On the influence of the amorphous phase on the stability of crystals in poly(cis-1,4-isoprene) networks

Abstract

Crosslinked natural rubber and synthetic rubber samples are additivated with up to 9 wt% stearic acid (StA) to better understand the influence of StA on the melting temperature Tm of strain-induced crystallized poly(cis-1,4-isoprene) crystals. To this end, lamellae thicknesses are determined from wide-angle x-ray patterns and used to calculate the crystal size dependent melting temperature Tm,calc. Comparing the measured Tm with Tm,calc reveals that Tm deviates downward from Tm,calc and converges Tm,calc with increasing StA concentration until it is identical to Tm,calc, in case of room temperature strain-induced crystallization. In case of strain-induced crystallization at 80°C, it was found that Tm is identical with Tm,calc without added StA and deviates upward from Tm,calc with increasing amount of added StA. We suggest that this is due to internal stress onto the polymer crystals exerted by highly strained macromolecules in the surrounding amorphous phase. Whether this stress has a stabilizing or destabilizing effect on the crystals is assumed to depend on its intensity and direction, which can be efficiently altered by the amount and the location of StA crystals in the amorphous phase.