Polymer chains rarely form a perfectly periodic crystal throughout an ordinary bulk sample.

An amorphous polymer has no long-range translational order. Chains adopt disordered conformations and are entangled with one another.

A semicrystalline polymer contains ordered crystalline regions embedded in an amorphous matrix. The crystalline fraction, crystal size and orientation depend on chain regularity and processing history.

Crystallization requires chains to pack into compatible arrangements. Regular, relatively linear chains can often crystallize more readily than strongly branched or irregular chains. Rapid cooling can also trap a polymer in a more amorphous state because chains do not have enough time to reorganize.

Crystallinity generally increases density, stiffness, chemical resistance and melting-related dimensional stability, but can reduce optical transparency or alter toughness.

Mechanical drawing can orient chains and crystalline regions along the deformation direction, producing anisotropic properties even without changing chemistry.

Polymer morphology therefore plays the same broad role that grains and phases play in metals: it is an intermediate structural scale through which processing history controls macroscopic properties.