Dictionary Definition
reticulation
Noun
1 (photography) the formation of a network of
cracks or wrinkles in a photographic emulsion
2 an arrangement resembling a net or network;
"the reticulation of a leaf"; "the reticulation of a photographic
emulsion"
User Contributed Dictionary
English
Noun
- A network of
criss-crossing
lines or strands.
- The students studied the reticulation of the veins in the leaf.
Extensive Definition
Cross-links are bonds that link 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 milieu, it can be in reference to its use as a probe
to link proteins together to check 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. 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 reactions 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 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 tires. This process is often called
sulfur curing, and the term vulcanization comes from
Vulcan,
the 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. 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
bonds) between protein chains in hair.
Physical cross-links
Chemical covalent cross-links are stable mechanically and thermally, so once formed are difficult to break. So cross-linked products like car tires cannot be recycled easily, for example. A new class of polymers known as thermoplastic elastomers rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as snowmobile tracks, and catheters 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, proteins (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 proteins 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
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 analogs was introduced in 2005 by
researchers from the Max
Planck Institute 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 angstroms
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 tires 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
- 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
reticulation in Arabic: تشابك
reticulation in German: Vernetzung
(Chemie)
reticulation in French: Réticulation
reticulation in Japanese: 架橋
reticulation in Polish:
Sieciowanie