Autohesion of Polyisoprene

Autohesion of polyisoprene rubber (Natsyn 2200, a synthetic high-cis-l,4-polyiso-prene) and styrene-butadiene copolymer has been studied. Both elastomers were reinforced by carbon black and crossHnked by a sulfur-based system (see Table 24.1) [3]. The glass transition temperatures of the elastomers were not significantly changed by crossHnking and were equal to -66°C and -53°C for the IR and SBR, respectively, as measured by DSC analysis. 

This effect is more pronounced for SBR because crosslinking by radicals can also occur for this elastomer. No reversion is observed over the times considered. 

The assembhes were prepared according to Gents procedure in two steps as described above. To obtain various states of crosslinking before assembly, the time of pre-crosslinking was changed. The degree of pre-crosslinking a was determined from Eq. (1). 

A comment should also be made on the locus of failure for IR, it was apparently interfacial (both peeled parts appeared smooth with a glossy aspect, as before assembly) for peel strengths up to about 2.5 kj nT, and cohesive above that value (the separation led to a more or less rough surface and finally became impossible). For SBR joints, the locus of failure also changed and an additional type of failure was observed. Samples showed stick-shp behavior between 7 and 15 kJ mT The peel force oscillated between a minimum and a maximum value. 

Diffusion coefficients are proportional to 1/M, the molecular weight of linear chains [7]. They are not well known and therefore neither is the interdigitated thickness. Hence, it is not possible to say whether the observed behavior has to be related to the interdiffusion depth, or to the number of crosslinks formed in the interfacial region or, most hkely, to both effects. These results given for the elastomer joints crosshnked by a sulfur-based vulcanizing system show that it is very difficult to separate interdiffusion and crosshnking mechanisms because the temperature influences both the chain mobihty and the kinetics of network formation. 

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