We make use of a specially synthesized linear high density polyethylene with a bimodal molecular weight distribution (MWD) to demonstrate that it is possible to produce a suspension of extended-chain (shish) crystals only. Such a suspension can be generated at high temperatures, above but close to the equilibrium melting temperature of the unconstrained extended-chain crystals (T(m)(0)=141.2 degrees C) and requires stretch of the longest chains of the MWD. After the application of a shear flow of 120 s(-1) for 1 s at 142 degrees C, x-ray scattering suggests the presence of a large number of metastable needlelike precursors with limited or no crystallinity. Precursors that are too small dissolve on a timescale that correlates perfectly with the reptation time of the longest polymer molecules. Whereas, precursors that exceed a critical size crystallize forming extended-chain shishes.

The dissolution of flow-induced nucleation precursors in isotactic polypropylene is investigated indirectly by means of in situ rheo-SAXS measurements. The progress of crystallization and the evolution of crystal orientation are recorded in isothermal conditions after a controlled shear step followed by an annealing step of different duration at various melt temperatures. The results confirm that the survival time of shear-induced nucleation precursors is extremely large compared to typical rheological relaxation times and it is longer for the precursors originated at higher shear rate. Most important, we show that the effect of flow on the development of oriented morphologies is lost much earlier than that on the overall crystallization kinetics. A schematic model for precursors' dissolution involving gradual transformation from row into point-like nuclei is proposed.

This paper deals with structural and morphological developments during flow-induced crystallization of molten isotactic polypropylene (iPP). Several authors have invoked the formation of precursors in the early stages of this process. However, it is not clear whether these precursors can be generated and can crystallize already during flow. We address this issue using X-ray scattering (SAXS and WAXD) with a high image capturing rate during and immediately after a strong shear pulse to the undercooled melt. Eventually, we provide the first in situ evidence of formation of flow-induced precursors (FIPs) of crystallization generated applying shear to a fast crystallizing melt of flexible macromolecules, like iPP. Moreover, it is shown that a rheological classification can be used to define the flow conditions promoting FIPs formation. In fact, when molecular stretch is achieved, we found that shear rate is the parameter dominating the formation of structures during shear. When the shear rate is high enough, crystals with a high degree of orientation are formed during a brief shear pulse. Whereas, for low shear rates, crystalline structures do not develop during a brief shear pulse. However, the equatorial streak of intensity in SAXS points to the formation of high density domains with fibrillar morphology. These dense and noncrystalline scatterers are metastable precursors of crystallization. After cessation of flow, they nucleate and assist the radial growth of stacks of lamellae. Eventually, this sequence of events leads to the well-known shishkebab morphology.