The Specific Work of Flow as a Criterion for Orientation in Polymer Crystallization

Mykhaylyk, Oleksandr O.; Chambon, Pierre; Graham, Richard S.; Fairclough, J. Patrick A.; Olmsted, Peter D.; Ryan, Anthony J.

doi:10.1021/ma702603v

Cited by 191 publications

(315 citation statements)

References 32 publications

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“…The route to crystallization during flow is generally accepted to be via the formation of 'shishkebab' morphology [6][7][8][9]. Proposed mechanisms and kinetics for the formation of shish-kebab morphology usually relate to data from shear flow conditions using custom made shearing devices [10][11][12][13][14][15][16]. However, during extrusion, the polymer melt exits the die head and begins to cool rapidly, and at the same time it is uniaxially elongated under constant force, between the die head and take-up device.…”

Section: Introductionmentioning

confidence: 99%

Effect of processing parameters on the morphology development during extrusion of polyethylene tape: An in-line small-angle X-ray scattering (SAXS) study

Heeley

Gough

Hughes

et al. 2013

Polymer

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Ray scattering (SAXS). AbstractThe in-line development of crystalline morphology and orientation during melt extrusion of low density polyethylene (LDPE) tape at nil and low haul-off speeds has been investigated using Small-Angle X-Ray Scattering (SAXS). The processing parameters, namely haul-off speed and distance down the tape-line have been varied and the resulting crystalline morphology is described from detailed analysis of the SAXS data. Increasing haul-off speed increased orientation in the polymer tape and the resulting morphology could be described in terms of regular lamellar stacking perpendicular to the elongation direction. In contrast, under nil haul-off conditions the tape still showed some orientation down the tape-line, but a shish-kebab structure prevails. The final lamellae thickness (~50 Å) and bulk crystallinity (~20%), were low for all processing conditions investigated, which is attributed to the significant shortchain branching in the polymer acting as point defects limiting lamellae crystal growth.2

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Section: Introductionmentioning

confidence: 99%

Effect of processing parameters on the morphology development during extrusion of polyethylene tape: An in-line small-angle X-ray scattering (SAXS) study

Heeley

Gough

Hughes

et al. 2013

Polymer

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“…For high deformation rates, the effect of flow is no longer limited to an increase in nucleation sites and can cause the appearance of oriented structures. The transition is quite sharp, and the critical flow conditions for the appearance of oriented structures has received attention from a number of authors [18,35,[43][44][45][46][47]. shish kebab / parent daughter Fig.…”

Section: State Of the Artmentioning

confidence: 99%

“…Reproduced with permission of the copyright owner [5] For such experiments, a shear cell is combined with in-situ X-ray diffraction (XRD) [4,[48][49][50][51][52][53], birefringence [18,23,43], or light scattering [46], or examined ex-situ with microscopy [4]. The critical flow conditions are usually expressed in terms of stress [4,44], mechanical work [35,45], or shear rate for a fixed strain [43]. It has been shown in a number of papers that long chains play a catalytic role in the formation of shish-kebabs [23,41,47,49,54].…”

Section: State Of the Artmentioning

confidence: 99%

Modeling Flow-Induced Crystallization

Roozemond

Drongelen

Peters

2016

Polymer Crystallization II

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A numerical model is presented that describes all aspects of flow-induced crystallization of isotactic polypropylene at high shear rates and elevated pressures. It incorporates nonlinear viscoelasticity, including viscosity change as a result of formation of oriented fibrillar crystals (shish), compressibility, and nonisothermal process conditions caused by shear heating and heat release as a result of crystallization. In the first part of this chapter, the model is validated with experimental data obtained in a channel flow geometry. Quantitative agreement between experimental results and the numerical model is observed in terms of pressure drop, apparent crystallinity, parent/daughter ratio, Hermans' orientation, and shear layer thickness. In the second part, the focus is on flow-induced crystallization of isotactic polypropylene at elevated pressures, resulting in multiple crystal phases and morphologies. All parameters but one are fixed a priori from the first part of the chapter. One additional parameter, determining the portion of β-crystal spherulites nucleated by flow, is introduced. By doing so, an accurate description of the fraction of β-phase crystals is obtained. The model accurately captures experimental data for fractions of all crystal phases over a wide range of flow conditions (shear rates from 0 to 200 s À1 , pressures from 100 to 1,200 bar, shear temperatures from 130 C to 180 C). Moreover, it is shown that, for high shear rates and pressures, the measured γ-phase fractions can only be matched if γ-crystals can nucleate directly on shish.

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“…2 Although a number of studies have focused on the issue that the _ c min relates to a specific relaxation time, the reptation time s d or the Rouse time s R , [6][7][8] it was recently confirmed experimentally in two studies that the _ c min correlates the inverse of s R . 9,10 Thus, stretching of the molecules, and not just orientation of the primitive path of the molecules, is required for the formation of oriented morphology under flow conditions. Consequently, the creation of the shish nuclei is a key element in forming oriented morphologies.…”

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confidence: 99%

“…9 Thus, the amount of mechanical work performed by flow on the polymer melt controls the formation of oriented shish nuclei via the shear-induced nucleation of stretched molecules. In this respect, it needs to be pointed out that neither a critical stress 15 nor a critical strain 16 should be used to describe the onset of oriented crystal morphology as these parameters relate to the later stage-transformation of shish nuclei rows into a fibrillar structure (shish).…”

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confidence: 99%

Using multimodal blends to elucidate the mechanism of flow‐induced crystallization in polymers

Okura

Chambon

Mykhaylyk

et al. 2011

J Polym Sci B Polym Phys

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The effect of polydispersity on the formation of flow-induced, oriented morphology in polyolefins is investigated by polarized light imaging and small angle X-ray scattering. A torsional shear flow was applied at different temperatures to model polyethylene blends (bimodal and trimodal hydrogenated polybutadienes) comprising of two kinds of long chains with different molecular weight (1080 and 1770 kDa) in a matrix of short chains (18 KDa), and the results were compared to those of polydisperse materials. While a single boundary associated with the threshold flow conditions for the onset of oriented morphology is observed in the bimodal blends, two boundaries corresponding to the orientation of the longest chains (1770 kDa) and next longest chains (1080 kDa) are detected in the trimodal blends. The results obtained, herein, are extended by inference to polydisperse polymers. It is demonstrated that the shear rate dependence of the critical specific work parameter for the onset of oriented morphology in polydisperse polymers is dictated by the molecular weight distribution and that the longest chains mainly control the process with some contribution from shorter chains involved in the formation of flow-induced precursors at higher flow rates. KEYWORDS: crystallization; orientation; SAXS; viscoelastic properties INTRODUCTION Polyolefins are the most widely used polymer nowadays due to their excellent cost-benefit performance. The typical processing methods for semicrystalline polyolefins take a melt and shape it by means of either an extrusion or molding technique and the shape stabilization process is crystallization by cooling.1 The flow conditions in the extruder, and die or mold system have a profound effect on the morphology of the crystalline material and introduce different forms of crystals from isotropic spherulites to highly oriented ''shish-kebab structure'' in the polyolefin with the significant effect on materials through their mechanical, thermal, and optical properties.2 As processing conditions (temperature, pressure, flow rate, and duration of flow) can be adjusted and are relatively easy to control, the relationship between this group of parameters and characteristics of the polymer (chemical composition and molecular weight distribution) could significantly improve the control of structural morphologies and widen an understanding of flowinduced phenomena in polymeric materials.

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The Specific Work of Flow as a Criterion for Orientation in Polymer Crystallization

Cited by 191 publications

References 32 publications

Effect of processing parameters on the morphology development during extrusion of polyethylene tape: An in-line small-angle X-ray scattering (SAXS) study

Effect of processing parameters on the morphology development during extrusion of polyethylene tape: An in-line small-angle X-ray scattering (SAXS) study

Modeling Flow-Induced Crystallization

Using multimodal blends to elucidate the mechanism of flow‐induced crystallization in polymers

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