The smallest, regularly repeating material portion contained in a parallelepiped from
which a crystal is formed by parallel displacements in three dimensions.
Unlike in the case of low molar mass substances, the unit cell of polymer crystals
usually comprises only parts of the polymer molecules and the regularity of the periodic
repetition may be imperfect.
In the case of parallel-chain crystals, the chain axis is usually denoted by c or, sometimes, b.
This definition applies to the so-called primitive unit cell. In practice, the effective
unit cell may consist of more than one primitive unit cell.
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook