Secondary or van der Waals Bonds

In general there are two types of bonds (1) primary or chemical bonds and (2) secondary or van der Waals bonds. Primary bonds are metallic, co-... 

By far the most important of the adhesive bonds are the secondary or Van der Waals" bonds that give rise to attraction between molecules. Most significant of these are the London or dispersion forces. They are responsible for virtually all the molar cohesion of nonpolar polymers such as polyethylene, natural rubber, SBR, and butyl rubber. These forces act at a distance of approximately 4 A, and fall off rapidly, as the sixth power of the distance between atoms. Consequently, molecules must be in close proximity for London forces to be effective. This helps to explain why a very flexible molecule such as natural rubber is a better adhesive than a moderately flexible molecule such as polystyrene. Low modulus, indicating free-... 

Non-epitaxial electrodeposition occurs when the substrate is a semiconductor. The metallic deposit cannot form strong bonds with the substrate lattice, and the stability conferred by co-ordination across the interface would be much less than that lost by straining the lattices. The case is the converse of the metal-metal interface the stable arrangement is that in which each lattice maintains its equilibrium spacing, and there is consequently no epitaxy. The bonding between the met lic lattice of the electrodeposit and the ionic or covalent lattice of the substrate arises only from secondary or van der Waals forces. The force of adhesion is not more than a tenth of that to a metal substrate, and may be much less. 

The molecular forces are secondary or van der Waals forces. It is also conceivable that primary valence forces form chemical bonds, either covalent or ionic, between adhesive and adherend. The contribution of covalent bonds to bond strength is a subject of great, if sometimes controversial, interest (6). 

Self-assembled structures are supramolecular assemblies of covalent backbones structured through intra- and interchain noncovalent interactions. These secondary structures arise from steric constraints and a network of weak interactions (i.e., hydrogen or Van der Waals bonding, dipole-dipole or amphiphilic interactions). Helical morphologies are stiU rarely represented in these artificial species but the control of the heHx sense, and a better knowledge of the chiral amplification mechanism, is highly desirable due to their potential use in many applications. For example, helically chiral polymers can be used as chiral stationary phases for HPLC or for catalysis. 

Polymer alloys are physical mixtures of structurally different homopolymers or copolymers. The mixture is held together by secondary intermolecular forces such as dipole interaction, hydrogen bonding, or van der Waals forces. 

Secondary Bonding. The atoms in a polymer molecule are held together by primary covalent bonds. Linear and branched chains are held together by secondary bonds hydrogen bonds, dipole interactions, and dispersion or van der Waal s forces. By copolymerization with minor amounts of acryhc (CH2=CHCOOH) or methacrylic acid followed by neutralization, ionic bonding can also be introduced between chains. Such polymers are known as ionomers (qv). 

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. 

In the cell walls of higher plants,178 hemicelluloses are found in close association with cellulose and lignin, and evidence for the distribution of these three main components in the various parts of the wall has been obtained by staining techniques.179 It is not certain, however, whether the close association of these substances can be accounted for entirely in terms of physical entanglement and secondary forces (van der Waals and hydrogen bonding) or whether primary chemical bonds unite some of the components. 

Fundamental adhesion is connected with the nature of the bonds producing cohesion between two media. These bonds may be classified into two categories, namely strong bonds (polar, covalent and metallic bonds) and secondary bonds (hydrogenous and Van der Waals bonds). Different atomic or molecular models have been proposed to describe the electronic structure of interfaces. None however, is sufficient for calculating the intensity of adhesion forces for systems of practical interest. 

The magnitude of adhesive forces that occur as the thin film is applied to the substrate and during drying or firing will depend on the nature of the film and the substrate surface. These adhesive forces can be generally classified as either primary interatomic bonds (ionic and covalent bonds) or secondary bonding (van der Waals bonding) (William and Callister, 1994 Kendall, 2001). 

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