Three-dimensional order in the arrangement of atoms and molecules within a chemical phase. Most chemical compounds of low molecular weight may be obtained in a state of virtually complete three-dimensional order. When polymers crystallize, in general the product consists of regions of high order (crystallites), regions of low order (amorphous regions), and regions of intermediate order. Different methods of measuring the degree of crystallinity (e.g., density, wide-angle X-ray scattering, enthalpy measurement) emphasize different aspects and therefore lead to quantitatively different values. In recent years the simple concept of crystalline and amorphous regions has been questioned and terms such as para-crystalline have been introduced
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