Semicrystalline polymers constitute the largest group of commercially useful polymers. These polymers exists as viscous liquids at temperatures above the melting point of the crystals. Upon cooling, crystals nucleate and grow to fill the available volume. The reason these materials are called "semicrystalline" is that some fraction of the polymer remains un-crystallized, or, amorphous when the polymer is cooled to room temperature. The amorphous polymer becomes trapped between the growing crystals. As a result of the highly entangled nature of the polymer chains, the movement of the amorphous polymer becomes restricted.