Effects of the degree of undercooling on flow induced crystallization in polymer melts

Coppola, Salvatore; Balzano, Luigi; Gioffredi, Emilia; Maffettone, Pier Luca; Grizzuti, Nino

doi:10.1016/j.polymer.2004.03.049

Cited by 84 publications

(80 citation statements)

References 27 publications

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“…Increasing the rate of deformation from zero (quiescent condition), a critical shear rate is observed after which effective flow-induced crystallization evolves (Coppola et al 2004). Since, very low shear rates cannot stimulate the melt efficiently, chain orientation does not occur extensively and the crystallization kinetics will be practically the same as that in quiescent crystallization.…”

Section: Flow-induced (Fic) Crystallization In Steady Shearmentioning

confidence: 99%

Flow-induced crystallization of high-density polyethylene: the effects of shear and uniaxial extension

Derakhshandeh

Hatzikiriakos

2011

Rheol Acta

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The effects of shear, uniaxial extension and temperature on the flow-induced crystallization of two different types of high-density polyethylene (a metallocene and a ZN-HDPE) are examined using rheometry. Shear and uniaxial extension experiments were performed at temperatures below and well above the peak melting point of the polyethylenes in order to characterize their flow-induced crystallization behavior at rates relevant to processing (elongational rates up to 30 s −1 and shear rates 1 to 1,000 s −1 depending on the application). Generally, strain and strain rate found to enhance crystallization in both shear and elongation. In particular, extensional flow was found to be a much stronger stimulus for polymer crystallization compared to shear. At temperatures well above the melting peak point (up to 25 • C), polymer crystallized under elongational flow, while there was no sign of crystallization under simple shear. A modified Kolmogorov crystallization model (Kolmogorov, Bull Akad Sci USSR, Class Sci, Math Nat 1:355-359, 1937) proposed by Tanner and Qi (Chem Eng Sci 64:4576-4579, 2009) was used to describe the crystallization kinetics under both shear and elongational flow at different temperatures.

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Section: Flow-induced (Fic) Crystallization In Steady Shearmentioning

confidence: 99%

Flow-induced crystallization of high-density polyethylene: the effects of shear and uniaxial extension

Derakhshandeh

Hatzikiriakos

2011

Rheol Acta

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“…[2][3][4][5][6] It is possible to control and predict the final morphologies and properties of semicrystalline polymers by controlling different shear flow parameters. [7][8][9][10][11][12][13][14][15] It is well known that isotactic polypropylene (iPP) exhibits pronounced polymorphic crystalline modifications, which are designated as a-form (a-iPP), b-form (b-iPP), and c-form (c-iPP). [16][17][18] In addition, three types of a spherulites and two types of b-spherulites might be formed, depending on different crystallization temperature's (T c 's); these include a I (radial spherulite), a II (negative radial spherulite), a m (mixed radial), and b III (negative radial spherulite), and b IV (negative ringed spherulite).…”

Section: Introductionmentioning

confidence: 99%

Crystal morphology and structure of the β‐form of isotactic polypropylene under supercooled extrusion

Zhang

Shen

2011

J of Applied Polymer Sci

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A supercooled melt of isotactic polypropylene (iPP) was extruded through a capillary die. Polarized light microscopy (PLM), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC) were used to investigate the effects of the relatively weak wall shear stress (r w ), extrusion temperature (T e ), and crystallization temperature (T c ) on the structure and morphology of b-form isotactic polypropylene (b-iPP). b-cylindrites crystals could be observed by PLM in the extruded specimen even at a lower r w 's (0.020 MPa), and the b-iPP content increased with decreasing T e . Under a given T e of 150 C, the increase in r w positively influenced the b-iPP content. The DSC and WAXD results indicate that the total crystallinity and b-iPP content increased when T c was set from 105 to 125 C; the other experimental parameters were kept on the same level. Although T c was above 125 C, the b-iPP content obviously decreased, and the total crystallinity continued to increase. On the basis of the influences of r w , T e , and T c on the b-iPP crystal morphology and structure, a modified model is proposed to explain the growing of shearinduced b-iPP nucleation.

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“…Both temperature and flow history can alter the crystallization behavior. Flow can increase the crystallization rate by decades [10][11][12][13][14][15] and, in addition, can dramatically change the crystalline morphology. [16][17][18][19][20][21] In quiescent or quasi-quiescent conditions, morphology is dominated by spherulites, three-dimensional assemblies of randomly oriented folded chain lamellae.…”

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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Effects of the degree of undercooling on flow induced crystallization in polymer melts

Cited by 84 publications

References 27 publications

Flow-induced crystallization of high-density polyethylene: the effects of shear and uniaxial extension

Flow-induced crystallization of high-density polyethylene: the effects of shear and uniaxial extension

Crystal morphology and structure of the β‐form of isotactic polypropylene under supercooled extrusion

Crystallization and Precursors during Fast Short-Term Shear

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