the spherulite

When a molten semicrystalline polymer cools and solidifies, its crystals do not grow as one big block. Instead they sprout outward from many separate starting points, each one growing into a little ball made of ribbon-like crystal sheets radiating in every direction — rather like a dandelion seed-head or a sea urchin. These balls are spherulites, and they grow until they bump into their neighbours and fill up all the space. Under a polarising microscope each spherulite shows a striking dark Maltese-cross pattern.

Zoom in and a spherulite is made of thin chain-folded crystalline lamellae (the ordered ribbons) growing radially from the central nucleus, with disordered amorphous polymer filling the gaps between them. So a spherulite is itself part crystalline and part amorphous. The spherulites nucleate at many points, grow outward, and stop where they meet their neighbours, tiling the whole solid — which makes them the polymer world's close cousin of the grains you find in a solidified metal. They range from a few microns to a few millimetres across depending on how the material was cooled.

It matters because spherulite size controls two everyday properties. Big spherulites are comparable in size to the wavelength of light, so they scatter it and make the plastic look hazy or opaque; small ones let the plastic stay clearer. And because the boundaries where spherulites meet can be regions of weakness, spherulite size also affects toughness. Engineers control it through cooling rate (fast cooling gives many small spherulites) and by adding nucleating agents. One honest clarification: a spherulite looks like a single crystal but is not one — it is an aggregate of many tiny lamellar crystals plus amorphous material, which is exactly why polymers are only ever semicrystalline.