Transient Elongational Viscosities and Drawability of Polymer Melts

Laun, H. M.; Schuch, Horst

doi:10.1122/1.550058

Cited by 317 publications

(149 citation statements)

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“…For higher deformation rates, the extension viscosity thins again as the steady-state extensional stress now grows only weakly. The qualitative similarity with extensional data on LDPE [11,19] is remarkable, and especially significant in occurring for both uniaxial and planar extension.…”

Section: Ds C Dtsupporting

confidence: 52%

“…Even the very general integral-type equation [8], containing arbitrary functions of the strain invariants, cannot combine the observed strain hardening in both uniaxial [8] and planar [10] extension together with the softening in shear. Such equations therefore cannot consistently account for the special behavior which occurs in complex flows of LDPE, when implemented in non-Newtonian flow solvers [11]. In particular, linear and branched melts of identical viscosity and terminal relaxation times exhibit very different flow fields in a contraction: Linear polymers mimic Newtonian fluids, while branched polymers set up a large rotating vortex in the corner of the contraction [12,13].…”

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confidence: 99%

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Theoretical Molecular Rheology of Branched Polymers in Simple and Complex Flows: The Pom-Pom Model

et al. 1997

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The nonlinear rheological constitutive equation of a class of multiply branched polymers is derived using the tube model. The molecular architecture may be thought of as two q-arm stars connected by a polymeric "crossbar." The dynamics lead to a novel integrodifferential equation which exhibits extreme strain hardening in extension and strain softening in shear. Calculations of flow through a contraction predict that the degree of long-chain branching controls the growth of corner vortices, in agreement with experiments on commercial branched polymers. [S0031-9007(97)

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Section: Ds C Dtsupporting

confidence: 52%

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confidence: 99%

Theoretical Molecular Rheology of Branched Polymers in Simple and Complex Flows: The Pom-Pom Model

et al. 1997

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“…The emergence in the late 1970s of devices that measured rheological behaviour in extensional flows in the melt, revealed that branched polymers show high elongational viscosity and a characteristic strain-hardening behaviour, which is not common in linear polymers [33][34][35][36][37][38][39].…”

Section: Conventional Polymersmentioning

confidence: 99%

Effect of long chain branching on linear-viscoelastic melt properties of polyolefins

et al. 2002

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Abstract:The aim of this review is to provide evidence that rheological testing is a potent tool for characterising polymers in the melt. An effort has been made in order to gather results in conventional and model polyolefins, and correlating them with phenomena occurring at the molecular level. We have focused our interest on long chain branching (LCB). In the case of materials containing long side-chain branches, strong effects on viscosity, elastic character and activation energy of flow are general features. Literature results mostly indicate that the effect of polydispersity on these parameters could be very similar to that expected due to the presence of LCB -notwithstanding that the effects of LCB seem to be stronger than those due to polydispersity for a given molecular weight. Different relaxation processes appear as a consequence of the presence of LCB: slower terminal relaxation behaviour than of linear counterparts, and faster additional branch relaxation at higher frequencies, clearly distinguishable from polydispersity effects. To measure the amount of LCB from limited viscoelastic data and molecular properties seems to be a suitable instrument to explain the rheological features of the different polymers, but it fails when the results are compared with measured values of LCB density in model polymers. The actual framework leads us to say that the number of branches is less important than the topology itself. Therefore, the position and architecture of the branches along the polymer main chain are the main factors that control the rheology of the material.

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“…To this, in addition to knowing the dependence of extensional behavior on external parameters ͑like applied strain rate, time, and temperature͒, one should also know how molecular-level parameters ͑e.g., chemical structure, molecular architecture, molecular weight, branching, and polydispersity͒ affect rheological response. Unfortunately, from an experimental point of view, measuring the ͑steady-state͒ extensional properties of polymers is an extremely difficult task due to a number of issues, 1,14,15 the most important of which being that extensional deformations are inherently unstable due to the formation of a neck in the sample undergoing elongation; furthermore, the material may not deform homogeneously. This significantly limits the range of operation of extensional rheometers and the maximum Hencky strain that can be probed, and much labor is required on the measurements to achieve experimental control; in most of the experiments and tests the material breaks before a ͑'true'͒ steady-state is attained.…”

Section: Introductionmentioning

confidence: 99%

“…Even in cases where steady-state elongational properties ͑especially for some branched polymers, e.g., LDPE͒ have been reported in the literature with advanced rheometers, 22 it is still debatable whether these are true steady-state extensional data or not. 1,15,19 From a theoretical point of view, recent studies 23,24 have shown that making use of reliable steady-state elongational properties is also essential in evaluating viscoelastic models or in developing new ones.…”

Section: Introductionmentioning

confidence: 99%

Tension thickening, molecular shape, and flow birefringence of an H-shaped polymer melt in steady shear and planar extension

Baig

Mavrantzas

2010

The Journal of Chemical Physics

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Articles you may be interested in Nonequilibrium molecular dynamics simulation of dendrimers and hyperbranched polymer melts undergoing planar elongational flow J. Rheol. 58, 281 (2014) Despite recent advances in the design of extensional rheometers optimized for strain and stress controlled operation in steady, dynamic, and transient modes, obtaining reliable steady-state elongational data for macromolecular systems is still a formidable task, limiting today's approach to trial-and-error efforts rather than based on a deep understanding of the deformation processes occurring under elongation. Guided, in particular, by the need to understand the special rheology of branched polymers, we studied a model, unentangled H-shaped polyethylene melt using nonequilibrium molecular dynamics simulations based on a recently developed rigorous statistical mechanics algorithm. The melt has been simulated under steady shear and steady planar extension, over a wide range of deformation rates. In shear, the steady-state shear viscosity is observed to decrease monotonically as the shear rate increases; furthermore, the degree of shear thinning of the viscosity and of the first-and second-normal stress coefficients is observed to be similar to that of a linear analog of the same total chain length. By contrast, in planar extension, the primary steady-state elongational viscosity 1 is observed to exhibit a tension-thickening behavior as the elongation rate increases, which we analyze here in terms of ͑a͒ perturbations in the instantaneous intrinsic chain shape and ͑b͒ differences in the stress distribution along chain contour. The maximum in the plot of 1 with occurs when the arm-stretching mode becomes active and is followed by a rather abrupt tension-thinning behavior. In contrast, the second elongational viscosity 2 shows only a tension-thinning behavior. As an interesting point, the simulations predict the same value for the stress optical coefficient in the two flows, revealing an important rheo-optical characteristic. In agreement with experimental indications on significantly longer systems, our results confirm the importance of chain branching on the unique rheological properties of polymer melts in extension.

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Transient Elongational Viscosities and Drawability of Polymer Melts

Cited by 317 publications

References 0 publications

Theoretical Molecular Rheology of Branched Polymers in Simple and Complex Flows: The Pom-Pom Model

Theoretical Molecular Rheology of Branched Polymers in Simple and Complex Flows: The Pom-Pom Model

Effect of long chain branching on linear-viscoelastic melt properties of polyolefins

Tension thickening, molecular shape, and flow birefringence of an H-shaped polymer melt in steady shear and planar extension

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