Effects of crystallization temperature and cooling rate on the polymorphism of trans-1,4-polyisoprene vulcanizate

The effects of crystallization temperature and cooling rate on the polymorphism of trans-1,4-polyisoprene vulcanizate (TPI-V) with different sulfur dosages were monitored by WAXD in real time. The results showed that both the crystallization temperature and the cooling rate play decisive roles in determining the polymorphism and their relative amounts, whereas their effects are interfered by the crosslinking density which restrains the crystallization kinetics. Only alpha crystal is formed when crystallization is conducted at 40 degrees C or greater and mixed alpha/beta crystals are obtained in the temperature range of 10-30 degrees C. beta crystal is predominate when crystallized at 10 degrees C while alpha crystal is limited at 30 degrees C. Both the lower temperature and increased crosslinking density are beneficial to beta crystal formation. A high crosslinking density causes a prolonged time is needed for pure alpha crystal formation, and hinders the TPI-Vs from forming pure alpha crystal even if crystallization is conducted at 45 degrees C. A lower cooling rate favors the formation of alpha crystal while a higher cooling rate favors beta crystal formation. The beta spherulites are much brighter than alpha spherulites under the observation of polarized microscopy.