A synthetic polymer may be loosely described as crystalline if it contains regions of three-dimensional ordering on atomic () length scales, normally arising from intramolecular folding and/or stacking of adjacent chains. Synthetic polymers could contain both crystalline and amorphous areas; the degree of crystallinity may be expressed in terms of a weight fraction or volume fraction of crystalline material.
The crystallinity of polymers is categorized by their degree of crystallinity, varying from zero for a completely non-crystalline polymer to one for a theoretical completely crystalline polymer. Polymers with microcrystalline regions are generally tougher and more impact-resistant than totally amorphous polymers.
Polymers with a degree of crystallinity approaching zero or one will tend to be transparent, while polymers with intermediate grade of crystallinity will tend to be opaque due to light scattering due to crystalline or glassy regions. Thus for many polymers, reduced crystallinity may also be associated with increased transparency.