Molecular Aspects of Flow-Induced Crystallization of Polypropylene

Abstract

Polyolefins, semicrystalline polymers also known as thermoplastics, are highly desirable because of their material properties, low cost, and ease in processing. The flow and thermal history experienced during processing are known to affect dramatic changes in crystalline kinetics and morphology, dictating the final material properties of solidified products. However, the underlying physics that control crystalline orientation and kinetics is not well understood. To optimize processing conditions and maximize material performance, it is desirable to understand how the interplay of molecular character and flow conditions shape crystalline microstructure. In the last decade, advances in catalyst technology have produced well defined materials enabling the systematic study of molecular influences on flow-induced crystallization. We investigate 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 examination (polarized light microscopy, TEM) of the final morphology. Blending a well-characterized high molecular weight isotactic polypropylene into a iPP at various concentrations (c), we determined that blends with less than 1% of with Mw five times larger than the Mw of the base resin profoundly affected the crystallization kinetics and crystalline morphology of a sheared melt. Beyond unambiguously demonstrating the important role of in the formation of anisotropic crystallization under flow, this approach allowed us to be specific about the length that is meant by long chains and the concentration of these in the melt. Varying the concentration from below to above c* revealed that the effect of the involves cooperative interactions, evident in the non-linear relationship of the chain concentration, particularly as c approaches the chain-long chain overlap concentration. The greatly enhance the formation of threadlike precursors but only mildly enhance the formation of pointlike precursors. In studying a series of blends in which the Mw of the chain mode was varied, we found that increasing the Mw of the chain portion of a bimodal blend increased the tendency to form threadlike precursors to oriented crystallization. This was highlighted by a marked decrease in the threshold stress necessary to induce oriented crystalline growth and is related to the separation in time scales between the slowest relaxing and the average. Thus, the propagation of shish varies strongly with the separation in time scales between the slowest relaxing and the average. Below a threshold ratio of relaxation times (tau_L/tau_S ~ 100) addition of did not change the behavior from that of Base-PP itself. Our analysis of real-time rheo-optical and rheo-WAXD experiments combined with dependent information from a novel depth sectioning analysis technique uncovers several keys to understanding how anisotropic crystallization is induced by flow. Threads first form near the channel wall, where stress is highest, and grow in length with prolonged flow. After sufficient time, thread length per unit volume saturates, perhaps due to collisions with other threads or crystalline overgrowth from those threads. Prior to saturation, when crystalline overgrowth is negligible, the thread propagation appears to be linear with shearing time. The propagation of threads varies in a nonlinear manner with stress. Finally, we identify a promising set of conditions that can be used to measure the thread propagation velocity for this material if the appropriate length scale can be assigned by microscopy. We examined the effects of chain regularity on the formation of threadlike precursors, showing that addition of molecular level defects to the high end of the molecular weight distribution effectively raises the threshold stress and mitigates the formation of oriented precursors induced by flow. Our study included a model bimodal blend of isotactic and atactic polypropylene as well as large scale bimodal blends of isotactic polypropylene and a propylene-ethylene copolymer fit for pilot-scale production of nonwoven fabrics. It is noteworthy that the qualitative behavior observed in the melt-spinning process accords well with the trends evident in isothermal shear-induced crystallization. This has value in two respects. Scientifically, it is significant that idealized flow and thermal conditions may well reveal the physics relevant to polymer processing, which involves mixed shear and extension under non-isothermal conditions. Technologically, the ability to screen different resin compositions on a small scale can be used to optimize flow-induced crystallization characteristics prior to scale up.