Transient Birefringence of Elastomeric Polypropylene Subjected to Step Shear Strain

The step shear strain behavior of elastomeric polypropylene (ePP) synthesized from an unbridged metallocene catalyst is studied using polarimetry. The elastomeric nature of ePP is presumed to arise from a multiblock structure of isotactic (iPP) and atactic (aPP) polypropylene blocks. At lower temperatures stereoregular blocks of iPP are able to crystallize while stereoirregular blocks of aPP remain amorphous. The flow behavior of ePP is compared with homopolymer blends of iPP and aPP that have the same isotactic content as ePP as well as solvent fractions of the parent ePP sample. Step shear strain experiments carried out on crystallized samples show that ePP does not completely relax from an applied strain and that a cross-linked network has formed. Imperfections in the network structure are revealed by birefringence measurements that show a partial relaxation of the polymer orientation. In contrast, crystallized homopolymer blend samples are able to completely relax after the application of a step shear strain. Step shear strain experiments of the solvent fractions of ePP suggest that the elastomeric behavior results from a combination of fractions that are able to cocrystallize to form a physically cross-linked network.