Semicrystalline polymers achieve rigidity by folding into highly ordered microscopic lamellae.

Some polymer chains are highly regular and symmetrical. This simplicity allows them to do something remarkable as they cool: they fold. The chains bend back and forth on themselves, creating flat, neat sheets called lamellae. These folded regions are highly ordered, acting like tiny, tough crystals inside the material. However, because these chains are so long, a single molecule might have one end trapped in a neat fold and the other end lost in a tangled, amorphous mess. No synthetic polymer is entirely crystalline. They are always semicrystalline, combining the flexibility of the tangled zones with the strength of the folded crystals.