Current problems and new directions

Some aspects of rubber-like elasticity that are clearly in need of additional research are listed below. 

An example of a cross-Unking technique currently under development is the preparation of triblock copolymers such as those of styrene-butadiene-styrene. This system undergoes phase separation in such a way that relatively hard polystyrene domains act as temporary, physical cross-links, as is shown in Fig. 1.43 [228]. The resulting elastomer is thermoplastic, and it is possible to reprocess it by simply heating it to above the glass-transition temperature of polystyrene. It is thus a repm-cessible elastomer. There is a need to develop thermoplastic elastomers that are less expensive than the Kraton styrene-butadiene-styrene triblock copolymers. The leading candidates are stereochemical copolymers of polypropylene, and chemical copolymers of ethylene and comonomers such as hexene-1 [229-231]. 

As has already been mentioned, more novel approaches could probably be learned by studying the cross-linking techniques used by nature in preparing bioelastomers. 

A particularly challenging problem is the development of a more quantitative molecular understanding of the effects of filler particles [232-234], in particular 

An example of an important future trend is the study of single polymer chains, particularly with regard to their stress-strain isotherms [235,236]. Although such studies are obviously not relevant to the many unresolved issues that involve the interactions among chains in an elastomeric network, they are certainly of interest in their own right. 

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