Chemical cross-linking, elastomeric networks

Vulcanization, or cross-linking of elastomers, is technically the most important process for conventional elastomers. During that process, strong chemical bonds are formed between molecules, thus restraining their mobility. As pointed out earlier, a three-dimensional network is formed. The cross-linking of elastomeric molecules is a random process typically, one cross-link is formed per 100 to 200 monomeric units. 

The second requirement for an elastomeric material is that the chains be cross-linked to form a loose three-dimensional intermolecular network. This cross-linking may be either by covalent chemical bonds or by strong non-covalent interactions such as hydrogen bonds or hydrophobic interactions. Cross-linking is essential for the restoring mechanism once the stress is removed from the material. If the material is not cross-linked, low stresses will straighten the polymer chains, but at higher sResses they will slip past each other and irreversibly deform. 

Styrenic block copolymer (SBS) TPEs are multiphase compositions in which the phases are chemically bonded by block copolymerization (see chapter Introduction to Plastics and Polymers). At least one of the phases is a hard styrenic polymer. This styrenic phase may become fluid when the TPE composition is heated. Another phase is a softer elastomeric material that is rubber-like at room temperature. The polystyrene blocks act as cross-links, tying the elastomeric chains together in a three-dimensional network. SBS TPEs have no commercial applications when the product is just a pure polymer. They must be compounded with other polymers, oils, fillers, and additives to have any commercial value. 

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