Randomly-linked elastomers

Table I. Threshold Tear Strength T0 of Randomly—linked and Endlinked Elastomers of Varying Mc...

A number of workers have treated non-Gaussian networks theoretically in terms of this finite extensibility problem. The surprising conclusion is that the effect on simple statistical theory is not as severe as might be expected. Even for chains as short as 5 statistical random links at strains of up to 0.25, the equilibrium rubbery modulus is increased by no more than 20-30 percent (typical epoxy elastomers rupture at much lower strains). Indeed, hterature reports of highly crosslinked epoxy M, calculated from equilibrium rubbery moduh are consistently reasonable, apparently confirming this mild finite extmsibiUty effect. 

In spite of this, the failure envelopes are normal. Thus Figure 1 shows the envelopes for several Solithane 113-300 compositions (lO), These envelopes can be fitted by the inverse Langevin approximation (ll) to the stress-strain curve, and from the curve fit both the number of effective chains per cm and the niimber of equivalent random links N can be determined (l2). The fit for two compositions is shown in Figure l8 and the results of such an analysis (13) are given in Table II. It can be seen that the chain concentration is almost constant but N increases, i.e, the chains effectively become stiffer as the concentration of prepolymer is increased. 05iis is -ttie only elastomer system we are aware of in which such a change can be effected at constant Ye ... 

Broad changes in the composition of the basic elastomer family do not influence the crosslink density or effective chain concentration but rather change the nature of the crosslink site so that the number of the equivalent random links per chain is changed. 

The reversible recovery of a deformed elastomer to its original (undeformed) state is due to an entropic driving force. The entropy of polymer chains is minimum in the extended conformation and maximum in the random coil conformation. Cross-linking of an elastomer to form a network structure (IX) is... 

Although hdpe and it-PP are crystalline, the commercial random copolymer of ethylene and propylene (EP) is an amorphous elastomer. The most widely used EP copolymer (EPDM) is produced by the copolymerization of ethylene and propylene with a small amount of an alkyldiene this permits cross-linking or vulcanization. 

The random copolymer of propylene and ethylene (EP) lacks the good symmetry of it PP and is a flexible elastomer. Since this copolymer is used as an elastomer, it is customary to add a small amount of a diene, such as ethylidene norbomene, to the monomer reactants before polymerization to allow subsequent cross-linking or curing. 

Cured polymers of butadiene with low cross-link density do not tend to cold flow and are useful elastomers. These vulcanized elastomers crystallize when stretched, but when the stress is removed, the restoring force is largely entropy and most of the crystals melt and the chains return to the random conformation.The tensile strength is increased dramatically when large amounts of carbon black or amorphous silica are added. 

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. 

When elastomer networks are formed, the segments of chains that are close to each other in space may be crosslinked, independently of their locations along the chain. Therefore, the network has a totally random structure in which the number of cross-linking points and their locations... 

One further requirement the long chains of an elastomer must be connected to each other by occasional cross-links enough of them to prevent slipping of molecules past one another not so many as to deprive the chains of the flexibility that is needed for ready extension and return to randomness. 

The properties of block copolymers differ from those of a blend of the correponding homopolymers or a random copolymer (Chapter 7) with the same overall composition. An important practical example is the ABA-type styrene/butadiene/styrene triblock copolymer. These behave as thermoplastic elastomers. Ordinary elastomers are cross-linked by covalent bonds, e.g., vulcanization (see Chapter 2) to impart elastic recovery property, as without this there will be permanent deformation. Such cross-linked rubbers are therraosets and so cannot be softened and reshaped by molding. However, solid thermoplastic styrene/butadiene/styrene triblock elastomers can be resoftened and remolded. This can be explained as follows. At room temperature, the triblock elastomers consist of glassy, rigid, polystyrene domains... 

An elastomer is a polymer that stretches and then reverts to its original shape. It is a randomly oriented amorphous polymer, but it must have some cross-linking so that the chains do not slip over one another. When elastomers are stretched, the random chains stretch out. The van der Waals forces are not strong enough to maintain them in that arrangement therefore, when the stretching force is removed, the chains go back to their random shapes. Rubber is an example of an elastomer. 

Copolymerization of ethylene with propylene results in a random, noncrystalline copolymer that is a chemically inert and rubbery material. EPM is a saturated copolymer that can be cross-linked through the combination of the free radicals generated by peroxides or radiation. To incorporate sites for vulcanization, an unsaturated terpolymer can be prepared from ethylene, propylene, and a small amount (3 to 9%) of a nonconjugated diene (EPDM). The diene is either dicyclopentadiene, ethylidene nor-bomene, or 1,4-hexadiene. The resulting unsaturated terpolymer can be vulcanized by traditional techniques. Each of the termonomers confers different characteristics on the final elastomer. 

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