Polymer

A polymer [ polymeric ː r] (poly of Greek πολύ much ' and μέρος meros, part ') is a chemical compound made ​​of chain or branched molecules ( macromolecules ), which in turn, consist of identical or similar units, the so-called constitutional repeating units or repeat units. The adjective corresponding polymer means " many equal parts built ". Synthetic polymers are plastics in general.

Alternative definition

" [ Polymers ] substances with molecular weights relate to each other with the same percentage atomic composition in an integer ratio and also have the same internal structure. The same internal structure over all the macromolecule is dispersed. Further, the definition of a polymer comprising the property that a polymer molecule consisting of n units can not be distinguished from a polymer molecule consisting of n-1 or n 1 building blocks. "

By this definition are inorganic macromolecules and inorganic compounds of the form [- O - B - A- - B -] recorded. Such inorganic polymers are formed by the elements boron, silicon ( silicones, polysiloxanes, and polysilazanes ), aluminum, titanium, germanium, phosphorus ( polyphosphazenes ), arsenic, sulfur, or oxygen.

Classification

Polymers may be classified according to the number of basic monomers from which they are generated. However, it is important that at least one monomeric substance builds up the chain.

• Copolymers are composed of different monomers, such as polyesters, polyurethanes, and some polyamides ( PA 66).
• Homopolymer: the polymer consisting of only one type of monomer as polyethylene, polypropylene, polyvinyl chloride or polyamide PA6.
• Polymer alloys produced by blending of different polymers and copolymers.

In addition to different organic and inorganic polymers. The organic polymers can be further subdivided into

• Biopolymers or "natural" polymers are the basic building blocks of living organisms.
• Chemically modified polymers resulting from the processing of biopolymers, such as nitrocellulose, celluloid or starch derivatives.
• Synthetic polymers are prepared by polymerization processes of industrial or laboratory scale, including polyethylene, polystyrene, polyvinyl chloride.

Mixtures of several polymers are referred to as polymer blend. These are defined as a macroscopically homogeneous mixture of two or more different polymers. They are made mostly by intensive mechanical mixing of molten polymers, resulting in a homogeneous material. Upon cooling the melt, the polymer chains remain finely divided and thus ensure that the property profile of the blend is a permanently get permanent superposition of the properties of the individual polymers.

A different classification leads to the group of geopolymers.

Polymer Physics

The classification of polymer materials according to DIN 7724 due to the temperature variation of the shear modulus, and of tension at room temperature. It is based on the mechanical behavior in the operating temperature and the existence of a melting range (flow range):

Moreover, a distinction between semi-crystalline and amorphous polymers. These properties are mainly influenced by the degree of branching. Amorphous polymers have the properties of polymer melts. In them, the polymers are present as a random coil. Below the glass transition temperature they go about in polymer glasses, the elastic properties are lost and the polymers are hard and brittle. The structural design of the semi-crystalline polymers is described in the article crystallization ( polymer). Amorphous polymers and melts are described theoretically by the Freely - Jointed Chain model or the wormlike chain model.

In polymer physics, dealing among other things with

Polymer Chemistry

The chain formation, so the connection of individual monomers with one another to form polymers is carried out by various types of polymerization reactions such as chain polymerisation, polycondensation or polyaddition. Polymers of different monomers built is called Straight polymers or copolymers. For most plastics, the polymer backbone of carbon chains is formed.

One distinguishes isotactic polymers in which all the substituents of a polymer chain have the same stereoelectronic conformation, such as isotactic polystyrene, with the configuration RRRRR - SSSSS ... or ... In atactic polymers the substituents are randomly ordered, as a kind of a racemate. As syndiotactic are polymers whose substituents alternately consist of R and S.

• Synthetic polymers based on carbon: Polyethylene (PE)
• Polypropylene (PP)
• Polyketone (PK) under the trade name Akrotek ® PK ( Akro -Plastic )
• Polyvinyl chloride ( PVC)
• Polystyrene (PS ), better known in expanded state as Styrofoam ® (trade name of BASF )
• Polytetrafluoroethylene ( PTFE), a trade name is Teflon ® ( EI Du Pont de Nemours and Company) or Tefal ®
• Polymethylmethacrylate ( PMMA), under the trade name Plexiglas ® (Evonik Industries AG )
• The group of the polyamides, as PA66 under the trademark nylon ® as PA6 under the trade name or Perlon ® PA12G under the trade name ® lauramide
• Polyester to this product group also includes Polycarbonate ( PC) with trade name Lexan or Makrolon ® (Bayer AG) and
• Polyethylene terephthalate ( PET)

• Silicones, more poly (organo ) siloxanes
• Melamine resin, a polymer based on melamine and formaldehyde

• Proteins such as enzymes, hair, silk
• DNA ( the genetic material )
• RNS
• Carbohydrates such as cellulose, wood, paper, starch, chitin
• Polyhydroxyalkanoates biopolyester as an energy and carbon storage of bacteria

Ecological considerations

Health risks are associated with the polymer itself practically never. The example is PVC, the poisonous and highly corrosive hydrogen chloride gas releases only during combustion, which dissolves in hydrochloric acid than in water. In smoldering fires occur in greater quantities polychlorinated dibenzodioxins and -furans. The polymer PVC itself is food compatible and due to its excellent gas tightness used in medicine, for example, for blood transfusions.

Other environmental problems can be caused by additives which are included in practically every plastic object, eg Plasticizers. These additives are mainly used in PVC to adjust its mechanical properties to the application. It also further developments may prove to be doubtful due to the massive use of PVC. For the soft PVC " Igelit " tricresyl phosphate was used in the 1940s. The replacement of this substance by phthalates, initially dibutyl phthalate ( DBP ), dioctyl phthalate later (DOP), led in recent times to the realization that also bring them under the environmental aspect by their quantitative presence of significant problems.

Temperature -resistant polymers

The thermal stability of a polymer depends on the structure of the monomers used, the stability of the bonds between the monomers, and the interactions of the polymer chains from each other. A high heat resistance can be achieved by increasing the enthalpy of fusion and a reduction in the entropy of fusion. For amorphous polymers the glass transition temperature and should the glass melting temperature and be as high as possible for semi-crystalline polymers. To achieve temperature stability CH bonds and carbon-carbon bonds can be replaced by bonds between carbon and hetero atoms such as fluorine, oxygen or nitrogen or by aromatic stable bonds. Another possibility is the construction of conductor polymers (polymers with two parallel and interconnected main chain ).

Conductive polymers and polymer electronics

A condition of the electrical conductivity of polymers is the presence of conjugated Pi electron systems. However, such polymers are initially still insulators, semiconductors at best. Conductivity comparable to that of metallic conductors, only starts when the polymers are oxidatively or reductively doped. The first studies in this field were carried out on polyacetylene, its conductivity was achieved by doping with arsenic pentafluoride and iodine. Moreover, conductivity increases with increasing crystallinity of the polymer. Other examples of conductive polymers are doped polypyrrole, polyphenylene, polythiophene and metal- organic complexes with macrocyclic ligands such as phthalocyanine. Oxidative doping is achieved with arsenic pentafluoride, titanium tetrachloride, bromine or iodine, by a reductive dopant, however, with sodium-potassium alloys or Dilithiumbenzophenonat. During doping caused charges on the polymer chains, which are delocalized over the π conjugation of the chains. The explanation for the conductivity of polymers is very complex, however. So you have tried to describe the charge transport along a polyene chain with the soliton concept or with the model of bipolaron ( held together in a small space charge pairs ).

Conductive, thus electrically active polymers can be used for the construction of polytronic applications. Other than in molecular electronics, the information is not processed in a single molecule, but differently doped volumes.

Such electronic applications are:

• Functional layers: UV Filter
• Displays: OFETs, OLEDs
• RFID tags
• Solar cells
• Sensors and Actuators
• Fuel cells
• Batteries
• Electrolytic capacitors

Another application is the processing of polymers using the electronics during electrospinning.

Supramolecular polymers

A relatively new area of ​​polymer chemistry includes supramolecular polymers, ie polymers in which the units are held together not by covalent bonds, but by comparatively weak intramolecular bonds such as hydrogen bonds, ionic bonds, metal -ligand interactions, van der Waals or hydrophobic interactions. These intramolecular bonds can be easily broken ( at elevated temperature), but can also quickly regress ( during cooling ). Because of this reversibility include supramolecular polymers as a new class of self-healing materials. A further consequence of the weak intramolecular bonds is the low viscosity of melts of supramolecular polymers, which can be in the preparation and processing of advantage, but also in certain applications, such as ink jet printing.

While covalently bonded polymers have a large role to play in nature (DNA, polypeptides, cellulose), relatively few naturally occurring supramolecular polymers are known. An example of supramolecular polymerization in nature is the self-assembly of tobacco mosaic virus.