Why do polymers crystallize

A good example is Kevlar which has a high degree of crystallinity. The polar amide groups in the backbone are strongly attracted to each other and form strong hydrogen bonds. This raises the glass transiton temperature and the melting point. The high crystallinity and strong intermolecular interactions also greatly increases the mechanical strength.

In fact, Kevlar fibers are some of the strongest plastic fibers on the market. Bulky side groups have the opposite effect on crystallinity. With increasing size of the side groups it becomes progressively more difficult for the polymer to fold and align itself along the crystal growth direction. Thus bulky side groups and branching reduce the ability and likelihood of a polymer to crystallize.

For example branched polyethylene has a low dregree of crystallinity, even though polyethylene itself easily crystallizes.

Similarly, most network polymers do not crystallize because the polymer subchains do not have the freedom to move. Crystalline polymers are characterized by a melting point T m and amorphous polymers are characterized by a glass transition temperature T g. For crystalline polymers, the relationship between T m and T g has been described by Boyer as follows.

Examples are poly methylene oxide , polyethylene, and polyacetal. These polymers are markedly crystalline. They can be highly crystalline if they have long sequences of methylene groups or are highly stereo-regular.

Relatively short polymer chains form crystals more readily than long chains, because the long chains tend to be more tangled. High crystallinity generally means a stronger material, but low molecular weight polymers usually are weaker in strength even if they are highly crystalline. Low molecular weight polymers have a low degree of chain entanglement, so the polymer chains can slide by each other and cause a break in the material.

Crystallinity is favored by strong interchain forces. The presence of polar and hydrogen bonding groups favors crystallinity because they make possible dipole-dipole and hydrogen bonding intermolecular forces. A polyester, such as poly ethylene terephalate , contains polar ester groups. Dipole-dipole forces between the polar groups hold the PET molecules in strong crystals.

Crystallinity in poly ethylene terephalate also is favored by the structural regularity of the benzene rings in the chain The benzene rings stack together in an orderly fashion.

Regular polymers with small pendant groups crystallize more readily than do polymers with large, bulky pendant groups. A major difference between small molecules and polymers is that the morphology of a polymer is dependent on its thermal history.

The crystallinity of a polymer can be changed by cooling the polymer melt slowly or quickly, and by "pulling" the bulk material either during its synthesis or during its processing. 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 below that the polar amide groups in the backbone chain of nylon 6,6 are strongly attracted to each other.

They form strong intermolecular hydrogen bonds. This strong binding holds chains together, and because those chains are so symmetrical, they're also form crystals. This raises the melting point of the crystals compared to polymers without such strong intermolecular interactions. That's why nylons have much higher melting points than, say, polyethylene or polypropylene. Polyesters are another example.

Let's look at the polyester we call poly ethylene terephthalate or PET. The polar ester groups make for strong interactions, just like the poles of a magnet pull toward each other.

In addition, the aromatic rings like to stack together in an orderly fashion remember pi-stackin? These strong interaction again raise the Tm of PET much higher than that of polyethylene. And now you might be asking yourself, "If those intermolecular forces affect crystallinity and Tm, don't they also affect Tg? Wouldn't stronger interactions causing a higher Tm also lead to a higher Tg?

You've come a long way already. Great questions and the answer to both is "Yes! Take a look at the plot below for some of the more common polymers that are at least semi-crystalline. And as you might have guessed, Tm is always greater than Tg. And just in case you're wondering about physical properties, here's a brief summary of several properties for a couple dozen common polymers.

These include some pretty high perfomance materials as well as some that aren't like low density PE. Brandup, E. How Much Crystallinity? Remember we said that 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 has its own page, and it's called differential scanning calorimetry. It uses an analytical instrument to actually measure the glass transition and melting temperatures. More importantly, it can measure how much of each is in a given sample. This kind of "quantitation" is very important in understanding how well any polymer will behave and what it can be used for. Lesson here is: the more you know, the more you understand. There are other methods that can tell you something about what kind of crystallinity is present, such as neutron and x-ray scattering.

Solid state NMR has recently become an important tool for looking at the types and amounts of crystalline and amorphous domains present. We don't have time or space to tell you all about these powerful tools here, but if you're interested, you can find tons of information on the web.

Using a combination of these techniques, it's even possible to differentiate amorphous domains from what's called rigid amorphous areas. The former makes a polymer tougher and more flexible while the latter makes it stronger. Didn't know there were that many different regions inside a perfectly ordinary polymer, did you? Factors favouring crystallinity In general, factors causing polymers to be more ordered and regular tend to lead to a higher degree of crystallinity.

Fewer short branches — allowing molecules to pack closely together Higher degree of stereoregularity - syndiotactic and isotactic polymers are more ordered than atactic polymers. More regular copolymer configuration — having the same effect as stereoregularity This topic is covered in the Crystallinity in Polymers TLP. Previous Next.