10 - Oriented polymers I – production and characterisation

Summary

Introduction – the meaning and importance of orientation

Consider a volume within a sample of polymer that is large compared with the size of individual chain segments, crystallites or spherulites, but small compared with the sample as a whole. If equal numbers of the axes of the chain segments or of the crystallites within this region point in every direction in space, the region is isotropic on a macroscopic scale. If all such regions within the sample are isotropic, the sample itself is said to be isotropic, randomly oriented or unoriented. If any region of the sample is not isotropic it is said to be oriented or partially oriented. The sample as a whole may be homogeneously oriented if all regions of it are oriented in the same way, or it may be inhomogeneously oriented. The various types and degrees of orientation that are possible are considered in detail in subsequent sections.

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.