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Crystalline domains are regions where polymer chains are aligned in an orderly manner and held together by intermolecular forces like van der Waals interactions or hydrogen bonding. Amorphous domains are regions where chains are randomly oriented and loosely packed, resulting in weak intermolecular interactions. Both types coexist in polymers due to diffusion limitations of long polymer chains.

Branching or large substituents in the polymer chain increase the proportion of amorphous domains, resulting in noncrystalline, amorphous polymers. These structural features prevent the tight, orderly packing of chains needed for crystalline domain formation. Examples include poly(methyl methacrylate), which is highly disordered and noncrystalline due to radical chain growth polymerization chain branching effects.

Crystalline domains provide toughness to polymers, while amorphous domains lend flexibility. However, both domains impart hardness and brittleness below the glass transition temperature. Above this temperature, amorphous domains become increasingly flexible, significantly improving the polymer's overall mechanical performance and usability.

Crystalline polymers exhibit a sharp melt transition temperature at which all crystalline regions become amorphous and fluid. Amorphous polymers gradually transform to a liquid state with no sharply defined melt transition temperature. This difference arises because crystalline domains have ordered structure that melts at a precise point, while amorphous domains lack this organization.

Polymer crystallinity can be controlled by varying the proportion of crystalline domains. Polyethylene terephthalate (PET) is manufactured in different grades with crystalline domain proportions ranging from 0% to about 55%. Less crystalline PET is used for plastic bottles, while highly crystalline PET is used as textile fiber for enhanced durability.

At the glass transition temperature, both highly crystalline and noncrystalline polymers transform from a hard solid to a flexible material. This transition occurs because polymer chains gain sufficient thermal energy to move more freely. Further heating causes crystalline polymers to reach their melt transition, while amorphous polymers continue to gradually soften.

Cross-linked polymers do not melt but directly decompose at extreme temperatures, unlike linear crystalline or amorphous polymers. The covalent bonds linking polymer chains throughout the network prevent the chain mobility required for melting, causing the material to break down chemically rather than transition to a liquid state.