a spherulite
/ SFEER-you-lite /
When a molten polymer cools and starts to crystallize, it does not make one big crystal. Instead, crystallization sparks at scattered points and grows outward from each one in every direction at once, so each seed blossoms into a tiny sphere of crystal that keeps swelling until it bumps into its neighbours. These spheres — anywhere from a few microns to a few millimetres across — are spherulites, and a melt-crystallized plastic is packed full of them, like a foam of frost flowers filling all the space. They are the largest structural feature of an ordinary semicrystalline polymer.
Zoom into one spherulite and it is not solid crystal but a spray of thin chain-folded lamellae radiating outward from the central seed like the ribs of a fan or the spokes of a dandelion clock, with disordered amorphous chain filling the wedges between the ribs. As the lamellae grow outward they often twist in step with one another, and this periodic twist produces the concentric dark rings you can see in some spherulites. Because the crystal ribbons are arranged with radial symmetry, a spherulite is optically birefringent, and viewed between crossed polarizers each one shows a striking dark cross — the Maltese cross — aligned with the polarizer axes, the classic fingerprint that tells a microscopist the sample is spherulitic.
Spherulites bridge the nanometre lamella and the millimetre-scale part, and their size runs the show for real properties. Big spherulites scatter visible light strongly, which is why a slowly cooled semicrystalline plastic looks milky or opaque, and the weak amorphous boundaries between large spherulites are where cracks like to start, so coarse-spherulite parts are brittle. That is why processors add nucleating agents: seeding many centres at once makes the spherulites small and numerous, which improves clarity and toughness. Note that a spherulite is a polycrystalline aggregate of countless lamellae, not one single crystal — and, confusingly, the chains themselves lie roughly tangential, folding within lamellae whose growth direction points radially outward.
Melt a thin film of polypropylene, cool it slowly on a microscope hot stage, and switch to crossed polarizers: dozens of bright discs bloom and grow until they jam against each other, each wearing a sharp dark Maltese cross. Those are spherulites caught in the act. Cool a second film fast, or add a nucleating agent, and the spherulites are far smaller — and the film is far clearer and tougher.
A spherulite is a sphere of radiating chain-folded lamellae; between crossed polarizers it shows a Maltese cross.
A spherulite is a polycrystalline aggregate, not a single crystal. And beware the geometry: the lamellae grow radially, but the chains inside them lie roughly tangential (perpendicular to the growth direction), folding across each ribbon.