Monodisperse macromolecules – A stepping stone to understanding industrial polymers

Mykhaylyk, Oleksandr O.; Fernyhough, Christine M.; Okura, Masayuki; Fairclough, J. Patrick A.; Ryan, Anthony J.; Graham, Richard S.

doi:10.1016/j.eurpolymj.2010.09.021

Cited by 43 publications

(42 citation statements)

References 72 publications

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“…Little or no effect was observed between extension rates above the critical extension rate. This observation is not completely unexpected because it has been shown in shear flow that for monodisperse polymers, the formation of shish-kebab morphologies are only possible for flows whereγ > 1/λ R (Mykhaylyk et al 2011;van Meerveld et al 2004) and all of the experiments here are for 1/λ d <γ < 1/λ R . However, for highly polydisperse systems like the poly-1-butylene melts studied by Chellamuthu et al (2011), shish-kebab morphologies can be generated if the specific work imposed by the shear or extensional flow is large enough to adequately deform the higher molecular weight fractions of the melt thereby generating the highly aligned shish nuclei on which the lower molecular weight fraction of the polymer can crystallize as kebabs (Mykhaylyk et al 2010).…”

Section: Crystallization Measurementssupporting

confidence: 49%

“…However, it is not until Wi > λ d /λ R = 3Z that the polymer chains within their tubes of entanglements are extended and stretched. It has been shown in shear flow that for monodisperse polymers, the formation of shishkebab morphologies are only possible for flows wherė γ > 1/λ R (Mykhaylyk et al 2011;van Meerveld et al 2004). However, for polydisperse systems, shish-kebab morphologies can be generated if the specific work imposed by the flow is large enough to deform the higher molecular weight fractions of the melt (Mykhaylyk et al 2010).…”

Section: Introductionmentioning

confidence: 98%

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Extensional-flow-induced crystallization of isotactic polypropylene

2011

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A filament stretching extensional rheometer with a custom-built oven was used to investigate the effect of uniaxial flow on the crystallization of polypropylene. Prior to stretching, samples were heated to a temperature well above the melt temperature to erase their thermal and mechanical histories and the Janeschitz-Kriegl protocol was applied. The samples were stretched at extension rates in the range of 0.01 s −1 ≤ε ≤ 0.75 s −1 to a final strain of ε = 3.0. After stretching, the samples were allowed to crystallize isothermally. Differential scanning calorimetry was applied to the crystallized samples to measure the degree of crystallinity. The results showed that a minimum extension rate is required for an increase in percent crystallization to occur and that there is an extension rate for which percent crystallization is maximized. No increase in crystallization was observed for extension rates below a critical extension rate corresponding to a Weissenberg number of approximately Wi = 1. Below this Weissenberg number, the flow is not strong enough to align the contour path of the polymer chains within the melt and as a result there is no change in the final percent crystallization from the quiescent state. Beyond this critical extension rate, the percent crystallization was observed to increase to a maximum, which was 18% greater than the quiescent case, before decaying again at higher extension rates. The increase in crystallinity is likely due to flow-induced orientation and alignment of contour path of the polymer chains in the flow direction. Polarized light microscopy verified an increase in number of spherulites and a decrease in spherulite size with increasing extension rate. In addition, small angle X-ray scattering showed a 7% decrease in inter-lamellar spacing at the transition to flowinduced crystallization. Although an increase in strain resulted in a slight increase in percent crystallization, no significant trends were observed. Crystallization kinetics were examined as a function of extension rate by observing the time required for molten samples to crystallize under uniaxial flow. The crystallization time was defined as the time at which a sudden increase in the transient force measurement was observed. The crystallization time was found to decrease as one over the extension rate, even for extension rates where no increase in percent crystallization was observed. As a result, the onset of extensional-flow-induced crystallization was found to occur at a constant value of strain equal to ε c = 5.8.

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Section: Crystallization Measurementssupporting

confidence: 49%

Section: Introductionmentioning

confidence: 98%

Extensional-flow-induced crystallization of isotactic polypropylene

2011

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“…It was observed that the w c required for the FIN increases with the temperature, especially in a range of temperatures close to, or above, the nominal melting point (the melting point of spherulites) [11]. This kind of behavior is predictable as the higher the processing temperature, the larger the critical size of the nucleus and, therefore, the more specific work is required to stabilize this nucleus.…”

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confidence: 96%

“…Recent advances in a combinatorial technique for studying FIC of polymers [8,9] combined with the results obtained in the previous studies have shown that FIC can be represented as a four-stage process [10,11], where each stage is associated with particular flow conditions. The process is initiated by (i) stretching of molecules at flow rates _ > _ LC min ¼ À1 R (where _ LC min is defined by Rouse time, R , of the longest chains, LC, in the polymer ensemble) [9,12,13], followed by (ii) the formation of persistent nuclei and (iii) their simultaneous alignment into rows in the melt occurring above a threshold of critical specific work (energy density) performed by the flow, w c , [9,14] and completed by the (iv) aggregation of the nuclei into fibrills (shishes) taking place above the critical strain, c [15][16][17].…”

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confidence: 97%

Effect of Matrix Polymer on Flow-Induced Nucleation in Polymer Blends

2013

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Long-chain molecules in polymers are responsible for flow-induced nucleation (FIN). The role of a short-chain matrix in FIN was investigated using bidisperse blends of polyethylenelike high molecular weight long chains in short-chain polymer matrices. It was found that the critical specific work, w(c), measured for the onset of FIN in the blends has a power law dependence on the molecular weight of the matrix polymer with w(c) proportional M(w)(2.59 ± 0.27) indicating that the matrix can affect the stretching of the long chains and limit their ability to act as nucleation sites for flow-induced crystallization.

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“…2 The formation of shish on its own is a multistage process, [3][4][5] which could be described as stretching of the long molecules present in polymer ensemble followed by the formation of stable point nuclei (shish nuclei) under the flow and, finally, alignment of the shish nuclei into rows transforming into fibrillar structure (shish). It is generally considered that the shear rate needs to surpass a critical value, _ c min , for the shish nuclei formation.…”

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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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Monodisperse macromolecules – A stepping stone to understanding industrial polymers

Cited by 43 publications

References 72 publications

Extensional-flow-induced crystallization of isotactic polypropylene

Extensional-flow-induced crystallization of isotactic polypropylene

Effect of Matrix Polymer on Flow-Induced Nucleation in Polymer Blends

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

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