Cross-linked

A small region in a macromolecule from which at least four chains
emanate, and formed by reactions involving sites or groups on existing
macromolecules or by interactions between existing macromolecules.

Notes

1. The small region may be an atom, a group of atoms, or a number of
branch points connected by bonds, groups of atoms, or oligomeric chains.

2. In the majority of cases, a crosslink is a covalent structure but the term
is also used to describe sites of weaker chemical interactions, portions of
crystallites, and even physical interactions and entanglements.

A cross-link is a bond that links one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers (such as proteins). When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of cross-links to promote a difference in the polymers' physical properties. When "crosslinking" is used in the biological field, it refers to the use of a probe to link proteins together to check for protein–protein interactions, as well as other creative cross-linking methodologies.

Cross-linking is used in both synthetic polymer chemistry and in the biological sciences. Although the term is used to refer to the "linking of polymer chains" for both sciences, the extent of crosslinking and specificities of the crosslinking agents vary. Of course, with all science, there are overlaps, and the following delineations are a starting point to understanding the subtleties.

When cross links are added to long rubber molecules, the flexibility decreases, the hardness increases and the melting point increases as well.

When polymer chains are linked together by cross-links, they lose some of their ability to move as individual polymer chains. How the crosslinking process works and how it improves the properties of polymers is nicely illustrated in this video-animation. For example, a liquid polymer (such as resin or even melted cheese which contains protein polymers) (where the chains are freely flowing) can be turned into a "solid" or "gel" by cross-linking the chains together.

In polymer chemistry, when a synthetic polymer is said to be "cross-linked", it usually means that the entire bulk of the polymer has been exposed to the cross-linking method. The resulting modification of mechanical properties depends strongly on the cross-link density. Low cross-link densities decrease the viscosities of polymer melts. Intermediate cross-link densities transform gummy polymers into materials that have elastomeric properties and potentially high strengths. Very high cross-link densities can cause materials to become very rigid or glassy, such as phenol-formaldehyde materials.

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