The step shear strain behavior of elastomeric polypropylene (ePP) synthesiz
ed 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 temper
atures stereoregular blocks of iPP are able to crystallize while stereoirre
gular 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 s
train experiments carried out on crystallized samples show that ePP does no
t completely relax from an applied strain and that a cross-linked network h
as formed. Imperfections in the network structure are revealed by birefring
ence measurements that show a partial relaxation of the polymer orientation
. In contrast, crystallized homopolymer blend samples are able to completel
y relax after the application of a step shear strain. Step shear strain exp
eriments of the solvent fractions of ePP suggest that the elastomeric behav
ior results from a combination of fractions that are able to cocrystallize
to form a physically cross-linked network.