Cross-links between elastomer molecules

Explain how cross-linking between the molecules affect the molecular mobility and elasticity of elastomers. 

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. 

Elastomers and duromers are characterised by additional covalent cross-links between the chain molecules. In the energy-elastic regime, these additional bonds do not influence the elastic properties significantly Young s modulus only increases slightly. 

Although thermoplastics comprise un-linked chain molecules, they can nevertheless have a branched structure. In contrast to elastomers and duromers, these branches do not cause cross-linking between the chains, but they can inhibit a geometrically dense packing necessary to form crystalline regions. This is sketched in figure 8.25. 

Elastomers like polybutadiene can also be attacked by oxidation. In this case, additional cross-links between the chain molecules are formed, causing embrittlement. 

Only when chemical bonds between neighboring molecules are introduced is a raw elastomer converted into a rubber vulcanizate, which is essentially a three-dimensional network structure (see Figure 5.3). The process is referred to as vulcanizahon or curing, or more accurately, as cross-linking. A cross-linked elastomer, or rubber vulcanizate, is capable of large reversible deformations within a broad temperature range and does not dissolve, but only swells in solvents and other liquids. 

Polymeric hydrocarbon elastomers, such as natural rubber, are cross-linked or vulcanized by the use of sulfur, which reacts with the carbon of the unsaturated bonds in polymer molecules to form a bridge between two molecules so that one polymer molecule is covalently bonded to a second polymer molecule (6). 

The possibility of employing block copolymers as materials that might possess desirable properties was originally considered by Mark In the first period the effort in preparing block copolymers was directed to radical polymerization and it was only in 1956 that Szwarc obtained well-defined block copolymers by anionic polymerization . In block copolymers, the incompatibility between polymeric chains becomes an advantage a phase separation of the blocks occurs leading to the formation of microdomains which are responsible for the ecific properties of block copolymers. For instance, the presence in a molecule of an elastomeric block linked by its ends to thermoplastic blocks generates a polymer in which reversible physical multifunctional cross-links allow the behaviour of conventional vulcanized elastomers at room temperature, but the material remains easily moldable at elevated temperature just as normal thermoplastic resins ° ... 

Silicones may be polymerized to molecules containing 2000 or more (0115)2810 units and then milled with inorganic fillers (such as zinc oxide or carbon black, used also for ordinary rubber), and "Vulcanized, by heating to cause cross-links to form between the molecules, bonding them into an insoluble, infusible three-dimensional framework. The resultant elastomers (silicone rubber) may be used for electrical insulation and for other purposes for which rubber is needed, especially at high temperature. 

The correlation of average molecular weight between branch points wdth the swelling of a urethane elastomer is shown in Figure 19. As the chain length between cross links increases, or cross linking decreases, swelling is increased, since the solvent molecule can penetrate more readily the three-dimensional polymer. 

The elastic behaviour of polymers is mainly determined by the intermolecular bonds between the chain molecules, not by the covalent bonds within. For elastomers and duromers, the covalent bonds linking the chains are also relevant. In the following, we will start by discussing the elastic properties of thermoplastics and afterwards study the influence of cross-linking. 

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