Strain-crystallization of guayule and hevea rubbers

A comparison was made of the room temperature strain-crystailization of naturally-occurring cis-1,4-polyisoprenes having varying non-rubber content. A variety of measurements were employed to assess crystallization, including stress relaxation, optical birefringence, and the infrared absorption spectrum. All methods yielded the same result: The strain required to induce crystallization is less for polyisoprenes having larger concentrations of impurities. The ability to crystallize at lower orientation presumably underlies the superior failure properties of guayule rubber and the poorer grades of natural rubber (NR) in comparison to deproteinized NR.