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| | Epoxy resins, also known as epoxy or casting resin, are pourable liquid plastic. This chemical is used as a bonding agent in home or office repairs, constructions, preservation, as well as in metal industries. This chemical when mixed with a catalyzing agent, commonly known as hardener, produces the properties of adhesiveness.<br><br>Epoxies have many properties that make their use versatile and useful. Some of the common features of this chemical substance are discussed below:<br><br>According to the expert experiment reports, this substance is a polymer that has two epoxy groups in the chemical structure on both ends, and has a smaller molecular weight.<br><br>The low-weighted molecules present in the polymer are generally known as di-epoxy, due to the presence of epoxy group at either end.<br><br>The casting resins generally come up as the result of a reaction that includes chemicals like, epichlorohydrin and bisphenol A, along with a catalyst, sodium hydroxide.<br><br>When epoxies are chemically combined with a diamine (a type of polyamine), the resulting chemical reaction produce a chemical with strong gluing properties and cannot be molded or melted.<br><br>If you liked this article and you simply would like to obtain more info regarding [http://randolphl4.Blog.com/2014/05/29/la-fibra-de-vidrio-un-elemento-provechoso-con-formidable-potencial-en-el-reciclaje/ resina epoxi] i implore you to visit our site. Apart from their use as a bonding or gluing agent, casting resins are resistant to heat and are used for electrical conductivity or insulation.<br><br>Due to various properties, epoxy resin can be used for different purposes, as mentioned below:<br><br>Adhesive: This chemical is a strong material that has properties to join two different objects together and results in the formation of a strong connection between them. Used in a variety of projects such as fixing of ceramic materials, or bike parts, they serve as a vital part for various industries.<br><br>Paint and coated epoxies have properties to protect the painted surfaces of materials. Because they are durable and resistant to fire. Sometimes, this chemical needs dilution before it is used for coating or painting.<br><br>Sealants: Are also used for sealing purposes, as the name shows. The epoxies act as a strong sealer on many items. The most common applications of this chemical include; reaping of damaged materials in order to glue the parts back together. It's also used for repairing household doors and windows. Its actual use differs depending on the type of items or materials to be used on, but basically they act as a filler to cover the damaged area.<br><br>After few hours of its application on the materials, they become dry. Once epoxies become dry they form the solid bond between material they are applied to and really make it difficult to separate them.<br><br>One important thing that must be kept in mind during the use of epoxies is to make sure that the surface that is to be glued is properly cleaned, because a dirty surface can result in a weaker bond. |
| A '''random coil''' is a [[polymer]] [[Chemical structure|conformation]] where the [[monomer]] subunits are oriented [[randomness|randomly]] while still being [[chemical bond|bonded]] to [[graph (mathematics)|adjacent]] units. It is not one specific [[shape]], but a [[statistics|statistical]] distribution of shapes for all the chains in a [[statistical population|population]] of [[macromolecule]]s. The conformation's name is derived from the idea that, in the absence of specific, stabilizing interactions, a polymer backbone will "sample" all possible conformations randomly. Many linear, [[Branching (polymer chemistry)|unbranched]] [[homopolymer]]s — in solution, or above their [[glass transition temperature|melting temperature]]s — assume ([[approximation|approximate]]) random coils. Even [[copolymer]]s with [[monomers]] of unequal [[length]] will distribute in random coils if the subunits lack any specific interactions. The parts of branched polymers may also assume random coils.
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| Below their melting temperatures, most [[thermoplastic]] polymers ([[polyethylene]], [[nylon]], etc.) have [[amorphous solid|amorphous]] regions in which the chains approximate random coils, alternating with regions that are [[crystal]]line. The amorphous regions contribute elasticity and the crystalline regions contribute strength and rigidity.
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| More complex polymers such as [[protein]]s, with various interacting chemical groups attached to their backbones, [[Molecular self-assembly|self-assemble]] into well-defined structures. But segments of proteins, and [[peptide|polypeptides]] that lack [[secondary structure]], are often assumed to exhibit a random-coil conformation in which the only fixed relationship is the joining of adjacent [[amino acid]] [[residue (chemistry)|residue]]s by a [[peptide bond]]. This is not actually the case, since the [[statistical ensemble (mathematical physics)|ensemble]] will be [[energy]] weighted due to interactions between amino acid [[side-chain]]s, with lower-energy conformations being present more frequently. In addition, even arbitrary sequences of amino acids tend to exhibit some [[hydrogen bond]]ing and secondary structure. For this reason, the term "statistical coil" is occasionally preferred. The [[conformational entropy]] associated with the random-coil state significantly contributes to its energetic stabilization and accounts for much of the energy barrier to [[protein folding]].
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| A random-coil conformation can be detected using spectroscopic techniques. The arrangement of the planar amide bonds results in a distinctive signal in [[circular dichroism]]. The [[chemical shift]] of amino acids in a random-coil conformation is well known in [[Protein NMR|nuclear magnetic resonance]] (NMR). Deviations from these signatures often indicates the presence of some secondary structure, rather than complete random coil. Furthermore, there are signals in multidimensional NMR experiments that indicate that stable, non-local amino acid interactions are absent for polypeptides in a random-coil conformation. Likewise, in the images produced by [[X-ray crystallography|crystallography]] experiments, segments of random coil result simply in a reduction in "electron density" or contrast. A randomly coiled state for any polypeptide chain can be attained by [[denaturation (biochemistry)|denaturing]] the system. However, there is evidence that proteins are never truly random coils, even when denatured (Shortle & Ackerman).
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| ==Random walk model: The Gaussian chain==
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| {{main|Ideal chain}}
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| [[Image:Ideal chain random walk.png|thumb|200px|Short [[ideal chain|random chain]]]]
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| There are an enormous number of different [[Ludwig Boltzmann|ways]] in which a chain can be curled around in a relatively compact shape, like an unraveling ball of twine with lots of open [[space]], and comparatively few ways it can be more or less stretched out. So, if each conformation has an equal [[probability]] or [[statistics|statistical]] weight, chains are much more likely to be ball-like than they are to be extended — a purely [[entropy|entropic]] effect. In an [[statistical ensemble (mathematical physics)|ensemble]] of chains, most of them will, therefore, be loosely [[sphere|balled up]]. This is the kind of shape any one of them will have most of the time.
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| Consider a linear polymer to be a freely-jointed chain with ''N'' subunits, each of length <math>\scriptstyle\ell</math>, that occupy [[0 (number)|zero]] [[volume]], so that no part of the chain excludes another from any location. One can regard the segments of each such chain in an ensemble as performing a [[random walk]] (or "random flight") in three [[dimension]]s, limited only by the constraint that each segment must be joined to its neighbors. This is the ''[[ideal chain]]'' [[mathematical model]]. It is clear that the maximum, fully extended length ''L'' of the chain is <math>\scriptstyle N\,\times\,\ell</math>. If we assume that each possible chain conformation has an equal statistical weight, it can be [[ideal chain|shown]] that the probability ''P''(''r'') of a polymer chain in the [[statistical population|population]] to have distance ''r'' between the ends will obey a characteristic [[Probability distribution|distribution]] described by the formula
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| : <math>P(r) = \frac{4 \pi r^2}{(2/3\; \pi \langle r^2\rangle)^{3/2}} \;e^{-\,\frac{3r^2}{2\langle r^2\rangle}}</math> | |
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| The ''average'' ([[root mean square]]) end-to-end distance for the chain, <math>\scriptstyle \sqrt{\langle r^2\rangle}</math>, turns out to be <math>\scriptstyle\ell</math> times the square root of ''N'' — in other words, the average distance scales with ''N''<sup>0.5</sup>.
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| Note that although this model is termed a "Gaussian chain", the distribution function is not a [[normal distribution|gaussian (normal) distribution]]. The end-to-end distance probability distribution function of a Gaussian chain is non-zero only for ''r'' > 0.
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| <ref>In fact, the Gaussian chain's distribution function is also unphysical for real chains, because it has a non-zero probability for lengths that are larger than the extended chain. This comes from the fact that, in strict terms, the formula is only valid for the limiting case of an infinite long chain. However, it is not problematic since the probabilities are very small.</ref> | |
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| ==Real polymers==
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| A real polymer is not freely-jointed. A -C-C- single [[chemical bond|bond]] has a fixed [[alkane#Molecular geometry|tetrahedral]] angle of 109.5 degrees. The value of ''L'' is well-defined for, say, a fully extended [[polyethylene]] or [[nylon]], but it is less than ''N'' x ''l'' because of the zig-zag backbone. There is, however, free rotation about many chain bonds. The model above can be enhanced. A longer, "effective" unit length can be defined such that the chain can be regarded as freely-jointed, along with a smaller ''N'', such that the constraint ''L'' = ''N'' x ''l'' is still obeyed. It, too, gives a Gaussian distribution. However, specific cases can also be precisely calculated. The average end-to-end distance for ''freely-rotating'' (not freely-jointed) polymethylene (polyethylene with each -C-C- considered as a subunit) is ''l'' times the square root of 2''N'', an increase by a factor of about 1.4. Unlike the zero volume assumed in a random walk calculation, all real polymers' segments occupy space because of the [[van der Waals radius|van der Waals radii]] of their atoms, including [[steric effects|bulky substituent groups]] that interfere with [[molecular geometry|bond rotations]]. This can also be taken into account in calculations. All such effects increase the mean end-to-end distance.
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| Because their polymerization is [[stochastic]]ally driven, chain lengths in any real population of [[chemical synthesis|synthetic]] polymers will obey a statistical distribution. In that case, we should take ''N'' to be an average value. Also, many polymers have random branching.
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| Even with corrections for local constraints, the random walk model ignores steric interference between chains, and between distal parts of the same chain. A chain often cannot move from a given conformation to a closely related one by a small displacement because one part of it would have to pass through another part, or through a neighbor. We may still hope that the ideal-chain, random-coil model will be at least a qualitative indication of the shapes and [[dimension]]s of real polymers in [[solution]], and in the amorphous state, as long as there are only weak [[intermolecular force|physicochemical interactions]] between the monomers. This model, and the [[Flory-Huggins Solution Theory]],<ref>Flory, P.J. (1953) ''Principles of Polymer Chemistry'', Cornell Univ. Press, ISBN 0-8014-0134-8</ref><ref>Flory, P.J. (1969) ''Statistical Mechanics of Chain Molecules'', Wiley, ISBN 0-470-26495-0; reissued 1989, ISBN 1-56990-019-1</ref> for which [[Paul Flory]] received the [[Nobel Prize in Chemistry]] in 1974, ostensibly apply only to [[ideal solution|ideal, dilute solutions]]. But there is reason to believe (e.g., [[neutron diffraction]] studies) that [[steric effects|excluded volume effects]] may cancel out, so that, under certain conditions, chain dimensions in amorphous polymers have approximately the ideal, calculated size <ref>"Conformations, Solutions, and Molecular Weight" from "Polymer Science & Technology" courtesy of Prentice Hall Professional publications [http://www.informit.com/content/images/chap3_0130181684/elementLinks/chap3_0130181684.pdf]</ref>
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| When separate chains interact cooperatively, as in forming crystalline regions in [[solid]] thermoplastics, a different mathematical approach must be used.
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| Stiffer polymers such as [[alpha helix|helical]] polypeptides, [[Kevlar]], and double-stranded [[DNA]] can be treated by the [[worm-like chain]] model.
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| ==See also==
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| *[[protein folding]]
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| *[[native state]]
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| *[[molten globule]]
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| *[[probability theory]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| *[http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.pc.25.100174.001143 polymer statistical mechanics]
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| *[http://www.iop.org/EJ/abstract/0305-4470/20/12/040/ A topological problem in polymer physics: configurational and mechanical properties of a random walk enclosing a constant are]
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| *[http://www.sciencemag.org/cgi/content/abstract/293/5529/487?view=abstract D. Shortle and M. Ackerman, Persistence of native-like topology in a denatured protein in 8 M urea, Science 293 (2001), pp. 487–489]
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| *[http://phptr.com/content/images/chap3_0130181684/elementLinks/chap3_0130181684.pdf Sample chapter "Conformations, Solutions, and Molecular Weight" from "Polymer Science & Technology" courtesy of Prentice Hall Professional publications]
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| {{DEFAULTSORT:Random Coil}}
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| [[Category:Polymer physics]]
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| [[Category:Physical chemistry]]
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Epoxy resins, also known as epoxy or casting resin, are pourable liquid plastic. This chemical is used as a bonding agent in home or office repairs, constructions, preservation, as well as in metal industries. This chemical when mixed with a catalyzing agent, commonly known as hardener, produces the properties of adhesiveness.
Epoxies have many properties that make their use versatile and useful. Some of the common features of this chemical substance are discussed below:
According to the expert experiment reports, this substance is a polymer that has two epoxy groups in the chemical structure on both ends, and has a smaller molecular weight.
The low-weighted molecules present in the polymer are generally known as di-epoxy, due to the presence of epoxy group at either end.
The casting resins generally come up as the result of a reaction that includes chemicals like, epichlorohydrin and bisphenol A, along with a catalyst, sodium hydroxide.
When epoxies are chemically combined with a diamine (a type of polyamine), the resulting chemical reaction produce a chemical with strong gluing properties and cannot be molded or melted.
If you liked this article and you simply would like to obtain more info regarding resina epoxi i implore you to visit our site. Apart from their use as a bonding or gluing agent, casting resins are resistant to heat and are used for electrical conductivity or insulation.
Due to various properties, epoxy resin can be used for different purposes, as mentioned below:
Adhesive: This chemical is a strong material that has properties to join two different objects together and results in the formation of a strong connection between them. Used in a variety of projects such as fixing of ceramic materials, or bike parts, they serve as a vital part for various industries.
Paint and coated epoxies have properties to protect the painted surfaces of materials. Because they are durable and resistant to fire. Sometimes, this chemical needs dilution before it is used for coating or painting.
Sealants: Are also used for sealing purposes, as the name shows. The epoxies act as a strong sealer on many items. The most common applications of this chemical include; reaping of damaged materials in order to glue the parts back together. It's also used for repairing household doors and windows. Its actual use differs depending on the type of items or materials to be used on, but basically they act as a filler to cover the damaged area.
After few hours of its application on the materials, they become dry. Once epoxies become dry they form the solid bond between material they are applied to and really make it difficult to separate them.
One important thing that must be kept in mind during the use of epoxies is to make sure that the surface that is to be glued is properly cleaned, because a dirty surface can result in a weaker bond.