Regime transitions in fractions of isotactic polypropylene

Cheng, Stephen Z. D.; Janimak, James J.; Zhang, Anqiu; Cheng, H. N.

doi:10.1021/ma00203a051

Cited by 139 publications

(128 citation statements)

References 15 publications

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“…Two main observations can be extracted from the experimental results: the growth rates of the LMW material are practically the same as the bimodal, whereas the HMW grade shows slightly slower growth in the investigated temperature range. The influence of molecular weight on the maximum growth rates has been investigated in literature [54][55][56], and a decrease with increasing molecular weight is expected, in line with our experimental observations. In this work we use an empirical relation to describe the growth rate of each crystal phase (α β , and γ), which, similarly to the widely used Lauritzen-Hoffman theory [57], translates to a bell-shaped curve to describe the temperature dependence of the growth rate:…”

Section: Crystallization Model Frameworksupporting

confidence: 89%

Application of a multi-phase multi-morphology crystallization model to isotactic polypropylenes with different molecular weight distributions

Troisi

Arntz

Roozemond

et al. 2017

European Polymer Journal

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A B S T R A C TFlow-induced crystallization at elevated pressure of a set of metallocene isotactic polypropylenes (iPP) possessing different molecular weight distributions is studied using extended dilatometry experiments in an apparatus able to apply elevated pressure (up to 1200 bar) and strong shear flow (shear rates up to 180 s −1 ). The effect of flow on the crystallization temperature was quantified and the samples were analyzed using X-ray diffraction to measure the relative amounts of different crystal phases.The experimental results were used to test a flow induced crystallization model framework recently developed in our group which can describe the complex crystallization behavior of iPP and includes formation of multiple morphologies (spherulites, shish-kebab structure with lamellar branching) and crystal phases (α β , and γ ). Almost all model parameters were left unchanged, except for the temperature description of the quiescent crystal growth rate and nucleation density (experimentally measured using optical microscopy) and an extra parameter which takes into account the fraction of high molecular weight for the creation of flow-induced nuclei.The model describes rather good the experimentally observed trends for what regards crystallization temperature and amount of α and γ -phase but substantial discrepancies were found in the amount of β-phase formed. This could be related to the presence of 2,1 insertion errors in metallocene samples which could inhibit the formation of this crystal phase.

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Section: Crystallization Model Frameworksupporting

confidence: 89%

Application of a multi-phase multi-morphology crystallization model to isotactic polypropylenes with different molecular weight distributions

Troisi

Arntz

Roozemond

et al. 2017

European Polymer Journal

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“…An increase in crystallinity can lead to an increase in the lamellar thickness which leads to higher storage modulus and stiffiness values [2][3][4] . The effects of molecular weight [5][6][7][8] , molecular weight distribution [9] and tacticity [10][11][12][13] on the crystallization have been investigated by several authors. Cheng et al [6] showed that the linear growth rate of crystals decreases with the increase of molecular weight, but the overall crystallization rate might increase because an increasing number of intramolecular folded-chain nuclei could result in a higher nucleation density [7] .…”

Section: Introductionmentioning

confidence: 99%

“…The effects of molecular weight [5][6][7][8] , molecular weight distribution [9] and tacticity [10][11][12][13] on the crystallization have been investigated by several authors. Cheng et al [6] showed that the linear growth rate of crystals decreases with the increase of molecular weight, but the overall crystallization rate might increase because an increasing number of intramolecular folded-chain nuclei could result in a higher nucleation density [7] . For samples with similar molecular weights and different tacticities the linear crystal growth rate might increase by three orders of magnitude when the isotacticity (mmmm %) of isotactic polypropylene increases from 78.7 to 98.8% [11,14] .…”

Section: Introductionmentioning

confidence: 99%

Molecular weight and tacticity effect on morphological and mechanical properties of Ziegler–Natta catalyzed isotactic polypropylenes

2015

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SbstractThe morphological and mechanical properties of Ziegler-Natta catalyzed isotactic polypropylenes as influenced by the molecular weight and tacticity was investigated. Polypropylene samples were injection moulded into standard disks with a HAAKE MiniJet II injection moulder at 190 °C and 200 bar for morphological and mechanical tests. The morphological and mechanical properties of specimens were investigated by means of optical microscope (OM), scanning electron microscopy (SEM), microhardness (MH) and dynamic mechanical analysis (DMA). The samples exhibited a typical α-modification spherulite structure of isotactic polypropylenes crystallized from the melt. It was found that the most important factor affecting the structure and properties of these polymers is the isotacticity content. A clear molecular weight effect was also found for samples with low molecular weights. The microhardness and storage modulus values increased as crystallinity did. Accordingly, isotacticity degree is considered as the main parameter affecting the crystallinity of samples.

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“…For iPP, a visible discontinuity, or more exactly an inflexion point of the growth rate curve is observed around 140 C. 24 The temperature at the discontinuity and the growth rate are strongly dependent on molecular weight 25 and isotacticity 26 of iPP. The discontinuity in the growth rate has been interpreted as a II-III regime transition.…”

mentioning

confidence: 99%

“…The discontinuity in the growth rate has been interpreted as a II-III regime transition. [24][25][26][27][28] However, the crystal growth rate is influenced by several factors, such as crystal phase transition, change in radial growth direction, tacticity and molecular weight. Among these factors, a change of the crystal phase has a major impact on the temperature dependence of the crystal growth rate.…”

mentioning

confidence: 99%

Temperature Dependence of Crystal Growth Rate for α and β Forms of Isotactic Polypropylene

Nakamura

Shimizu

Umemoto

et al. 2008

Polym J

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The crystal growth rate of the 1 , 2 and forms of isotactic polypropylene (iPP) and their morphological changes were studied in a wide range of crystallization temperatures. The temperature dependence of the crystal growth rate of the 1 form showed a bell-shaped curve with the maximum growth rate (G max ) at 70 C. The crystal growth rate of the form induced by calcium pimelate also showed a bell-shaped curve with G max at 79 C. Two crossover points on the growth rate curve from the 1 form to the form or vice versa were observed at 90 C and 133 C, respectively. Tear-drop shaped spherulites were observed at temperatures between the two crossover points. Electron diffraction patterns showed the high ordered structure of the 2 form for thin films crystallized at 140 C. The 2 fraction became detectable in X-ray diffraction pattern for temperatures above 110 C. The 2 fraction increased with crystallization temperature and saturated to 100% at about 140 C. In the temperature region from 110 C to 140 C, several crystal growth rates were found at the same crystallization temperature. The growth rate variations could be associated with the co-crystallization of the 1 and 2 forms. Above 140 C, the growth rate variations disappeared, since the fraction of the 2 reached to 100%. The extrapolated G max of the 2 showed at 72 C.KEY WORDS: Isotactic Polypropylene / -Form / -Form / Crystal Growth Rate / Crossover / Isotactic polypropylene (iPP) is one of the most widely used commercial polymeric materials and has been extensively investigated for it academic and industrial interest. iPP displays different structural polymorphism, such as form (monoclinic), form (trigonal), form (orthorhombic) and smectic form, which all consist of 3 1 helical conformations with a 0.65 nm chain axis periodicity. The form is the most common crystal structure, which is produced by crystallization from the melt or solution.1-5 The form can be obtained from temperature gradient crystallization, 6 crystallization under molecular orientation 7 or can be induced by selective -nucleating agent. 8-10The crystal structure of the form was first determined by Natta and Corradini. 11 Mencik 12 and Hikosaka and Seto 13showed that the structure has two limiting modifications named 1 and 2 forms. Extensive works on these crystalline forms have been also reported. 14-16 The 1 form space group (C2/c) is the limit-disordered structure and the 2 form (P2 1 /c) is the limit-ordered structure. They are characterized by either regular alternation of the up and down orientations of the methyl group along the chain ( 2 ) or random orientation. A transition from the 1 form to the 2 form is observed by annealing at a higher temperature. [17][18][19][20][21][22] The crystal structure of the form has been analyzed by Meille et al. 23 and refined by D. L. Dorset et al. 9Many studies about the crystallization behavior and morphology of iPP have been reported. When iPP was crystallized from the melt in a wide temperature range, Keith and Padden found four distinct spherulites,...

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Regime transitions in fractions of isotactic polypropylene

Cited by 139 publications

References 15 publications

Application of a multi-phase multi-morphology crystallization model to isotactic polypropylenes with different molecular weight distributions

Application of a multi-phase multi-morphology crystallization model to isotactic polypropylenes with different molecular weight distributions

Molecular weight and tacticity effect on morphological and mechanical properties of Ziegler–Natta catalyzed isotactic polypropylenes

Temperature Dependence of Crystal Growth Rate for α and β Forms of Isotactic Polypropylene

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