Crystallization and strength of natural rubber and synthetic cis-1,4-polyisoprene

Crosslinked samples of natural rubber (NR, SMR-5L) and a synthetic cis-1,4-polyisoprene (IR, Natsyn 2200, The Goodyear Tire & Rubber Company) were crystallized at -25 degrees C at various strains, and then slowly melted. Crystallization was somewhat slower for IR under all conditions, taking two to three times as long as for NR crosslinked to the same degree. The melting temperature was also a few degrees lower. These effects are consistent with the presence of a small fraction of non-cia material in the IR molecule. Measurements of tensile strength and tear strength were made over a wide temperature range. The results were closely similar for the two materials at low rates of stretching although the abrupt fall in tensile strength associated with a failure to crystallize(1) occurred at slightly lower temperatures for IR. Tear strength at low speeds showed ii steady fall on raising the temperature, with little difference between the two materials. However, at high rates of strain the IR materials appeared to have significantly lower tensile strength, and at high tear speeds the tear strength was markedly lower, about one-half of that for NR. Thus, the minor structural irregularities in IR appear to be of little consequence at low rates of strain, but cause significant weakening at high rates, when the lower rate of crystallization is inferred to become a critical factor.