Molecular aspects of flow-induced crystallization of polypropylene

Using a novel shearing device, we investigate-flow-induced crystallization of bimodal blends of polypropylenes (PP) in which we vary the molecular character (concentration, molecular weight, regularity) of the high molecular weight mode. We apply a number of in situ characterization tools (rheo-optics, rheo-WAXD) to the development of transient structure and interpret our findings in light of ex situ microscopic examination of the final morphology.; Blending a well-characterized high molecular weight isotactic polypropylene into a "base iPP" at various concentrations (c), we determined that less than 1% of long chains with Mw five times larger than the Mw of the base resin profoundly affected the flow-induced crystallization kinetics and morphology. Varying the concentration from below to above c* indicated that the effect of the long chains involves cooperative interactions enhanced by long chain-long chain overlap. The long chains particularly influence the formation of anisotropic nuclei.; Studying a series of bimodal blends in which the long chain molecular weight (ML) was varied, we found that increasing ML increased the tendency to form threadlike precursors to oriented crystallization. This was highlighted by a marked decrease in the threshold stress (sigma*) necessary to induce oriented crystallization. A minimum separation in relaxation time scales (∼100 times slower) between the long chains and the average was necessary to form long lived oriented precursor structures.; A novel "depth sectioning" analysis technique allowed us to gain depth dependent information from real-time rheo-optical and rheo-WAXD experiments. We identify a promising set of conditions that may be used to measure the thread propagation velocity for this material if the appropriate length scale can be assigned by microscopy. Threads first form near the channel wall and grow in length with prolonged flow until thread length per unit volume saturates. Prior to saturation, the thread propagation appears to be linear with shearing time.; We examined the effects of long chain regularity on the formation of threadlike precursors, showing that addition of molecular level defects (comonomers or stereo-errors) to the high end of the molecular weight distribution effectively raises sigma* and mitigates the formation of oriented precursors induced by flow.