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July 31, 2001

10 Min Read
The Materials Analyst, Part 46: I can't believe it's not crystalline (Part 3)

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This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure problem. Michael Sepe is our analyst and author. He is the technical director at Dickten & Masch Mfg., a molder of thermoset and thermoplastic materials in Nashotah, WI. Mike has provided analytical services to material suppliers, molders, and end users for 15-plus years.

To this point we have discussed the two most important properties that promote crystallization in a polymer: a chain with a streamlined and regular shape and sufficient opportunity for motion during the cooling process. We have defined the temperature range over which this motion may occur. We have outlined a method of measurement for determining the results of our efforts to produce a crystalline product. And we have touched on some of the practical consequences of failing to control the development of an adequately crystalline structure. But we have limited our detailed discussions to polyethylene. This material provides a great model because it has a very simple structure and it can be made in a variety of densities. But if polyethylene were the only polymer available we would be limited to application temperatures below 130C, yield strengths of 3000 psi, and modulus values of about 200,000 psi. 

So how do we boost the properties of materials? There are several ways to achieve property improvements, but the two that are most applicable to semicrystalline polymers involve increasing the number of attachments that develop between the polymer chains or increasing the strength of the individual attachments. We will consider the first of these this month and save the latter for next month. 

Let's return to the polyethylene molecule for a moment and create a picture of a small segment of the polymer chain (Figure 1). This pattern can be repeated many times to produce materials of varying molecular weights. In the shorthand of chemistry, the carbon atoms in the polymer backbone are not drawn but are inferred to be at the up/down vertices. Attached to each carbon atom are two hydrogen atoms as we have described previously. The zigzag shape of the structure is our attempt to show a three-dimensional shape on a flat piece of paper. Notice that this unit is completely symmetrical. We can turn and twist it in a variety of ways and it will still look the same. 

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Figure 1. Structure of high-density polyethylene.


Atom Juggling 
There are a variety of materials that can be created by replacing one of the hydrogen atoms with something else. Figure 2 shows a general pattern for a single repeating unit of a particular type of polymer chain. If the polymer is polyethylene, all four positions represented by W, X, Y, and Z will be a hydrogen atom. If we replace a single hydrogen atom with a chlorine atom we get PVC. If we substitute a more complicated structure known as an acrylate group, we get acrylic. But neither of these materials is semicrystalline. In both of these cases we have broken up the natural symmetry required for crystallization. 

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Figure 2. General pattern of placement for side groups.

However, it is possible to perform such a replacement without losing crystallinity entirely. If we replace one hydrogen with a larger hydrocarbon group known as a methyl group, we obtain polypropylene. We know that polypropylene is semicrystalline. So how is it that some groups can be substituted for hydrogen to maintain a degree of crystallinity and others cannot? 

It comes back to the importance of position. When polypropylene was first synthesized it was useless as a load-bearing material. It would soften near room temperature and was only used commercially as an adhesive. This was because the original poly-propylene polymers were amorphous. Since amorphous materials contain no significant amount of crystalline structure, they soften when they reach the glass transition temperature (Tg). The Tg of a polypropylene homopolymer is right around room temperature, 15 to 25C (59 to 77F). If polyethylene is at least partially crystalline, why did the original polypropylenes fail to form crystals? 

The answer lies in the placement of the methyl group. The methyl group is a carbon attached to three more hydrogens. In the general scheme shown in Figure 2, the repeating unit for polypropylene will contain three hydrogens and one methyl group. The methyl group creates the same problem for symmetry that the chlorine does in PVC; it is huge compared to the hydrogen atom it replaces, and this size creates a problem for the streamlined shape that polyethylene is known for. 

The problem with the original polypropylenes had to do with the exact position of the methyl group along the polymer backbone. Sometimes it would be in the W position, other times in the X, Y, or Z, in no particular pattern. This type of pattern, or lack thereof, is referred to as atactic, and it effectively prevented crystallization in polypropylene. PVC and acrylic also contain an atactic arrangement. But because they have glass transition temperatures well above room temperature, they are useful materials in their own right without the benefit of crystallinity. 

Order in the Chain 
The solution to the polypropylene problem came from the same breakthrough that created high-density polyethylene. The new catalysts that allowed polymer chemists to reduce the branching in polyethylene also resulted in an increased level of control over the placement of the methyl group. As the chemists refined their tools they developed a material where the methyl group would appear in the same position almost every time. Such an arrangement is called isotactic. Once the position of the methyl group was regulated, the material formed crystals. 

Because the methyl group provided the opportunity for more interactions per unit of chain length, the crystals had a higher melting point, resulting in a material that was potentially stiffer and stronger than polyethylene. Figure 3 (below) shows a short segment of the isotactic structure. In this sketch the chemist's shorthand has been further simplified by drawing only the methyl group. The carbon atoms are still at the vertices of the zigzags in the backbone, and the hydrogen atoms are still in the same places they were in Figure 1. It is fairly easy to imagine that with this type of regularity, the polymer chains will have a better chance of fitting together in a jigsaw puzzle manner as they fold on themselves. 

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Figure 3. Structure of isotactic polypropylene.


There's the Rub 
Now here is the paradox. Despite the improved properties of the isotactic polypropylene, the degree of crystallinity is not as great as that achieved in a high-density polyethylene. Remember that ideal value of 290 J/g needed to melt the ideal 100 percent crystalline polyethylene? Well, for polypropylene that same value is only 194 J/g. And while some polyethylenes can achieve as much as 90 percent of that ideal value, with polypropylenes even the high-crystallinity materials rarely exceed 65 percent. This is a consistent pattern. As we change the chemistry of the side groups to achieve stronger attachments, we tend to lose some of the structural regularity needed for crystallization to occur. The individual crystals are harder to break up, as indicated by the higher melting point. But there are fewer of these higher-melting-point crystals. 

Polystyrene is an amorphous material that has changed its stripes through a third variation of side group placement. This has only become possible since the commercialization of the breakthrough metallocene chemistry of the 1990s. For 50 years polystyrene has been one of the workhorse materials of the commodity realm; it offers strength and rigidity coupled with high gloss and transparency for a price comparable to polyethylene and polypropylene. Even though it is amorphous, polystyrene did not suffer the commercial fate of amorphous polypropylene because its Tg is near 100C. Between room temperature and 100C, therefore, polystyrene has useful load bearing properties. Above Tg it softens and loses all load-bearing capabilities. 

Amorphous polystyrene is atactic; the group that substitutes for the fourth hydrogen in polystyrene is a huge hexagonal ring called a benzene ring or a phenyl group. Each vertex of the hexagon contains a carbon atom, so it is much bigger than the methyl group in polypropylene. As you might imagine, such an imposing presence destroys all hope of achieving any kind of structural regularity. 

Benzene Control 
Or does it? The new catalysts actually make it possible for chemists to control the placement of the benzene ring just as the chemists of the 1950s controlled the placement of the methyl group. Isotactic polystyrene is now a physical possibility, but the massive proportions of the benzene ring cause the molecule to become overcrowded if all of the rings are placed on one side of the chain. 

But there is a third type of arrangement that promotes the regularity required for crystallinity, an arrangement where the placement repeats every other monomer unit, as shown in Figure 4. This is known as syndiotactic, often called a head-to-tail arrangement. It relieves the overcrowding of the isotactic structure while preserving the orderly pattern needed to promote crystallinity. If we run a DSC on a "normal" polystyrene we can heat it indefinitely and all we will find is the glass transition, right where we expect it at 100C. But if we heat this syndiotactic polystyrene to a sufficiently high temperature, we find a melting point, hard evidence of crystal formation. (This is not be confused with crystal polystyrene, a layman's term that refers to the crystal-like clarity of amorphous polystyrene.) 

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Figure 4. Syndiotactic structure of polystyrene.


Consider the consequences of such a creation. If the Tg of polystyrene is 100C, and the melting point of a semicrystalline material is 150 to 175 deg C above its Tg, this would place the melting point of semicrystalline polystyrene somewhere between 250 and 275C (it's actually 270C). The physical consequences of such a material are best illustrated by mapping the stiffness of the amorphous and the semicrystalline variety as a function of temperature. Figure 5 shows a dynamic mechanical analysis for an amorphous and a semicrystalline polystyrene. 

When creating a semicrystalline polystyrene we are still working with nothing more than carbon and hydrogen.

Both of these materials contain 30 percent glass fiber for purposes of an apples-to-apples comparison. The implications of this comparison are clear. While both types of polystyrene experience a decline in stiffness as they pass through the glass transition, the amorphous material softens completely while the semicrystalline version maintains its solid state at a reduced but useful level until the crystals melt. It is important to note that in creating semicrystalline polystyrene we are still working with nothing more than carbon and hydrogen. 

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Figure 5. Modulus vs. temperature plot for amorphous and semicrystalline PS.


The individual attractions within the molecules are of the same strength as in polyethylene and polypropylene. All three polymers are nonpolar and require no drying. But because of the size of each benzene ring we have increased the number of these attractions. In the process we have progressed from a material with a melting point of 135C to one with a melting point of 270C. We have also introduced a new challenge to achieving the desired crystalline structure. 

Look carefully at Figure 5, because in the next two months we are going to discuss the problems that occur in high-performance materials such as semicrystalline polystyrene when the crystals fail to form during the molding process. It can lead to a bewildering variety of failures that often remain a mystery until the mechanism of poor crystallinity development is understood. 

Editor's note: Mike Sepe will speak on "If This Stuff is 50 Percent of Our Cost Structure, Why Are We Paying So Little Attention?" at IMM's technology conference, Oct. 1-2 in Chicago, preceding Plastics USA. For more information, go to www.immconference.com. 

Contact information
Dickten & Masch Mfg. Co.
Nashotah, WI
Mike Sepe
(262) 369-5555, ext. 572
www.dicktenplastics.com
[email protected]

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