Mitología céltica

The polymeric word comes from the Greek “polus” several, and “meros” left.

A polymer is a Macromolécule, organics or inorganic, made up of the repeated sequence of the same reason, the Monomère (of the Greek monos: only one or only one, and meros; part), connected ones to the others by covalent bonds.

A formed polymer body can present in form solid Liquide or to room temperature.

A polymer can be natural (e.g.: Polysaccharide S, DNA), obtained by chemical modification of a natural polymer (e.g.: Methyl cellulose), or entirely synthesized by chemical way (e.g.: Polystyrene, Polyisoprene) by a reaction of Polymerization.

The sequence of the monomers can be made in a linear way (linear polymers), to present random ramifications (polymeric connected) or systematic and regular (Dendrimère S).

Because of the degrees of freedom of the Conformation of each monomer, the tridimentionnelle conformation of polymer results from this sequence but also from the interactions between monomers.

General information

The polymers can be manufactured starting from one only type of monomer (one speaks then about Homopolymère, such as for example the Polystyrène ); or starting from several types of monomers (one speaks then about Copolymère S, such as for example the Acrylonitrile butadiene styrene).

One distinguishes two main categories of chemical reactions allowing the preparation from polymers: the polymerization chains or Polyaddition of it (to produce for example the polyethylene , the polystyrene , the polypropylene …) and the polymerization by stages or Polycondensation (to produce for example the FART or poly (ethylene terephthalate) ).

The polymers are often classified according to their thermomechanical properties. Let us quote in particular:

• the Thermoplastique S, which become malleable when they are heated what allows their implementation;
• the Thermodurcissable S, which harden under the action of the heat or by addition of an additive;
• the elastomer S, which are deformable in a reversible way.

The polymers became the essential component of a very significant number of objects, in which they often replaced natural substances. The term indicates abundant and varied matters: the thinnest Protein S with fibers of Kevlar high strength. Certain polymers are used in solution such as for example in the Shampooing S; others form solid materials. For these applications, the polymers are generally mixed with other substances - loads such as the Silice, of the additives such as the Antioxydant S - in operations of Formulation. The manufacture of the objects themselves results most of the time from an operation of Mise in work in a proceeded industrial which comes under the field of the Plasturgie.

The description of polymers as a physical object making it possible to include/understand their properties raises of the physical statistics.

History

The concept of macromolecule appeared only tardily in the history of chemistry. Although predicted by Wilhelm Eduard Weber at the beginning of the 19th century, of many researchers only aggregates or micelles see there. It is necessary to wait the years 1920 - 1930 so that the idea of macromolecule is accepted, in particular thanks to work of Hermann Staudinger. The consecutive industrial development of macromolecular science was accelerated then by the Second world war. The the United States of America were private at the time of their entry in war of their provisioning of natural rubber coming from Southeast Asia. They then launched an immense research program aiming at finding substitutes of synthesis.

Structure and conformation

Primary sequence

The polymers are macromolecules, resulting from the covalent sequence (see Covalent bond) of " reasons for répétition" identical or different from/to each other. The molar mass of these molecules often exceeds 10.000 g/mol, to compare for example with the 18 g/mol water molecule. The covalent bonds constituting the macromolecular skeleton are generally connections carbon-carbon (case of the Polyéthylène, Polypropylène…), but can also result from the connection of carbon atoms with other atoms, in particular oxygen (case of the Polyéther S and Polyester S) or nitrogenizes it (case of the Polyamide S). There exist also polymers for which the sequence results from connections not comprising carbon atoms (Polysilane S, Polysiloxane S, etc…)

This sequence of repeated reasons has at polymers simplest one linear structure, a little like a collar of pearls. One can also meet side branches (they same more or less connected), resulting either from a parasitic chemical reaction during the synthesis of polymer (for example in the case of polyethylene low density or PEBD), or of a reaction of grafting practiced voluntarily on polymer to modify the physicochemical properties of them.

If the macromolecule is made up of the repetition of only one reason - what generally results from the polymerization of only one type of monomer -, one speaks about homopolymers . When several different reasons are repeated one speaks about copolymers . One distinguishes then several types of copolymers according to the organization between the various monomers. In the most frequent case, there is a statistical copolymer where the various monomers mix according to the reactivity and of the concentration of those. The mechanical properties are then realized. On the other hand, in a copolymer sequence (the Anglicism copolymer with blocks is sometimes used) or alternate copolymer , it can have combination of the mechanical properties there.

There exist sometimes covalent bonds towards other pieces of polymeric chains. One speaks then about molecules " branchées" or ramified . One can synthesize for example molecules in “comb” or “star”. When many chains or links were joined together by a certain number of covalent bonds (the branchpoints are called points or nodes of Réticulation), they form nothing any more but one gigantic macromolecule; one then speaks about macromolecular network or freezing .

Linear polymers

During the reaction of polymerization, when each monomer is likely to bind to two others the reaction produces a linear chain. Typically this case is that of polymers known as " thermoplastiques".

Because of the degrees of freedom of the Conformation of each monomer, the way in which the chain occupies space is however not rectilinear.

Concept of link statistical

Each monomer has a certain rigidity. Often, this rigidity influences the orientation of the close monomer. However, this influence grows blurred as one moves away from the initial monomer and ends up disappearing beyond a distance $l\_0$, known as length of a statistical link of the chain. Technically this length is the length of correlation of the orientation of a link. It names length of persistence of polymer and can be seen as the length in lower part of which the polymer is rigid and beyond which it is completely flexible.

Having introduced this concept, it then possible of renormalier the chain by now regarding the statistical link as being its elementary reason. To describe the conformation of this chain, the characteristics suitable for the chemical structure of the monomer do not intervene any more. Generic, valid laws of behavior whatever the chemical nature of polymer, emerge.

Ideal chain (or Gaussian)

The simplest case is that of the linear sequence of links not exerting an interaction between-them. In the liquid state, the chain adopts in space a conformation which for a given molecule unceasingly changes because of thermal agitation. In an amorphous state of solid or at a moment given in the case of a liquid, the conformation of the chains is different from one molecule to another. This conformation obeys nevertheless statistical laws.

Maybe in the primary sequence of the chain a given link taken for origin. When the links do not interact, the probability that the $n^\; \{ieme\}$ chain link is at a distance $r$ of the origin obeys a normal law or law Gaussienne of null average and variance $n$. A length characteristic of the chain is the distance $R$ between its two ends (known as " outdistance end-with-bout"). The arithmetical average of $R$ is null. Thus, to thus characterize the size of the ball which form the chain, it is necessary to consider the quadratic average , noted here . Because of the normal law, this average varies like the square root of the $N$ number of links.

$\backslash \; mathcal\; \{R\}\; =\; l\_0\; \backslash \; cdot\; naked\; N^\; \{\backslash \}\; \backslash \; quad\; \backslash \; textrm\; \{with\}\; \backslash \; quad\; \backslash \; nu=\; \backslash \; frac\; \{1\}\; \{2\}$

The statitistic conformation of such a chain is the analog of the trace left by a random walker where $N$ represents the number of steps of walk, $l\_0$ their length and $\backslash \; mathcal\; \{R\}\; ^2$ the average square displacement of the walker.

In the Seventies, Pierre-Gilles of Genoa showed the analogy between the description of a polymeric chain and the phenomena critical. Also, the use of the letter $\backslash \; nu$ to appoint the exhibitor, obeys the nomenclature of the critical exhibitors. The objects met in the critical phenomena have properties of autosimilarity and can be described in term of geometry of the fractals, in this case the exhibitor $\backslash \; nu$ represents the reverse of the dimension fractale $d\_f$:

$d\_f\; =\; \backslash \; frac\; \{1\}\; \{\backslash \; naked\}$

A Gaussian conformation of the chains meet in two cases:

1. When the chains many and are intermingled (think of a dish of spaghetti). A given link is then surrounded indifferently by the chain links to which it belongs and by the links of the close chains. The interactions of the ones and others with this link are compensated exactly.

2. When a chain is alone at a particular temperature called température-$\backslash \; theta$ to which gravitational attractions of van der Waals between two links are compensated exactly by the repelling powers known as of excluded volume (cettre repelling power comes owing to the fact that two links cannot physically be at the same time at the same place).

Chain inflated (or with " exclu" volume;)

When the repulsive interactions between links of the same chain dominate (typically at temperature higher than the température-$\backslash \; theta$), the conformation of the chain is " of it; gonflée" compared to its ideal conformation. In this case the value of the exhibitor $\backslash \; nu$ and size characteristic of the ball which form the chain are higher than that of an ideal chain:

$\backslash \; naked\; \backslash \; simeq\; \backslash \; frac\; \{3\}\; \{5\}$

The approximate value of this exhibitor was established by Paul Flory in the Forties. Although it was shown since the reasoning utlized at the time was erroneous, the value $3/5$ is surprisingly close to the exact value $\backslash \; naked\; \backslash \; simeq\; 0.588$ found since by the methods more sophisticated much group of renormalization.

In random term of walk, an inflated chain corresponds to the trace left by a walker carrying out a random walk autoévitante.

Globular chain

When the gravitational interactions between links of the same chain dominate (typically at temperature lower than the température-$\backslash \; theta$), the chain crumbles on itself and adopts compact a conformation known as " globulaire" (to be opposed at the end " pelote" used for conformations ideal or inflated). In this case:

$\backslash \; nu=\; \backslash \; frac\; \{1\}\; \{3\}$

The " term; conformation compacte" includes itself/understands better by writing the relation " cut characteristic-number of maillons" in the form: $\backslash \; mathcal\; \{R\}\; ^3\; \backslash \; propto\; N$, which expresses that the volume of the globule is proportional to the number of links. This behavior is that of a homogeneous object whose density is a constant independent of its size.

Polymers in solution

Implicitly we considered up to now a chain alone whose links would be like the molecules of a gas. In practice, the chains are in the presence of their similar, very close from/to each other and is entremélées (evoked case with the paragraph " chain gausienne"), that is to say in the presence of a solvent. This last case is that of a solution of polymer.

In solution, the conformation of polymer results from the assessment of the interactions " monomer-monomère" , " monomer-solvant" and " solvent-solvant". It is possible to give an account of this assessment by means of an actual parameter of interaction, called parameter of Flory-Huggins. Three cases are possible:

1. Good solvent: the couple polymer-solvent is such as a monomer minimizes its free energy when it is surrounded by solvent molecules. The effective interactions between chain links are thus repulsive, thus supporting the dispersion of polymers and their solubilization (from where the " term; good solvant"). It is the case of a polymer in a solvent made up of monomers, for example the polystyrene in solution in the styrene.

2. Solvant-$\backslash theta$: the assessment of the interactions between chain links is null. This case meets at a precise temperature (température-$\backslash \; theta$) which is not always accessible in experiments. When this temperature is accessible, the solvent is described as $\backslash \; theta$. For example, the cyclohexane is a solvant-$\backslash \; theta$ polystyrene with 35°C.
3. Bad solvent: the effective interactions between chain links are gravitational. In this case, the solubilization (dispersion) of polymers is not directly realizable, it can be done at sufficiently high temperature so that the solvent is " bon". While cooling, the polymers can be in bad solvent but remain dispersed if the solution is sufficiently diluted. This case can also meet for polymers sequences of which one of the sequences (majority) would be in situation of good solvent, thus allowing solubilization and forcing another sequence to be in situation of bad solvent.

In sufficiently diluted solution, the chains are quite separate from/to each other. The conformation of a chain does not depend whereas assessment on the effective interactions between its own links. In solvant-$\backslash \; theta$, conformation is ideal ($\backslash \; nu=1/2$), in good solvent it is inflated ($\backslash \; naked\; \backslash \; simeq\; 3/5$) and in bad solvent it is globular ($\backslash \; nu=1/3$).

Connected polymers and transition ground-freezing

Certain molecules have the property to be able to bind by chance to at least three others during their reaction of polymerization. The polymers which result from it linear but are connected and réticulés and their size very widely distributed. The average of this distribution increases with the advance of the reaction. The whole of the population of the molecules is soluble (one indicates it by the term ground ) until the largest molecule is of macroscopic size and connects the two edges of the container containing the bath of reaction. This molecule is called the freezing . Typically, this type of reaction is at the base of the resins Thermodurcissable S.

The appearance of freezing confers on the bath reaction, initially liquid, a elasticity which is the characteristic of a solid. This transition from phase is well described by a model of percolation (conjecture emitted in 1976 independently by Pierre-Gilles of Genoa and well checked in experiments since) which envisages the form of the function of distribution, $p\; (NR)$, of the number of monomers of each molecule and the way in which they occupy space. To the largest molecule, $p\; (NR)$ is a law of power of the type:

$p\; (NR)\; \backslash \; propto\; N^\; \{-\; \backslash \; tau\}\; \backslash \; quad\; \backslash \; textrm\; \{with\}\; \backslash \; quad\; \backslash \; tau=2.20$

A characteristic size, $\backslash \; mathcal\; \{R\}$, of each molecule can be defined by the quadratic average of the distances between monomers, one speaks about radius of gyration. The relation between this length and the number of monomers are also a law of power:

$NR\; \backslash \; propto\; \backslash \; mathcal\; \{R\}\; ^\; \{d\_f\}\; \backslash \; quad\; \backslash \; textrm\; \{with\}\; \backslash \; quad\; d\_f=2.50$

where $d\_f$ is dimension fractale molecules. The particular values of these exhibitors make that they obey the relation known as of hyperéchelle connecting the critical exhibitors to the dimension of space $d$: $d/d\_f=\; \backslash \; tau-1$. The major implication of this relation is that the connected polymers occupy space the made-to-order of the Russian Poupées, the small ones inside the volume occupied by largest.

Experimental aspects

The structural characteristics of polymers are accessible in experiments by experiments from: diffusion of the light, diffusion to the small angles of and Des.

Principle of an experiment of diffusion of radiation

During an elastic experiment of diffusion of radiation, a wave planes incidental, of vector of wave $\backslash \; mathbf\; \{k\_i\}$, (light, X, neutrons) lights a sample. The atoms contained in each element of volume of the sample diffuse part of this wave. The wave diffused according to an angle $\backslash \; theta$ of vector of wave $\backslash \; mathbf\; \{k\_d\}$ is supposed of the same wavelength. The intensity, $I$, of the radiation diffused in this direction is the sum of the interferences of the waves diffused by each element of volume taken two by two. These interferences are expressed according to the vector of diffusion: $\backslash \; mathbf\; \{Q\}\; =\; \{\backslash \; mathbf\; \{k\_d\}\}\; -\; \{\backslash \; mathbf\; \{k\_i\}\}$, like the transformed of Fourier of the function of Autocorrelation of the density of the sample (or spectral concentration of power):

Nomenclature

The nomenclature UICPA recommends to leave the basic reason for the repetition. However of very many polymers have usual names not respecting nomenclature UICPA. An example: The polymer of formula $(CH\_2-CH\_2)\; \_n$ is usually called Polyéthylène. However, if the nomenclature is respected, it should be named polymethylene because the constitutive reason is not the ethylene $CH\_2=CH\_2$ but methylene $CH\_2$.

Here some other examples of case where usual name and nomenclature UICPA differ:

Examples

External bonds

• Abbreviations of more than 230 industrial polymers

Simple: Polymer

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