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TECHNICAL

NEWSLETTER

Crystallinity in Plastics
Introduction
In previous Technical Newsletters, we mentioned the effect of crystallinity in plastics but never delved deeper into this topic. This Technical Newsletter will more closely consider crystallinity in plastics: what it is, and more importantly, how it affects the properties of plastics. Most people are familiar with crystals only because of salt and perhaps growing crystals as part of a school science project. Therefore, talking about crystals and plastics together is a new concept to many people. In reality, polymers are not like salt, which is totally crystalline, but are semi-crystalline. Unlike crystals like salt, polymers have only short-range order and a much looser organization.

Crystallinity is one of the great divisions of the complete family of plastics. Amorphous polymers (those with no significant degree of crystallization) behave very differently than crystalline polymers (those with a significant degree of crystallization). Understanding crystallization and the effect it has on the properties of polymers can make understanding the behavior of polymer families much easier.

Amorphous and crystalline polymers:
Amorphous polymers
Amorphous polymers are those where the polymer chains have no well-defined order in either the solid or liquid states. The model that is most often associated with this characterization is a bowl of cooked spaghetti, where the long strands of spaghetti are both flexible and slide over one another. This is similar to a molten amorphous polymer as it is being processed. If the hot spaghetti is tipped into a bowl, then it will take the shape of the bowl, just as a plastic does when it fills a mold. After the spaghetti has cooled, the long strands tend to stick to one another, which is very similar to the behavior of an amorphous polymer after it has cooled. The classic two-dimensional representation of an amorphous polymer is shown below:

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Amorphous polymer - random molecular orientation in both molten and solid phases

Examples of amorphous polymer are PMMA, PS, PVC and ABS. As a rule, any polymer that can be produced in a transparent form is an amorphous polymer. Crystalline polymers
In the molten phase, crystalline polymers closely resemble amorphous polymers. However, the real difference between the two exhibits itself during cooling. As crystalline polymers cool, small areas of short-range order form. These are highly organized and closely packed areas of polymer molecules and are recognized as the “polymer crystals” in crystalline polymers. However, they are not at all like the crystals in salt or other inorganic materials. There are several models and theories regarding crystal formation, but the most popular is the “fringed micelle” model. Shown below is a two-dimensional representation of molecules in a crystalline polymer according to the fringed micelle theory. It shows the ordered regions or crystallites (in red) embedded in an amorphous matrix.

Crystalline polymer - random molecular orientation in molten phase but densely packed crystallites in solid phase

The red ordered regions show above are made up of polymer chains that are folded into highly ordered regions as shown in the figure below.

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Folded lamellae crystals in a crystalline polymer, color coded for clarity.

A single polymer backbone chain may be present in sections of different crystallites, which are generally small and separated by areas without a short-range order. Larger structures, made up of bundles of crystallites, are termed spherulites and these are formed in the bulk of the material. These are formed and grow in a similar manner to the formation and growth of grains in a metal.

Spherulites in poly-3-hydroxy butyrate (PHB)
Source: Prof T. W. Clyne, Department of Materials Science and...
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