Shi-qing Wang | The University of Akron (original) (raw)

Papers by Shi-qing Wang

Bulletin of the American Physical Society, Mar 20, 2013

During uniaxial extension, polymer glasses undergo elastic deformation, yielding, strain softenin... more During uniaxial extension, polymer glasses undergo elastic deformation, yielding, strain softening, neck propagation, and "strain hardening". Both plasticity and anelasticity emerge under the large deformation, making the origin of the mechanic stress elusive to identify. The present work employs an IR camera to make in situ temperature measurements on the extending specimen along with the conventional force measurements. To demonstrate the generality of our findings we studied the ductile polycarbonate as well as two brittle polymers, i.e., PS and PMMA, which can be made ductile by melt extension [1]. We found that the rate of heat generation is only a small fraction of the mechanical power involved in the uniaxial extension of these polymer glasses. Thus, it seems that the origin of the tensile stress is largely intrachain, stemming from straining of the chain network.

Bulletin of the American Physical Society, Mar 20, 2013

Bulletin of the American Physical Society, Mar 17, 2016

yielding shows up when a considerable retractive stress rises from a piece of cold-drawn polymer ... more yielding shows up when a considerable retractive stress rises from a piece of cold-drawn polymer glass during annealing at temperatures above storage temperature .............[1,2]. This phenomenon indicates significant chain tension built up during cold drawing. To explore the role of chain networking, we applied γ-irradiation to produce chain scission and cause partial breakdown of the chain network in the pre-necked polymer glasses. To demonstrate universality, four different glasses, i.e., polycarbonate (PC), polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2,6-dimethyl-1,4-phenylene oxide) (PPE) were first subjected to uniaxial extension at room temperature before the irradiation. Our data shows that the retractive stress significantly decreases in magnitude with increasing dosage of the γ-irradiation. The diminishing elastic yielding effect may be due to the loss of chain tension by chain scission brought about by the irradiation. .[1] S.

Bulletin of the American Physical Society, Feb 29, 2012

Submitted for the MAR12 Meeting of The American Physical Society Understanding failure behavior o... more Submitted for the MAR12 Meeting of The American Physical Society Understanding failure behavior of polymer glasses: a molecular viewpoint GREGORY ZARTMAN, SHIWANG CHENG, SHI-QING WANG, The University of Akron-In surveying the vast literature we note that a unified simple picture appears to be lacking to account for all the known facts on failure behavior of polymer glasses. In this work, we first apply the fresh insight we have gained from studying nonlinear extensional rheology of entangled melt to show why a meltstretched polystyrene turns ductile. We further show that the ductile polycarbonate can also turn brittle upon pre-melt-stretching. Finally, blending oligomeric PC into an originally ductile PC also causes the mixture to become brittle. All these old and new phenomena point to the fact that the strength of the load bearing chain network dictates at a given temperature whether the polymer glass would undergo ductile failure through shear yielding or brittle fracture via crazing. This presentation will provide a description of how the entanglement structure can be altered by melt deformation or the choice of different chemical specificity (that influences the packing length) to affect the strength of polymer glasses. It is this network strength relative to the yield strength associated with the inter-segmental van der Waals interactions that determines how strain localization (shear yielding-necking vs. crazing) takes place.

Bulletin of the American Physical Society, Feb 27, 2012

There is ample evidence to show that the essential physics governing yielding of entangled polyme... more There is ample evidence to show that the essential physics governing yielding of entangled polymers is the same, independent of the mode of deformation, e.g., shear versus extension. In either of these two most commonly studied forms of deformation, the elastic retraction force associated with the chain deformation cannot grow without bound during continuous deformation. In practice, a transition from the initial dominantly elastic deformation to flow (irreversible deformation) inevitably takes place. Such yielding can produce strain localization in large deformation of well entangled polymer melts. Apart from the superficial difference related to the confusion about the "strain hardening" behavior, a true difference in the respective responses of entangled melts to shear and extension arises when the strain rate is sufficiently high. The entanglement network can still yield on its path to the eventual flow state upon startup shear. However, startup extension could cause the entanglements to lock in, and the melt undergoes rubber-like rupture instead of yielding. This presentation raises the question of whether shear is an intrinsically different deformation from uniaxial extension in the extremely high rate limit.

Bulletin of the American Physical Society, Mar 17, 2016

Submitted for the MAR16 Meeting of The American Physical Society To explore the nature of mechani... more Submitted for the MAR16 Meeting of The American Physical Society To explore the nature of mechanical stress of polymeric glass by stress relaxation tests 1 XIAOXIAO LI, JIANNING LIU, PANPAN LIN, SHI-QING WANG, University of Akron-In a glassy polymer intermolecular interactions glue all segments into one single macroscopic piece thanks to attractive van der Waals bonding. The cohesive strength of such a primary structure is rather weak. If the molecular weight is sufficiently high, the covalent bonding can "magically" take part in the cohesion of the polymer glass through formation of a chain network. This picture of hybrid structure enables us to delineate the nature of mechanical stress [1]. Under either extension or compression, we performed stress relaxation experiments in both pre-yield and post-yield regimes to illustrate how inter-segmental and intra-segmental components of stress emerge in the different regimes. [1] S.-Q.

Bulletin of the American Physical Society, Feb 27, 2012

ABSTRACT Small angle neutron scattering (SANS) has been used to measure entangled chains in polym... more ABSTRACT Small angle neutron scattering (SANS) has been used to measure entangled chains in polymer melts upon uniaxial stretching. The degree of anisotropy and the coil dimension are discussed in the light of the applicability of the tube model.

Journal of Rheology, Nov 1, 1993

We present rheological evidence which demonstrates that dissolution of a side-chain liquid-crysta... more We present rheological evidence which demonstrates that dissolution of a side-chain liquid-crystal polymer with a methyl methacrylate backbone (MSHMA) in a flow-aligning low molar mass nematogen (LMMN) pentylcyanobiphenyl (5CB) produces a director-tumbling response. For comparison, we also provide the rheological behavior of pure octylcyanobiphenyl (8CB), a LMMN which exhibits director tumbling. 8CB and the MSHMA/5CB mixture each show a similar pattern of shear stress oscillations, both in flow startup and flow reversal, characteristic of director tumbling, whereas pure 5CB has no oscillation response. Our results indicate that addition of a side-chain liquid-crystal polymer to a shear-aligning nematic solvent changes the sign of the Leslie viscosity coefficient α3 from negative (shear-aligning) to positive (director-tumbling). This is consistent with a theoretical discussion of Brochard (1979), provided that the polymer has an oblate configuration.

Journal of Rheology, 1998

... respectively, with two dies of the same dimensions that differ only in the surface condition.... more ... respectively, with two dies of the same dimensions that differ only in the surface condition. ... Theadjustable pressure on the piston in the barrel is generated by a nitrogen tank. ... where the interfacial friction coefficient β characterizes viscous interactions at the boundary and η is the ...

Bulletin of the American Physical Society, Mar 22, 2013

Submitted for the MAR13 Meeting of The American Physical Society How melt stretching affect the b... more Submitted for the MAR13 Meeting of The American Physical Society How melt stretching affect the brittle-ductile transition temperature of polymer glasses SHIWANG CHENG, SHI-QING WANG, University of Akron-Upon increasing temperature a brittle polymer glass can turn ductile. PMMA is a good example. For a while this brittle-ductile transition (BDT) was thought to be determined by the emergence of a secondary relaxation.... 1−3 On the other hand, it has been known for a long time... 4−6 that predeformation in the melt state (e.g., melt stretching) can also make brittle glasses behave in a ductile manner. This transformation has recently received a satisfactory explanation based on a picture of structural hybrid for polymer glasses.... 7 It appears that BDT is dictated by the relative mechanical characteristics of the primary structure (due to the van der Waals bonds) and the chain network. The present work, based on conventional Instron tensile extension tests and DMA tests, shows that melt stretching does not alter the secondary relaxation behavior of PMMA and PC yet can turn them the brittle PMMA ductile and the ductile PC brittle. Moreover, sufficient melt stretching makes the brittle PS ductile although it does not produce any secondary relaxation process.

Bulletin of the American Physical Society, Mar 20, 2013

Journal of Rheology, Sep 1, 2015

We point out, based on confirmation from in situ IR thermal imaging measurements of the sample te... more We point out, based on confirmation from in situ IR thermal imaging measurements of the sample temperature, that many previous studies of melt extension behavior of polymer melts were not carried out under isothermal condition. Our estimates show considerable temperature rises during the fast melt stretching that produces rather high tensile stress at appreciable Hencky rates, where air convection is not sufficient to maintain the prescribed temperatures.

Macromolecules, Mar 24, 1999

ABSTRACT

Science China-chemistry, Feb 20, 2012

This work extends our previous understanding concerning the nonlinear responses of entangled poly... more This work extends our previous understanding concerning the nonlinear responses of entangled polymer solutions and melts to large external deformation in both simple shear and uniaxial extension. Many similarities have recently been identified for both step strain and startup continuous deformation, including elastic yielding, i.e., chain disentanglement after cessation of shear or extension, and emergence of a yield point during startup deformation that involves a deformation rate in excess of the dominant molecular relaxation rate. At a sufficiently high constant Hencky rate, uniaxial extension of an entangled melt is known to produce window-glass-like rupture. The present study provides evidence against the speculation that chain entanglements tie up into "dead knots" in constant-rate extension because of the exponentially growing chain stretching with time. In particular, it is shown that even Instron-style tensile stretching, i.e., extending a specimen by applying a constant velocity on both ends, results in rupture. Yet, in the same rate range, the same entangled melt only yields in simple shear, and the resulting shear banding is clearly not a characteristic of rupture. Thus, we conclude that chain entanglements respond to simple shear in the manner of yielding whereas uniaxial extension is rather effective in causing some entanglements to lock up, making it impossible for the entanglement network to yield at high rates.

Bulletin of the American Physical Society, 2015

Nearly all polymeric materials are of high molecular weight and therefore entangled in their liqu... more Nearly all polymeric materials are of high molecular weight and therefore entangled in their liquid state. Significant melt elasticity arises from the transient networking due to chain entanglement. All rheological behavior stems from how the entanglement responds to external deformation of various forms. Unfortunately, the concept of entanglement still remains theoretically elusive to describe. On other hand, modeling the evolution of chain entanglement is the key to answering the core questions in polymer rheology: a) where chain deformation comes from? b) when affine-like elastic molecular deformation ceases? In other words, yielding at both macroscopic (which is obviously taking place, e.g., signified by the stress overshoot response to startup shear) and molecular levels (through chain disentanglement) is an essential ingredient of any theoretical description of nonlinear polymer rheology. Macroscopic observations are valuable to afford useful insights, but it is the molecular dynamics simulations that are expected to address the foundational issues. This presentation will attempt to make a coherent discussion of what is known and where we are going from here.

Bulletin of the American Physical Society, 2017

Nonpolymeric organic glasses rarely yield during external deformation. In contrast, polymeric gla... more Nonpolymeric organic glasses rarely yield during external deformation. In contrast, polymeric glasses of high molecular weight can always undergo ductile deformation during which segmental mobility greatly increases [1]. We carried out molecular dynamics simulation based on a coarse-grained model [2] to investigate how polymeric glasses of high and low molecular weights respond differently to large deformation. In both uniaxial extension and simple shear, we observed inhomogeneous responses, e.g., spatially varying segmental mobility and strain localization. The presentation reports such inhomogeneities at different temperatures and rates for both long and short chain systems. [1]

Bulletin of the American Physical Society, Mar 3, 2015

Submitted for the MAR15 Meeting of The American Physical Society What deformation does and does n... more Submitted for the MAR15 Meeting of The American Physical Society What deformation does and does not do in ductile polymer glasses JIANNING LIU, SHI-QING WANG, Department of Polymer Science, University of Akron-Entangled polymeric liquids have so far only shown strain softening, signified by stress overshoot upon startup shear. We show for the first time that entangled solutions of polymers with high glass transition temperature undergoes non-Gaussian chain stretching, losing its ability to yield through chain disentanglement. The stronger than linear increase of the shear stress with the strain ends with a sharp decline, forming a cusp. In situ visualization by particle-tracking velocimetry confirms that the solution undergoes abrupt rupture at a shearing plate in the sample interior. The rapid sample recoils elastically, producing the observed stress drop.

nonlinear rheology of entangled linear polymers has been fully explored in recent years, the effe... more nonlinear rheology of entangled linear polymers has been fully explored in recent years, the effect of chain architecture remains the last frontier in polymer rheology. Here we study two H-shape and one grafted-polyisoprene (3 branches) using startup and step extension and shear. Long chain branches (LCB) impede yielding and prevent entangled network from full disentanglement. Thus, nonlinear rheological behavior of LCB polymers forms a sharp contrast to that of linear chains. We will demonstrate these striking differences.

Bulletin of the American …, 2005

Bulletin of the American Physical Society, Mar 20, 2013

Bulletin of the American Physical Society, Mar 20, 2013

During uniaxial extension, polymer glasses undergo elastic deformation, yielding, strain softenin... more During uniaxial extension, polymer glasses undergo elastic deformation, yielding, strain softening, neck propagation, and "strain hardening". Both plasticity and anelasticity emerge under the large deformation, making the origin of the mechanic stress elusive to identify. The present work employs an IR camera to make in situ temperature measurements on the extending specimen along with the conventional force measurements. To demonstrate the generality of our findings we studied the ductile polycarbonate as well as two brittle polymers, i.e., PS and PMMA, which can be made ductile by melt extension [1]. We found that the rate of heat generation is only a small fraction of the mechanical power involved in the uniaxial extension of these polymer glasses. Thus, it seems that the origin of the tensile stress is largely intrachain, stemming from straining of the chain network.

Bulletin of the American Physical Society, Mar 20, 2013

Bulletin of the American Physical Society, Mar 17, 2016

yielding shows up when a considerable retractive stress rises from a piece of cold-drawn polymer ... more yielding shows up when a considerable retractive stress rises from a piece of cold-drawn polymer glass during annealing at temperatures above storage temperature .............[1,2]. This phenomenon indicates significant chain tension built up during cold drawing. To explore the role of chain networking, we applied γ-irradiation to produce chain scission and cause partial breakdown of the chain network in the pre-necked polymer glasses. To demonstrate universality, four different glasses, i.e., polycarbonate (PC), polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2,6-dimethyl-1,4-phenylene oxide) (PPE) were first subjected to uniaxial extension at room temperature before the irradiation. Our data shows that the retractive stress significantly decreases in magnitude with increasing dosage of the γ-irradiation. The diminishing elastic yielding effect may be due to the loss of chain tension by chain scission brought about by the irradiation. .[1] S.

Bulletin of the American Physical Society, Feb 29, 2012

Submitted for the MAR12 Meeting of The American Physical Society Understanding failure behavior o... more Submitted for the MAR12 Meeting of The American Physical Society Understanding failure behavior of polymer glasses: a molecular viewpoint GREGORY ZARTMAN, SHIWANG CHENG, SHI-QING WANG, The University of Akron-In surveying the vast literature we note that a unified simple picture appears to be lacking to account for all the known facts on failure behavior of polymer glasses. In this work, we first apply the fresh insight we have gained from studying nonlinear extensional rheology of entangled melt to show why a meltstretched polystyrene turns ductile. We further show that the ductile polycarbonate can also turn brittle upon pre-melt-stretching. Finally, blending oligomeric PC into an originally ductile PC also causes the mixture to become brittle. All these old and new phenomena point to the fact that the strength of the load bearing chain network dictates at a given temperature whether the polymer glass would undergo ductile failure through shear yielding or brittle fracture via crazing. This presentation will provide a description of how the entanglement structure can be altered by melt deformation or the choice of different chemical specificity (that influences the packing length) to affect the strength of polymer glasses. It is this network strength relative to the yield strength associated with the inter-segmental van der Waals interactions that determines how strain localization (shear yielding-necking vs. crazing) takes place.

Bulletin of the American Physical Society, Feb 27, 2012

There is ample evidence to show that the essential physics governing yielding of entangled polyme... more There is ample evidence to show that the essential physics governing yielding of entangled polymers is the same, independent of the mode of deformation, e.g., shear versus extension. In either of these two most commonly studied forms of deformation, the elastic retraction force associated with the chain deformation cannot grow without bound during continuous deformation. In practice, a transition from the initial dominantly elastic deformation to flow (irreversible deformation) inevitably takes place. Such yielding can produce strain localization in large deformation of well entangled polymer melts. Apart from the superficial difference related to the confusion about the "strain hardening" behavior, a true difference in the respective responses of entangled melts to shear and extension arises when the strain rate is sufficiently high. The entanglement network can still yield on its path to the eventual flow state upon startup shear. However, startup extension could cause the entanglements to lock in, and the melt undergoes rubber-like rupture instead of yielding. This presentation raises the question of whether shear is an intrinsically different deformation from uniaxial extension in the extremely high rate limit.

Bulletin of the American Physical Society, Mar 17, 2016

Submitted for the MAR16 Meeting of The American Physical Society To explore the nature of mechani... more Submitted for the MAR16 Meeting of The American Physical Society To explore the nature of mechanical stress of polymeric glass by stress relaxation tests 1 XIAOXIAO LI, JIANNING LIU, PANPAN LIN, SHI-QING WANG, University of Akron-In a glassy polymer intermolecular interactions glue all segments into one single macroscopic piece thanks to attractive van der Waals bonding. The cohesive strength of such a primary structure is rather weak. If the molecular weight is sufficiently high, the covalent bonding can "magically" take part in the cohesion of the polymer glass through formation of a chain network. This picture of hybrid structure enables us to delineate the nature of mechanical stress [1]. Under either extension or compression, we performed stress relaxation experiments in both pre-yield and post-yield regimes to illustrate how inter-segmental and intra-segmental components of stress emerge in the different regimes. [1] S.-Q.

Bulletin of the American Physical Society, Feb 27, 2012

ABSTRACT Small angle neutron scattering (SANS) has been used to measure entangled chains in polym... more ABSTRACT Small angle neutron scattering (SANS) has been used to measure entangled chains in polymer melts upon uniaxial stretching. The degree of anisotropy and the coil dimension are discussed in the light of the applicability of the tube model.

Journal of Rheology, Nov 1, 1993

We present rheological evidence which demonstrates that dissolution of a side-chain liquid-crysta... more We present rheological evidence which demonstrates that dissolution of a side-chain liquid-crystal polymer with a methyl methacrylate backbone (MSHMA) in a flow-aligning low molar mass nematogen (LMMN) pentylcyanobiphenyl (5CB) produces a director-tumbling response. For comparison, we also provide the rheological behavior of pure octylcyanobiphenyl (8CB), a LMMN which exhibits director tumbling. 8CB and the MSHMA/5CB mixture each show a similar pattern of shear stress oscillations, both in flow startup and flow reversal, characteristic of director tumbling, whereas pure 5CB has no oscillation response. Our results indicate that addition of a side-chain liquid-crystal polymer to a shear-aligning nematic solvent changes the sign of the Leslie viscosity coefficient α3 from negative (shear-aligning) to positive (director-tumbling). This is consistent with a theoretical discussion of Brochard (1979), provided that the polymer has an oblate configuration.

Journal of Rheology, 1998

... respectively, with two dies of the same dimensions that differ only in the surface condition.... more ... respectively, with two dies of the same dimensions that differ only in the surface condition. ... Theadjustable pressure on the piston in the barrel is generated by a nitrogen tank. ... where the interfacial friction coefficient β characterizes viscous interactions at the boundary and η is the ...

Bulletin of the American Physical Society, Mar 22, 2013

Submitted for the MAR13 Meeting of The American Physical Society How melt stretching affect the b... more Submitted for the MAR13 Meeting of The American Physical Society How melt stretching affect the brittle-ductile transition temperature of polymer glasses SHIWANG CHENG, SHI-QING WANG, University of Akron-Upon increasing temperature a brittle polymer glass can turn ductile. PMMA is a good example. For a while this brittle-ductile transition (BDT) was thought to be determined by the emergence of a secondary relaxation.... 1−3 On the other hand, it has been known for a long time... 4−6 that predeformation in the melt state (e.g., melt stretching) can also make brittle glasses behave in a ductile manner. This transformation has recently received a satisfactory explanation based on a picture of structural hybrid for polymer glasses.... 7 It appears that BDT is dictated by the relative mechanical characteristics of the primary structure (due to the van der Waals bonds) and the chain network. The present work, based on conventional Instron tensile extension tests and DMA tests, shows that melt stretching does not alter the secondary relaxation behavior of PMMA and PC yet can turn them the brittle PMMA ductile and the ductile PC brittle. Moreover, sufficient melt stretching makes the brittle PS ductile although it does not produce any secondary relaxation process.

Bulletin of the American Physical Society, Mar 20, 2013

Journal of Rheology, Sep 1, 2015

We point out, based on confirmation from in situ IR thermal imaging measurements of the sample te... more We point out, based on confirmation from in situ IR thermal imaging measurements of the sample temperature, that many previous studies of melt extension behavior of polymer melts were not carried out under isothermal condition. Our estimates show considerable temperature rises during the fast melt stretching that produces rather high tensile stress at appreciable Hencky rates, where air convection is not sufficient to maintain the prescribed temperatures.

Macromolecules, Mar 24, 1999

ABSTRACT

Science China-chemistry, Feb 20, 2012

This work extends our previous understanding concerning the nonlinear responses of entangled poly... more This work extends our previous understanding concerning the nonlinear responses of entangled polymer solutions and melts to large external deformation in both simple shear and uniaxial extension. Many similarities have recently been identified for both step strain and startup continuous deformation, including elastic yielding, i.e., chain disentanglement after cessation of shear or extension, and emergence of a yield point during startup deformation that involves a deformation rate in excess of the dominant molecular relaxation rate. At a sufficiently high constant Hencky rate, uniaxial extension of an entangled melt is known to produce window-glass-like rupture. The present study provides evidence against the speculation that chain entanglements tie up into "dead knots" in constant-rate extension because of the exponentially growing chain stretching with time. In particular, it is shown that even Instron-style tensile stretching, i.e., extending a specimen by applying a constant velocity on both ends, results in rupture. Yet, in the same rate range, the same entangled melt only yields in simple shear, and the resulting shear banding is clearly not a characteristic of rupture. Thus, we conclude that chain entanglements respond to simple shear in the manner of yielding whereas uniaxial extension is rather effective in causing some entanglements to lock up, making it impossible for the entanglement network to yield at high rates.

Bulletin of the American Physical Society, 2015

Nearly all polymeric materials are of high molecular weight and therefore entangled in their liqu... more Nearly all polymeric materials are of high molecular weight and therefore entangled in their liquid state. Significant melt elasticity arises from the transient networking due to chain entanglement. All rheological behavior stems from how the entanglement responds to external deformation of various forms. Unfortunately, the concept of entanglement still remains theoretically elusive to describe. On other hand, modeling the evolution of chain entanglement is the key to answering the core questions in polymer rheology: a) where chain deformation comes from? b) when affine-like elastic molecular deformation ceases? In other words, yielding at both macroscopic (which is obviously taking place, e.g., signified by the stress overshoot response to startup shear) and molecular levels (through chain disentanglement) is an essential ingredient of any theoretical description of nonlinear polymer rheology. Macroscopic observations are valuable to afford useful insights, but it is the molecular dynamics simulations that are expected to address the foundational issues. This presentation will attempt to make a coherent discussion of what is known and where we are going from here.

Bulletin of the American Physical Society, 2017

Nonpolymeric organic glasses rarely yield during external deformation. In contrast, polymeric gla... more Nonpolymeric organic glasses rarely yield during external deformation. In contrast, polymeric glasses of high molecular weight can always undergo ductile deformation during which segmental mobility greatly increases [1]. We carried out molecular dynamics simulation based on a coarse-grained model [2] to investigate how polymeric glasses of high and low molecular weights respond differently to large deformation. In both uniaxial extension and simple shear, we observed inhomogeneous responses, e.g., spatially varying segmental mobility and strain localization. The presentation reports such inhomogeneities at different temperatures and rates for both long and short chain systems. [1]

Bulletin of the American Physical Society, Mar 3, 2015

Submitted for the MAR15 Meeting of The American Physical Society What deformation does and does n... more Submitted for the MAR15 Meeting of The American Physical Society What deformation does and does not do in ductile polymer glasses JIANNING LIU, SHI-QING WANG, Department of Polymer Science, University of Akron-Entangled polymeric liquids have so far only shown strain softening, signified by stress overshoot upon startup shear. We show for the first time that entangled solutions of polymers with high glass transition temperature undergoes non-Gaussian chain stretching, losing its ability to yield through chain disentanglement. The stronger than linear increase of the shear stress with the strain ends with a sharp decline, forming a cusp. In situ visualization by particle-tracking velocimetry confirms that the solution undergoes abrupt rupture at a shearing plate in the sample interior. The rapid sample recoils elastically, producing the observed stress drop.

nonlinear rheology of entangled linear polymers has been fully explored in recent years, the effe... more nonlinear rheology of entangled linear polymers has been fully explored in recent years, the effect of chain architecture remains the last frontier in polymer rheology. Here we study two H-shape and one grafted-polyisoprene (3 branches) using startup and step extension and shear. Long chain branches (LCB) impede yielding and prevent entangled network from full disentanglement. Thus, nonlinear rheological behavior of LCB polymers forms a sharp contrast to that of linear chains. We will demonstrate these striking differences.

Bulletin of the American …, 2005

Bulletin of the American Physical Society, Mar 20, 2013