Blends That Aren't Really
What Is an Immiscible Blend?
Morphology
Properties of Immiscible Blends
What Is an Immiscible Blend?
Long ago someone came up with the idea of taking two polymers and mixing them together in order to get a material with properties somewhere between those of the two polymers mixed. Materials made from two polymers mixed together are called blends. But if you read the blends page, you'll know that it's not very often that two polymers will mix with each other. Most of the time, if you try to mix two kinds of polymers, you'll end up with something that looks like chicken soup. Take a look at a bowl of good chicken soup, and you'll see that it has two phases: a water phase and a chicken fat phase. The chicken fat is insoluble in water, so it forms little blobs in the soup separate form the water phase. So we say mixtures like chicken soup are phase-separated.Phase-separated mixtures are just what you get when you try to mix most polymers. But strangely enough, the phase-separated materials often turn out to be rather nifty and useful. We even have a name for them. We call them immiscible blends. Ok, I have to admit, that name is an oxymoron. These materials aren't really blends. They can't be if they're immiscible, but that's the name people use.
Anyway...immiscible blends turn out to be useful, as I was saying. Would you like an example? Alright, then consider polystyrene and polybutadiene. These two polymers are immiscible, totally. When you mix polystyrene with a small amount of polybutadiene, the two polymers won't blend and form a homogeneous material. Instead, the polybutadiene will separate from the polystyrene into tiny spherical blobs, just like the chicken fat in your soup separates from the water into little blobs. If you looked at the mixture under an electron microscope you'd see something that looks like the picture below.
The phase morphology of HIPS.
The little spheres of polybutadiene do a lot for the material, however. You see, polystyrene is a rather brittle material. It's stiff, but you can break it easily if you try to bend it. But those little polybutadiene spheres are rubbery, remember, and they can absorb energy under stress. This keeps the polystyrene from breaking. This immiscible blend has more ability to bend instead of breaking than regular polystyrene. That is, it's tougher and more ductile. Immiscible blends of polystyrene and polybutadiene are sold commercially under the name high-impact polystyrene, or HIPS for short. And that name points to another improve property for this type of blend: impact resistance or strength. You can hit it pretty hard and it just laughs at you.
Another immiscible blend you may be familiar with is one made from a
polyester called poly(ethylene terephthalate) and
poly(vinyl alcohol). We call them PET and PVA, respectively, for short.
If you put just the right amounts of the two polymers together under the
right conditions you'll get something that looks like the picture on the
left, when you look at it under an electron microscope.
In this material, PET and PVA separate into sheetlike layers called lamellae. We call the resulting arrangement lamellar morphology. This particular immiscible blend is used to make plastic bottles for carbonated beverages. The PET makes the bottle strong, while the layers of PVA do something very important if you want your sodas to stay fizzy. Carbon dioxide can't pass through PVA. If the carbon dioxide in your soda leaked out (it can pass easily through plain PET), your soda would go flat.
Morphology
I hope you folks are asking a question right now. Did you notice a difference between the two immiscible blends we just talked about? Did you notice that in HIPS, one polymer forms into little spheres dispersed in the polystyrene? Did you also notice that in the PET-PVA system, the two polymers separated into layers? Why are the two different? Why do they separate in different fashions?We call the shape made by the two phases, and the arrangement of the two phases morphology. The biggest thing one can do to affect the morphology of an immiscible blend is to control the relative amounts of the two polymers one is using. Let's say you're trying to make an immiscible blend from two polymers, polymer A and polymer B. If you have a lot more of polymer A than polymer B, polymer B separate into little spherical globs. The spheres of polymer B will be separated from each other by a sea of polymer A, like you see in the picture below. In such a case we call polymer A the major component and polymer B the minor component.
relative amount of polymer B in the immiscible blend
But if you put more polymer B into the immiscible blend, the spheres will get bigger and bigger, until they get so big that they become joined together. Now they aren't isolated spheres anymore, but a continuous phase. The immiscible blend now looks like the middle picture above. It might help to think of a block of colby-jack cheese. The domains of polymer B are now joined together, but so are the domains of polymer A. When this happens we say that the polymer A phase and the polymer B phase are co-continuous.
But if we keep adding more polymer B, eventually there will be so much more polymer B in the immiscible blend that polymer A will become nothing but isolated spheres surrounded by a continuous phase of polymer just like you see in the picture above on the right. Polymer B is now the major component and polymer A is the minor component, and the situation is reversed from what we had at first.
Spheres, co-continuity, then more spheres...so how do we get the neat
layers that you get in the PET-PVA immiscible blend? Sometimes the way in
which a product is processed affects the morphology of the material. Soft
drink bottles are made by a technique called blow molding. To make
a bottle we take a small piece of plastic that looks like a test tube,
about 1 inch (2.5 cm) in diameter and maybe 6 inches (15 cm) long. We
heat the tube up, then inflate it like a balloon until it is the size we
want it. This whole procedure puts the material under stress. Think about
a section of the skin of the bottle. When it is being inflated, it is put
under stress in two directions, like you see in this picture on the right.
This is called biaxial stress, and it causes the domains of PET and the domains of PVA to flatten out, just like pizza dough does when you roll it. This is how we get flat layers instead of spheres in our immiscible blend.
Processing under flow in one direction turns the spheres into rods.
Another interesting morphology you can get is one of rod-like domains of one polymer surrounded by a continuous phase of the other. This happens when the immiscible blend is put under stress in only one direction, such as during extrusion.
Ok, we have to talk about one last thing when we're talking about morphology, and that is size. Let's go back to that simple case we talked about earlier, where we had spheres of polymer B surrounded by continuous phase of polymer A. How big are the spheres? How far apart are they? Could you see them if you looked at a sample of an immiscible blend?
I hate to disappoint everyone, but in most cases you're not going to be able to see the two separate phases with your own eyes. In fact, it usually takes an electron microscope. So the phase domains, spherical or otherwise, are very small.
But the domains do try to be as big as they can. Take our spheres for example. The bigger the spheres are, the less surface area they will have. A few bigger spheres have less surface area than a bunch of little ones. The less surface area, the better. Remember, the two polymers in an immiscible blend don't like each other, and the smaller the surface area of the spheres, the less the two polymers have to touch each other.
I figure you're going to want some numbers, so I'll tell you some specifics on an 80:20 immiscible blend of high density polyethylene and polystyrene. Polystyrene is the minor component here, so it will form the separated spherical domains, and they tend to be in the range of 5-10 X 10-6 meter in diameter.1
Properties of Immiscible Blends
How do these immiscible blends behave? They have to behave in some interesting manner, or else nobody would make them, and people like me wouldn't write about them.One unusual property of immiscible blends is that one made from two amorphous polymers has two glass transition temperatures or Tgs for short. Since the two components are phase separated, the retain their separate Tgs. In fact, scientists often measure the Tg of a blend to find out if it is miscible or immiscible. If two Tgs are found, then the blend is immiscible. If only one Tg is observed, then the blend is likely to be miscible.
But what about mechanical properties? Let's consider an immiscible blend of a major component polymer A and a minor component polymer B, whose morphology is that of spheres of polymer B dispersed in a matrix of polymer A. The mechanical properties of this immiscible blend are going to depend on those of polymer A, because the polymer A phase is absorbing all the stress and energy when the material is under load. In addition, the immiscible blend is going to be weaker than a sample of pure polymer A.
So why make immiscible blends then, if separate materials are stronger?
It turn out there are some tricks one can do to make immiscible blends
strong. One is to process them under flow. If we process them under flow
in one direction, the minor component will form rods instead of spheres,
Like you see in the picture on your right. These rods act like the fibers of a reinforced composite material. They make the material
stronger in the direction of the rods.
Another way to make a strong immiscible blend is to use more equal amounts of the two polymers. Remember, when the relative amounts of the two polymers are equal, we get a different morphology than when one is in large excess. When polymer A and polymer B are present in roughly equal amounts, they form two co-continuous phases. This means both phases will be bearing the load of any stress on the material, so it will be stronger.
But one of the most interesting ways to make immiscible blends stronger is to use a compatibilizer. So what is a compatibilizer? A compatibilizer is anything that helps bond the two phases to each other more tightly. You see, in an immiscible blend, the two phases are not bonded very strongly to each other. Remember, they don't like each other, and that is why they are immiscible in the first place. But if stress and energy are going to be transferred between the components, they have to be bound to each other in some fashion.
That's where compatibilizers come in. Often times a compatibilizer is a
block copolymer of the two components of the
immiscible blend. Let's take our example of an immiscible blend of
polymer A and polymer B again. Let's make polymer A the major component
and polymer B the minor component, and then let's throw in a block
copolymer of A and B. For those of you who may not no, a block copolymer
of A and B is a polymer with one long segment of polymer A joined to
another long segment of polymer B, like you can see above left.
Of course, the A block is going to want to be in the polymer A phase, and
the B block is going to want to be in the polymer B phase. So the
copolymer molecule has to sit right on the phase boundary between the
polymer A and the polymer B phases. The A block can then be happy sitting
in the polymer A phase, and the B block is happy because it can stay in
the polymer B phase, as you can see in the picture on the right.
The block copolymers tie the two phases together, and allow energy to be transferred from one phase to the other. This means that the minor component can improve the mechanical properties of the major component rather than worsen them.
Graft copolymers are also used as compatibilizers. HIPS contains graft copolymers of polystyrene grafted onto a polybutadiene backbone chain. These graft copolymers allow stress to be transferred from the polystyrene phase to the polybutadiene phase. Since polybutadiene is rubbery, it dissipates the energy which would otherwise cause the brittle polystyrene phase to break. This is why HIPS is tougher than regular polystyrene.
But a compatibilizer lowers the energy of the phase boundary, as we say. What we mean is that the two phases can stand each other a little better when there is a compatibilizer present. So the need to minimize contact between the two phases isn't as great. So when a compatibilizer is used, our spheres don't need to be as big. Remember our 80:20 immiscible blend of high density polyethylene and polystyrene? The polystyrene spheres were about 5-10 X 10-6 meter in diameter. When enough of a polystyrene-polyethylene block copolymer (enough being 9%) is added to the immiscible blend, the size of the polystyrene spheres drops to about 1 X 10-6 meter.
This is good for the mechanical properties of the immiscible blend. The smaller the spheres, the greater the area of the phase boundary between the two phases, of course. The greater the area of the phase boundary, the more efficiently energy can be transferred from one phase to the other, meaning better mechanical properties. So we have a paradox: the separate phases want to be as large as possible but have the greatest impact on properties when they're as small as we can get them to be. Weird, huh?
References
1. Fayt, R., Hadjiandreou, P. and Teyssie, P., J. Polym. Sci. Polym. Chem. Ed., 1985, 23, 337.
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