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In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
chain to another. These links may take the form of
covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they s ...
s or
ionic bond Ionic bonding is a type of chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the Coulomb's law, electrostatic force of attraction be ...
s and the polymers can be either synthetic polymers or natural polymers (such as
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s). In
polymer chemistry Polymer chemistry is a sub-discipline of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...
"cross-linking" usually refers to the use of cross-links to promote a change 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 interaction Protein–protein interactions (PPIs) are physical contacts of high specificity established between two or more protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemist ...
s, as well as other creative cross-linking methodologies. 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 greatly. As with all science, there are overlaps, and the following delineations are a starting point to understanding the subtleties.


Polymer chemistry

Crosslinking is the general term for the process of forming covalent bonds or relatively short sequences of chemical bonds to join two polymer chains together. The term '' curing'' refers to the crosslinking of
thermosetting A thermosetting polymer, resin, or plastic, often called a thermoset, is a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecule File:Pentacene on Ni(111) STM.jpg, ...
resins, such as unsaturated
polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
and
epoxy Epoxy is the family of basic components or cured A cure is a completely effective treatment for a disease. Cure, or similar, may also refer to: Places * Cure (river), a river in France * Cures, Sabinum, an ancient Italian town * Cures, Sa ...

epoxy
resin, and the term ''
vulcanization Vulcanization (British: Vulcanisation) refers to a range of processes for hardening rubber Rubber is also called India rubber, latex, Amazonian rubber, ''caucho'' or ''caoutchouc'', as initially produced, consists of polymer A poly ...

vulcanization
'' is characteristically used for
rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much" ...

rubber
s. When polymer chains are crosslinked, the material becomes more rigid. 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 increase the viscosities of
polymer melt A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inco ...
s. Intermediate cross-link densities transform gummy polymers into materials that have
elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repeat unit, repe ...
ic properties and potentially high strengths. Very high cross-link densities can cause materials to become very rigid or glassy, such as phenol-formaldehyde materials.


Formation

Cross-links can be formed by
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and t ...

chemical reaction
s that are initiated by heat, pressure, change in pH, or
irradiation Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a ...

irradiation
. For example, mixing of an unpolymerized or partially polymerized
resin In polymer chemistry and materials science, resin is a solid or highly Viscosity, viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focus ...

resin
with specific chemicals called crosslinking reagents results in a chemical reaction that forms cross-links. Cross-linking can also be induced in materials that are normally
thermoplastic A thermoplastic, or thermosoft plastic, is a plastic polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, ...
through exposure to a radiation source, such as
electron beam Cathode rays (electron beam or e-beam) are streams of electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical ...
exposure,
gamma radiation A gamma ray, or gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuc ...
, or
UV
UV
light. For example,
electron beam processing Electron-beam processing or electron irradiation Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiat ...
is used to cross-link the C type of
cross-linked polyethylene Cross-linked polyethylene, commonly abbreviated PEX, XPE or XLPE, is a form of polyethylene with cross-links. It is used predominantly in building services pipework systems, hydronic radiant heating and cooling systems, domestic water piping, and ...
. Other types of cross-linked polyethylene are made by addition of peroxide during extruding (type A) or by addition of a cross-linking agent (e.g.
vinylsilane Vinylsilane refers to an organosilicon compound with chemical formula CH2=CHSiH3. It is a derivative of silane (SiH4). The compound, which is a colorless gas, is mainly of theoretical interest. Substituted vinylsilanes More commonly used than the ...

vinylsilane
) and a catalyst during extruding and then performing a post-extrusion curing. The chemical process of
vulcanization Vulcanization (British: Vulcanisation) refers to a range of processes for hardening rubber Rubber is also called India rubber, latex, Amazonian rubber, ''caucho'' or ''caoutchouc'', as initially produced, consists of polymer A poly ...

vulcanization
is a type of cross-linking that changes
rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much" ...

rubber
to the hard, durable material associated with car and bike
tire A tire (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States ...

tire
s. This process is often called sulfur curing; the term
vulcanization Vulcanization (British: Vulcanisation) refers to a range of processes for hardening rubber Rubber is also called India rubber, latex, Amazonian rubber, ''caucho'' or ''caoutchouc'', as initially produced, consists of polymer A poly ...

vulcanization
comes from
Vulcan Vulcan may refer to: Mythology * Vulcan (mythology), the god of fire, volcanoes, metalworking, and the forge in Roman mythology Arts, entertainment and media Film and television * Vulcan (Star Trek), Vulcan (''Star Trek''), name of a fictional rac ...
, the
Roman Roman or Romans most often refers to: *, the capital city of Italy *, Roman civilization from 8th century BC to 5th century AD *, the people of ancient Rome *', shortened to ''Romans'', a letter in the New Testament of the Christian Bible Roman ...
god of fire. This is, however, a slower process. 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. Accelerators increase the rate of cure by catalysing the addition of sulfur chains to the rubber molecules. Cross-links are the characteristic property of
thermosetting plastic A thermosetting polymer, resin, or plastic, often called a thermoset, is a polymer that is irreversibly hardened by curing (chemistry), curing from a soft solid or viscous liquid prepolymer or resin. Curing is induced by heat or suitable radiation ...
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 A permanent wave, commonly called a perm or permanent (sometimes called a "curly perm" to distinguish it from a " straight perm"), is a hairstyle consisting of waves or curls set into the hair. The curls may last a number of months, hence the n ...
solutions, for example, break and re-form naturally occurring cross-links (
disulfide bond In biochemistry Biochemistry or biological chemistry, is the study of chemical process In a scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-Europe ...
s) between protein chains in
hair Hair is a protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Ph ...

hair
.


Physical cross-links

Where chemical cross-links are covalent bonds, physical cross-links are formed by weak interactions. For example, sodium
alginate Alginic acid, also called algin, is a polysaccharide distributed widely in the cell walls of brown algae that is hydrophilic and forms a viscous natural gum, gum when hydrated. With metals such as sodium and calcium, its salts are known as alginat ...
gels upon exposure to calcium ion, which allows it to form ionic bonds that bridge between alginate chains.
Polyvinyl alcohol Poly(vinyl alcohol) (PVOH, PVA, or PVAl) is a water-soluble synthetic polymer. It has the idealized formula H2CH(OH). It is used in papermaking, sizing, textile warp sizing, as a thickener and emulsion stabilizer in polyvinyl acetate, PVAc adhe ...

Polyvinyl alcohol
gels upon the addition of
borax Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron chemical substance, compound, a mineral, and a salt (chemistry), salt of boric acid. Powdered borax is white, consisting of soft colorless c ...

borax
through hydrogen bonding between
boric acid Boric acid, also called hydrogen borate, boracic acid, and orthoboric acid is a weak, monobasic Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is ...

boric acid
and the polymer's alcohol groups. Other examples of materials which form physically cross-linked gels include
gelatin Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...
,
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
,
agarose Agarose is a polysaccharide, generally extracted from certain Red algae, red seaweed. It is a linear polymer made up of the repeating unit of agarobiose, which is a disaccharide made up of D-galactose, D-galactose and 3,6-anhydro-L-galactopyranose. ...

agarose
, and
agar agar Agar ( or ), or agar-agar, is a jelly-like substance, obtained from red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of ...
. Chemical covalent cross-links are stable mechanically and thermally, so once formed are difficult to break. Therefore, cross-linked products like car
tire A tire (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States ...

tire
s cannot be recycled easily. A class of polymers known as
thermoplastic elastomer A thermoplastic, or thermosoftening plastic, is a plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular mass, molecular weight. The polyme ...
s rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as
snowmobile A snowmobile, also known as a Ski-Doo, snowmachine, sled, motor sled, motor sledge, skimobile, or snow scooter, is a motorized vehicle designed for winter travel and recreation on snow Snow comprises individual ice crystals that grow whi ...

snowmobile
tracks, and
catheter In medicine Medicine is the science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity, awareness, or understanding of someone or something, such as facts ( descriptive knowledge) ...

catheter
s for medical use. They offer a much wider range of properties than conventional cross-linked elastomers because the domains that act as cross-links are reversible, so can be reformed by heat. The stabilizing domains may be non-crystalline (as in styrene-butadiene block copolymers) or crystalline as in thermoplastic copolyesters. Note: A rubber which cannot be reformed by heat or chemical treatment is called a thermoset elastomer. On the other hand, a thermoplastic elastomer can be molded and recycled by heat.


Oxidative cross-links

Many polymers undergo oxidative cross-linking, typically when exposed to atmospheric oxygen. In some cases this is undesirable and thus polymerization reactions may involve the use of an antioxidant to slow the formation of oxidative cross-links. In other cases, when formation of cross-links by oxidation is desirable, an oxidizer such as hydrogen peroxide may be used to speed up the process. The aforementioned process of applying a permanent wave to hair is one example of oxidative cross-linking. In that process the disulfide bonds are reduced, typically using a mercaptan such as ammonium thioglycolate. Following this, the hair is curled and then "neutralized". The neutralizer is typically an acidic solution of hydrogen peroxide, which causes new disulfide bonds to form under conditions of oxidation, thus permanently fixing the hair into its new configuration. It also happens for
gluten Gluten is a protein naturally found in some grains including wheat, barley, and rye. Although, strictly speaking, "gluten" pertains only to wheat proteins, in the medical literature it refers to the combination of prolamin Prolamins are a group ...

gluten
which change structure of foods.


In biology

Proteins Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

Proteins
naturally present in the body can contain crosslinks generated by
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
-catalyzed or spontaneous reactions. Such crosslinks are important in generating mechanically stable structures such as
hair Hair is a protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Ph ...

hair
,
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have differ ...

skin
, and
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

cartilage
.
Disulfide bond In biochemistry Biochemistry or biological chemistry, is the study of chemical process In a scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-Europe ...
formation is one of the most common crosslinks, but
isopeptide bond An isopeptide bond is an amide bond that can form for example between the carboxyl group A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COO ...
formation is also common. Proteins can also be cross-linked artificially using small-molecule crosslinkers. Compromised
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
in the cornea, a condition known as
keratoconus Keratoconus (KC) is a disorder of the eye that results in progressive thinning of the cornea The cornea is the transparent front part of the eye that covers the iris, pupil The pupil is a black hole located in the center of the Iris (an ...

keratoconus
, can be treated with clinical crosslinking. In biological context crosslinking could play a role in
atherosclerosis Atherosclerosis is a disease in which the wall of the artery An artery (plural arteries) () is a blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels tr ...

atherosclerosis
through
advanced glycation end-product Advanced glycation end products (AGEs) are proteins or lipids that become Glycation, glycated as a result of exposure to sugars. They are a bio-marker implicated in aging and the development, or worsening, of many degenerative diseases, such as diab ...
s (AGEs), which have been implicated to induce crosslinking of collagen, which may lead to vascular stiffening.


Use in protein study

The interactions or mere proximity of
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

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 Protein–protein interactions (PPIs) are physical contacts of high specificity established between two or more protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemist ...
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
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s,
protein interaction Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing m ...
s, and various parameters of protein function by using differing crosslinkers, often with diverse spacer arm lengths. Subunit structure is deduced, since crosslinkers bind only surface residues in relatively close proximity in the
native state In biochemistry, the native state of a protein or nucleic acid is its properly folded and/or assembled form, which is operative and functional. The native state of a biomolecule may possess all four levels of biomolecular structure, with the seco ...
. Protein interactions are often too weak or transient to be easily detected, but by crosslinking, the interactions can be stabilized, captured, and analyzed. Examples of some common crosslinkers are the
imidoester
imidoester
crosslinker dimethyl suberimidate, the
N-Hydroxysuccinimide
N-Hydroxysuccinimide
-ester crosslinker BS3 and
formaldehyde Formaldehyde ( , also ) (systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics and general chemist ...
. Each of these crosslinkers induces nucleophilic attack of the amino group of
lysine Lysine (symbol Lys or K) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, ...

lysine
and subsequent covalent bonding via the crosslinker. The zero-length
carbodiimide In organic chemistry, a carbodiimide (systematic IUPAC name: methanediimine) is a functional group In organic chemistry, a functional group is a substituent or moiety in a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling ...
crosslinker
EDC
EDC
functions by converting carboxyls into amine-reactive isourea intermediates that bind to lysine residues or other available primary amines. SMCC or its water-soluble analog, Sulfo-SMCC, is commonly used to prepare antibody-hapten conjugates for antibody development. ''In-vitro'' cross-linking method, termed PICUP ( photo-induced cross-linking of unmodified proteins), was developed in 1999. They devised a process in which
ammonium persulfate Ammonium persulfate (APS) is the inorganic compound with the formula (NH4)2S2O8. It is a colourless (white) salt that is highly soluble in water, much more so than the related potassium salt. It is a strong oxidizing agent that is used in polyme ...

ammonium persulfate
(APS), which acts as an electron acceptor, and tris(bipyridine)ruthenium(II) chloride, tris-bipyridylruthenium (II) cation () are added to the protein of interest and irradiated with UV light. PICUP is more expeditious and high yielding compared to previous chemical cross-linking methods. ''In-vivo'' crosslinking of protein complexes using photo-reactive amino acid analogs was introduced in 2005 by researchers from the Max Planck Institute of Molecular Cell Biology and Genetics. In this method, cells are grown with photoreactive diazirine analogs to leucine and
methionine Methionine (symbol Met or M) () is an essential amino acid An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come ...

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
ångström The angstromEntry "angstrom" in the Oxford online dictionary. Retrieved on 2019-03-02 from https://en.oxforddictionaries.com/definition/angstrom.Entry "angstrom" in the Merriam-Webster online dictionary. Retrieved on 2019-03-02 from https://www.me ...

ångström
s of the photo-reactive amino acid analog (UV cross-linking).


Uses for crosslinked polymers

Synthetically crosslinked polymers have many uses, including those in the biological sciences, such as applications in forming
polyacrylamide Polyacrylamide (abbreviated as PAM) is a polymer with the formula (-CH2CHCONH2-). It has a linear-chain structure. PAM is highly water-absorbent, forming a soft gel when hydrated. In 2008, an estimated 750,000,000 kg were produced, mainly fo ...

polyacrylamide
gels for
gel electrophoresis File:Gel Electrophoresis in DNA Fingerprinting.svg, 200px, Gel electrophoresis is a process where an electric current is applied to DNA samples creating fragments that can be used for comparison between DNA samples. 1) DNA is extracted.2) Isola ...

gel electrophoresis
. Synthetic rubber used for
tire A tire (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States ...

tire
s is made by crosslinking rubber through the process of
vulcanization Vulcanization (British: Vulcanisation) refers to a range of processes for hardening rubber Rubber is also called India rubber, latex, Amazonian rubber, ''caucho'' or ''caoutchouc'', as initially produced, consists of polymer A poly ...

vulcanization
. This crosslinking makes them more elastic. Hard-shell kayaks are also often manufactured with crosslinked polymers. Other examples of polymers that can be crosslinked are ethylene-vinyl acetate–as used in
solar panel A solar cell panel, solar electric panel, photo-voltaic (PV) module or just solar panel is an assembly of photo-voltaic cells mounted in a framework for installation. Solar panels use sunlight as a source of energy to generate direct current e ...

solar panel
manufacturing – and
polyethylene Polyethylene or (incorrectly) polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most common plastic in use today. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and container ...

polyethylene
. Alkyd enamels, the dominant type of commercial oil-based paint, cure by oxidative crosslinking after exposure to air. In many hydraulic fracturing treatments, a delayed gel-cross-linker fluid is used to carry out fracture treatment of the rock. The earliest examples of crosslinking, linking long chains of polymers together to increase strength and mass, involved tires. Rubber was vulcanized with sulfur under heat, which created a link between latex models. Novel uses for crosslinking can be found in regenerative medicine, where bio-scaffolds are crosslinked to improve their mechanical properties. More specifically increasing the resistance to dissolution in water based solutions.


Measuring degree of crosslinking

Crosslinking is often measured by swelling tests. The crosslinked sample is placed into a good solvent at a specific temperature, and either the change in mass or the change in volume is measured. The more crosslinking, the less swelling is attainable. Based on the degree of swelling, the Flory Interaction Parameter (which relates the solvent interaction with the sample), and the density of the solvent, the theoretical degree of crosslinking can be calculated according to Flory's Network Theory. Two ASTM standards are commonly used to describe the degree of crosslinking in thermoplastics. In ASTM D2765, the sample is weighed, then placed in a solvent for 24 hours, weighed again while swollen, then dried and weighed a final time. The degree of swelling and the soluble portion can be calculated. In another ASTM standard, F2214, the sample is placed in an instrument that measures the height change in the sample, allowing the user to measure the volume change. The crosslink density can then be calculated.


See also

*
Branching (polymer chemistry) In polymer chemistry, branching occurs by the replacement of a substituent A substituent is one or a group of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance th ...
*
Cross-linked enzyme aggregateIn biochemistry, a cross-linked enzyme aggregate is an immobilized enzyme prepared via cross-linking of the physical enzyme aggregates with a difunctional cross-linker. They can be used as stereoselective industrial biocatalysts. Background Enzymes ...
*
Cross-linked polyethylene Cross-linked polyethylene, commonly abbreviated PEX, XPE or XLPE, is a form of polyethylene with cross-links. It is used predominantly in building services pipework systems, hydronic radiant heating and cooling systems, domestic water piping, and ...
(PEX) *
Crosslinking of DNA In genetics, crosslinking of DNA occurs when various exogenous or endogenous agents react with two nucleotides Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid poly ...
*
Fixation (histology) In the fields of histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross a ...
*
Phenol formaldehyde resin Phenol formaldehyde resins (PF) or phenolic resins are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Used as the basis for Bakelite, PFs were the first commercial synthetic resins (plastics). They h ...
(phenolic resin)


References

{{Reflist, 30em


External links


Application note on how to measure degree of crosslinking in plastics
Ageing processes Polymer chemistry Protein–protein interaction assays Rubber properties