Cross-linking elastomeric networks

Definition of Vulcanization. Vulcanization is a process generally applied to rubbery or elastomeric materials, which forcibly retract to their approximately original shape after a rather large mechanically imposed deformation. Vulcanization can be defined as a process that increases the retractile force and reduces the amount of permanent deformation remaining after removal of the deforming force. Vulcanization increases elasticity while it decreases plasticity. It is accomplished by the formation of a cross-linked molecular network (Fig. 4.13). 

Rubbers are materials which display elastomeric properties, i.e. they can be stretched easily to high extensions and will spring back rapidly when the stress is released. This extremely important and useful property is a reflection of the molecular structure of the polymer which consists of a lightly cross-linked macromolecular network. The molecules slide past each other on deformation, but the cross-links prevent permanent flow and the molecules spring back to their original position on removal of the... 

Characterization of elastomeric networks by swelling equilibrium measurements may take advantage of the applicability of the constrained junction theory already demonstrated for mechanical testing. Use of the interpenetration concept (equation 124 or 125), and of topological expressions for (equations 47 or 60) cause equation (156) to depend only on and x for a perfect network, or on M, M , and x for a randomly cross-linked network. An example of such an application was given by Erman and Baysal. Two cross-linked polystyrene networks were... 

The segments derived from the condensation reaction of the butanediol and the diisocyanate agglomerate into separate phases, which are hard and crystalline. The elastomeric chains are thus cross-linked to form a network similar in many ways to that given by the simple... 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

There are now a number of techniques which may be used to prepare elastomeric networks of known structure Q-8). Two particularly useful and convenient ones involve the multi-functional end-linking of hydroxyl-terminated (4-16) or vinyl-terminated polydimethylsiloxane (PDMS) chains (3,17-21), and the cross-linking of PDMS chains through vinyl side groups present in known amounts and in known locations along the chains (4,18,22-25). A typical reaction of this type is... 

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. 

In the affine model of network deformation, the cross-links are viewed as firmly embedded in the elastomeric matrix, and thus as moving linearly with the imposed macroscopic strain [1-4, 20]. In the alternative phantom model, the chains are treated as having zero cross-sectional areas, with the ability to move through one another as phantoms [2-4]. The cross-links in this model undergo consider-... 

As already described, the upper three portions of Figure 2 summarize the differences in the way the constraints are applied in the constrained-junction theory, constrained-chain theory, and the diffused-constraints theory, respectively [4], Additional comparisons between theory and experiment for a variety of elastomeric properties should be very helpful [20], Also, neutron-scattering measurements conducted on series of networks having different values of the junction functionality , which is the number of chains emanating from a junction (cross-link), would be extremely useful in suggesting how to position the constraints along a chain in refining such models, since should have a pronounced effect on the... 

These observations can be qualitatively explained in terms of the constrained-junction theory. If a network is cross-linked in solution and the solvent then removed, the chains collapse in such a way that there is reduced overlap in their configurational domains. It is primarily in this regard, namely reduced chain-junction entangling, that solution-cross-linked samples have simpler topologies, and these diminished constraints give correspondingly simpler elastomeric behavior. 

This trapping technique can also be used to form networks with no cross-links. Mixing the same types of linear chain with large amounts of the cyclics and then functionally end-linking them could give sufficient cyclic interlinking to yield an Olympic or chain-mail network [3, 193, 200, 203], as is illustrated in Figure 5 [193], Attempts have been made to prepare and characterize such materials, because they could well have unusual elastomeric properties [204],... 

Figure 5. Preparation of a chain mail or Olympic network consisting entirely of interlooped cyclic molecules, without any cross-links [193]. Linear chains passing through the cyclics are difunctionally end-linked at the regions shown by the rectangles. The result is a series of interpenetrating cyclics, which would function as an elastomeric network.

In any case, if this polymerized form of elemental sulfur is quenched (cooled rapidly), it becomes a solid. This solid is glassy at very low temperatures, but becomes highly elastomeric above its glass-transition temperature of approximately -30 °c.6 8 14 30 The situation is complicated by the presence of unpolymerized S8 molecules which would certainly act as plasticizers. So far, attempts to cross-link the elastomeric form into a network structure suitable for stress-strain measurements have not been successful. The polymer is unstable at room temperature, gradually crystallizing, and eventually reverting entirely to the S8 cyclics. 

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