was the First Vice President in 1940-41. You knew him. And Ewell was Second Vice President.

Mark:

Once when there was also the Gibson Conference, we were sitting at the bar talking and Mooney complained — he said, “I’m not a man, I’m a test.” There exists a cold Mooney, a warm Mooney, a fast Mooney, a slow Mooney...

Marvin:

Well, it became very famous.

Mark:

Yes. It still is the best existing test for whatever behavior a rubber company needs to know.

Marvin:

Yes, I guess so.

Mark:

Whatever it is.

Marvin:

It kind of interested me, what was it that attracted you to the Society of Rheology particularly? I would have thought with your background that you might have become more active in the American Chemical Society. Well, I suppose you were, also.

Mark:

Yes, but the ACS was, still is, a very large operation, and polymers at that time weren’t that much of it. As an example, in 1946, after the war, I talked with W. A. Noyes, who was Editor of the Journal of the American Chemical Society, and I told him that there is already, and there will be more, publications in the field of large molecules. Whether one should not make a division of the Journal and call it the Journal of Polymer Science or to call it something. And he said, “Look here, I was often asked, well, how about Inorganic Chemistry, how about Organic Chemistry, how about Physical Chemistry, how about and Biochemistry. They are all large branches of chemistry. Should each of them have its own journal? My attitude is ‘no.’ Because if that is so, then the people who read the Journal of Inorganic Chemistry will never know anything about Organic Chemistry.” So he said “no.” I think he was correct from his point of view. So we founded the Journal of Polymer Science. Now, five or six years ago, the A.CS felt that this was now such a large group and such a big industry; meanwhile, they had founded a Journal of Organic Chemistry They had founded a Journal of Physical Chemistry. They had founded a Journal of Inorganic Chemistry. So now they founded Macromolecules.

Marvin:

I guess in those days a lot of people did try to keep the societies together.

Mark:

Up to a certain extent it works. And the journal still exists.

Marvin:

Of course, now it’s so big that it’s impossible...

Mark:

On the other hand, the Journal of Rheology, which was my field, only contained papers and articles that were 100 percent in the domain of my interest. That was the reason why I got interested in the journal.

Marvin:

Of course, by that time the Society no longer had its Journal of Rheology, and at that time they were publishing in the Journal of Applied Physics.

Mark:

Yes, but still the Society was still the Society of Rheology.

Marvin:

The Society itself was small, and its meetings were small. So that was really the thing that attracted you. The fact that you could have a small group all interested in some of the same problems.

Mark:

It’s a little bit like, why does everyone like the Gordon Conferences? Because he goes only to one or two of them, but they fill out 100 percent what he wants to hear.

Marvin:

And was it perhaps also the fact that the Society pulled together a lot of people with different backgrounds — universities, industrial?

Mark:

Of course. I knew Dillon from the Rubber Project. I knew Hunter because I was a consultant with DuPont. I knew Mooney from the Rubber Project. I knew another man from DuPont, who was a President or something. As I said, in 1940, when I came down from Canada, I persuaded Interscience to start this series.

Marvin:

Yes, High Polymer Series.

Mark:

And immediately after the war I persuaded them to start Advances in Colloid Science, and the first volume, this is already Volume 3. Volume 3 appeared in 1950 and the man I meant was Elmer Kraemer. He was the Editor of the first volume and I was the Editor of the second volume. And Verwey and I were the Editors of the third volume. So, I started with Interscience something which would practically amount to a journal of colloid science. Advances — we didn’t want to call it a journal because with a journal you are committed to bring it out every year. But if you call it Advances, you can skip one year and so on. .1 think the first volume came out in 1944. And Kraemer, unfortunately died very early. I’ll just read you this. Dillon wrote an article, Hawser wrote an article, deBoer wrote an article, and Alexander wrote an article. So, I presume the fact that early in 1940 this started. I started to invite people to write articles for this thing. They felt that I was a colloid chemist, you see.

Marvin:

Well, when you first, became associated here with the Society, and when you were President…

Mark:

Was it ‘42?

Marvin:

You were President 1942-46, two terms during the war.

Mark:

What was the name of the 'Secretary' at that time?

Marvin:

Dow.

Mark:

Dow, sure.

Marvin:

What was the condition of the Society during those, years? Our records are quite incomplete, unfortunately. I have talked to Dr. Dow, and his recollection is that the Society really wasn’t very active after Bingham became ill — until after you started reactivating things.

Mark:

I remember — Dow came to me, sometime during the war, and asked ‘me whether I would be President and, of’ course, I said I would be delighted, and Hunter was also there, and Kraemer, and we met here in my office, not this off ice, but in my older office, and said since Bingham isn’t interested anymore, I think we have to give the whole thing a new slant. And we have to beat the drum and see to it that some new activity starts. And then I think our joint idea was that this is an area which should be an ideal overlap of fundamental science and practical application. Because everything at the end of it was either a fiber or a film, or it was an injection — molded piece or it was an emulsion, a piece of rubber and so on. On the other hand, in order to intelligently understand what’s going on in spinning and casting, and extruding and in blow-molding, and so on, one would have to be fundamental.

Marvin:

Of course, that was as far as polymers were concerned, but as I gather from looking at the papers presented, the emphasis on polymers didn’t really become dominant until sometime after 1940 — sometime after you became President.

Mark:

Well, it took a while before these people would start publishing.

Marvin:

Another question that I really had down for later was, why was it that the people who had been quite active before, those interested in lubrication and geology and petroleum and metals seemed to almost completely drop out of their activity in the Society, and for quite a while there polymers were very much the dominant theme of publications and the meetings? Not exclusively. There were still some from the other fields.

Mark:

Well, it may have been our joint responsibility that we just scared them out. But I know, the Gibson Island Conferences in those days, they started in 1937, and in 1937 Carothers was one of the speakers. Bakeland was one, and Wakefield was one. It was a very small group, a kind of a family affair. And then in 1940, I attended — it was the first time I attended. At that time, S. S. Kistler was the Chairman of that conference. And then polymers became more and more in the foreground. To a certain extent because, at the same time, there was a Petroleum Conference, also at Gibson Island. Gus Egloff was the Chairman. They took over most of the oil and even a little bit of the rubber. So, there were all the groups who were interested in lubricating oils and in greases. And that was the petroleum chemistry. There is still a Gordon Conference on Petroleum Chemistry and, of course, there is now a Gordon Conference on Colloid Chemistry. More than one. So it was a kind of a push and pull operation. The polymers became preponderant — that was the push — and other people wanted to have the rheology of lubricants in their domain — and that was the pull.

Marvin:

Well, at that time, the Society seemed to be having a very bad time in its publications — publishing in the Journal of Applied Physics, but as far as —

Mark:

I think I published several articles at that time.

Marvin:

You probably did. But as far as I can tell, there were no separate rheology issues. There were from 1933-36 in Physics, but starting in 1937 and through 1946 the papers seemed, as far as I can tell, to have been published at random. I have not been able to find any —

Mark:

That was one reason why we started these Advances in Colloid Science.

Marvin:

I suppose that that was an unsatisfactory arrangement from the Society’s point of view, because we had no identifiable publications. And I suppose that was the reason we eventually switched over to the other publication schemes. Cost certainly had something to do with it.

Mark:

Yes. Well it was Interscience who eventually published a journal.

Marvin:

It got very complicated. From 1947 to 1952 they were published in special Rheology issues of the Journal of Colloid Science. And then from 1953 through 1956 in the Journal of Applied Physics again, but those were Rheology Symposia. Those issues were identified. Then starting in 1957, we picked up the Transactions of the Society of Rheology, which has grown into our present publication. That was, and has been, published by Interscience.

Mark:

I remember well that while I was here during the war, we felt that something should be done for the colloid chemists, that somewhere if it’s not original articles, somewhere a colloid chemist should find review papers on the most recent progress.

Marvin:

On that Journal of Colloid Science publication, according to the Bulletin of May of 1946, Dr. Proskauer of Interscience in 1945 was interested in publishing the Society papers in a new Journal of Polymer Research, and at the same time —

Mark:

It was called Journal of Polymer Science.

Marvin:

I see. At one time it was referred to as the Journal of Polymer Research.

Mark:

That must be a mistake, but leave it as it is. In 1946, it was founded by Interscience, the Journal of Polymer Science.

Marvin:

But why, I wonder then, did we go to Journal of Colloid Science. Do you know? Interscience did publish our Bulletin in 1945 for one year, and apparently this was preliminary. Now this was just at the end of your term as President when all these things were taking place I wonder if you recall why the choice was made to use Journal of Colloid Science at the time, rather than Journal of Polymer Science.

Mark:

I really don’t remember. When we founded it, the first Editors of the Journal of Polymer Science were Marvel, Doty, and I. Of course, we were not at all sure whether the Journal would survive. Because the first year we were very short of articles. So maybe I wasn’t too confident in the survival of that young little flower. But I really don’t remember. It may be that all three of us somehow felt that the word “polymer” should be on the cover of the journal in order to emphasize that this is an important field and it will be more and more important. But, I don’t know. Today I’ll say that might have been a possibility because you know I was a kind of an apostle for the word “polymer.” Because most universities — if you came to Harvard or to the University of California or to Columbia or any of the important universities, if you said, “I’m working in the polymer field,” they would say, “What is that?” Some still do it today.

Marvin:

Do you think perhaps that some members of the Society of Rheology did not want to emphasize “polymer” quite that much in their papers?

Mark:

That may very well be.

Marvin:

It may be that there were still enough of the old colloid chemists who still felt more comfortable with colloid chemistry?

Mark:

I think there is a philosophical difference. Polymer is a generic term for a group of substances. Rheology is a generic term for a group of methods. So one is .a material and the other is methodological. All of us said, well, those are two different things, and you know the German is now Kolloid Wissenschaften and polymers are colloid substances. And, of course, just in these years, the whole problem of rheology that means a phenomenon or flow regardless of the substance, went through an important development, all though Riseman, and Debye, and Brinkman, all the additional theoretical work on flow is here. And here you have it all combined. But this really is now. Look at those equations. This is certainly higher mathematics.

Marvin:

This is the book Polymer Rheology by Lenke.

Mark:

Then, of course, in those years the Weissenberg rheogoniometer became better known and used.

Marvin:

It was used in England. Did word of that reach here?

Mark:

It was used in England. It only came over after the ‘40’s.

Marvin:

In the late ‘40’s, after the war. Yes. It was probably classified during the war.

Mark:

It may have been classified during the war —

Marvin:

I assume so. He was working, I believe, on flame thrower materials.

Mark:

It had a lot to do with his gels. Then the whole problem of light scattering exploded during the war. In one direction — to measure the molecular weight of polymeric materials — and in the other, to measure the density of fogs. Aerosols. If a warship saw another warship and wanted to get away, it had to produce as much fog as possible as fast as possible with a minimum of material. And that was part of colloid science. No question about that. Particulate science, particulate solutions. LaMer had worked on that — Victor LaMer.

Marvin:

Well, during this period when you were President, was there a shift in the balance between industrial and academic people in the Society? Do you remember?

Mark:

I think more and more academic people came in. Because before that, when Dillon was in industry, and Hunter was in industry, and Kraemer was in industry —

Marvin:

The founders were mostly academic people. At that time, Mooney, Mayo Hersey, who moved back and forth (was partly in industry and partly at Brown), Fulweiler, R. L. Peek was at Bell Labs., Bingham, of course, and Mooney and Hunter and Wheeler Davey, who was academic, and again Peek, then Hunter, Dillon, Davey, Wakefield. Then when you were President in 1942 through 1946, your Vice President was Bill Fair who was in industry, and Paul Flory, and again Wakefield and Dow, N. Taylor with 3M. It’s interesting to see Paul Flory in there. Did you have a role in attracting him to the Society?

Mark:

Yes.

Marvin:

He was Vice President there for three terms.

Mark:

Wasn’t he later President?

Marvin:

No. After that, he was not particularly active. His interests seemed to shift to other areas.

Mark:

Another academic man, of course, who was very important in those years, was Eyring, Henry Eyring.

Marvin:

Yes. He was a Vice President in 1946 and ‘47, as was J. W. McBain. Was he in Stanford at that time?

Mark:

Yes. That was before he went to India.

Marvin:

Also, Henry Eyring was active. He gave papers at several of the meetings earlier and —

Mark:

He was a very important contributor. His theory of viscosity.

Marvin:

Yes. His hyperbolic sine relations.

Mark:

His theory of viscosity is still the best thing we have.

Marvin:

And Tobolsky worked with him on some of these things.

Mark:

And Tobo, you know, was here during the war and then he went to Princeton.

Marvin:

Is that the way it went? He took his doctor’s degree at Princeton, did he not?

Mark:

He got his Ph.D. at Princeton.

Marvin:

Did he do his undergraduate work here?

Mark:

No. When he was already a Ph.D., we started to assemble here this big group on the Rubber Project. He worked here for four or five years.

Marvin:

Oh, I see. He got his Ph.D. before the war, then came here, and then went back to Princeton on the staff.

Mark:

Then, when I thought that a new edition of this book here should be prepared, I asked him to help me. So, the second edition of the second volume is Mark-Tobolsky. Now, this came out in 1950, just ten years after the first edition. So, the first was 1940 and the second 1950.

Marvin:

I see. So you think the balance rather shifted toward the academic group while you were President.

Mark:

Yes. You could say that I tried to lure as many academic people into the Society and its activities as possible. Certainly, Flory was one of them, certainly Eyring was one of them, and Tobolsky was one of them. Maybe there were others — Eirich. Of course, he is now the undisputed Pope of rheology.

Marvin:

Of course, his famous series.

Mark:

Also, there was Kirkwood. Was he never actively engaged?

Marvin:

He was never an officer and I haven’t run across his name in committees.

Mark:

You know there exist three famous papers, one by Kirkwood and Auer, and two by Kirkwood and Riseman. And Ferry was another one.

Marvin:

Of course, he stayed very active and was President and so on. Did you attract Ferry into the group?

Mark:

I don’t remember whether I attracted him, the atmosphere probably attracted him.

Marvin:

Probably…

Mark:

It’s a kind of a chain reaction. If you have three good men, ten more will come.

Marvin:

Of course, during the war, you had these special Saturday seminars here. They are still going on. I’m sure those special programs helped to build up interest in the Society and its meetings, also. How did the Society manage to grow and maintain its activity during the war? I would have thought there would have been restrictions on travel and people getting to meetings and people being disrupted and pulled away from their regular activities.

Mark:

Yes, but New York was a good place. Because from where would the people come? From Princeton, Wilmington, and New Jersey. And in the north from maybe as far away as Albany. Because the travel from most of the places where people came from could be done by train. There was no civilian plane travel at that time. So, we had our clients, essentially; Washington was the southern-most and Boston was the northern-most.

Marvin:

Can you think of other individuals besides yourself who were responsible for the Society’s growth and activities during your Presidency?

Mark:

I would say certainly that Tobolsky was one. Eyring was one. Kraemer was very active, a DuPont man.

Marvin:

Alfrey?

Mark:

Alfrey wrote a big book on mechanical behavior of polymers.

Marvin:

I was thinking more of the Society there he was Editor for a while, Publishing Editor. I suppose you had kind of twisted his arm to get him to take that job.

Mark:

Yes. But I think another man who always helped greatly was Boyer.

Marvin:

Ray Boyer. Yes.

Mark:

He was always interested in phenomena. There were people more interested in materials and people more interested in phenomena. Boyer was always interested in phenomena, and now he is really the center of gravity of everything that has to do with second-order transitions and the glass point. And the whole phenomenon of the glass point was discovered during the war. It was really this fact that this coefficient of expansion has an abrupt change at a temperature way below the melting point. Alfrey was one. And Boyer. Boyer was always a very prominent figure, trying to understand what is really molecularly going on at that temperature. Is it a phase transition, is it thermodynamically justified because of the phase transition? You know, all these questions are still not completely solved.

Marvin:

Well, I guess there are still some arguments about it.

Mark:

Certainly, Boyer still argues about it.

Marvin:

Do you remember, what were our relations with the American Institute of Physics, between the Society of Rheology and the American Institute of Physics.

Mark:

They were always very good and very friendly, and I think the main reason why they were good and friendly was K. K. Darrow.

Marvin:

He was the Physical Society.

Mark:

He was running everything in physics. And, he was the Secretary of the Physical Society and of the Institute of Physics.

Marvin:

When I talked to Dr. Dow, he sort of implied that at least some of the Society members had a little bit of an inferiority complex-didn’t feel that they were quite real physicists and I wondered if you have any recollection of any feeling like that.

Mark:

They had the same inferiority complex which we all have. I’m not a real chemist. If you go and ask the seven most prominent chemists in this country, organic chemistry, physical chemistry, biochemistry, and ask them if I am a chemist, they’ll say, “No. He is a polymerist.” And it was the same thing, physics was developing in entirely different directions — there was either nuclear physics or solid state physics, and maybe there was spectroscopy, and there was particle physics and there was solid state physics — those two things, and there still are.

Marvin:

But, as far as getting along with the American Institute of Physics, you had no problems.

Mark:

No. We had a lot of meetings in their office “in mid-Manhattan. I remember we always had to do with K. K. Darrow.

Marvin:

We’ve already talked about the Journal of Colloid Science. Did you work out those arrangements, or was that somebody else, or don’t you recall?

Mark:

I don’t recall. But it was with Interscience —

Marvin:

No. The first one was with Journal of Colloid Science. There was a proposal to publish with Interscience, with your new Journal of Polymer Science, but that somehow fell through. They did publish the Bulletin for one year in 1945 and then the American Institute of Physics published it for one year and then we dropped that Bulletin for two years because we thought this new Physics Today would take care of all the new publications, but it was not satisfactory. We didn’t get enough coverage in that, so after two years the Society went back to its own Bulletin Unfortunately, during those two years there were a number of rather important things that happened so I have no record of some of the detail’s for that period. It’s too bad this happened. Do you remember when the Rheology Bulletin was being published, before when the Society was publishing that itself? They had a rather extensive section of rheology abstracts that a lot of people contributed to — people like Alfrey, Tobolsky, Ferry, Bondi, Flory — a great many people made contributions. Do you know why that was discontinued later? Was it simply a matter of the cost, which certainly was an important point, or did people think that it was no longer necessary to have the Rheology Abstracts?

Mark:

Well, I think it was probably both. A little later, Interscience published for a number of year’s abstracts for fibers, rubbers, and plastics. Proskauer and I published them — all abstracts. After five or six years Interscience gave it up. And, the reason was that, first of all, there was Chemical Abstracts, which started to cover everything —for quite a while it did not cover everything. Then they were so well organized they really did start to cover everything. Then I think that because these Advances started to appear — there was an Advance in Colloid Science, there was an Advance in Polymer Science, there was an Advance in Biochemistry, there were Advances in Adhesion, and all kinds — there are now at least a dozen of such Advances. Why are they really? They are abstracts, reviews. We have the Macromolecular Reviews — about fifteen volumes of Macromolecular Reviews. They really covered the field. You find there all the literature. Very much of it. So, I think the specialized abstract services have not really survived in any of those fields.

Marvin:

So, it was not just money that led to that discontinuance. It was the fact that it was not needed so much.

Mark:

Not just money, no. You see, at the same time there existed Rubber Chemistry and Technology.

Marvin:

When did that start?

Mark:

Oh, in the late forties. It still exists. It’s more than an abstract; it’s a review, review papers.

Marvin:

Reprints.

Mark:

Not only, but essentially reprints.

Marvin:

One question on the side. In 1945 to 1951 there was some discussion of an International Union of Rheology Burgers and Scott Blair, and a number of others were on a special committee set up by ICSU. Were you involved in those discussions at all?

Mark:

No, I wasn’t really involved, but attended those meetings. You see, at the same time, 1946 and 1947, when I was a visiting Professor in Belgium in Liege, Smetz, Houwink, Rideal, and I, we advocated to set up a Commission on Polymer Science within the framework of the physical chemical department, or whatever the name was, of IIJPAC Division In the Division of Physical Chemistry (Hugh Taylor was the Chairman at the time, I think) we advocated a Commission on Polymer Science. And it was granted. And we started to have IUPAC Polymer Symposia. They went on for years and years. Three years ago, a special Division of Polymer Science was established. In other words, polymer science was then lifted from a Commission to be a Division. So today there is Inorganic Chemistry, Organic Chemistry, Biochemistry, Polymer Chemistry, and Physical Chemistry. But it started as a Commission. And it may have been since those things kind of overlapped, that the management of IUPAC decided to put it all together in the Commission of Polymer Science.

Marvin:

Well, this was a joint group of ICSU. A Committee on Rheology, with representatives from IUPAC and IUPAP and IUTAM… It was not just IUPAC.

Mark:

Oh, well, we were part of IUPAC.

Marvin:

I see. So your discussion was of that group. The joint committee I was talking about had, well; J. M. Burgers and G. W. Scott Blair were the leading figures. Eventually this did grow into the International Committee on Rheology. They decided that the group was really too small to set up a separate union, which was certainly true then. And I think still too small. Are there any other things you can think of about the history you would like to add? I thought maybe we could conclude by asking you to polish your crystal ball and tell me what you see in the future. But before we go into that, are there any other aspects of the Society you remember that you think I ought to know about?

Mark:

Well, altogether, looking back over the years, the ups and downs, I think certainly the people who were representing colloid science and, of course, rheology, were very resilient, and tough enough so they survived all these ups and downs, and they were not absorbed by someone. There were certain shocks. We refused to be swallowed by any of those shocks. The polymer people were shocks. So I think it was very much to the benefit of science, and it was a tribute to those people that they felt that the phenomenological aspects of how to measure something better and better, and how to interpret it, regardless of whether it was a suspension or an oil, or a metal, or a polymer solution, or a polymer melt. Methods, methods are important. And interpretation. And, of course, certainly within this framework, tremendous progress was made.

Marvin:

Yes. What do you consider the outstanding problems in rheology today?

Mark:

Well, I think there are several. From the fundamental point of view, my personal opinion is that we are now entering the period of refinements. I don’t think that any new, unexpected, fundamental law will emerge. After all, it all amounts to molecular interpretation of hydrodynamics or aerodynamics. There is a set of differential equations. They are, and always will be, the framework because all they really contain is the laws of the conservation of matter and the conservation of energy. And thermodynamics. In other words, first principles. But the molecular interpretation was really what interested us all the time. And my personal opinion is that this is pretty well in hand. There will be refinements. For instance, I’m certain that the Weissenberg effect, the anomalous behavior in the Weissenberg rheogoniometer, will need some additional refinement.

Marvin:

Well, there are still some serious questions of measurement. The hole error, for instance, in normal stress measurements, which I think are really not resolved. I don’t think it’s clear whether we understand our normal stress measurements as yet.

Mark:

Well, that’s why I’m starting to talk about the fundamental aspects. Not yet the methods — we’ll talk about the methods right away. So I think that in the fundamental aspects, in the merging of molecular aspects with the fundamental differential equations of either thermodynamics or aerodynamics, I feel that we are entering now an area of refinement. But from the point of view of measurement, I think, for instance, we know very little, let us say, about what happens if a bullet hits a solid body. That’s rheology. It’s a kind of special rheology. In other words, very fast. Very fast. Of liquids, of solids, or gases. Well, you know shock waves. That is really a part of rheology I mean, it’s aerodynamics But certainly something rhes, something flows, something moves. So that still is a big field, And supersonic phenomena. Measurements, as you said, measurements. One doesn’t know how to make the measurements.

Marvin:

Well, they’ve been touched on in rheology, but for the most part, people in that field have not considered themselves as rheologists.

Mark:

Yes, I agree with you that they don’t, but —

Marvin:

Of course, the definition of rheology keeps changing. I guess there’s no clear agreement on what rheology is; never has been.

Mark:

Well, there’s no clear agreement on what physical chemistry is. And then, I think there will be many important practical expansions. Maybe in the deformation of, not so much metals perhaps, but plastics. Then, of course – another — many people would say forget it, this has nothing to do with rheology — is plasma physics. How do charged particles move when they are compressed and when they are under very high temperature?

Marvin:

Of course, this would include problems of strength and all that that we were talking about. I suppose I should have asked this earlier.

Mark:

Well, first of all, I think they have been enormous. In the merger of molecular aspects and phenomenology. Classical hydrodynamics, classical aerodynamics, and its molecular interpretation. The gaseous state, the liquid state, and the solid state. And practically they have been even more important. In 1940, spinning speeds of a nylon filament was about 60 feet a minute. Today, it’s 3000. The casting speed, the extrusion speed, the injection molding, the entire technology of forming either a thermoplastic or a thermosetting material, and even metals, has been developed during this time. And it was all rheology. Practical rheology. Deformation science, the science of deformation.

Marvin:

How much influence have the basic studies, things like say, linear viscoelastic theory that’s been very thoroughly worked out during that time, how much real influence have those things had on these industrial advances?

Mark:

Well, I think in general terms, the first big step in introducing a new method into practice is intuition, careful observation, and tedious but tough development. And then one ends at a plateau. But then, from this plateau on there are many refinements. And they need a complete understanding of what’s going on. And they’re important. Because the plateau is eventually reached by everybody, and the man who has the refinement will make the money. Because he’ll make something which nobody else can make. For instance, the DuPont Company has been always outstanding, and still is. They always are better than their competitors. Why? Because of really good scientists in their ranks. And on top of that, they work with a lot of consultants, and believe what these people tell them. Paul Flory. He certainly was influential in a lot that went on in spinning inside of the DuPont Company. And Marvel. He certainly was influential in a lot that went on in their synthesis. And now still is there. Overberger. I was a consultant, and still am a consultant of DuPont’s, since 1940. That is almost forty years. Imagine a company that can stand my consultant ship for forty years!

Marvin:

Well, you mean by this that these theories are applied directly, or that they give the industrial development people a clue to what change to try next?

Mark:

What change to try next?

Marvin:

You mean even if you can’t really calculate the final answer in your process…

Mark:

Well, in any event it tells them in what direction, so it will be a step in the right direction rather than the wrong direction. You know Boltzmann once told us, when somebody once told him, “Look here, these theories of yours, they are not really practical.” And he said, “Well, look here, nothing is more practical than a good theory, because it not only explains to you what you have done in the past, but it also tells you what you ought to do in the future.”

Marvin:

Yes. So I guess you’ve really answered the last question I had jotted down, which was whether you thought rheology will continue as a separate discipline or whether we’ve reached the point where the major advances will come only in the rheology of specific materials, like polymer rheology, metal rheology. You think I guess that there are still some advances needed in our basic understanding of flow processes.

Mark:

I think, yes. Particularly of extravagant flow processes. Say you want to build up a bullet-proof vest. How must we do that? Several layers. To give a composite material. Well, what must each layer do? How do they eventually contribute to the fact that this thing goes back rather than goes on? And I think the tremendous importance of composite systems right now, in airplanes, in automobiles, everywhere is a challenge for rheology.