Crystallization and Precursors during Fast Short-Term Shear

Abstract

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.

Influence of Nanoparticles on the Rheological Behaviour and Initial Stages of Crystal Growth in Linear Polyethylene

“…The growth of shish-kebab structures is enhanced in this flow protocol, an observation in agreement with earlier reported findings. [26] Similar to other studies our results also suggest the role of a critical shear rate below which the streak-like scattering in the equator doesn't occur. If it does happen it is beyond the detection limit of the applied experimental conditions and the presence could be realized by the appearance of the intensity along the meridian during formation of the kebabs.…”

“…They claimed the presence of oriented nuclei under shear in the melt because of the distribution of relaxation time. Balzano et al [26] recently studied precursor formation in the early stages of the flow induced crystallization of iPP from the melt during and immediately after the application of strong shear. They reported the nucleation of crystalline structures already during shear, which further directed the development in structure and morphology.…”

Beyond a Model of Polymer Processing-Triggered Shear: Reconciling Shish-Kebab Formation and Control of Chain Degradation in Sheared Poly(l-lactic acid)

“…Since the early 1960s, formation mechanisms for shear-induced shish-kebabs have been widely illustrated in an assortment of polyolefins, which has yielded seemingly controversial suppositions on the critical parameters of the shear flow targeted for shish-kebab formation. 37−41 Three distinguished hypotheses have been proposed to describe the critical flow parameters for shish formation, i.e., critical shear rate larger than the Rouse time to sufficiently stretch chains, 42 critical strain to fully extend chain segments, 43 and critical specific work of flow that integrates the multiple factors of time, flow geometry, and molecular weight distribution. 44,45 The fundamental understanding has been totally based on the polyolefin models.…”

Flow‐Induced Morphology of iPP Solidified in a Shear Device

“…In this way, little or no material changes take place during flow. Therefore, the flow conditions can be characterized assuming a melt‐like rheological behavior 11. We find that shear rate, time, and temperature affect the position of the boundary between skin and core and the distributions of spherulite dimensions inside the core layer.…”