The structure of polymers, which are built from basic units of long chain macromolecules, differs very significantly from that of metals, the basic units of which are atoms and molecules. Most metals are polycrystalline materials while most polymers are amorphous, in which the chains are not arranged in a regular, ordered manner. Polymers exhibiting crystallinity are really semicrystalline materials in which crystalline structures are embedded in amorphous matrices. And, in contrast to metals, the crystalline structures of polymers are very complex. These basic differences between metals and polymers translate into significant qualitative and quantitative differences between their properties.