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The Structure of Polymers.
Thermosoftening Polymers and Crystallinity.

Polymers may be classified as thermosoftening or thermosetting.
Thermosoftening polymers are sometimes called thermoplastic.

Thermosoftening polymers include poly(ethene), poly(propene),
poly(styrene) and poly vinyl chloride.

What is the Structure of a Thermosoftening Polymer?

A polymer molecule is a long chain of carbon atoms
which are held together by strong covalent bonds.
The forces in between the molecules are relatively weak but
sections of the chains can sometimes line up side by side to
form tiny crystals. These crystals form links between the chains,
hold the structure together and make the polymer solid.

What is the Crystallinity of a Polymer?

The amount of the polymer that forms crystals is called its
crystallinity. The higher the cystallinity of the polymer, the stronger
and stiffer the material becomes and the melting point gets higher
(see HDPE and LDPE). The melting point of the polymer depends
on the strength of the forces of attraction between the molecules
(the melting point of the crystals).

When the polymer is heated the crystals will melt, the material
will become very soft and can flow slowly like a thick liquid.
In this state the polymer can fill a mould and be cast into a shape.
When the polymer cools down, new crystals can form between
the chains and the new shape is fixed. The same polymer can be
reheated and remoulded. Such polymers are called thermosoftening
(meaning that they go soft when you heat them).

The picture below shows the tangled polymer chains
which have lined up and crystallised in the pink region.
Thermosoftening Polymer

Some polymers do not form crystals. They soften
when heated and harden when cooled down again.
When cold, they are not crystalline but glassy.
These polymers are also called thermosoftening.

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