Introduction - the meaning and importance of orientation

It is important to realise that, even in a sample that is randomly oriented on a macroscopic scale, there may be regions, such as crystallites, in which the molecular chains are oriented on a microscopic scale. 

Polymers are not the only materials that can exhibit orientation. Any polycrystalline material, such as a metal, can exhibit orientation and this can confer desirable or undesirable properties on the material. The effects are, however, often much greater for polymers than they are for other materials. Table 10.1 compares the effects of the forging of steel and the extrusion of polypropylene. In addition to the other changes shown in table 10.1, the elongation that takes place before breaking occurs, often called the elongation to break, is substantially reduced for the polymer. 

Other desirable changes in properties may be produced by orientation, for example polystyrene sheet that is biaxially oriented (see section 10.2.2) can be highly flexible, whereas unoriented sheet is brittle oriented sheet exhibits substantial increases in impact strength, in tensile-yield strength and in resistance to stress crazing. It should not, however, be assumed that such improvements will always result from orientation. 

Undesirable properties due to orientation may include anisotropy in properties and dimensional instabilities at elevated temperatures in thermoplastic materials, e.g. shrinkage of textile fibres, due to randomisation of the orientation of the amorphous regions on heating above the glass transition temperature. 

Natural polymers often owe their important properties to the fact that they are highly oriented, e.g. cellulose, in the form of wood and cotton, and silk fibre, which is a protein. 

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