Cross-link 1089106 224354581 2008-07-08T14:08:32Z SmackBot 433328 Date the maintenance tags or general fixes {{Refimprove|date=July 2008}} [[Image:Vulcanization.png|thumb|right|340px|[[Vulcanization]] is an example of cross-linking.]] '''Cross-links''' are bonds that link one [[polymer]] chain to another. They can be [[covalent bond]]s or [[ionic bond]]s. "Polymer chains" can refer to synthetic polymers or natural polymers (such as [[protein]]s). 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 milieu, it can be in reference to its use as a probe to link proteins together to check [[protein-protein interaction]]s, as well as other creative cross-linking methodologies. Cross-linking is used in both synthetic polymer chemistry and in the biological sciences. While 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 stated as a starting point to understanding the subtleties. ==Crosslinks in synthetic polymer chemistry== When polymer chains are linked together by crosslinks, they lose some of their ability to move as individual polymer chains. For example, a liquid polymer (where the chains are freely flowing) can be turned into a "solid" or "gel" by crosslinking the chains together. In polymer chemistry, when a synthetic polymer is said to be "crosslinked", it usually means that the entire bulk of the polymer has been exposed to the crosslinking method, resulting in fairly extensive crosslinking treatment. Crosslinking inhibits close packing of the polymer chains, preventing the formation of crystalline regions. The restricted molecular mobility of a crosslinked structure limits the extension of the polymer material under loading. ===Formation of crosslinks=== Cross-links can be formed by [[chemical reaction]]s that are initiated by heat, pressure, or radiation. For example, mixing of an unpolymerized or partially polymerized [[resin]] with specific chemicals called '''crosslinking reagents''' results in a chemical reaction that forms crosslinks. Cross-linking can also be induced in materials that are normally [[thermoplastic]] through exposure to a radiation source, such as [[electron beam]] exposure, gamma-radiation, or UV light. For example, [[electron beam processing]] is used to cross-link the C type of [[PEX|cross-linked polyethylene]]. Other types of cross-linked polyethylene are made by addition of peroxide during [[extruding]] (type A) or by addition of a cross-linking agent (eg. [[vinylsilane]]) and a catalyst during extruding and then performing a post-extrusion curing. The chemical process of [[vulcanization]] is a type of cross-linking and it changes the property of [[rubber]] to the hard, durable material we associate with car and bike [[tire]]s. This process is often called sulfur curing, and the term [[vulcanization]] comes from [[Vulcan (mythology)|Vulcan]], the [[Roman mythology|Roman]] god of fire. However, this is a slow process, taking around 8 hours. A typical car tire is cured for 15 minutes at 150°C. However, the time can be reduced by the addition of accelerators such as 2-benzothiazolethiol or tetramethylthiuram disulfide. Both of these contain a sulfur atom in the molecule that initiates the reaction of the sulfur chains with the rubber. [[Accelerator (chemistry)|Accelerators]] increase the rate of cure by catalysing the addition of sulfur chains to the rubber molecules. Crosslinks are the characteristic property of [[thermosetting plastic]] materials. In most cases, cross-linking is irreversible, and the resulting thermosetting material will degrade or burn if heated, without melting. Especially in the case of commercially used plastics, once a substance is cross-linked, the product is very hard or impossible to recycle. In some cases, though, if the cross-link bonds are sufficiently different, chemically, from the bonds forming the polymers, the process can be reversed. [[Permanent wave]] solutions, for example, break and re-form naturally occurring cross-links ([[disulfide bond]]s) between protein chains in [[hair]]. ===Physical cross-links=== Chemical covalent cross-links are stable mechanically and thermally, so once formed are difficult to break. Therefore, cross-linked products like car [[tire]]s cannot be recycled easily. A new class of polymers known as [[thermoplastic elastomer]]s rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as [[snowmobile]] tracks, and [[catheter]]s for medical use. They offer a much wider range of properties than conventional cross-linked elastomers because the domains which act as cross-links are reversible, so can be reformed by heat. The stabilising domains may be non-crystalline (as in styrene-butadiene block copolymers) or crystalline as in thermoplastic copolyesters. ==Crosslinks in the biological milieu== In the biological sciences, crosslinking typically refers to a more specific reaction used to probe molecular interactions. For example, [[protein]]s (a type of natural polymer) can be cross-linked together to probe molecular interactions using small-molecule crosslinkers. === Crosslinker use in protein study === The interactions or mere proximity of [[protein]]s can be studied by the clever use of crosslinking agents. For example, protein A and protein B may be very close to each other in a cell, and a chemical crosslinker could be used to probe the [[protein-protein interaction]] between these two proteins by linking them together, disrupting the cell, and looking for the crosslinked proteins. A variety of crosslinkers are used to analyze [[subunit]] structure of [[proteins]], [[protein interactions]] and various parameters of protein function. Subunit structure is deduced since crosslinkers only bind surface amino residues in relatively close proximity in the [[native state]]. Protein interactions are often too weak or transient to be easily detected, but by crosslinking, the interactions can be captured and analyzed. Examples of some common crosslinkers are the [[imidoester]] crosslinker dimethyl suberimidate, the [[NHS-ester]] crosslinker BS3 and [[formaldehyde]]. Each of these crosslinkers induces nucleophilic attack of the amino group of [[lysine]] and subsequent covalent bonding via the crosslinker. The zero-length [[carbodiimide]] crosslinker [[Carbodiimide#EDC|EDC]] functions by converting caboxyls into amine-reactive isourea intermediates that bind to lysine residues or other available primary amines. ''In-vivo'' crosslinking of protein complexes using [[photo-reactive amino acid analog]]s was introduced in 2005 by researchers from the [[Max Planck Society|Max Planck Institute]] <ref>Suchanek, M., Radzikowska, A., and Thiele, C. (2005) Photo-leucine and photo-methionine allow identification of protein-protein interactions in living cells. Nature Methods. 2, 261 – 268.</ref> In this method, cells are grown with [[photoreactive]] [[diazirine]] analogs to [[leucine]] and [[methionine]], which are incorporated into proteins. Upon exposure to ultraviolet light, the diazirines are activated and bind to interacting proteins that are within a few [[angstrom]]s of the photo-reactive amino acid analog. ==Uses for crosslinked polymers== Synthetically crosslinked polymers have many uses, including those in the biological sciences, such as applications in forming [[polyacrylamide]] gels for [[gel electrophoresis]]. Synthetic rubber used for [[tire]]s is made by crosslinking rubber through the process of [[vulcanization]]. Also most rubber articles are cross-linked to make them more elastic. == See also == * [[Branching (chemistry)|branching]] *[[Cross-linked polyethylene]] *[[Phenol formaldehyde resin]] and [[Phenolic resin]] * Application in enzyme catalysis: [[Cross-linked enzyme aggregate]] * [[Crosslinking of DNA]] *[[Fixation (histology)]] *[[Photo-reactive amino acid analog]] ==References== {{Reflist}} [[Category:Polymer chemistry]] [[ar:تشابك]] [[de:Vernetzung (Chemie)]] [[fr:Réticulation]] [[ja:架橋]] [[pl:Sieciowanie]]