Three-dimensional network of polymer molecules

ELASTICITY OF A THREE-DIMENSIONAL NETWORK OF POLYMER MOLECULES... 

III. Elasticity of a Three-Dimensional Network of Polymer Molecules... 

In step 1, functional monomer and template are connected by a covalent linkage (in covalent imprinting ) or they are placed nearby through non-covalent interactions (in non-covalent imprinting ). In step 2, the structures of these conjugates (or adducts) are frozen in a three-dimensional network of polymers. The functional residues (derived from the functional monomers) are topographically complementary to the template. In step 3, the template molecules are removed from the polymer. Here, the space in the polymer originally occupied by... 

A linear polymer may also become cross-linked, which is the binding together of polymer chains at various places by means of strong covalent bonds, usually through the presence of foreign atoms. The result is a three-dimensional network of polymer chains in which the movement of the individual molecules is quite restricted. Network and cross-lined polymers are rarely crystalline, even to a limited extent, and may be treated as amorphous. 

A polymer is a large molecule built up by the repetition of small, simple chemical units. In some cases the repetition is linear while in other cases the chains are branched or interconnected to form three-dimensional networks. The polymer can be formed not only through linear addition, but also through condensation of similar units as well. 

There are a number of methods of classifying polymers. One is to adopt the approach of using their response to thermal treatment and to divide them into thermoplastics and thermosets. Thermoplastics are polymers which melt when heated and resolidify when cooled, while thermosets are those which do not melt when heated but, at sufficiently high temperatures, decompose irreversibly. This system has the benefit that there is a useful chemical distinction between the two groups. Thermoplastics comprise essentially linear or lightly branched polymer molecules, while thermosets are substantially crosslinked materials, consisting of an extensive three-dimensional network of covalent chemical bonding. 

Heavily crosslinked polymers, by contrast, tend to be very brittle and, unlike thermoplastics, this brittleness cannot be altered much by heahng. Heavily crosslinked materials have a dense three-dimensional network of covalent bonds in them, with little freedom for motion by the individual segments of the molecules involved in such structures. Hence there is no mechanism available to allow the material to take up the stress, with the result that it fails catastrophically at a given load with minimal deformation. 

Entrapment involves the capture of the biocatalyst in a three-dimensional network of a polymer. This can be achieved by forming the polymer by a polymerization in the presence of the biocatalyst, or by mixing the biocatalyst and soluble polymer molecules. The latter are cross-linked in a subsequent step. In... 

Three-dimensional networks of polyethylene are manufactured through peroxide-initiated covalent bonding between preformed linear molecules. These peroxide-thermal-decomposition reactions lead to free radical Intermediates which abstract hydrogen atoms from the polyethylene backbone to produce long chain polymer radicals. Combinations of these chain polymer radicals lead to a crossllnked network. ( ) (Figure 1)... 

The linking of plastics molecules by primary valencies into a mainly three-dimensional network. Suitable polymers can also be cross-linked chemically, by adding corresponding bridge-building molecules... 

Figure 1.5. Branched and crosslinked polymer molecules. While a branched polymer molecule retains finite size and an identifiable molecular weight, the crosslinked polymer forms a three-dimensional network of macroscopic proportions. (A rubber band is essentially one molecule, since any two atoms are connected ultimately by covalent bonds.)...

Afterwards, based on the hypothesis that the elasticity development in plasticized PVC was due to the enhancement of micro-Brownian motions of the polymer, Doolittle, Stickney and Cheyney and Alfrey et developed the Gel Theory. They considered that the mechanical properties of a polymer were due to the elastic resistance of entangled segments of the macromolecules structured as a three-dimensional network. The plasticizer molecules which are dissolved into the polymer matrix break some points of attachment between macromolecules leading to an easier glide. Moreover,... 

There are two main classes of hydrogels (Figure 2.1), those composed of three-dimensional networks of cross-linked polymer chain strucmres that are insoluble in water chemical hydrogels) and those produced by the self-assembly of (macro) molecules to form noncovalent structures physical hydrogels). 

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