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Effect of Temperatures on Polymers:
Solids on heating
eventually melt to form a liquid. With polymers it is not so
simple. Rubber on cooling (in liquid nitrogen) becomes brittle or
glassy. Many polymers have a mixture of ordered
(crystalline) regions and random (amorphous) regions. In the glassy
state the tangled chains in the amorphous region are frozen so
movement of chains is not possible. The polymer is brittle.
If the glassy material is
heated, the chains reach a temperature at which they can move. This
temperature is called the glass transition temperature Tg.
Above this temperature the polymer is flexible. At the melting
point, the crystalline regions break down and the polymer becomes a
viscous liquid.
The glass transition
temperature of a polymer can be changed by two different ways:
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Copolymerisation. Ethene can be
polymerised with propene to give a new polymer with different
properties.
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Plasticisers. PVC is quite
brittle. Its Tg can be lowered, making it less
brittle, by introducing a substance between the polymer chains,
allowing the chains to slide over each other more easily. Such a
substance is called a plasticiser.
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