Polydienes crosslinking

The use of lightly crosslinked polymers did result in hydrophilic surfaces (contact angle 50°, c-PI, 0.2 M PhTD). However, the surfaces displayed severe cracking after 5 days. Although qualitatively they appeared to remain hydrophilic, reliable contact angle measurements on these surfaces were impossible. Also, the use of a styrene-butadiene-styrene triblock copolymer thermoplastic elastomer did not show improved permanence of the hydrophilicity over other polydienes treated with PhTD. The block copolymer film was cast from toluene, and transmission electron microscopy showed that the continuous phase was the polybutadiene portion of the copolymer. Both polystyrene and polybutadiene domains are present at the surface. This would probably limit the maximum hydrophilicity obtainable since the RTD reagents are not expected to modify the polystyrene domains. 

Crosslinking occurs by the corresponding reaction between XIX and the polydiene ... 

Polydiene rubbers can also be crosslinked by heating with p-dinitrosobenzene, phenolic resins, or maleimides [Coran, 1978 Gan and Chew, 1979 Gan et al., 1977, 1978 Sullivan, 1966]. The crosslinking mechanism is similar to that for accelerated sulfur vulcanization, for example, for vulcanization by p-dinitrosobenzene... 

These products find use in specialty applications requiring better thermal stability than available in the sulfur vulcanized elastomers. Other processes are also used to crosslink polydiene rubbers (Secs. 9-2c and 9-2d). 

Examples are the introduction of carboxyl or hydroxy groups to increase hy-drophilicity, crosslinking (vulcanization) of polydienes with sulfur, or conversions that proceed only at the terminal groups under retention of the molecular backbone (chain-analogous conversion). 

Of technical importance are radically crosslinking reactions on finished articles of polyolefins by means of electron beams in order to increase, for example, the thermostability. The technical importance of networks consisting of polydienes and other rubbers, polyurethanes, formaldehyde resins, alkyd resins, and silicones has already been explained in Sects. 1.3.3.3,4.1 and 4.2. 

Hydrolysis procedures have been, developed for the removal of the acetal protecting groups without accompanying crosslinking of polydiene backbones. Dilute solution hydrolyses are preferred over bulk methods ( ). (Eq. 6-9). 

THe network structure of these block polymers can also be altered by actually crosslinking the elastomeric polydiene chains, thus introducing a chemical network. In this approach it was necessary to use a crosslinking method which would not result in any measurable chain scission. Dicumyl peroxide (Dicup) was chosen for crosslinking an SIS polymer since this peroxide is known to accomplish exclusive crosslinking without any observable chain scission (3). The Dicup was dissolved in a THF solution of the polymer, and a cast film was prepared which was then vulcanized in a press at 155 °C for 35 min. A control sample, without Dicup was treated in the same way. 

Crosslinking of EPDM and Polydiene Rubbers Studied by Optical Spectroscopy... 

The aim of this chapter is to review optical spectroscopy studies on sulfur and peroxide crosslinking of polydiene rubbers, such as NR and BR (Sections 6.2.1 and 6.3.1, respectively), and to discuss in detail recent FT-Raman and FT-IR spectroscopy studies into the sulfur and peroxide crosslinking of EPDM (Sections 6.2.2 and 6.3.2, respectively). The results of optical spectroscopy studies will also be discussed in the light of results obtained with other techniques. Finally, the elucidation of the chemical structures of the crosslinks formed will allow enhanced understanding of the mechanisms of crosslinking and some preliminary insight into the structure/property relationships of crosslinked rubber. 

The mechanism of the accelerated sulfur vulcanisation of EPDM is probably similar to that of the highly unsaturated polydiene rubbers. The vulcanisation of EPDM has been studied with emphasis on the cure behaviour and mechanical and elastic properties of the crosslinked EPDM. Hardly any spectroscopic studies on the crosslinking chemistry of EPDM have been published, not only because of the problems discussed in Section 6.1.3 but also because of the low amount of unsaturation of EPDM relative to the sensitivity of the analytical techniques. For instance, high-temperature magic-angle spinning solid-state 13C NMR spectroscopy of crosslinked EPDM just allows the identification of the rubber type, but spectroscopic evidence for the presence of crosslinks is not found [72]. 

In view of the abundance of unsaturation in polydiene elastomers it may be expected that this type of elastomers can be cured very efficiently with peroxides. This section will show that a very high crosslinking efficiency (= moles of crosslinks formed per mole of peroxide decomposed) can indeed be obtained. However, the substitution pattern of the unsaturation plays a major role in the actual efficiency found. 

Molded foam copolymers with 10-30% glycidyl acrylate may be crosslinked by heating in the presence of polyamines. Heat resistance may be increased to as high as 150°C. by this treatment (25). Use of certain allyl or diallyl esters to control the crosslinking of polystyrene to the desired very light extent has been disclosed as a means of obtaining foams of improved and regular cell structure (135). Block copolymers of polystyrene, polydiene (particularly polyisoprene), and polystyrene... 

In the literature, two additional reactions following addition esterification have been treated using the cascade theory the addition esterification followed by polyetherification with epoxide groups in excess (a reaction used for crosslinking of carboxyl terminated polydienes) and addition esterification followed by transesterification. Transesterification often interferes wherever hydroxyester groups are formed, for example, in synthesis of linear oligomeric polyesters from diepoxide and acids. As has been explained before, polyetherification is an initiated reaction and, therefore, the statistical treatment offerend in Refs. should be revised. Below we show the treatment of transesterification for a system composed of a diepoxide and a dicar boxylic acid. 

Hydroxyl-terminated polydiene resins gelled by the reaction with orthosilicate esters have increased thermal stability. These polymeric gels, like silicone rubbers, exhibit outstanding electrical properties. The polymeric gels crosslinked at ambient temperature are castable as self-curing liquids. For example, they are used as binders for rocket solid fuels, in coatings for pipes, tanks, etc. They can be mixed with rubbers. 

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