Polymer network

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Polymer networks are three-dimensionally linked polymer chains. The chains are linked to one another via networking points.

Differentiation according to types of chemical bond

Depending on the chemical bond, three different basic types can be distinguished:

  • permanent networks: the chains are connected via chemical cross-linking points, mostly sigma bonds . It is not possible to loosen and re-create cross-linking points here. Strictly speaking, the term polymer network only denotes this type;
  • Temporary chemical networks: The chains are linked by weaker chemical bonds that are not permanent because the bond is relatively weak. Typical examples are hydrogen bonds and interactions between cations and polar organic groups. These differ from the former in the fact that the connection points can dynamically separate and reconnect. As a result, in contrast to the former, the materials are plastically deformable and often have self-healing properties . Technically, these materials are used as ionomers .
  • Temporary physical networks: the chains are not chemically but only physically connected to one another. This is done by entangling , whereby 2 or more chains are crossed at one point and thus restrict their mobility. With this type of bond, the entanglements are continuously formed above the glass transition temperature and are then lost again. Below the glass transition temperature, the molecular mobility is so low that the entanglement network can be viewed as permanent.

Occasionally, temporary chemical networks are referred to as physical networks and the temporary physical networks as entanglement networks, so that there is a risk of confusion.

Chemical cross-linking options

Chemical cross-links are in principle a polymerization reaction, so that networks can be generated by using trifunctional monomers in polyaddition and polycondensation reactions . This is used, for example, with polyurethane foams (construction foam, foam rubber), where water serves as a blowing agent (water splits off parts of the isocyanate groups from the polyurethane, which leads to the development of gas).

Alternatively, (mostly relatively short) polymer chains can also be linked to one another by crosslinkers. This principle is used, for example, in vulcanization, the chemical reaction between sulfur and natural rubber , whereby the chains are connected to one another by means of short sulfide bridges .

Another possibility is by introducing radicals, e.g. B. by irradiation with ionizing rays, peroxides or great heat to bring about statistical crosslinks. These then lead to a crosslinking of the material. An example of this is radiation- crosslinked polyethylene (XPE).

Differentiation according to the degree of networking

  • strongly cross-linked polymers, so-called thermosets or duromers , have a very high density of cross-linking points, which leads to a high degree of rigidity. The glass transition temperature is relatively unimportant with these materials , since the materials show only a slight drop in stiffness. Typical examples are synthetic resins , e.g. B. epoxy resin , phenolic resin , unsaturated polyester resin . The area of ​​application here is structural components and adhesives. Advantages of this group of materials compared to normal thermoplastics are primarily the better mechanical strength and the higher maximum operating temperature, disadvantage is the slower processing and the poor recyclability .
  • Slightly crosslinked polymers, the elastomers, are significantly less rigid than thermosets and are generally used technically well above the glass transition temperature . This group is colloquially known as rubber. From a chemical point of view, these are primarily natural rubber , styrene-butadiene rubber , but also a number of fluorine and chlorine rubbers and silicones .
  • uncrosslinked polymers (in technical applications almost only looped types are used, otherwise the mechanical properties are too poor), unlike the two groups mentioned above, they have no chemical bonds, but only loopings , so that they are above the glass transition temperature and possibly the Melting temperature have a pronounced tendency to creep and are therefore not technically usable in this temperature range.

Determination of the crosslink density

In many cases the question of how strongly a material is networked is of great technical importance. There is a method to determine the degree of crosslinking, which only works with materials without fillers , as these significantly influence the rigidity . For this purpose, the stiffness in shear or elongation must be measured and the measured value obtained can be used with

Convert into the entanglement density n , where E - modulus of elasticity , G - shear modulus , k - Boltzmann's constant , 1.38 · 10 −23 J / K, T - temperature in Kelvin and n - entanglement density per unit volume [1 / m³].


  • JD Ferry, Viscoelastic Properties of Polymers. John Wiley and Sons: New York, 1980.
  • MR Tant, KA Mauritz, GL Wilkes, Ionomers - synthesis, structure, properties and applications. Blackie Academics & Professional: London, 1997.