Mechanisms of flow induced crystallization

McHugh, A. J.

doi:10.1002/pen.760220104

Cited by 72 publications

(52 citation statements)

References 95 publications

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“…9,10 However, despite the large amount of experimental works investigating the flow induced crystallization (FIC) phenomena, an effective modeling of the FIC kinetics was not achieved. Starting from early work by McHugh 10 and until more recent results, the proposed models are generally difficult to apply to practical cases 11 or require a large number of adjustable parameters.…”

Section: Introductionmentioning

confidence: 99%

Orientation and crystallinity in film casting of polypropylene

Lamberti

Brucato

Titomanlio

2002

J of Applied Polymer Sci

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ABSTRACT:In the cast film process a polymer melt is extruded through a slit die, stretched in air, and cooled on a chill roll. During the path in air the melt cools while being stretched. Film casting experiments were carried out with an isotactic polypropylene resin. The temperature and width distributions were measured along the draw direction. Further, the crystallinity and Hermans orientation factor were measured on the final film. The process was described by a simple thermomechanical model derived elsewhere. The evolution of the molecular orientation parameters was calculated on the basis of a dumbbell model coupled with velocity and temperature distributions provided by the thermomechanical model. The experimental crystalline orientations of the final films collapsed into a single step-shaped curve (from low to high orientation) if plotted versus the stress calculated by the model at the frozen line. The experimental values of the crystallinity and Hermans orientation factors are discussed on the basis of predictions of the dumbbell model for melt orientation at the frozen line and the crystallinity data obtained in quiescent conditions under the same cooling rate.

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Section: Introductionmentioning

confidence: 99%

Orientation and crystallinity in film casting of polypropylene

Lamberti

Brucato

Titomanlio

2002

J of Applied Polymer Sci

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“…22 For strong enough flows, spherulites are replaced by shish-kebabs, composite crystallites with an extended chain fibrillar core (shish) dressed with disklike folded chain lamellae (kebabs). [23][24][25][26][27][28][29] The origin of the shishkebab morphology is the topic of a long-standing discussion 28,[30][31][32][33][34] that begun around the 1960s. It is known that stretch of the longest (highest molecular weight) molecules in a undercooled melt promotes shish-kebab formation, but the events in the early stages of the process are not yet fully clarified and certainly not quantified, an important issue for modeling.…”

Section: Introductionmentioning

confidence: 99%

Crystallization and Precursors during Fast Short-Term Shear

2009

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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.

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“…Shish-kebab crystallites, a special kind of polymer assembly, are frequently observed under these circumstances. [3][4][5][6][7] Such crystallites exhibit a heterogeneous structure: a stack of foldedchain lamellae that form around a central fiber. Assemblies of this kind were first observed as row structures in crystallization of polyethylene from the melt under stress.…”

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Simulation of Shish-Kebab Crystallite Induced by a Single Prealigned Macromolecule

2002

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Crystal nucleation is an activated process. In a supersaturated solution, small crystallites may dissolve again. Crystals can only be formed when a (rare) fluctuation leads to the formation of a crystallite with a size that exceeds a threshold. The probability to form such a "critical" crystal nucleus is determined by its excess free energy. For a simple molecular crystal, 1 the free energy change for forming a small crystallite from the liquid is approximately given by where n is the number of particles in the nucleus and ∆µ is the difference between the chemical potentials of the particles in the (metastable) liquid phase and in the (stable) crystal phase. The surface area of the crystallite is denoted by A, and the solid-liquid interfacial free energy is denoted by γ. The free energy of a crystal nucleus contains two competing contributions: a (negative) bulk term favoring the formation of crystallites is counteracted by a (positive) surface term. For small crystallites, the surface term dominates, but for larger crystallites the bulk term becomes more important.In the nucleation of chainlike polymers, we must consider the anisotropy introduced by the shape of the molecules. Assuming that a bundle of chain stems form a cylindrical polymer crystallite with a radius r and a thickness l, the free energy change for forming such a crystallite becomes where n ) πr 2 l is the total number of units in the crystallite; γ e and γ l are the surface free energies corresponding to the stem ends and the lateral surface, respectively. The most favorable size of critical nucleus can be calculated from ∂∆F(n)/∂r ) ∂∆F(n)/∂l ) 0 as l crit ) 4γ e /|∆µ| and r crit ) 2γ l /|∆µ|. Note that as the lateral surface free energy can be smaller by 1 order of magnitude than that of the stem-end surface of polymer crystallites, the critical thickness should be larger by 1 order of magnitude than the critical radius of nucleus. In other words, the most favorable shape of critical nuclei will be fiberlike. 2 This shape requires a large degree of chain alignment, which is a very rare event in thermal fluctuations for an isotropic liquid, especially at high temperatures. As the supercooling is increased, the critical nucleus becomes smaller and less elongated. Therefore, polymer nucleation usually occurs via chain folding and requires a much higher degree of supercooling than is needed to induce crystallization of simple molecular crystals.There are several methods to increase the rate of polymer nucleation. All methods rely in one way or another on reducing the free energy barrier of crystal formation. The simplest method would be to decrease the temperature T and thereby increase the thermodynamic driving force for crystallization. However, from a practical point of view this is often not a viable optionsfor instance, the polymer liquid may vitrify on cooling and results in poor crystallinity. A more practical method is to facilitate the incorporation of polymer chains in a crystallite by prealigning and stretching them in a flow field (ext...

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Mechanisms of flow induced crystallization

Cited by 72 publications

References 95 publications

Orientation and crystallinity in film casting of polypropylene

Orientation and crystallinity in film casting of polypropylene

Crystallization and Precursors during Fast Short-Term Shear

Simulation of Shish-Kebab Crystallite Induced by a Single Prealigned Macromolecule

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