Crystallizable elastomers

Recent advances in the synthesis of crystallizable elastomers based on high cistactic and transtactic polybutadiene structures are reviewed. 

The third route can be considered a variation of the first one, in which a monomer couple performs as a new monomer "unit". Apart from the route chosen, the melting point is so designed as to fall in a limited range of temperatures (295 20°K). In our laboratories, we have followed the first two routes for obtaining crystallizable elastomers and we have found both suitable for the purpose. 

Let us review now the results obtained in our laboratories in the synthesis of crystallizable elastomers by following the second route indicated in the Introduction. As previously... 

The copolymerization of butadiene in trans configuration with suitable comonomers represents a second route for obtaining a wide range of strain induced crystallizable elastomers, with melting point tailorable in a wide range of temperatures. These copolymers can be used, in particular, in blends with other crystallizable rubbers (e.g. synthetic cis-l,4-polyisoprene) in order to improve their "green strength". 

Fig. 1.12. A sketch explaining the increase in melting point with elongation in the case of a crystallizable elastomer [65].

A strain-crystallizing material like NR shows much better autohesion. It can be processed to a relatively low viscosity for easy wetting on contact, and still exhibit green strength due to strain-induced crystallization. Several other strain-crystallizable elastomers have been synthesized and shown to exhibit autohesion and green strength comparable or superior to that of NR. These include rran.s-polypen-tenamer, fran -butadiene-piperylene elastomers, and uranium-catalyzed polybutadiene. 

Another thermo-elastic effect that was mentioned earlier is the tendency of an elastomer to become warm when stretched rapidly. This type of behaviour is illustrated in Fig. 5.19 for the adiabatic extension of an elastomer. Some of the heating can be explained by crystallization which can also occur when natural rubber is stretched (Section 4.5) but a temperature rise is found even in non-crystallizable elastomers. This can be explained by consideration of the thermodynamic equilibrium. The rise in... 

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