Unlike ionic or small covalent molecules, polymers do not form crystalline solids due to the diffusion limitations of their long-chain structures. However, polymers contain microscopic crystalline domains separated by amorphous domains.

Crystalline domains are the regions where polymer chains are aligned in an orderly manner and held together in proximity by intermolecular forces. For example, chains in the crystalline domains of polyethylene and nylon are bound together by van der Waals interactions and hydrogen bonding, respectively.

Amorphous domains are the regions where the chains are randomly oriented and loosely packed, resulting in weak intermolecular interactions. The presence of branching or large substituents in the polymer chain further increases the likelihood of forming amorphous domains. Highly disordered polymer chains can result in a noncrystalline polymer; for example, poly(methyl methacrylate).

Figure 1: A crystalline domain (rectangle) and an amorphous domain (oval) of a polymer.

Crystalline domains impart toughness to a polymer, whereas amorphous domains lend flexibility. Poly(ethylene terephthalate), or PET, is manufactured in different grades varying the proportion of crystalline domains from 0% to about 55%. Less crystalline PET is used for making plastic bottles. Highly crystalline PET is used as a textile fiber.

The thermal properties of highly crystalline polymers and noncrystalline polymers are different. At the glass transition temperature, both polymers transform from a hard solid to a flexible material. Upon heating further, only crystalline polymers exhibit a sharp melt transition temperature—at which the polymer transforms to a liquid. Noncrystalline polymers possess no definite melt transition temperature (crystalline melting temperature). Cross-linked polymers do not melt but directly decompose at extreme temperatures.