Lecture 29: Structure-property Relationships in Polymers, Crystalline Polymers

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OK, let's get down to business. One announcement: tomorrow will be weekly quiz number 11 based on homework 11. And then, of course Thursday and Friday there will be no academic exercises in connection with the Thanksgiving day holiday.

So, last day we started talking about polymers, macromolecules. These are chemicals that have a very high molecular weight with a repeating chemical structure. I've given an example here of vinyl chloride as the individual molecule.

And, by addition polymerization, we can break the double bond, have it propagate, and we end up with something which is the polyvinyl chloride. N numbers can be very large, and we can get molecular weights in the vicinity of 1,000,000 grams per mole.

We further looked at tailoring properties by control of molecular architecture. We saw that we could look at bulk composition and form such things as copolymers, which are to polymers what alloys are to metals.

Another way of tailoring properties is to vary the arrangement of the side groups on the backbone. This is the tacticity of the polymer. And it is to a polymer what the stereoisomerism is to individual molecules.

And that lastly, we looked at the shape of the backbone itself, different conformations, and this is owing to the rotational capability of the carbon-carbon bond. And, at the very end of the lecture, we looked at two forms of formation.

One is addition polymerization, which is shown here. We have the three stages. We start with initiation here. You see hydrogen peroxide, which has turned into the radical. And that triggers this reaction, which then moves the unpaired electron to the end here.

And this is sticking out like a hot poker. And other gas molecules in this case, ethylene that come in contact with it will, in turn, have their double bond broken, and so on, and so on. And finally, we get to the length that we want.

We terminate the reaction, and that's the end of the process. The second one was condensation polymerization. And condensation polymerization is stepwise, where you take two different mer types. And here's a cartoon showing an example of the formation of PET.

This is the material that's used for soda bottles, and many of the clear plastic containers that you find in the supermarket or the drugstore. So, this one vividly indicates that it, in fact, is an acid-base reaction.

I said last day, you had an R1, which is a proton donor, and an R2, which is a hydroxyl donor. Those are classical Arrhenius definitions. And so, in this case, you start with terephthalic acid, which is basically benzene, with this carboxylic acid on the end.

The COOH is carboxylic acid. We'll see that repeatedly. It's a basic group. Basic is the wrong term. It's a group intrinsic to proteins. It's an acidic group. It's not a basic group. But, basic meaning a different way.

So, here's terephthalic acid. Here's ethylene glycol. This is what you put in your cooling system. This is the same thing as Prestone 2 Antifreeze. It's a double alcohol. It's got hydroxyls at both ends, it gives up one of those, combines with the proton here to form water.

And then we form the amide bond, which then starts the polymer here. And, it in fact is a polyester because in the end, you've got this COO that the O acts as a bridging oxygen. So, bridging oxygens we've seen in silicates, borates, as critical in covalent network formation.

And, they are operative here. When they are operative in polymer systems, that type of polymer is termed a polyester because the ester linkage is this oxygen with a double bond. So, this is polyethylene terephthalate, PET as you know it.

And, the other thing that I wanted to point out is that if you look carefully at the structure of these, last day we studied cross-linking. And, cross-linking depended upon a covalent bridge between two chains.

And in this case, it's typically made with sulfur. So, we have sulfur attacking residual double bonds in the backbone to form single bonds between backbones. So, we have two chains linking. Well, if you look at the features of the polymers that are made by condensation polymerization, you'll notice that in many instances, I think here's a good one to look at, Kevlar.

You see the carbonyl group here, the C with the double bond to the oxygen. And, on this side, you have an amino group: nitrogen with a hydrogen. And, what can happen is adjacent chains can hang the aminos on one side opposite carbonyls from the other side.

And, look at the situation that we've set up. Here's the oxygen off the carbonyl group. And, it's sitting opposite the hydrogen off the amino group. And, you know what that can lead to: hydrogen bonding.

And so, in many of these condensation polymers, nylons, silicones, hydrogen bonding acts almost as a pseudo-cross-linking. It's nowhere near as strong as a true covalent cross link. But what we do observe is that the nylons, Kevlars, things like this have much, much higher strength.

And, we're going to learn a little bit later. They have a higher melting point owing to this hydrogen bonding. So, I think we want to just document that: hydrogen bonding operative in nylons and silicones.

And, this leads to some additional strengthening of the material. Well, what I wanted to do today was to take up this concept that we talked about with regard to ordering versus disordering last day.

We talked about low density polyethylene versus high density polyethylene. So, I want to just revisit influences on ordering, and actually what favors crystallization, factors favoring crystallization.

And, remember, we are not talking about perfect crystals here. We're talking about local ordering over certain lengths of chain which give rise to some strengthening of the material. And so, I'm just going to go back and revisit one, two, and three above.

I'm going to ask, how does composition, how does tacticity, and how does conformation enhance crystallization? So, first thing: let's look at composition. The way to favorite crystallization is to make materials more nearly uniform.

The higher the degree of uniformity in the polymer, the greater the chances are that it can zigzag back and forth and in a predictable manner, set up these zones of crystallinity. So, if this is a straight chain or a linear chain polymer, if it's made homogeneous of the same composition, then the chances of its folding back and forth on itself and setting up this local sort of semi-crystalline region are greater than if the polymer has some random variation in composition.

So, the factor that favors crystallization is having single composition. So, homo-polymers are much more easily crystallized. So, as an example, polyethylene oxide can form crystalline polymer, whereas if you take the polyethylene oxide, block copolymer with, say, polypropylene oxide.

The second case is more likely to be amorphous. The second factor we want to look at is tacticity. You've got the three forms of tacticity: isotactic, syndiotactic, and atactic. And, the one that's more readily willing to pack is going to be isotactic.

So, isotactic version will be favored from the standpoint of crystallization. And, we have evidence that isotactic polystyrene is highly crystalline. So, when you see some of these hard polystyrene containers, they are predominately isotactic, whereas atactic polystyrene is dominantly amorphous.

So, there's the influence of tacticity. And the third one is conformation. How does conformation act? Well, we've got the linear chains. We've got branched chains, and we've got cross-linked chains.

And clearly, following this paradigm, the linear chains are favored when it comes to the formation of ordered structures. So, if you look at something like high density polyethylene, that's shown here, this is the opaque milk jug.

This has a high degree of linearity in the chains, whereas branching or cross-linking are going to make things very difficult from the perspective of forming crystalline structures. And, in fact, here's something that's volume versus temperature.

It should remind you of the unit that we have just covered in silicate glasses. We see the cooling curves here at two different rates. And, fast cooling as a knee in the curve at a higher glass transition temperature than slow cooling.

And, similarly, with fast cooling, there is a higher degree of quenched in free volume. So, all those ideas apply. The other thing I want to look at now is on the basis of what we've seen so far, what are the properties of polymers? We can tailor things to a great extent.

But there are certain properties that they all share. So, let's document those. First one: electrical insulators. Why? Whenever I ask why, we have to look at electronic structure. Here, we have strong covalent bonds up and down the backbone.

And, we have relatively weak van der Waals bonds with some hydrogen bonding in between. In no case are we looking at free electrons. So: strong covalent bonds internally. These are intramolecular bonds, no free electrons.

With covalent bonds, we have electron sharing and we have octet stability and the electrical insularity that accompanies that. Second, optically, optical behavior, while they're transparent to visible light, again, the high-strength covalent bonds transparent to visible light, and in the case of amorphous, when amorphous, they are clear.

When amorphous, they are clear. And, when we have the semicrystalline setup, regions of higher atom density, which translate into regions of higher refractive index. And since the refractive indices are not matched between the amorphous and the semicrystalline regime, this acts effectively as an interface that can scatter light.

So, the semi-crystalline, although they are formally transparent to visible light, these in fact prove to be opaque as you can see from the appearance of that milk jug. Now, the question you've got to ask is, could you generate semicrystalline zones that were either index matched or on a length scale shorter than that of visible light? We saw that in the inorganic case with the difference between Pyroceram glass and visions.

The only difference was that the crystallites in visions were much, much smaller. And as a result, we were able to get the strengthening effect without incurring the penalty of opacity. The third one is chemically inert.

Polymers are chemically inert. Again, the strong covalent bonds, octet stability, same reasons. And this leads to their extensive use in packaging, packaging especially of foods and beverages. Foods and beverages exploit this property.

And, lastly, they are solid at room temperature. The long molecules, even though they're only weak van der Waals bonds operative between molecules, remember, the strong covalent bonds are within the molecule.

If you want to ask the question, is something solid, liquid, or gas, you have to ask, how does one molecule bond to another one of identical composition? And for that, we got not strong covalent bonds, but we've got weak van der Waals bonds.

But, they are operating over such large contact areas. These molecules are very, very long. So, the long molecule presents a high surface to volume ratio, very high surface to volume ratio. And, that leads to strong bonding.

And ultimately, it leads to entanglement. So, we have entanglement. We have weak van der Waals bonds over high surface to volume ratio. And, that leads to solidity at room temperature. And, we recognized last day that absent cross-linking can reprocess these by reheating.

This gives us thermoplastics that can be reprocessed. And even in the case of hydrogen bonding, although the nylons have to go slightly higher temperatures. We are able to reprocess the nylons. In the case of cross-linking, of course, we have to go to other measures because heating to very, very high temperatures capable of breaking the cross links would also break the backbone.

So, these things become not very easily recyclable. And, in fact, what I've got here is the recycle codes which you see on products today. The first one is the polyethylene terephthalate which we just saw the reaction for.

And that's used in these clear plastics for many of the things such as, in this case, it's dishwasher detergent. The two-liter soda bottles and so on are this. High-density polyethylene, that's this one here.

The next one, V, is the reaction I showed at the beginning of the lecture, polyvinyl chloride. The fourth one is low density polyethylene, which is the stretch and seal and so on. And, I want you to note that all of these have names associated with them.

And, these are inventions. All of these materials that I'm showing you are inventions of the modern era. They all came as a result of research, patenting, etc. So, you see this one: 1940, 1951. These are very, very recent materials.

Polypropylene: 1951. Polystyrene technically was isolated by a German apothecary in 1839 who didn't realize what he had. He had some material that he got out of this resin, and it was around 1922 when Herman Staudinger reasoned that in fact this was as long chain of styrene.

It was polystyrene. He wrote extensively on it, and eventually won the Nobel Prize for it. Technically, Simon isolated polystyrene in 1839. And then, seven is everything else. So if you see a seven at the bottom, it's, who knows? So, that's the chain of events.

So, what I'd like to do now is I want to show you a few demonstrations. And in years past, I had someone come in here and do this. But, she has since graduated and there is a whole bunch of other factors involved.

So, what I want to do is I've got some video footage of the demonstration as it was done. So, what you're going to see is several things. The first thing I'm going to show you is the synthesis of nylon.

And, this is beaker synthesis of nylon 6,6. It's condensation polymerization. And, we're going to use hexamethylene diamine. So, that's the base, and adipic acid, which is obviously an acid. So, acid plus base gives water, plus in this case the neutralized polymer.

This is only done for demonstration purposes. In industry, nylon is not synthesized this way. They typically go up to around 300°C. So, they're going to take each of these ingredients. And, they're going to dissolve them in different solvents.

So, in one case, we're going to dissolve the hexamethylene diamine in an aqueous solution, and then the adipic acid is going to be dissolved in hexane. Hexane is aliphatic. It's nonpolar. It's immiscible with water.

So, you're going to see two liquid phases: an aqueous phase and an organic phase. So, one ingredient is in an aqueous phase. The other ingredient is in the organic phase. The only place they come in contact with one another is at the interface.

So, you're going to see nylon synthesized at the interface between the two liquids. And, here's the reaction. This is Heidi Birch speaking. [VIDEO BEGINS] Oh, shut up. [LAUGHTER] Train your eye right here.

[VIDEO ENDS] I'm going to show you, we had a close-up of a previous year when she did it and we actually had a camera focused right on the beaker. So, you get a better scene of the formation of the nylon.

It's happening right here at the interface between the hexane and the aqueous solution. So, the nylon is forming right here. And, as soon as the nylon forms, and seals off the reaction so no more reactants can get here.

What she does is she pulls this up. And as she pulls the nylon up, she exposes fresh interface. So, everything is occurring at this interface. It's a self-limiting reaction. So this next one, I think, shows it just a little bit better.

[VIDEO PLAYS] OK, the second one I'm going to show you is a demonstration of the role that the glass transition temperature plays in rubbers. Rubbers are materials that are cross-linked. And, they gain their bounce from the fact that if you pull on the rubber these cross links will cause you to have to recoil.

But if you cool the rubber below the glass transition temperature, the entire structure becomes a rigid solid. And so, the rubber loses its bounce. In our lab, if we have to make, for example, a hole through a rubber stopper, if you take that rubber stopper, put it on a drill press and try to drill, it's very dangerous to do because the rubber will grip on to the drill bit.

If you quench the rubber in liquid nitrogen, take it below its glass transition temperature, the rubber behaves as a brittle solid. And then, you just drill right through, get a nice, clean hole, and then the material heats back up above its glass transition temperature.

So, Heidi is going to show you two different rubbers, one that has a glass transition temperature above room temperature. So, room temperature is already down here, and the rubber ball, I'm saying in quotation marks, the ball is going to act like a billiard ball.

It's solid, very poor bounce. The other one has a glass transition temperature of negative degrees C. and so, at room temperature it bounces well. So, she's going to, in one case, heat both of them above the glass transition temperature, put them in boiling water.

The other case, she's going to put them in liquid nitrogen. So, she'll turn both of them into solids, and both of them into liquids. [VIDEO BEGINS] He did. He did. Did you see that? Pay attention.

[VIDEO ENDS] OK, the last thing I am going to show you is something very peculiar with macromolecules. I'm going to show you two beakers that contain liquids that are clear colorless, but in this case, green food coloring has been added.

The first one is water. And, when the propeller spins, you get a vortex. In the case of polymers, at low spin rates, the same thing happens. At a low spin rate, you'll get a vortex. At a high spin rate, when the spin rate goes faster, and these are liquids.

We are above glass transition temperature. But if you try to spin too quickly, you get shear dependent behavior. It's peculiar with long chain molecules. And what happens is, at very, very high shear rates, the molecules actually won't slip fast enough and they essentially behave as an elastomer.

So, at very high shear rates, instead of seeing the vortex trough, you see quite the opposite. And this is called the Weisenberg effect. And it has to do with the inability of the molecules to move quickly enough over one another.

Let's see what this one looks like. This is a short one.  [VIDEO BEGINS] That's water. So there's the Weisenberg effectct. And this is a very, very high shear rates so, the one thing I wish she had done is turn the speed down.

At a very, very low speeds you would actually see the vortex form because you're asking the molecules to shear over one another in a slow enough rate that they can keep pace. So, this is very different from Newtonian fluids.

This is non-Newtonian flow. In this, you have to understand if you're doing such things as injection molding of polymers because they do weird things at very, very high shear rates. And let's face it, if you're in business to make money, you're not going to be gently plunging the piston into the mold.

You want to pop these things out quickly. So, there's a lot of esoteric physics involved. OK, I'm going to hold that there. What I want to talk about now for the last few minutes as the cultural impact of the advent of polymers.

In fact, many of these come under the general rubric of synthetics. These are man-made materials. These are synthetics. And the real burst occurred at the dawn of the 20th century. And what I'd like to talk about is the pattern of adoption.

And this is something that occurs with other materials, but certainly in the case of polymers. At first, it's wonder. They have these strange properties that we've seen some of here. And then, people go crazy with substitution.

And what we find is that, for example, the first commercial polymer was Bakelite. And, Bakelite was used for such things as billiard balls, piano keys, and so on. So you have what I call the fake everything.

So you have faux ivory, which then means you can have ivory free billiard balls, ivory free piano keys and so on. With the advent of nylon, you have fake silk. And so, you have, instead of using silk, silk stockings were a luxury afforded only by the rich.

With the advent of nylon stockings, there's a democratization. Price plummets. Huge social changes take place. So, all sorts of things take precedence that previously couldn't happen. I think I've got some shots here.

This is an ad from something like a Look magazine. This is Bakelite. And if you go to grandma's house, you may even see some of these large coffee urns. And the handles on the side are typically sort of brownish.

These are made of Bakelite. And it's a phenolic resin that's made by carbolic acid with formaldehyde. So, it's a condensation polymerization. And, the note the symbol here for Bakelite. Bakelite, there's a B with an infinity sign underneath it, the material of a thousand uses.

And, these are all substitute parts that were previously made of metal or wood now being made of Bakelite. By now, I mean 1910, 1915. So: big change. And, plus, certain parts that used to be made of metal are now being made of Bakelite, which is an electrical insulator.

And for electrical applications in certain instances, you do not want the ability of that part to conduct electricity. So, that's the sort of thing that takes place. And then, we have new uses, new products, new devices.

And then, here's a shot from Fortune Magazine, 1940, referring to them as plastics. And I'll explain why in a second. But all of these new products, you've got the telephone here, fashion, auto parts, footwear, all of these different things.

There is LP records. You know this is, but years ago there was an electromechanical device called a tone arm with a needle in it. And it used to play records. Yeah, well they were made of vinyl. Actually they were made of polyvinyl chloride.

And look at this thing here. You see this thing? That's a film strip. Photography had been invented back in the early 1800s. The mere fact that you can go to the movies is a result of the advent of macromolecular chemistry.

Without celluloid, there's no way you could take glass plates and move them at 24 frames a second and watch Star Wars. You just can't do that. You have to have the ability to put halide on something that is flexible and can be made in long strips.

And, there was no material until the advent of polymers. Why plastic? The general public refers to these as plastics. If I go to Shaws, and I'm checking out and ask me what kind of bag I want, sometimes I slip and I say I'd like the polymer bag.

And they look at me and I say that one. [LAUGHTER] So, it comes from the Greek word meaning to shape. It's a cognate of the word potter. So, plastikos really means something that can be shaped. And, many of these products are made by processes that involve starting with polymer that the shape is changed.

And that's it makes it so easy to stamp things out. This is also from the 1940 Fortune Magazine. This is A Whole New World. I don't know if you can read this definition, but here's an area called cellulose.

There is Nylon Island over here. There's cast phenolic. I mean, the whole world: there's the Acrylic Mountains. Everything is polymer. I recall seeing in an antique shop a ladies Waltham watch made here in Waltham, MA in 1922: platinum case encrusted with diamonds.

The crystal was made of acrylic. Acrylic has very poor scratch resistance, but acrylic was an advanced material. And by golly, there were going to make it out of acrylic. It's an advanced material.

And it keeps going and going. Here we are after World War II. This is Plastics: the Future Has Arrived. First, it doesn't look like anything. It looks like abstract art. Actually, this is a room.

This is an entire room made of sprayed polyurethane foam. This is 1968, people going crazy over polymers. But then, people come to their senses and this book here documents quite nicely sort of the next stage which is the chagrin.

And for that I want to take you to a piece of iconic filmmaking when people are starting to wonder what's the real future of polymers? [VIDEO BEGINS] This could be you three years from now. He's just graduated from college and he's coming home.

[LAUGHTER] That's a young Dustin Hoffman in 1968. OK, now this is the scene. Watch. That's Angela Lansbury. [VIDEO ENDS] Well, in point of fact, that's so ironic. By 1968, it's not true to say that there was such a great future in plastics because people started to realize the next thing about, which is the concern.

And, what's the concern? Well, I've already talked about recycling. But, there are other concerns. And they all come from this: environmental safety and health. We know about the environmental, the difficulty to recycle.

And, that's a challenging problem to this day when it comes to thermosets. And, even the ones that are recycled, there's this concept: recycle as opposed to reuse. We recycle polyethylene terephthalate.

But I hope you didn't think that this goes back on the supermarket shelves in about 20 days as another bottle containing dish wash detergent. There is a cascade of utility. So, you start off with something of high-value, and it gets shredded maybe as carpet fiber.

Then you end up with something that could get molded into a park bench. But, the value keeps going lower, and lower, and lower. So, this is still a thorny issue when it comes to much of polymers. And then, of course, they are not degradable in landfill.

Why? Strong covalent bonds. Very difficult: so that's the E. And, let's talk about the H - health. We saw the Weisenberg effect. In order to facilitate the processing of polymers, we add lubricants, what is effectively a lubricant to enable the polymer, the long chain molecules to more easily slip over one another.

And, these additives are called plasticizers. So, they facilitate slip. And, these are low molecular weight organics. So, if they are low molecular weight, and they are organic, they have vapor pressure.

They have a very, very low melting point, low boiling point. So, they are volatile. And, when you are finished processing, you like to get most of them out. But, they are embedded inside macromolecular solids.

And they take time. That new car smell that you smell, that's plasticizer. And, you'll smell it until the last of the plasticizer is out. And the trouble with some of these plasticizers is that they have structures that are similar to certain hormones.

And, so they can be ingested and over certain periods of time could, I underline could, it's very difficult to prove direct cause-and-effect, but they are becoming increasingly suspect in genetic disorders.

So down the road, 25 years down the road, you might find that people who have been exposed to these could be the bearers of more tumors than the general population. One commonly used one dioctyl pthalate.

This is one of the most commonly used lubricants. And, I talked about the democratization, the faux silk, giving rise to nylon stockings for everybody, but those early pictures of polymers as this wonder material, by the time this movie was made, people were starting to use the term plastic to be synonymous with cheap, with shoddy, with junk.

Plastic didn't have the connotation that it had when the watch with the acrylic crystal was made. Things had changed a lot. And part of it had to do with this recyclability. Look at this. This is the annual production of polymers: about 100 billion pounds, 50 million tons, and only about three to four million tons recycled.

And, by recycled I mean returned. You might ask me, well, why is it that if you can't reuse this directly do they make it recyclable? And, the answer is if this were not recyclable and the aluminum can is recyclable, people would not buy aluminum cans bearing soda.

They would buy only polymer because they wouldn't want to pay the five cent deposit. This is human nature. So, once we decide to recycle any beverage container we must demand a five cent, or whatever it is, deposit is across the board.

Otherwise people will base their purchases on the one that is least environmentally friendly because it doesn't have the deposit on it. That's a policy issue for you. Three to four million tons recycled.

Steel, US annual production is 140 million tons, about three times that. 80 million tons is virgin metal, and about 60 million tons recycled scrap, a lot of recycling. Steel is the most recycled material by far.

You might say, well, what about aluminum? I heard aluminum was. Yeah, you heard it from the aluminum industry. There is 4 million tons of aluminum capacity in the United States, about one million tons recycled.

And most of it is thanks to the used beverage containers. But this is a very special situation because this is made of three different alloys that were designed for the environment, DFE, designed for the environment.

They were chosen, the constituents of those alloys were chosen to allow for recycling. So, they contain only aluminum, silicon, and magnesium. And these will melt down and allow to be reprocessed very, very easily.

But, I like to give the example, I could take the door off a 1928 model T Ford, throw it into an electric arc furnace, melt it down, reshape it, and put it on a 2005 t-bird, the same steel. I cannot take the wing off a McDonnell Douglas DC-3 aircraft, it's aluminum, melt it down, and put it on tomorrow's Boeing 777 because different alloys are used in the top of the wing and the bottom of the wing.

One set contains copper aluminum. The other set contains zinc aluminum. And when you co-melt that, you end up with a mix that's neither fish nor fowl, and aluminum's reactivity is so great that to separate the copper and the zinc is either impossible or too costly.

So, too costly is equivalent to impossible, OK? So, what's the conclusion? You end up with the virgin metal. So, I think this is a good place to stay with these wonderful materials and I'll see you on Wednesday.

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