Shear-Mediated Crystallization of Isotactic Polypropylene:  The Role of Long Chain−Long Chain Overlap

The role of long chains in shear-mediated crystallization was studied by in situ rheo-optical measurements and ex situ microscopic observations. To elucidate the effects of long chains, we prepared model blends in which fractionated isotactic polypropylene (iPP) (denoted L-PP) with high molecular weight (MW) and narrow molecular weight distribution was blended with a metallocene iPP (Base-PP) with lower molecular weight. The concentration of L-PP (c) was varied ranging from 0 to twice the concentration (c*) at which L-PP coils overlap. The crystallization of all blends after cessation of transient shearing was accelerated, while the quiescent crystallization kinetics were not affected by the addition of L-PP. A distinctive change in the development of birefringence after shearing was observed when the wall shear stress (σw) exceeded a critical value (σ*). Below σ*, irrespective of c, the birefringence after transient shearing increased gradually, reaching a small value at the end of crystallization. Above σ*, a brief interval of shear induced highly oriented growth, manifested in the birefringence after cessation of flow and growing stronger and reaching a large value as crystallization proceeded. Further, the rate of growth of the birefringence exhibited a strong, nonlinear c dependence. The morphology of the skin layer showed a shish kebab type structure observed by TEM for samples subjected to stresses above σ*. The number density and thickness of shish were affected by c and changed drastically at c near the overlap concentration of the long chains. This indicates that the role of long chains in shear-induced oriented crystallization is cooperative (rather than a single chain effect), enhanced by long chain−long chain overlap.