Rheo-Optics and X-Ray Scattering Study of Elastomeric Polypropylene

Elastomeric polypropylenes (ePP) have been known since the early history of polypropylene. Elastomeric polypropylenes were first synthesized by Natta using heterogeneous Ziegler-type catalysts; he attributed the elastomeric behavior to their low crystallinity and stereoisomer blocks consisting of alternating isotactic crystalline and atactic amorphous segments. Subsequent to Natta's work, there have been a number of reports of the production of related materials produced with different catalysts, with notable advances by Collette, Chien, and Collins. Recently, we reported that metallocene catalysts derived from 2-arylindene ligands produce elastomeric polypropylenes which can be separated into fractions differing in tacticity and crystallinity. Studies of the elastomeric polypropylenes using rheological and optical-polarimetry methods reveal evidence of a physical network in contrast to the blend of atactic and isotactic fractions. Dynamic infrared (IR) polarimetry studies provide evidence of cocrystallization of solvent fractions of the materials subjected to step-shear flows. In the current study, we examine the tensile flow properties of elastomeric polypropylene and its solvent fractions derived from 2-arylindene metallocenes catalysts. The origin of tensile set is studied by simultaneously measuring the tensile stress and optical birefringence. The birefringence follows the flow-induced anisotropy arising from amorphous chain orientation and oriented crystallites. Wide Angle and Small Angle X-raymore » Scattering (WAXS and SAXS, respectively) were performed to investigate the crystallite orientation as well as strain-induced crystallinity.« less