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This is about science produced by the California Institute of Technology and originally broadcast by station KPCC in Pasadena California. The programs are made available to this station by national educational radio. This program is about plastics with host Dr. Albert Hibbs and his guest Dr. Paul Blatz associate professor of materials science. Here now is Dr. hit those plastic products such as rubber of long been known and they've been used for many years but it's only within the last 30 years or so that this field of research called polymer science has become a systematic effort and the results of this effort are not only the development of a wide variety of new plastics and rubber like product but also a better understanding of how to improve their strength and he properties and many other their properties. To talk about this line of research we have as our guest Paul Blatz an associate professor of materials science at Caltech and Paul let me start out by asking for a definition of what is a plastic. Well a plastic
is. Collection of very long spaghetti like fiber like molecules in a configuration that renders them relatively immobile and stiff. But it's the way it's both the blank of the molecules and their connection together that makes it a plastic clam. This is correct. If I had a random orientation of long molecules would that qualify as a plastic or do they have to be joined in some lined up all the same way as a bundle of fibers something like that. Well now we get into further distinctions among various types of plastic materials. Plastic is a very general type of term now associated with plastic materials. There is a characteristic temperature known as a glass transition temperature. It is the temperature at which if for example a plastic is heated through
it the plastic suddenly becomes soft. Another way of characterizing this temperature is a softening temperature. And this as I guess is called Glass translation because that's where glass behaves is correct. So for example if one starts with a material which is hard at room temperature like polystyrene which may be exemplified by a letter opener then as one heats or warms this material at a temperature of about 200 Fahrenheit or somewhere is there about the material suddenly softens very markedly and becomes rubbery or conversely if one starts with a rubber like material at room temperature like for example a rubber that you find in a synthetic tire cools it down then it's a temperature of let's say about minus 60 Fahrenheit or minus 80 Fahrenheit. It suddenly becomes very hard becomes classy What is this the essential difference between what I might call a plastic white polish styrene or something of the
sort and an artificial rubber this temperature. This is it exactly correct. What do you do. Rephrasing it again. A rubber at room temperature is above its class to temperature and the plastic at room temperature is below its class temperature. But a lot of projects get very sticky when you heat them up a rubber doesn't it's. This is correct. This depends on other factors now such as for example the length of the molecules and secondly how the molecules are tied together or coming back to the long molecule for just a minute. Since we've never been able to see a single molecule How was it discovered that there were such things as long molecules at all. To say nothing of the fact that all that is known about their properties now. Well the origin of this the field of polymer science actually goes back. Into the roots of coal oil and science. Now
coal oil refers to originally as it was thought of it referred to an aggregate of small molecules and no clear understanding at the time and I'm referring now to perhaps 50 years ago no clear understanding was then had of the nature of this aggregation. But it was known that the particles the coital particles are large in the sense that a beam of light impinging upon such a particle would be markedly scattered. And it is a basic Well physics that. Scattering phenomena are brought about by particles are roughly the same size as light or the light that is scattered. So there's a way of measuring the size. It was known that there that the organic chemists were extracting from various natural products materials which would scatter light markedly. These materials were originally thought to be colonial in
nature but gradually due to the pioneer work of stoning or a German organic chemist. It was established that these materials indeed were connected one to another in a chemical fashion rather than some sort of loose physical fashion as it was originally thought. Well this would give a measure of simply that there was a big molecule but what about the long thin. How could you find out it was long and thin rather than begun around for example. Well let me talk around that in the following fashion. First of all. Come back to your point about the softening of some materials which become gooey and sticky whereas others do not do this but remain retain their resilience it was found for example that in the case of some rubber like materials. Such as I said brains or natural rubber that the addition of a small amount of sulfur would set the rubber like materials into some sort of a
permanent chemical structure called an EIS and this is known as vocalization across like you're right and it was then found that such materials did not become soft and go and sticky upon warming but would retain their dimensional stability and also would be able to even see very market elongation. Now when such materials were studied under the X-ray it was found that the molecules would give rise to regular Crystal and rays and from such studies estimates have been able to be made of the earth. It says size distribution up to Christmas lights and from further studies one was able to assess and obtain some indications of the fact that these molecules work long and screen. I see so without using the same techniques as in Crystal analysis is correct and this is a but
this is only part of the story. In addition another approach to the question of the sighting how the molecules are actually shaped is provided by studying the molecules when I thin an isolated form. And this is achieved by dissolving the material before it is balkanized or a material which is on vulcanized you say by dissolving it in some sort of a suitable solvent solvent isolates the molecules one from another and then by performing various types of tests which may involve again either a light scattering or might involve. Some sort of measurement of a vapor pressure or a freezing point or a boiling point or a dielectric constant or a specific e or some other type of physical property one can infer back from these properties that the molecules are indeed wrong. I see it's complex not a single measure mass a complicated set of anything you have to do both on
the material in a solid and dispersed in a in a solvent. But the real difference then between step plastic and the rubbery ones is this melting point this glass is what is very key differences yes. Well what is if you have the long molecules. I suppose that it now becomes rather important as to exactly how they are arranged through the properties of the plastic the pan and the detailed arrangement of the minus. Yes let's talk about that a little bit. There are several factors. First of all one of the primary factors is just the length of the molecule. And let me make a comparison when we talk of a really long polymer molecule. We're talking of something that has a weight or a molecular weight of the order of several million. And by way of comparison such a molecule would perhaps be about as long as a red light wave. I'm seeing a red light wave is
of the order of several thousand maybe 6000 angstroms on the other hand there are a millionth of a meter. I have my thought on it why this is about correct. Yes on the other hand as one goes through the spectrum of lengths of molecules you get all the way down to the polymer is which have their legitimate use in various structural properties but perhaps are only in the molecular weight of 10000. And this might be exemplified by us a molecule known as synthetic polyurethane rubber on out as does the length of these. Excuse me does the length of these go were pretty much along with the molecular weight. Yeah the ratios gives these precise relation and almost very precise relation is that the the molecular weight goes is the square of the end and distance. I see. Now in addition to the primary factor of length or
molecular weight of the molecule. Another important factor. Has to do with the actual detailed chemical structure of the backbone of the molecules of every single one of these whether it's a repetition of a given mine American unit or whether it is a so-called co polymer of 2 Type 2 units. And then when you get into such a structure you are concerned with whether the individual units of two types occur at random in sequence or whether they occur in blocks of one type in a block of another type you see. But in all cases these molecules are build up of sub units. He's not likely to go to the joined up sort of end to end one following another is correct and the properties that one can achieve merely by making a molecule on he on the one hand which is a random mixture of two types of units as opposed to another which is a sequence of blocks.
Today different types of units are amazingly different but working always with the same units just different arrangements along the length of a single molecule. Now let me elaborate further on that for example one can make and we do today make synthetic rubbers in which one of the blocks is a material which is glassy had room temperature or plastic. Do you wish in the other block it's rubbery at room temperature and by combining blocks of this sort when you Jeeves not only be the air but the arithmetic average of the two types of properties but also synergistic effects superimposed on top of that you see. So do you get a way critical of the glass conversion point sort of midway between the two. Yes yes and as a matter of fact you not only get that but you also get the right to regions of behavior each appearing as if they had their own glass transition temperature associated with it.
How would that appear when you work with it. It is a quite separate zones of properties it gives the temperatures and this can be picked up for example merely by warming the material on for example plotting its volume as a function of temperature and looking at the shape of the curve. Well what about arrangements between molecules. Well this is also important I think a logical connection between discussing that point and the point we have been talking about up to this time is provided by adding one other item to the picture and that is the question of branching. Not all polymer molecules are single line entities and many of them are bifurcated or proliferated in a in a branched sense very much like a tree with with branches and twigs you see. But the same units and I say Man that's right all of the can this is correct now.
Another factor that enters immediately is the length of the branch of a molecule with the short chain branches is again very markedly different from one with long chain branches molecules with short chain branches in general do not appear at least according to current thinking to entangle one with another markedly which is now leading into the next question. You brought up the same sort of fuzzy but they don't they don't stick together acted be very much like or like the logs you seen a lodge and which the loggers have taken off the major big branches but left on some of the twigs honey whereas on the other hand molecules with very long branches do in tangle markedly. And this phenomenon of entanglement is a very important one it's not completely understood but it is no Ned for example of one measures the viscosity which is simply to resistance to
flow that down as one increases the branching of a molecule and you can do this by controlled experiment in the lab. As you increase the branching you increase the viscosity in an abnormal fashion so you can actually measure the amount of the sticking together. The resistance as a rule yes as a result of the branching. This is Connect the two together with measure. They are there when the branches do stick together. Is this a chemical reaction that one branch has with another or is it just they come they're sort of intertwined it is hard to pull apart like two paper fibers. This is I will say quote correct unquote reason for this is that people are not quite agreed on how much force or energy activation energy of Huish is involved in pulling in tangled molecules apart. So it's hard to judge exactly what the nature of the binding force is between the branches for example.
One can envisage two very slippery pieces of spaghetti which no matter how markedly they are intertwined can be pulled apart very easily. This happens to be we think the cage in under certain conditions for entangled polymer molecules. On the other hand again there are some conditions in which the unraveled it is very difficult and then these appear appear then to be stuck together with some other kind of force shouldn't lose him from the slippery ones. Well does this determine the state of the overall strength of the plastic this well you have now I think we've already brought a number of items into the conversation we've discussed entanglement. We have discussed Vulcan ization And so we already have two factors that are going to contribute markedly to his going through Parliament. Now there is a third factor and that is a so-called domain structure very much like theories that concern the evolution of the galaxy The They're resigned to stand or that cosmic dust
gradually separated into separate galactic entities which gradually spiraled and condensed and reduced what we have today and I think you have very much the same situation if you know a polymer which is condensing out of the molten state. It condenses into domains domains are are microscopic but relatively large in a molecular scale a perhaps include anywhere from 10 to 10 did and to 15 molecules and are separated one from another by relatively weak forces and in the regions between needs domains we have a discontinuity which may appear as either pinholes or Boy George. This is again very much like a crystal structure then you have a billion years of molecules in one clump and then a little bit of space and another few more billion and so on. Something like this yes except not quite as regular as in a in a crystal which might be represented by a metal or salt like sodium to write just clumps some cells
are different one from the other then you know size and shape. But there still is a sort of a tendency to be a boundary between one class. Yes. OK so now we have the entanglement The balkanization and this domain business. The main structure OK in all of these things now bear on the strength of the finished. This is correct. The finished hunk of plastic. So when you if you now go to take a test would you start pulling the plastic apart. I suppose that it would give along the first on the divisions between these crops are and on the domain boundaries that would that be typical. Yes let me elaborate a little bit on that one can do the following experiment. For example take a slab of plastic and cut a very small notch in it. OK. On this side and then pull it in the direction normal up perpendicular to the notch and then at some critical stress the notch begins
to grow propagates across the specimen of course of this process is allowed to occur especially separates and breaks into two parts. Now one can plot or correlate stress at which the crack runs as a function of the crack depth. The words for initial starting crack depths. There will be and a correlated value of the stress which is needed to make that crack run all the way across to make it run to make it stop. You see I started to run. I see in the downs on how deep you make Got enjoin how deep you make that crack. Now the interesting point is that if this crack dept is made small enough. So that it becomes comparable to the micro cracks that are already in the material from it's Nashton see comparable decisively as what we call the fix then the relation between the stress order force needed to make the
crack run and the crackling deviates markedly from theories which do not take account of the fact that there are these micro cracks present in the material and therefore we conclude they were these are deficient in the sense that one has to go back and revise that theory to account for the presence of these micro cracks. What are these micro cracks connected to the domain distribution of molecules. Yes yes so that does this. Then the way in which the the existence of domains can be proven. This is one way that information has been shared on that score. Well is there anything that can be done in the design or manufacture of plastics to do away with the micro cracks OS and cancel out those who want what there are a number of very interesting things that are being done. Well first of all the obvious thing. Put in another type of mine to marry. You know the state of working with
a single homo polymers that is cool. One can go to a COBOL or an or if you've already got the cracks and COBOL and we might go to inter polymer. The presence of a different type of chemical environment in the chain very often for reasons that we don't completely yet understand can can lead to improvement of the general backing of the of the molecules. It was a do away completely with this domain structured as a chain just change the size of a domain. We don't know the answers to this computed yet but it's in the in the direction of improving the situation. Now another procedure would be to actually physically blend two types of materials together. And this is done for example in. In fabricating impact plastics in which small pills of synthetic rubber are physically at mixed or blended into a
plastic such as polystyrene the improvement in impact strength of a polystyrene which has small bales of bitter die eat at MIX is markedly improved over that of ordinary polystyrene. How small are these pills. Oh they'd be in the order of microns or tense of microns and perhaps the same about the same water size as the domain's kind yes yes. And you're working yes. What do the synthetic rubbers like rubber tires for example. Do they have on unusual properties besides just strength. I seem to recall there are other things like abrasive resistance to abrasion and so on and these are the properties like that also connected to these same physical characteristics of the plastic that domain. I would say to mark a difference between a rubber and a plastic is the resilience of the rubber and its ability to even so high heat
on gay shades. Last again General will we even see you in a Geisha and by that I mean breaking the screen or breaking defamation of the order of a few percent. Of course there are many plastics that we know today in which we've been able to. To carry out this modification of properties by Kopel him or ization and it would have been able to obtain even geishas of the order of 50 to 100 percent and that becomes a synthetic rubber when you can watch something on the other hand through a synthetic rubber that is one in which the glass transition at is above or in which is at room temperature that Europe is above its class temperature. Such material can even several hundred percent. Oh I see OK should I see the fact that I was in percent is not uncommon. What's typical for Natural Rubber by the way about natural rubber I'd say about 800 percent. So the artificial rubbers are quite comparable and with natural rubbers and us
all the characteristics that are important in artificial good in making tires of synthetic rubber. Coming back to tires again are those characteristics amenable to this kind of modification of polymers adding different kinds of polymers into it can you. Oh change the embroidery properties of a tire would vary markedly. Another factor that is important in connection with the behavior and usefulness of a tire over and above and brazen which you mentioned is heat build up heat build up has to do with essentially friction between molecules to put it very simply. It's a viscous process of one molecule rubbing against as the tires flexed as the tire was flexed which it does periodically and sure as it churns around and around usually And in this case this process of heat buildup can markedly be reduced by using the all American units them on American units properly in the right proportions.
You know also with the right amount of trust what happens if the heat build up gets out of hand as this destroyed the plastic structure is breaking down these bonds are what what happens with the rise in temperature. It edged to the boss of abrasion resistance of the material can slip along. In addition it probably also increases Duraid which the rubber will oxidize all robbers with very few exceptions oxidize at certain slow rate in air in order to prevent this oxidation. Various types of antioxidants have to be added. Of course the situation happens to be a little worse in the law thank you and Aki air right the oxygen has a better chance or at least on the rubber in the ocean. Yes but then the heat. This is it heat that makes that go more quickly. Yes so if you cut down the internal heating process you can preserve it against oxygen what happens with oxidized by the way. What.
Well one of the steps involved has to do with an attack on a very key element of all chemical backbones and that is known as the double bond. There is always a double bond present in a beauty DIYing type structure which is the backbone of many synthetic routers that's along the single molecule. This is a particular type of chemical bond in the in the backbone of the molecule which is amenable to attack by oxygen and a type of degradation product known as a hydro peroxide is formed. And once this is formed I don't proxy decomposes relatively rapidly. I see this breaks and a lot of molecule breaks cleans or degrades or separates the molecule. What are the kinds of plastics that use Fred he seems the same have the same general properties as the kind we've been talking about as a robber like well they don't they have the properties that they have first of all are rendered
fluid by a suitable solvent a vehicle for carrying them to a position on an inherent and then they have these armed Lehi of waiting for the particular surface that they're destined to act as an adhesive to now stick to it as they run with it right and then in addition to very many plastics today with plastic it he says I should say today have the additional property that as they age they become harder and set by virtue of a post curing process which takes place right in connection with the adherent In other words they actually wind up as chemically bound to the adhering surface. I say dualistic to the hearing surface more strongly than I stick to themselves. In some cases this is actually the case yes. In fact there are quite a number of good it he says on the market today that will I stick to a surface so strongly that one cannot remove the
he said without removing it. So what if you break that you are going to break the material rather than the other. Rather than leave it heaved a can is it possible to consider the future that will ever make perfect plastics that have no cracks and are infinitely stronger Strongs of theoretical limits and so on. I for one I don't think this is a by itself a goal to shoot for in the sense that no surface is perfect you see the surface consist of many ridges in valleys and actually the the upper limit of use of the plastic will be deemed limited by how well one can get the plastic into these ridges and valleys and little by little and I suppose over the detail of my OSOs a pulse thank you very much for talking to us today about plastics. My pleasure. This was about science with host Dr Albert him and his guest Dr. Paul Bland's associate professor of materials science at the
California Institute of Technology. Join us again for our next program when another subject of interest to scientist and layman will be discussed. About science is produced by the California Institute of Technology and is originally broadcast by station KPCC Pasadena California. Programs are made available to the station by a national educational radio. This is the national educational radio network.
About science
About plastics
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California Institute of Technology
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University of Maryland (College Park, Maryland)
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This program focuses on the science behind plastics. The guest for this program is Paul J. Blatz.
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Interview series on variety of science-related subjects, produced by the California Institute of Technology. Features three Cal Tech faculty members: Dr. Peter Lissaman, Dr. Albert R. Hibbs, and Dr. Robert Meghreblian.
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Guest: Blatz, Paul J.
Host: Hibbs, Albert R.
Producing Organization: California Institute of Technology
Producing Organization: KPPC
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University of Maryland
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