Spherulite (polymer physics)

Spherulites are composed of highly ordered lamellae, which result in higher density, hardness, but also brittleness when compared to disordered regions in a polymer.

If a molten linear polymer (such as polyethylene) is cooled down rapidly, then the orientation of its molecules, which are randomly aligned, curved and entangled remain frozen and the solid has disordered structure.

The largest surfaces of the lamellae are terminated by molecular bends and kinks, and growth in this direction results in disordered regions.

Strong supercooling or intentional addition of crystallization seeds results in relatively large number of nucleation sites; then spherulites are numerous and small and interact with each other upon growth.

[7] Alignment of the polymer molecules within the lamellae results in birefringence producing a variety of colored patterns when spherulites are viewed between crossed polarizers in an optical microscope.

[8][9] This effect results in the dark perpendicular cones (Maltese cross) and colored brighter regions in between them in the front and right pictures.

Any dark or light spots are dependent on the angle made with the polarizer, which results in a symmetrical image due to the spherical shape.

When spherulites were rotated in their plane, the corresponding Maltese cross patterns did not change, indicating that the molecular arrangement is homogeneous versus the polar angle.

Principle of lamellae formation during the crystallization of polymers. Arrow shows the direction of temperature gradient. [ 1 ]
Schematic model of a spherulite. Black arrows indicate direction of molecular alignment
Strain at failure vs. spherulite size. [ 7 ]
A spherulite embedded into a mosaic mesogen viewed between crossed polarizers.
A schematic of Maltese cross formation
Spherulites embedded into a mosaic mesogen viewed between crossed polarizers.