The crystallization behavior of Cis‐polyisoprenes extracted from the guayule plant

Abstract

AbstractThe crystallization kinetics and resultant morphologies of thin films of cis‐polyisoprenes from guayule rubber have been studied using osmium tetroxide staining and transmission electron microscopy. Polymers used were extracted from the plant either on a laboratory scale or in a pilot plant operation. The polymers show significantly different behavior with regard to both morphological features and to lamellar growth rates. Freshly extracted cis‐polyisoprene as well as freshly prepared specimens of pilot plant rubber show only α growth, as in cis‐polyisoprene from hevea, but a slightly faster lamellar growth rate. Aged pilot plant rubber shows both α and β growth, both at higher rates than in natsyn, with the rate of α growth being lower than in hevea. Freshly extracted and aged specimens show a maximum lamellar growth rate at −20°C, whereas the maximum for pilot plant rubber occurs at −24°C. The data are interpreted in terms of oxidation of double bonds during aging to produce pendant ester groups. Lamellar growth rates and infrared analyses are both consistent with 1.2–1.5% of the double bonds being affected. Data from guayule and natsyn are now consistent in showing that the effects of microstructural impurities on the crystallization of cis‐polyisoprene are to reduce the rates of α‐lamellar growth and to induce the β growth, which would not otherwise exist. Secondary nucleation analyses show the regime II–III transition, as in hevea; however, the temperature of the transition, as well as the ratio of the slopes, is different for this linear polymer.