Effect of Temperatures on Polymers:

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 T_g.  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:

1. Copolymerisation.  Ethene can be polymerised with propene to give a new polymer with different properties.

2. Plasticisers.  PVC is quite brittle.  Its T_g 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.