About science; About plastics

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. Hitz 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 their 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 of plastic is heated through it the plastic suddenly becomes soft. Another way of characterizing this

temperature is a softening temperature. And this is 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 somewheres thereabouts 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 and cools it down and 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 like 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 classed temperature. Any plastic at room temperature is below its class temperature but a lot of plastics get very sticky when you heat them up. 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 molecules 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 is known about their properties now. Well the origin of this the field of polymer science actually goes back into the roots of. Call it science now call oit 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 cool little 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 law of physics that the 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 it. 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 colloidal in nature but gradually due to the pioneer work of stunning earth 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 was originally thought. Well this. Give a measure of simply that there was a big molecule but what about the long run. 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 as 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 sizing this is known as vocalization are crushed like you 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. Is 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 well using the same techniques as a crystal analysis is correct. 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 can in an isolated form. And this is achieved by dissolving the material before it is vulcanized 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 key 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 measurement is a complicated set of anything you have to do both on the material in a solid and dispersed in a in a solvent. But they real difference and between step plastic and the rubbery ones as a smoking point this class is what is very key. Differences Yes. Well what is if you have the long

molecules. I suppose that 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 and 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 doubt on it why this is about correct. Yes on the other hand as one goes through the spectrum of lengths of

molecules you get it all the way down to the polymers 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 sounds gives me just a like of these go pretty much along with a molecular weight. Yeah and the ratios gives these precise relation and almost very precise relation is that the the molecular weight goes is the square of the end 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 mommies whether it's a repetition of a given

mana merit 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 built up of sub units. These molecules are going to be 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 the on one hand which is a random mixture of two types of units as opposed to another which is a sequence of blocks of two different types of units are amazingly different. But working always of 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 at room temperature or blasting. Do you wish any other block is rubbery at room temperature. And by combining blocks of this sort when you Jeeves not only be the average be arithmetic average of the two types of properties but also hold 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 and that is 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 same units are all of the again this is correct now. Another factor that enters immediately is the length of the branch 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 us they say they're sort of fuzzy but they don't they don't stick together acted be very much like or like the logs you seen a logjam 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 known that for example of one measures the viscosity which is simply to resistance to flow through the head 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 sticking together.

The resistance of love 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. The 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 case in under certain conditions for entangled polymer molecules. On the other hand again there are some conditions in which the unraveling is very difficult.

Then these appear appear then to be stuck together with some other kind of force shooting 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. They are resigned to stand or that cosmic dust gradually separated into separate day. Galactic entities which gradually spiraled and condensed and reduced what we have today and I think you have very much the same situation as 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 to 10 to 15 molecules and are separated one from another by relatively weak forces and in the regions between these domains we have a discontinuity which may appear as either pinholes or voids or even this is again very much like a crystal structure then you have existence of molecules in one clump and then a little bit of space and another clump of a few more billion and so on. Something like this yes except not quite as regular as in a in a crystal way which might be represented by a metal or Sol like Saudi. The writers clumps themselves 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. Filming structure OK and 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 have 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 word for

initial starting cracked 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 an start you see started to run. I see in the how hands on how deep you make Got enjoin how deep you make that crack. Now the interesting point is that if this crack depth is made small enough so that it becomes comparable to the micro cracks that are already in the material from its Nason see comparable to the size of these what we call the fix. Then the relation between the stress order of 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 there he's 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 non-American into stead 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 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 as small bales of bitter die he had mixed 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 same water size as the domain's kind of yesteryears. And you're working yes. What do the synthetic rubbers like rubber tires for example. Do they have an unusual properties besides just strength that I seem to recall there are other things like abrasive resistance to abrasion and so on of these other 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 say hi long Asians last again General will even see you in a Geisha and by that I mean breaking strain 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 and Marie's Asian and have been able to obtain even geisha is of the order of 50 to 100 percent and that becomes a synthetic rubber when you can watch something on the other hand through 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 sure I see in 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 Open the characteristics that are important in artificial it 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 will 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 it 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 did and Stu the boss of abrasion resistance of the material can slap on the head. 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. Course this situation happens to be a little worse in the Los Angeles hockey area right. The oxygen has a better chance here at least on the rubber and the ozone. 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 view to dying type structure which is the backbone of many

synthetic runners 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 i don't peroxide is formed. And once this is formed I don't proxy the composer's relatively rapidly. I see this breaks that a long molecule breaks cleans your degrades or separates the molecule. What are the kinds of plastics they use for adhesives the same have the same general properties as the kind we've been talking about as a rubber like. Well they have and they have the properties that they first of all are rendered fluid by a suitable solvent the 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

to grow it right and then in addition there are very many plastics today with plastic adhesives 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 to him Do they stick to the appearing surface more strongly than they stick to themselves. In some cases this is actually the case yes. In fact that 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 heat without removing it. So if you break that you're going to break the material rather than the rather than the adhesive. 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 don't 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 with little and I suppose all the detail in my post showed 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 Blatz associate professor of materials science at the California Institute of Technology. Join us again for our next program when two more members of the counter-act faculty will discuss a subject of interest about science is produced by the California Institute of Technology and is originally broadcast by station KPCC Pasadena California. The programs are made available to the station by national educational radio.

This is the national educational radio network.