Cross-linked polymers vulcanized polyisoprene

The polymer industry traces its beginning to the early modifications of shellac, natural rubber (NR, an amorphous cis-l,4-polyisoprene), gutta-percha (GP, a semicrystalline fran -l,4-polyisoprene), and cellulose. In 1846, Parkes patented the first polymer blend NR with GP partially co-dissolved in carbon disulfide. Blending these two polyisoprene isomers resulted in partially cross-linked (co-vulcanized) materials whose rigidity was controllable by composition. The blends had many applications ranging from picture frames, tableware, ear trumpets, to sheathing the first submarine cables. 

An example of a nonlinear polymer derived by cross-linking an initially linear polymer is afforded by vulcanized natural rubber. In the usual vulcanization procedure involving the use of sulfur and accelerators, various types of cross-linkages may be introduced between occasional units (about one in a hundred) of the polyisoprene chains. Some of these bonds are indicated to be of the following type ... 

A good elastomer should not undergo plastic flow in either the stretched or relaxed state, and when stretched should have a memory of its relaxed state. These conditions are best achieved with natural rubber (ds-poIy-2-methyl-1,3-butadiene, ds-polyisoprene Section 13-4) by curing (vulcanizing) with sulfur. Natural rubber is tacky and undergoes plastic flow rather readily, but when it is heated with 1-8% by weight of elemental sulfur in the presence of an accelerator, sulfur cross-links are introduced between the chains. These cross-links reduce plastic flow and provide a reference framework for the stretched polymer to return to when it is allowed to relax. Too much sulfur completely destroys the elastic properties and produces hard rubber of the kind used in cases for storage batteries. 

The second example of a polymer reaction is the industrial cross-linking of rubber by vulcanization sketched in Fig. 3.50. The process was invented already in 1839 by C. N. Goodyear without knowledge of its chemical stracture. Natural rubber is cis-poly(l-methyl-1-butenylene) or polyisoprene with a low glass transition temperature of about 210 K. Its structure and those of other rubbers are given in Fig. 1.15. The addition of sulfur in the form of Sg rings and heating causes the vulcanization. Of the listed cross-hnks in Fig. 3.50, only the left example is an efficient network former. The sulfur introduces about 1 cross-link for each of 50 S-atoms used. Modem vulcanization involves activators and accelerators for increased efficiency. The detailed mechanism is rather complicated and not fully understood. 

The conducted tests leaded to developing grounds for the technology for dynamic vulcanization of materials with thermo-elastoplastic properties, in which a thermoplastic polymer constitutes a continuous phase, whereas the dispersed phase consists of cross-linked elastomer particles. Basic elastomers are polyisoprene with isotactivity level of 85% or higher and copolymer EOE containing over 30% of n-octene. 

Both CIS- and trans-polybutadienes present a somewhat different picture since efficiencies much greater than unity have been observed. This high efficienc tas been found to increase with an increase in the vinyl (1,2-) content. J or example it has been found (Kraus, 1963) that whereas a poly butadiene with a 10% vinyl content had a cross-linking efficiency of about 2, a 98% 1,2- polymer had a value in excess of 100 It is reasonable to presume that this high efficiency is due to a polymerization process initiated by reaction (A) but it is to be noted that there is much evidence to show that this polymerization cross-linking occurs via main chain double bonds as well as on the pendent vinyl groups. As with accelerated sulphur vulcanization there are important, but not well understood differences between polybutadiene and polyisoprene. 

The sulfur reacts with polyisoprene to replace some C—H bonds with disulfide bonds. As a result, the polymer chains become connected by cross-hnks that may contain one, two, or more sulfur atoms (Figure 28.9). These cross-links increase the rigidity of the rubber because most of the chains are linked into a larger molecule. The freedom of movement of one chain relative to another is diminished. After distortion, the vulcanized rubber returns to its original molded shape. The amount of sulfur—3-10% by weight—controls the flexibility of the rubber. 

Like all polydienes, the polyisoprene chain is highly flexible. This property, which relates to the weakness of molecular interactions, maintains this polymer in an amorphous state and is responsible for its marked elastomeric character. However, strain deformations are totally reversible only after vulcanization (cross-linking) of the corresponding material (see Section 9.3). The glass transition temperature of NR is very low (-72°C). 

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