Deliberate elastomers

The development of anionic chemistry over the past 30 years has led to the emergence of new processes and products of Industrial Importance, the most significant being a family of thermoplastic elastomers. These unique elastomers are presently commercialized by Shell Chemical Company as Kratons and by Phillips Chemical Company as Solprenes. Their uniqueness is the result of deliberate design of the polymeric structure and composition. 

Properties. One of the characteristic properties of the polyphosphazene backbone is high chain flexibility which allows mobility of the chains even at quite low temperatures. Glass-transition temperatures down to —105°C are known with some alkoxy substituents. Symmetrically substituted alkoxy and aryloxy polymers often exhibit melting transitions if the substituents allow packing of the chains, but mixed-substituent polymers are amorphous. Thus the mixed substitution pattern is deliberately used for the synthesis of various phosphazene elastomers. On the other hand, as with many other flexible-chain polymers, glass-transition temperatures above 100°C can be obtained with bulky substituents on the phosphazene backbone. 

Thus, substantial polymer molecular weight modification (reduction) is seen to be a consequence of deliberate stoichiometric reactant imbalance via MDI deficiency. This not only represents a method of thermoplastic polyurethane elastomer molecular weight control but also demonstrates what inadvertent reactant imbalance can do to the polymerization and the polymer product. 

To fully exploit the beneficial properties of PIB in, for example, polyurethanes, thermoplastic elastomers, and sealants, controlled polymerization methods are required that provide access to controlled end-group functionalities. The controlled polymerization of isobutene was first reported by Kennedy, which was based on the inifer technique. This so-called inifer that is added to provide control over the polymerization is a compound that acts both as the initiator and the transfer agent. The inifer deliberately induces chain transfer to control the molecular weight of the PIB, whereby each transfer step generates... 

In the early 1950 s the emerging aerospace industry developed a critical need for an elastomer with good heat and fuel resistance for use in seals and hose in military iet engines. Available rubbers, even the silicones, were just not adequate to meet the stringent new requirements. The studies which followed, unlike the early work, were not motivated by scientific curiosity but Involved deliberate use of the knowledge gained from past work on polymer structure to produce desired results. A number of attractive candidates soon emerged. 

Sometimes, the elastomer phase is deliberately cross-linked during the intensive mechanical mixing. This is described as dynamic vulcanization [27,34], It produces a finely dispersed, discontinuous, cross-linked elastomer phase (Fig. 5.5). The products are called dynamic vulcanizates or thermoplastic vulca-luzates. This process is more complex than simple mixing, but the products have two important advantages. First, the cross-Unked elastomer phase is insoluble and so oil and solvent resistance is improved. Second, cross-linking reduces or eliminates the flow of this phase at high temperatures and/or imder stress. This improves resistance to compression set. 

The principles covering the design of extraction tests proposed by various authorities are reviewed in Chapter 1. Chapters 2 and 3 review the types of polymers and elastomers, respectively, used in the packaging and the types of deliberately added or adventitious compounds that can occur in plastics and can be extracted into the packaged commodity. 

The broad range of various approaches to the preparation of thermoplastic polyurethane elastomers is quite obvious and the possibilities of obtaining a specific structure of the macromolecular chain are numerous. Therefore, polyurethanes should be considered not only as a class of polymers they could be also very efficient in the deliberate creation of macrochains with desired structures [1]. 

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