Spherulite (polymer physics)

In polymer physics, spherulites are spherical semi-crystalline regions inside non-branched linear polymers.

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When a liquid linear polymer (such as polyethylene) is cooled down slowly, the polymer strings take on a certain orderly configuration: they align themselves in plates of orthorhombic unit cells called crystalline lamellae. These highly ordered plates are interrupted by amorphous regions, so that even a completely spherulized polymer is still far from being fully crystalline.

Due to the high degree of order and crystallinity contained within the spherulite structure they exhibit birefringence. By placing the polymer spherulite between crossed polarisers different colours can be observed. See image "A PEO spherulite..."

The process of spherulization starts on a nucleation site and continues to extend radially outwards until a neighbouring spherulite is reached. This explaines the spherical shape of the spherulite.

Spherulites, when formed, impact some properties of the polymer material: crystallinity, density (crystalline lamellae are more dense than surrounding amorphous material), tensile strength and modulus of elasticity all increase during spherulization.

Sources

(1992) Introduction to engineering Materials: The bicycle and the walkman. Philadelphia: Enfield Publishing, pp 277 - 283. ISBN 0-9646598-0-8.
http://www.pslc.ws/mactest/crystal.htm

Category: Polymers