Secondary valencies

The atoms of a molecule are held together by primary bonds. The attractive forces which act between molecules are usually referred to as secondary bonds, secondary valence forces, intermolecular forces or van der Waals forces. 

The term secondary valency has been superseded by the term coordination number . This may be defined as the number of donor atoms associated with the central metal atom or ion. For many years a distinction was made between coordination number in this sense and in... 

TPEs are materials that possess, at normal temperatures, the characteristic resilience and recovery from the extension of crosslinked elastomers and exhibit plastic flow at elevated temperatures. They can be fabricated by the usual techniques such as blow molding, extrusion, injection molding, etc. This effect is associated with certain interchain secondary valence forces of attraction, which have the effect of typical conventional covalent crosslinks, but at elevated temperatures, the secondary... 

Cleanness of the base, i.e. freedom from grease, which improves the wettability of the metal surface, and the removal of oxides, dust or loose paint, etc. already described. The closer the surfaces of paint film and metal, the more secondary valencies originating in the polar constituents of the medium are brought into play. 

Alfred Werner. His theory of coordination chemistry was published in 1893 when Werner was 26 years old. In his paper Werner made the revolutionary suggestion that metal ions such as Co3+ could show two different kinds of valences. For the compound Co(NH3)eCI3, Werner postulated a central Co3+ ion joined by "primary valences" (ionic bonds) to three Cl- ions and by "secondary valences"... 

Moreover, he made the inspired guess that the six secondary valences were directed toward the corners of a regular octahedron. 

These results indicate clearly the usefulness of the CP/MAS NMR method in analysing the structure of secondary valence bonds in polymers by using relaxation T1 data. Similarly useful data may be obtained by the Tip parameter. 

Nylon fibers are semicrystalline, that is, they consist of crystallites separated by amorphous regions. Hydrogen bonding is an important secondary valence interaction in nylon-6 and nylon-6,6. Individual chains in the microcrystalline regions of nylons are held together by hydrogen bonds. Nylons are resistant to aqueous alkali but deteriorate more readily on exposure to mineral acids. 

Again, remember that coordination number is equivalent to Werner s secondary valence. 

Reversible gels which are dispersible on heating and are soluble in excess. These have secondary valence or ionic bonds... 

Bonding forces Primary valence forces (intramolecular forces) Secondary valence forces (intermolecular forces)... 

Amylase operates exclusively on nonreducing, terminal units in amylose or on the branches in amylopectin, to produce maltose directly, and its hydrolytic action on 4 — 1-a-D linkages is stopped by any branch points. Enzyme action is greatly impeded by secondary valence forces, as retro-gradation, for example, is accompanied by an increased resistance to j8-amylolysis.19 The enzyme can be crystallized relatively easily. The mode... 

In polymer science and technology, linear, branched and crosslinked structures are usually distinguished. For crosslinked polymers, insolubility and lack of fusibility are considered as characteristic properties. However, insoluble polymers are not necessarily covalently crosslinked because insolubility and infusibility may be also caused by extremely high molecular masses, strong inter-molecular interaction via secondary valency forces or by the lack of suitable solvents. For a long time, insolubility was the major obstacle for characterization of crosslinked polymers because it excluded analytical methods applicable to linear and branched macromolecules. In particular, the most important structural characteristic of crosslinked polymers, the crosslink density, could mostly be determined by indirect metho ds only [ 1 ], or was expressed relatively by the fraction of crosslinking monomers used in the synthesis. 

Various diverse systems qualify as gels if one assumes that in these systems the common features are the solid-like behavior and the presence of a continuous structure of macroscopic nature (6,7). For the purpose of the discussion in this paper, we describe a gel as a colloidal system comprised of a dispersed component and a dispersion medium both of which the junction points are formed by covalent bonds, secondary valence bonds, or long range attractive forces that cause association between segments of polymer chains or formation of crystalline regions which have essentially infinite life time (8). 

Systems such as the concentrated solution of the UHMWPE in paraffin oil (2-8% w/w) contain a three-dimensional molecular network in which the junction points are produced by secondary valence bonds which cause crystalline regions and by physical entanglements of different life times. Entanglements that are trapped between crystallites have, like the crystallites, essentially infinite life times. 

It is well known from very early studies on lignin reactivity that hydrogen bond formation and other secondary valence forces strongly affect the... 

The polar groups in ionomers are suppressing the tendency of crystallization. Moreover, a ionic crosslinking is effected. Thus, both secondary valency forces and ionic forces are active. The special types of bonds effect a special toughness of the materials. However, ionomers are true thermoplastic materials. 

Plasticization is the process in which the plasticizer molecules neutralize the secondary valence bonds, known as van der Waal s force between the polymer molecules. It increases the mobility of the polymer chains and reduces the crystallinity. These phenomena become evident in reduced modulus or stiffness, increased elongation and flexibility, and lowering of the brittle or softening temperature of the plasticized product. The effect of plasticizers on polymers is the subject of the first chapter by E. H. Immergut and H. F. Mark. 

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