Crystallization of cis- and trans-1,4-polyisoprene dispersed in SBR

Crystallization of natural rubber (NR) and trans-1,4-polyisoprene (TPI) in blends with styrene-butadiene rubber (SBR) has been studied dilatometrically. For TPI, the melting temperature and Avrami index of crystallization kinetics were unchanged by blending with SBR. However, the rate of crystallization was decreased significantly when only 1 wt % of SBR was added, and much more so for amounts large enough to constitute the continuous phase. These reductions in rate are attributed to two effects: chemical changes in TPI during mechanical mixing, and restricted nucleation in small TPI particles. Similar effects were noted for NR blended with SBR. When solution-blended as a 30/70 mixture with SBR, crystallization of the NR fraction was about 9 times slower than for NR itself, and the rate was reduced still further when mixing was more thorough. However, when NR and SBR were mixed mechanically, the rate of crystallization of the NR fraction was not reduced much although the Avrami index and final degree of crystallinity were significantly lower. These effects suggest that the NR particles are strained during mechanical mixing and do not recover afterwards.