Yr. Hu et al., Elastomeric polypropylenes from unbridged 2-phenylindene zirconocene catalysts: Temperature dependence of crystallinity and relaxation properties, MACROMOLEC, 32(10), 1999, pp. 3334-3340
The solid-state structure and physical behavior of an elastomeric polypropy
lene (PP1) synthesized with unbridged 2-phenylindene zirconocene catalyst w
ere examined using solid-state C-13 NMR spectroscopy, differential scanning
calorimetry, wide-angle X-ray powder diffraction, and birefringence measur
ements of relaxation behavior. Extraction of these elastomeric polypropylen
es with boiling ether and heptane yielded three fractions of different mole
cular weight and tacticity. The ether-soluble fraction is the lowest in iso
tactic pentad content and exhibits no detectable crystallinity. For the oth
er fractions and the parent homopolymer, the degrees of crystallinity estim
ated from DSC and WAXD results are similar. The amount of the immobile phas
e determined by solid-state NMR, however, appears much higher than the degr
ee of crystallinity. An increase in temperature from 20 to 80 degrees C res
ults in a loss in the relative fraction of mobile and immobile phase as mea
sured by solid-state NMR and a corresponding loss in the percent crystallin
ity as measured by DSC. The relaxation of polymer samples subjected to step
shear as a function of increasing temperature was investigated using polar
ized optical birefringence measurements. These studies revealed that the el
astic network remains stable up to temperatures of 80 degrees C. Blends of
atactic and isotactic polypropylene with similar average isotactic pentad c
ontents as PP1 and its heptane-insoluble fraction were examined in comparis
on. Despite their similar taciticities and degrees of crystallinity. the bl
ends show no evidence of a network structure after being subjected to a ste
p shear but instead relax back to an isotropic orientation.