Structure and Rheology of Molten Polymers

Dealy, John M.; Larson, Ronald G.

doi:10.3139/9783446412811.fm

Cited by 152 publications

(249 citation statements)

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“…20,21,[51][52][53] The effect of shear on the configuration of the molecules is often classified with the Deborah number (De). 51 With the shear rate γ , the disengagement time ( [54][55][56] (neglecting a factor of the order of unity). In an entangled polymer melt, Z .…”

Section: Resultsmentioning

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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Section: Resultsmentioning

confidence: 99%

Crystallization and Precursors during Fast Short-Term Shear

2009

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“…The tube constraint, on the other hand, acts along the whole chain; locally, it is thought that confinement is applied collectively by many overlapping surrounding chains, and not by specific pairwise interactions. Any gain or loss in the local pairwise interaction density does not necessarily affect the tube diameter or length, as it does the local entanglement density of slip-link models 19 (see p 552 in ref 20).…”

Section: Introductionmentioning

confidence: 99%

“…The real chain is represented by the corresponding PP, which is a series of strands of average molar mass M e connecting the links. The tube model provides a basis for working out analytically the linear and nonlinear viscoelasticty of polymer melts 9,[16][17][18][19][20] and networks 10,15,[21][22][23][24] at a molecular level. Slip link models 19,20,24 also allow for analytical 8,[11][12][13]15,25,26 or stochastic simulation 14,[27][28][29][30][31][32][33][34][35][36][37] treatments with a molecular representation, at the single or many-chain level.…”

Section: Introductionmentioning

confidence: 99%

Microscopic Description of Entanglements in Polyethylene Networks and Melts: Strong, Weak, Pairwise, and Collective Attributes

2012

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Abstract:We present atomistic molecular dynamics simulations of two Polyethylene systems where all entanglements are trapped: a perfect network, and a melt with grafted chain ends. We examine microscopically at what level topological constraints can be considered as a collective entanglement effect, as in tube model theories, or as certain pairwise uncrossability interactions, as in slip-link models.A pairwise parameter, which varies between these limiting cases, shows that, for the systems studied, the character of the entanglement environment is more pairwise than collective. We employ a novel methodology, which analyzes entanglement constraints into a complete set of pairwise interactions, similar to slip links. Entanglement confinement is assembled by a plethora of links, with a spectrum of confinement strengths, from strong to weak. The strength of interactions is quantified through a link 'persistence', which is the fraction of time for which the links are active. By weighting links according to their strength, we show that confinement is imposed mainly by the strong ones, and that the weak, trapped, uncrossability interactions cannot contribute to the low frequency modulus of an elastomer, or the plateau modulus of a melt. A selfconsistent scheme for mapping topological constraints to specific, strong binary links, according to a given entanglement density, is proposed and validated. Our results demonstrate that slip links can be viewed as the strongest pairwise interactions of a collective entanglement environment. The methodology developed provides a basis for bridging the gap between atomistic simulations and mesoscopic slip link models.

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“…Here, p CR−CLF [k] (x i ,t) is the survival probability of the initial tube segments at the position x i at time t defined for the CR-CLF process of the chain component k. For the short chain, the CRactivated CLF process is negligible, and thus we set p CR−CLF [1] (x i ,t) = 1. In contrast, for the long chain, this process should be important, and the probability for this chain is modeled in terms of the CR-CLF time (eq 12) as…”

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confidence: 99%

Dynamic Dilution Effect in Binary Blends of Linear Polymers with Well-Separated Molecular Weights

et al. 2014

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We investigate and model the viscoelastic properties of binary blends composed of linear chains. These systems are indeed very suitable to test the validity and the limit of the constraint release (CR) process and dynamic tube dilution and to determine the value of the dynamic dilution exponent α. We first focus on binary blends containing barely entangled short chains. In such a case, we show that the tension re-equilibration process previously observed [ van Ruymbeke van Ruymbeke Macromolecules2012452085] can be correctly described as a CR-activated contour length fluctuation (CLF) process, which takes place along the fully dilated tube and is governed by the intrinsic Rouse time of a long–long entanglement segment. In addition, reptation is considered to take place along the fully dilated tube. We also show that this CR-activated CLF process speeds up the relaxation of the long chains, which naturally leads to an effective dilution exponent α equal to 4/3, despite the fact that the modeling is based on α = 1. This result is in agreement with the experimental data. Then, we analyze the rheological behavior of binary blends containing entangled short chains. In such a case, we show that the CR-activated CLF also takes place, but with a delay time being necessary for the long chain to explore the dilated tube. This approach is tested for several binary blends, showing an improved quality of the predictions compared to previous tube modeling on the same blends. Indeed, by considering the chain motions on two different length scales, a larger fraction of the long chains relax at intermediate frequencies (through the tension re-equilibration) before their reptation, leading to an effective dilution exponent larger than 1 as determined from the low-frequency plateau modulus.

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Structure and Rheology of Molten Polymers

Cited by 152 publications

References 0 publications

Crystallization and Precursors during Fast Short-Term Shear

Crystallization and Precursors during Fast Short-Term Shear

Microscopic Description of Entanglements in Polyethylene Networks and Melts: Strong, Weak, Pairwise, and Collective Attributes

Dynamic Dilution Effect in Binary Blends of Linear Polymers with Well-Separated Molecular Weights

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