PPPolymers (acknowledgements to Dr. Lon Mathias, Department of Polymer Science, Univ. of Southern Missisipi) oollyymmeerrss

Polymers used as plastics

include: Polymers used as fibres

include: Polyethylene Polyethylene Polypropylene Polypropylene Polyesters Polyesters PVC Nylon Nylon Kevlar and Nomex Polystyrene Polyacrylonitrile Polycarbonate Cellulose Poly(methyl methacrylate) Polyurethanes Amorphous Polymers may exhibit various forms: Elastomers Thermoplastics Thermosets

Elastomers Elastomer essentially means "rubber". Some polymers which are elastomers include polyisoprene or natural rubber, polybutadiene, polyisobutylene, and polyurethanes. What makes elastomers special is the fact that they can be stretched to many times their original length, and can bounce back into their original shape without permanent deformation. Why ? The answer is in the thermodynamic principle of Entropy.

Entropy is disorder. Things in our universe tend toward high entropy, that is they tend to become more disordered. That's why keeping your house messy is easier than keeping it neat. Polymer molecules are the same way. The molecules in a piece of rubber, any kind of rubber, have no order to them. They tend to wind and tangle around each other in one jumbled mess, exhibiting high entropy.

But now the piece of rubber is stretched, the molecules are forced to line up in the direction in which the rubber is being pulled. When the molecules line up like this they become more ordered (lower entropy). If you stretch it far enough the chains may line up straight enough to crystallize.

Now when you let go of this stretched rubber sample, the molecules return to their tangled and disordered state, (a state of higher entropy) and the sample pops back to its original shape.

Glass or rubber?

Not all amorphous polymers are elastomers. Some are thermoplastics. Whether an amorphous polymer is a thermoplastic or an elastomer depends on its glass transition temperature, or Tg. This is the temperature above which a polymer becomes soft and pliable, and below which it becomes hard and glassy. If an amorphous polymer has a Tg below room temperature, that polymer will be an elastomer, because it is soft and rubbery at room temperature. If an amorphous polymer has a Tg above room temperature, it will be a thermoplastic, because it is hard and glassy at room temperature. So a general rule of thumb is that for amorphous polymers, elastomers have low Tg's and thermoplastics have high Tg's. However this is not true for crystalline polymers.

Crosslinking

To help elastomers bounce back even better it helps to crosslink them. Crosslinking forms covalent links between the polymer chains, joining them all into a single networked molecule. Most objects made of rubber contain only one molecule! When the polymer chains are joined together like this, it is even harder to pull them out of their original positions, and so it bounce back even better when

stretched. This is what Charles Goodyear did in 1839, inventing vulcanized rubber from natural rubber. The brittle fracture at low temperatures was also dramatically reduced.

Thermoplastics Everyone knows what plastic is. We call them plastics because they are pliable, that is, they can be shaped and moulded easily. As plastics become easier to mould and shape when they're hot, and melt when they get hot enough, we call them thermoplastics. This name can help you tell them apart from crosslinked materials that don't melt, called thermosets. Why do we call a material a plastic and not a rubber, or elastomer? The answer is in the bouncing. You can stretch an elastomer, and it bounces back. Plastics tend to either deform permanently, or just plain break, when you stretch them too hard. It takes a lot more energy to stretch them in the first place. Plastics resist deformation better than elastomers do. This is good if we are looking for high tensile modulus and good strength at moderate strain. Plastics are also much more pliable than polymers formed as fibres. Fibres stretch very little when you pull on them. This makes them good for things like rope or woven reinforcement geotextiles.

Hard Plastic and Soft Plastic

We've all seen plastics that are hard, and some that are soft. The plastic keys on your keyboard are hard, while the plastic around the cables of the same computer is soft. All platics have a certain temperature, the glass transition temperature or Tg, above which they are soft and pliable, and below which they are hard and brittle. Tg is different for each plastic. At room temperature, some plastics are below their Tg, and are hard. Other plastics are above their Tg at room temperature, and are soft.

Sometimes additives are added to a plastic to make it softer and more pliable. These additives are called plasticizers. Some polymers used as plastics are:

Polyethylene Polypropylene Polystyrene Polyesters Polycarbonate PVC Nylon Poly(methyl methacrylate)

Glass Transition If you leave a plastic object outside during the winter, you may find that it cracks or breaks more easily than it would in the summer time. This is the phenomenon known as the glass transition. The glass transition is something that only happens to polymers, and is one of the things which make polymers unique. The glass transition is pretty much what it sounds like. There is a certain temperature(different for each polymer) called the glass transition temperature, or Tg for short. When the polymer is cooled below this temperature, it becomes hard and brittle, like glass. Some polymers are used above their glass transition temperatures, and some are used below. Hard plastics like polystyrene and poly(methyl methacrylate), are used below their glass transition temperatures; that is in their glassy state. Their Tg's are above room temperature, both at around 100°C. Rubber elastomers like polyisoprene and polyisobutylene, are used above their Tg's, that is, in the rubbery state, where they are soft and flexible.

Amorphous and Crystalline Polymers

The glass transition is not the same thing as melting. Melting is a transition which occurs in crystalline polymers. Melting happens when the polymer chains fall out of their crystal structures, and become a disordered liquid. The glass transition is a transition which happens to amorphous polymers; that is, polymers whose chains are not arranged in ordered crystals, but are randomly ordered even in the solid state. BUT All crystalline polymers will have a some amorphous portion. This portion usually makes up 40-70% of the polymer sample. Therefore the same sample of a polymer can have both a glass transition temperature and a melting temperature. But the amorphous fraction undergoes the glass transition only, and the crystalline fraction undergoes melting only. When the temperature is warm, the polymer chains can move around easily. So you can take a piece of the polymer and bend it; the molecules, being in motion already, have no trouble moving into new positions to relieve the stress you have placed on them. If you try to bend the same polymer sample below its Tg, the polymer chains won't be able to move into new positions to relieve the stress which you have placed on them. So if the chains are strong enough to resist the force you apply, the sample won't bend. If the force you apply is too much for the motionless polymer chains to resist, being unable to move around to relieve the stress, the polymer sample will break or shatter in your hands. This change in mobility with temperature happens because "heat" is really a form of kinetic energy; that is, the energy of the random motion of molecules. Things are "hot" when their molecules have lots of kinetic energy and move around very fast. Things are "cold" when their molecules lack kinetic energy and move around slowly, or not at all.

Adjusting the Glass Transition

Sometimes, a polymer has a Tg that is higher than we'd like for a particular application. During manufacture, Tg may be adjested by the addition of a plasticizer. This is a small molecule which will get in between the polymer chains, and space them out from each other, increasing the free volume. As a result, they can slide past each other more easily at lower temperatures than they would without the plasticizer. Thus the Tg of a polymer can be lowered, to make a polymer more pliable, and easier to work with. Poly (vinyl chloride) or PVC, for example, may make rigid pipe (relatively less plasticizer) or plastic films (higher plasticizer content). Some commonly-used plasticizers:

In PVC Geomembranes, phthalate esters are commonly used, and in some older products, constituted almost half the mass of the finished product. The problem was that the phthalates leached out, and the integrity of the liner may be compromised. What about the “new car smell"? It’s the plasticizer evaporating from the plastic parts on the inside of your car. After many years, if enough of it evaporates, your dashboard will no longer be plasticized. The Tg of the polymers in your dashboard will rise above room temperature, and the dashboard becomes brittle and cracks.

The Glass Transition vs. Melting

Melting is something that happens to a crystalline polymer, Glass transition happens only to polymers in the amorphous state.

A given polymer will often have both amorphous and crystalline domains within it, so the same sample can often show a melting point and a Tg. But the chains that melt are not the chains that undergo the glass transition. Crystalline Polymers. When you heat a crystalline polymer at a constant rate, the temperature will increase at a constant rate. The heat amount of heat required to raise the temperature of one gram of the polymer one degree Celsius being the heat capacity. The temperature will continue to increase until the polymer reaches its melting point. When this happens, the temperature will hold steady, even though you're adding heat to the polymer, until the polymer has completely melted. Then the temperature of the polymer will begin to increase once again. The increase in temperature stops because melting requires energy. The energy added to a crystalline polymer at its melting point goes into melting, and is called the latent heat of melting. Now once the polymer has melted, the temperature begins to rise again, but now it rises at a slower rate. The molten polymer has a higher heat capacity than the solid crystalline polymer, so it can absorb more heat with a smaller increase in temperature. So two things happen when a crystalline polymer melts: It absorbs a certain amount of heat, the latent heat of melting, and it undergoes a change in its heat capacity. Any change brought about by heat, whether it is melting or freezing, or boiling or condensation, which has a change in heat capacity, and a latent heat involved, is called a first order transition. Amorphous Polymers When you heat an amorphous polymer to its Tg, something different happens. First you heat it, and the temperature goes up at a rate determined by the polymer's heat capacity. But when the sample reaches the Tg, the temperature doesn't stop rising. There is no latent heat of glass transition.

But the temperature doesn't go up at the same rate above the Tg as below it. The polymer does undergo an increase in its heat capacity when it undergoes the glass transition. Because the glass transition involves change in heat capacity, but it doesn't involve a latent heat, this transition is called second order transition.

The plot on the left shows what happens when you heat a 100% crystalline polymer. At the melting temperature (first order transition), the discontinuity represents the heat added without any temperature increase, the latent heat of melting. The slope is steeper on the high side of the break, reflecting the higher heat capacity above the melting point. The plot on the right shows a 100% amorphous polymer. There is no latent heat. The only change at the glass transition temperature (second order transition) is an increase in the slope, again the increase in heat capacity.

FIBRES A polymeric fibre is a polymer whose chains are stretched out straight (or close to straight) and lined up next to each other, all along the same axis.

Polymers arranged in fibres like this can be spun into threads and used as textiles. The clothes you're wearing are made out of polymeric fibres. So is carpet. So is rope. Here are some of the polymers which can be drawn into fibres:

Polyethylene Kevlar and Nomex Polypropylene Polyacrylonitrile Nylon Cellulose Polyester Polyurethanes

Fibres are always made of crystalline polymers. In fact, fibres are essentially a very long kind of crystal. We can show this by taking a closer look at the way nylon 6,6 packs into a crystalline fibre.

The hydrogen bonds and other secondary interactions between the individual chains hold the chains together very tightly. So tightly that they don't particularly like to slide past one another. This means that when you pull on the nylon fibre it won't stretch very much, if at all. This is why fibres are good for using as rope and thread. As every engineer knows, you can’t push a rope. So fibres have their drawbacks. While they have good tensile strength, that is, they're strong when you pull or stretch them, they usually have bad

compressional strength, that is, they're weak when you try to squash or compress them. Fibres tend to be strong only in one direction, the direction in which they're oriented. If you pull in them in the direction at right angles to their orientation, they tend to be weak.

Because of this combination of strengths and weaknesses, fibres are often used with another material, such as a thermoset. Fibres may reinforce thermosets, compensating for the thermoset's weaknesses, but the thermoset's strengths make up for the fibre's weaknesses as well. When a thermoset or any other polymer is reinforced with a fibre like this, it's called a composite material.

Vinyl Polymers Vinyl polymers are polymers made from vinyl monomers; that is, small molecules containing carbon-carbon double bonds. They make up largest family of polymers. Let's see how we get from a vinyl monomer to a vinyl polymer using for an example the simplest vinyl polymer, polyethylene. Polyethylene is made from the monomer ethylene, which is also called ethene. When polymerized, the ethylene molecules are joined along the axes of their double bonds, to form a long chain of many thousands of carbon atoms containing only single bonds between atoms.

More sophisticated vinyl polymers are made from monomers in which one or more of the hydrogen atoms of ethylene has been replaced by another atom or groups of atoms. Let's see what we can do by replacing just one of those hydrogen atoms. We can get a number of common plastics: polypropylene

Polystyrene

poly(vinyl chloride)

Replacing two hydrogen atoms, on the same carbon atom, we can get polyisobutylene, which is natural rubber.

Not many monomers in which hydrogen atoms have been replaced on both carbon atoms will polymerize. But one polymer that is made from a monomer substituted on both carbon atoms is polytetrafluoroethylene, which DuPont makes and calls Teflon.

Polyethylene

Polyethylene is probably the polymer you see most in daily life. Polyethylene is the most popular plastic in the world. This is the polymer that makes grocery bags, shampoo bottles, children's toys, and even bullet proof vests. For such a versatile material, it has a very simple structure, the simplest of all commercial polymers. A molecule of polyethylene is nothing more than a long chain of carbon atoms, with two hydrogen atoms attached to each carbon atom. That's what the picture above shows, but it might be easier to draw it like the picture below, only with the chain of carbon atoms being many thousands of atoms long:

Sometimes it's a little more complicated. Sometimes some of the carbons, instead of having hydrogens attached to them, will have long chains of polyethylene attached to them.

This branched PE or LDPE is not as strong as the Linear or HDPE but is cheaper and easier to make.

Linear polyethylene is normally produced with molecular weights in the range of 200,000 to 500,000, but it can be made even higher. Polyethylene with molecular weights of three to six million is referred to as ultra-high molecular weight polyethylene, or UHMWPE. UHMWPE can be used to make fibres which are so strong they replaced Kevlar for use in bullet proof vests. Large sheets of it can be used instead of ice for skating rinks.

By copolymerizing ethylene monomer with a alkyl-branched comonomer such as one gets a copolymer which has short hydrocarbon branches. Copolymers like this are called linear low-density polyethylene, or LLDPE. BP produces LLDPE using a comonomer with the catchy name 4-methyl-1-pentene, and sells it under the trade name Innovex®. LLDPE is often used to make things like plastic films.

Polypropylene

Polypropylene is one of those rather versatile polymers out there. It serves double duty, both as a plastic and as a fibre. As a plastic it is used to make things like dishwasher-safe food containers. It can do this because it doesn't melt below 160°C. Polyethylene, a more common plastic, will anneal at around 100°C, which means that polyethylene dishes will warp in the dishwasher. As a fibre, polypropylene is used to make indoor-outdoor carpeting, the kind that you always find around swimming pools and miniature golf courses. It works well for outdoor carpet because it is easy to make colored polypropylene, and because polypropylene doesn't absorb water, like nylon does. Structurally, it is a vinyl polymer, and is similar to polyethylene, only that on every other carbon atom in the backbone chain has a methyl group attached to it.

Polypropylene can be made with different tacticities. Most polypropylene we use is isotactic. This means that all the methyl groups are on the same side of the chain, like this:

But sometimes we use atactic polypropylene. Atactic means that the methyl groups are placed randomly on both sides of the chain like this:

The polypropylene commercially available today has about 50 - 60% crystallinity, too much for it to behave as an elastomer.

Polyesters

Polyesters are the polymers, in the form of fibres, that have been used for clothing, industrial fabrics, and literally hundreds of uses. One special form of polyester is Mylar®. Another special family of polyesters are polycarbonates. Polyesters have hydrocarbon backbones which contain ester linkages, hence the name.

The structure in the picture is called poly(ethylene terephthalate), or PET for short, because it is made up of ethylene groups and terephthalate groups.

The ester groups in the polyester chain are polar, with the carbonyl oxygen atom having a somewhat negative charge and the carbonyl carbon atom having a somewhat positive charge. The positive and negative charges of different ester groups are attracted to each other. This allows the ester groups of nearby chains to line up with each other in crystal form, which is why they can form strong fibres.

Two questions;

Why can't you return plastic soft drink for refilling like you can with glass beer bottles?

How come peanut butter comes in shatterproof plastic jars but jelly doesn't? These two questions, as it turns out, have the same answer. The answer is that PET has too low a glass transition temperature, that is the temperature at which the PET becomes soft. Now reusing a soft drink bottle requires that the bottle be sterilized before it is used again. This means washing it at really high temperatures, too high for PET. Filling a jelly jar is also carried out at high temperatures. Down at your local jelly factory, the stuff is shot into the jars hot, at temperatures which would cause PET to become soft. So PET is no good for jelly jars.

Nylons

Nylons are one of the most common polymers used as a fibre. Nylon is found in clothing all the time, but also in other places, in the form of a thermoplastic. Nylon's first real success came with it's use in women's stockings, in about 1940. They were a big hit, but they became hard to get, because the next year the United States entered World War II, and nylon was needed to make war materials, like parachutes and ropes. But before stockings or parachutes, the very first nylon product was a toothbrush with nylon bristles.

Nylons are also called polyamides, because of the characteristic amide groups in the backbone chain. Proteins, such as the silk nylon was made to replace, are also polyamides. These amide groups are very polar, and can hydrogen bond with each other. Because of this, and because the nylon backbone is so regular and symmetrical, nylons are often crystalline, and make very good fibres.

The nylon in the pictures below is called nylon 6,6, because each repeat unit of the polymer chain has two stretches of carbon atoms, each being six carbon atoms long. Other nylons can have different numbers of carbon atoms in these stretches.

Another kind of nylon is nylon 6. It's a lot like nylon 6,6 except that it only has one kind of carbon chain, which is six atoms long.

Nylon 6 doesn't behave much differently from nylon 6,6. The only reason both are made is because DuPont patented nylon 6,6, so other companies had to invent nylon 6 in order to get in on the nylon business. Another family of nylons are aramids such as Kevlar®.

Poly(vinyl chloride) is the plastic known at the hardware store as PVC. This is the PVC from which pipes are made, and PVC pipe is everywhere. The plumbing in your house is probably PVC pipe, unless it's an older house. PVC pipe is what rural high schools with small budgets use to make goal posts for their football fields. But there's more to PVC than just pipe. The "vinyl" siding used on houses is made of poly(vinyl chloride). Inside the house, PVC is used to make linoleum for the floor. In the seventies, PVC was often used to make vinyl car tops. PVC is useful because it resists two things: fire and water. Because of it's water resistance it is used to make raincoats and shower curtains, and of course, water pipes. It has flame resistance, too, because it contains chlorine. When you try to burn PVC, chlorine atoms are released, inhibiting combustion. Structurally, PVC is a vinyl polymer, similar to polyethylene, but on every other carbon in the backbone chain, one of the hydrogen atoms is replaced with a chlorine atom. It is produced by the free radical polymerization of vinyl chloride.

PVC was one of those odd discoveries that actually had to be made twice. It seems around a hundred years ago, a few German

entrepreneurs decided they were going to make loads of cash lighting people's homes with lamps fueled by acetylene gas. Wouldn't you know it, right about the time they had produced tons of acetylene to sell to everyone who was going to buy their lamps, new efficient electric generators were developed which made the price of electric lighting drop so low that the acetylene lamp business was finished. That left a lot of acetylene laying around.

So in 1912 one German chemist, Fritz Klatte decided to try to do something with it, and reacted some acetylene with hydrochloric acid (HCl). Now this reaction will produce vinyl chloride, but at that time no one knew what to do with it, so he put it on the shelf, where it polymerized over time. Not knowing what to do with the PVC he had just invented, he told his bosses at his company, Greisheim Electron, who had the material patented in Germany. They never figured out a use for PVC, and in 1925 their patent expired. Wouldn't you know it, in 1926 the very next year, and American chemist, Waldo Semon was working at B.F. Goodrich when he independently invented PVC. But unlike the earlier chemists, it dawned on him that this new material would make a perfect shower curtain. He and his bosses at B.F. Goodrich patented PVC in the United States (Klatte's bosses apparently never filed for a patent outside Germany). Tons of new uses for this wonderful waterproof material followed, and PVC was a smash hit the second time around.

Polymer Crystallinity A crystal is any object in which the molecules are arranged in a regular order and pattern. Ice is a crystal. In ice all the water molecules are arranged in a specific manner. So is table salt, sodium chloride. (Oddly, your mother's good crystal drinking glasses are not crystal at all, as glass is an amorphous solid, that is a solid in which the molecules have no order or arrangement.) Polymers may be arranged in a neat orderly manner; when this is the case, we say the polymer is crystalline. If there is no order, and the polymer chains just form a big tangled mess, we say the polymer is amorphous. So what kind of arrangements do the polymers like to form? They like to line up all stretched out, kind of like a neat pile of new boards down at the lumber yard. But they can't always stretch out that straight. In fact, very few polymers can stretch out perfectly straight, and those are ultra-high molecular weight polyethylene, and aramids like Kevlar and Nomex. Most polymers can only stretch out for a short distance before they fold back on themselves. You can see this in the picture.

For polyethylene, the length the chains will stretch before they fold is about 100 angstroms. But not only do they fold like this. Polymers form stacks of these folded chains. There is a picture of a stack, called a lamella, right below.

Of course, it isn't always as neat as this. Sometimes part of a chain is included in this crystal, and part of it isn't. When this happens we get the kind of mess you see below. our lamella is no longer neat and tidy, but sloppy, with chains hanging out of it everywhere!

Of course, being indecisive, the polymer chains will often decide they want to come back into the lamella after wandering around outside for awhile. When this happens, we get a picture like this:

This is the switchboard model of a polymer crystalline lamella. Because we like you, we're going to tell you that when a polymer chain doesn't wander around outside the crystal, but just folds right back in on itself, like we saw in the first pictures, that is called the adjacent re-entry model.

Amorphousness and Crystallinity

Are you wondering about something? If you look at those pictures up there, you can see that some of the polymer is crystalline, and some is not! Yes folks, most crystalline polymers are not entirely crystalline. The chains, or parts of chains, that aren't in the crystals have no order to the arrangement of their chains. So a crystalline polymer really has two components: the crystalline portion and the amorphous portion. The crystalline portion is in the lamellae, and the amorphous potion is outside the lamellae. If we look at a wide-angle picture of what a lamella looks like, we can see how the crystalline and amorphous portions are arranged.

As you can see, lamella grow like the spokes of a bicycle wheel from a central nucleus. (Sometimes we bigshot scientists like to call the "spokes" lamellar fibrils.) The fibrils grow out in three dimensions, so the whole thing really looks more like a sphere than a wheel. The whole assembly is called a spherulite. In a sample of a crystalline polymer weighng only a few grams there are many billions of spherulites. In between the crystalline lamellae there are regions when there is no order to the arrangement of the polymer chains. These disordered regions are the amorphous regions we were talking about. As you can also see in the picture, a single polymer chain may be partly in a crystalline lamella, and partly in the amorphous state. Some chains even start in one lamella, cross the amorphous region, and then join another lamella. These chains are called tie molecules. So you see, no polymer is completely crystalline. If you're making plastics, this is a good thing. Crystallinity makes a material strong, but it also makes it brittle. A completely crystalline polymer would be

too brittle to be used as plastic. The amorphous regions give a polymer toughness, that is, the ability to bend without breaking. But for making fibres, we like our polymers to be as crystalline as possible. This is because a fibre is really a long crystal Many polymers are a mix of amorphous and crystalline regions, but some are highly crystalline and some are highly amorphous. Here are some of the polymers that tend toward the extremes: Some Highly Crystalline Polymers:

Some Highly Amorphous Polymers:

Polypropylene Poly(methyl methacrylate) Syndiotactic polystyrene Atactic polystyrene Nylon Polycarbonate Kevlar and Nomex Polyisoprene Polyketones Polybutadiene

Why?

So why is it that some polymers are highly crystalline and some are highly amorphous? There are two important factors, polymer structure and intermolecular forces.

Crystallinity and polymer structure

A polymer's structure. affects crystallinity a goo deal. If it is regular and orderly, it will pack into crystals easily. If not, it won't. It helps to look at polystyrene to understand how this works.

As you can see on the lists above, there are two kinds of polystyrene. There is atactic polystyrene, and there is syndiotactic polystyrene. One is very crystalline, and one is very amorphous.

Syndiotactic polystyrene is very orderly, with the phenyl groups falling on alternating sides of the chain. This means it can pack very easily into crystals. But atactic styrene has no such order. The phenyl groups come on any which side of the chain they please. With no order, the chains can't pack very well. So atactic polystyrene is very amorphous. Other atactic polymers like poly(methyl methacrylate) and poly(vinyl chloride) are also amorphous. And as you might expect, stereoregular polymers like isotactic polypropylene and polytetrafluoroethylene are highly crystalline. Polyethylene is another good example. It can be crystalline or amorphous. Linear polyethylene is nearly 100% crystalline. But the

branched stuff just can't pack the way the linear stuff can, so it is highly amorphous.

Crystallinity and intermolecular forces

Intermolecular forces can be a big help for a polymer if it wants to form crystals. A good example is nylon. You can see from the picture that the polar amide groups in the backbone chain of nylon 6,6 are strongly attracted to each other. They form strong hydrogen bonds. This strong binding holds crystals together.

Polyesters are another example. Let's look at the polyester we call poly(ethylene terephthalate).

The polar ester groups make for strong crystals. In addition, the aromatic rings like to stack together in an orderly fashion, making the crystal even stronger.

How Much Crystallinity?

Remember many polymers contain lots of crystalline material and lots of amorphous material. There's a way we can find out how much of a polymer sample is amorphous and how much is crystalline. This method is called differential scanning calorimetry.

Differential Scanning Calorimetry Differential scanning calorimetry is a technique we use to study what happens to polymers when they're heated. We use it to study what we call the thermal transitions of a polymer. And what are thermal transitions? They're the changes that take place in a polymer when you heat it. The melting of a crystalline polymer is one example. The glass transition is also a thermal transition. So how do we study what happens to a polymer when we heat it? The first step would be to heat it, obviously. And that's what we do in differential scanning calorimetry, or DSC for short.

We heat our polymer in a device that looks something like this:

It's pretty simple, really. There are two pans. In one pan, the sample pan, you put your polymer sample. The other one is the reference pan. You leave it empty. Each pan sits on top of a heater. Then you tell the nifty computer to turn on the heaters. So the computer turns on the heaters, and tells it to heat the two pans at a specific rate, usually something like 10 oC per minute. The computer makes absolutely sure that the heating the rate stays exactly the same throughout the experiment. But more importantly, it makes sure that the two separate pans, with their two separate heaters, heat at the same rate as each other. Huh? Why wouldn't they heat at the same rate? The simple reason is that the two pans are different. One has polymer in it, and one doesn't. The polymer sample means there is extra material in the sample pan. Having extra material means that it will take more heat to keep the temperature of the sample pan increasing at the same rate as the reference pan. So the heater underneath the sample pan has to work harder than the heater underneath the reference pan. It has to put out more heat. By measuring just how much more heat it has to put out is what we measure in a DSC experiment. Specifically what we do is this: We make a plot as the temperature increases. On the x-axis we plot the temperature. On the y-axis we plot difference in heat output of the two heaters at a given temperature.

Heat Capacity

We can learn a lot from this plot. Let's imagine we're heating a polymer. When we start heating our two pans, the computer will plot the difference in heat output of the two heaters against temperature. That is to say, we're plotting the heat absorbed by the polymer against temperature. The plot will look something like this at first.

The heat flow at a given temperature can tell us something. The heat flow is going to be shown in units of heat, q supplied per unit time, t. The heating rate is temperature increase T per unit time, t.

Let's say now that we divide the heat flow q/t by the heating rate T/t. We end up with heat supplied, divided by the temperature increase.

Remember from the discussion of glass transition, that when you put a certain amount of heat into something, its temperature will go up by a certain amount, and the amount of heat it takes to get a certain temperature increase is called the heat capacity, or Cp. We

get the heat capacity by dividing the heat supplied by the resulting temperature increase. And that's just what we've done in that equation up there. We've figured up the heat capacity from the DSC plot.

The Glass Transition Temperature

Of course, we can learn a lot more than just a polymer's heat capacity with DSC. Let's see what happens when we heat the polymer a little more. After a certain temperature, our plot will shift upward suddenly, like this:

This means we're now getting more heat flow. This means we've also got an increase in the heat capacity of our polymer. This happens because the polymer has just gone through the glass transition. And polymers have a higher heat capacity above the glass transition temperature than they do below it. Because of this change in heat capacity that occurs at the glass transition, we can use DSC to measure a polymer's glass transition temperature. You may notice that the change doesn't occur suddenly, but takes place over a temperature range. This makes picking one discreet Tg kind of tricky, but we usually just take the middle of the incline to be the Tg.

Crystallization

Above the glass transition, the polymers have a lot of mobility. They wiggle and squirm, and never stay in one position for very long. When they reach the right temperature, they will have gained

enough energy to move into very ordered arrangements, which we call crystals, of course. When polymers fall into these crystalline arrangements, they give off heat. When this heat is dumped out, the little computer-controlled heater under the sample pan doesn't have to put out much heat to keep the temperature of the sample pan rising. You can see this drop in the heat flow as a big dip in the plot of heat flow versus temperature:

This dip tells us a lot of things. The temperature at the lowest point of the dip is usually considered to be the polymer's crystallization temperature, or Tc. Also, we can measure the area of the dip, and that will tell us the latent energy of crystallization for the polymer. But most importantly, this dip tells us that the polymer can in fact crystallize. If you analyzed a 100% amorphous polymer, like atactic polystyrene, you wouldn't get one of these dips, because such materials don't crystallize. Because the polymer gives off heat when it crystallizes, we call crystallization an exothermic transition.

Melting

Heat may allow crystals to form in a polymer, but too much of it can be their undoing. If we keep heating our polymer past its Tc, eventually we'll reach another thermal transition, one called melting. When we reach the polymer's melting temperature, or Tm, those polymer crystals begin to fall apart, that is they melt. The chains

come out of their ordered arrangements, and begin to move around freely. And we can spot this happening on a DSC plot. Remember that heat that the polymer gave off when it crystallized? There is a latent heat of melting as well as a latent heat of crystallization. When the polymer crystals melt, they must absorb heat in order to do so. Remember melting is a first order transition. This means that when you reach the melting temperature, the polymer's temperature won't rise until all the crystals have melted. This means that the little heater under the sample pan is going to have to put a lot of heat into the polymer in order to both melt the crystals and keep the temperature rising at the same rate as that of the reference pan. This extra heat flow during melting shows up as a big peak on our DSC plot, like this:

We can measure the latent heat of melting by measuring the area of this peak. And of course, we usually take the temperature at the top of the peak to be the polymer's melting temperature, Tm. Because we have to add energy to the polymer to make it melt, we call melting an endothermic transition.

Putting It All Together

So let's review now: we saw a step in the plot when the polymer was heated past its glass transition temperature. Then we saw big dip when the polymer reached its crystallization temperature. Then finally we saw a big peak when the polymer reached its melting temperature. To put them all together, a whole plot will often look something like this:

Of course, not everything you see here will be on every DSC plot. The crystallization dip and the melting peak will only show up for polymers that can form crystals. Completely amorphous polymers won't show any crystallization, or any melting either. But polymers with both crystalline and amorphous domains, will show all the features you see above. If you look at the DSC plot you can see a big difference between the glass transition and the other two thermal transitions, crystallization and melting. For the glass transition, there is no dip, and there's no peak, either. This is because there is no latent heat given off, or absorbed, by the polymer during the glass transition. Both melting and crystallization involve giving off or absorbing heat. The only thing we do see at the glass transition temperature is a change in the heat capacity of the polymer. Because there is a change in heat capacity, but there is no latent heat involved with the glass transition, we call the glass transition a second order transition. Transitions like melting and crystallization, which do have latent heats, are called first order transitions.

How much crystallinity?

DSC can also tell us how much of a polymer is crystalline and how much is amorphous. Recall that many polymers contain both amorphous and crystalline material. But how much of each? DSC can tell us. If we know the latent heat of melting, Hm, we can figure out the answer. The first thing we have to do is measure the area of that big peak we have for the melting of the polymer. Now our plot is a plot of heat flow per gram of material, versus temperature. Heat flow is heat given off per second, so the area of the peak is given is units of heat x temperature x time-1 x mass-1. We usually would put this in units such as joules x kelvins x (seconds)-1 x (grams)-1:

We usually divide the area by the heating rate of our DSC experiment. The heating rate is in units of K/s. So the expression becomes simpler:

Now we have a number of joules per gram. But because we know the mass of the sample, we can make it simpler. We just multiply this by the mass of the sample:

Now we just calculated the total heat given off when the polymer melted. Neat, huh? Now if we do the same calculation for our dip that we got on the DSC plot for the crystallization of the polymer, we can get the total heat absorbed during the crystallization. We'll call the heat total heat given off during melting Hm, total, and we'll call the heat of the crystallization Hc, total. Now we're going to subtract the two:

Why did we just do that? And what does that number H' mean? H' is the heat given off by that part of the polymer sample which was already in the crystalline state before we heated the polymer above the Tc. We want to know how much of the polymer was crystalline before we induced more of it to become crystalline. That's why we subtract the heat given off at crystallization. Now with H', we can figure up the percent crystallinity. We're going to divide it by the specific heat of melting, Hc*. The specific heat of melting? That's the amount of heat given of by a certain amount, usually one gram, of a polymer. H' is in joules, and the specific heat of melting is usually given in joules per gram, so we're going to get an answer in grams, which we'll call mc.

This is the total amount of grams of polymer that were crystalline below the Tc. Now if we divide this number by the weight of our sample, mtotal, we get the fraction of the sample that was crystalline, and then of course, the percent crystallinity:

And that's how we use DSC to get percent crystallinity.