The development of early knowledge about crystals of flexible macromolecules (usually called simply “polymers”) and their defects was reviewed in Reference. Much of the initial research followed the information gathered on crystals of small molecules, metals, salts, and ceramics. Very quickly, however, the special nature of polymers became obvious. It was suggested some time ago to describe semicrystalline polymers with the help of zero- to three-dimensional defects. The present status of this scheme is summarized in this article.

Crystallized polymers are rarely in equilibrium. Therefore, the phase rule, which permits the presence of only one phase with two degrees of freedom (temperature and pressure) for a onecomponent material, does not apply. This nonequilibrium state is kept metastable by restrictions caused by the molecules that cross the interfaces (two-dimensional defects). The strain propagated to the noncrystalline areas produces three-dimensional defects of limited mobility. The overall structure can also be described as being microphase-separated.

Often the sizes of the different phases can be as small as the nanometer scale. One-dimensional or line defects, also called dislocations, became obvious in polymers with the first observations of solution-grown crystals of polyethylene that showed frequent growth spirals. Because of the molecular chain struc ture, many of these dislocations are sessile (immobile) and cannot participate for example, in deformation mecha nisms. Until recently, zero-dimensiona or point defects were understood largely because of molecular-mechanics calculations.