| Abstract: | The 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. |
