In the initial manufactured state, most polymers consist of amorphous randomly-orientated molecular chains. However, if the plastic material is stretched in one direction, such as during the drawing of spun fibres, this causes an alignment of the molecular chains, leading to partial formation of crystalline regions and an associated anisotropy (Fig. 10.6).

Low-density polythene - branched chained Fig. 10.4 Straight- and branched-chain polymers

{□ and ^ represent two different monomer units) Fig. 10.5 Random, alternate and block copolymers

Crystalline regions may also be produced during the solidification of simple polymers such as polyethylene, but they will be limited in their extents due to the general entanglement of the molecular chains.

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