Dynamic Response of Stereoblock Elastomeric Polypropylene Studied by Rheooptics and X-ray Scattering. 1. Influence of Isotacticity

Abstract

The dynamic response of elastomeric polypropylene (ePP) prepared by metallocene 2-arylindene hafnium catalyst was investigated by rheooptical birefringence as well as wide- and small-angle X-ray scattering (WAXS and SAXS). Solvent extraction of ePP ([mmmm] = 34%) results in three fractions with increasing tacticity, crystallinity, and molecular weight in the following order: ether soluble ([mmmm] = 21%), heptane soluble (44%), and heptane insoluble (76%). Unstretched ePP reveals a crystalline phase of the α-form isotactic polypropylene (i-PP). Tensile stretching of ePP yields three sets of scatterings (equatorial, off-axis diagonal, and meridional arc). High-tacticity fractions contribute to the equatorial and off-axis diagonal scatterings, revealing molecular-scale orientation parallel to the strain axis and crystalline phase transformation from the α-form to the mesomorphic form. The meridional arc is contributed by the low-tacticity ether-soluble fraction with crystalline chains oriented with a preferred direction orthogonal relative to the strain direction. SAXS, which probes long-range ordering, exhibits a broad and diffuse meridional peak for the intermediate-tacticity fraction. This implies that lamellae are oriented orthogonal to the strain direction upon stretching. Permanent deformation of ePP after stretching, as measured by the residual strain (tensile set), originates from permanently oriented crystallites and chains pinned within crystalline networks.