Covalent bonds strong

Chemical reaction between different polymeric molecules to form a covalently bonded strong three-dimensional systems. During crosslinking, the consistency of material passes from liquid to solid or from a softer solid to a much harder one. (see also CURING.)... 

Covalent bond Strong bond holding atoms or molecules together by sharing pairs of electrons one pair, single bond two pairs, double bond three pairs, triple bond. 

Covalent Bonds = strong attractions between atoms (e.g.. O and H) within a molecule (e.g., H2O)... 

The successful preparation of polymers is achieved only if tire macromolecules are stable. Polymers are often prepared in solution where entropy destabilizes large molecular assemblies. Therefore, monomers have to be strongly bonded togetlier. These links are best realized by covalent bonds. Moreover, reaction kinetics favourable to polymeric materials must be fast, so tliat high-molecular-weight materials can be produced in a reasonable time. The polymerization reaction must also be fast compared to side reactions tliat often hinder or preclude tire fonnation of the desired product. 

In most covalent compounds, the strong covalent bonds link the atoms together into molecules, but the molecules themselves are held together by much weaker forces, hence the low melting points of molecular crystals and their inability to conduct electricity. These weak intermolecular forces are called van der WaaFs forces in general, they increase with increase in size of the molecule. Only... 

The unequal distribution of charge produced when elements of different electronegativities combine causes a polarity of the covalent bond joining them and, unless this polarity is balanced by an equal and opposite polarity, the molecule will be a dipole and have a dipole moment (for example, a hydrogen halide). Carbon tetrachloride is one of a relatively few examples in which a strong polarity does not result in a molecular dipole. It has a tetrahedral configuration... 

Ceramics and metals are entirely held together by primary bonds - the ionic and covalent bond in ceramics, and the metallic and covalent bond in metals. These strong, stiff bonds give high moduli. 

Fig. 23.2. A schematic of o linear-amorphous polymer, showing the strong covalent bonds (full lines) and the weak secondary bonds (dotted lines). When the polymer is loaded below Tg, it is the secondary bonds which stretch.

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