Shi-qing Wang - Profile on Academia.edu (original) (raw)

Papers by Shi-qing Wang

Two inorganic-organic hybrid polymers, Al(OH) 3 -polyacrylamide (Al-PAM) and Fe(OH) 3 -polyacryla... more Two inorganic-organic hybrid polymers, Al(OH) 3 -polyacrylamide (Al-PAM) and Fe(OH) 3 -polyacrylamide (Fe-PAM) were synthesized and used in flocculating model tailings (5 wt% kaolin suspensions) and laboratory extraction tailings. For comparison, a commercial anionic flocculant, partially hydrolyzed polyacrylamide or Magnafloc 1011 (MF 1011), was also examined. Moreover, a fundamental understanding on flocculation dynamics of model and laboratory extraction tailings by different polymers as well as adsorption kinetics of these polymers on different surfaces was established using focused beam reflectance measurement (FBRM) and quartz crystal microbalance with dissipation (QCM-D), respectively.

Bulletin of the American Physical Society, Mar 6, 2007

A new molecular theory beyond tube model to describe cohesive breakdown in nonlinear flow of enta... more A new molecular theory beyond tube model to describe cohesive breakdown in nonlinear flow of entangled polymers. SHI-QING WANG, University of Akron -When an entangled polymer is subjected to shear or extensional flow at a rate of deformation (RD) much greater than its dominant relaxation rate (dRR), it may not flow homogenously all the way to the limiting strain of (RD/dRR) before it suffers cohesive failure. What keeps the chains entangled is an essential question to answer before an appropriate theory of polymer flow can be established. Unlike the tube model that assumes presence of an infinitely high energy barrier preventing escape of chain entanglement, our theory [1] recognizes a finite barrier height given by kT(M/Me) for a polymer whose number of entanglements per chain is (M/Me). A second essential ingredient is to realize that a sufficiently high level of elastic force can be generated per chain by the externally imposed flow. This elastic force can overcome the entanglement (cohesive) force as a rate-activation process, leading to the onset condition for the cohesive breakup either during flow or upon cessation of flow. Flow produces frictional inter-chain interactions among all entangling chains. These interactions also resist constitutive disintegration, delaying the onset of cohesive collapse to a larger strain. A higher level of cohesive strength results from the very flow deformation that could eventually produce enough internal (elastic) forces to destroy the cohesive structure made of chain entanglement.

Bulletin of the American Physical Society, Mar 13, 2017

The Kremer-Grest bead-spring model has been the standard model in molecular dynamics simulation o... more The Kremer-Grest bead-spring model has been the standard model in molecular dynamics simulation of polymer glasses. However, due to current computational limitations in accessing relevant time scales in polymer glasses in a reasonable amount of CPU time, simulation of mechanical response of polymer glasses in molecular dynamic simulations requires a much higher quenching rate and deformation rate than used in experiments. Despite several orders of magnitude difference in time scale between simulation and experiment, previous studies have shown that simulations can produce meaningful results that can be directly compared with experimental results. In this work we show that by tuning the quenching rate and deformation rate relative to the segmental relaxation times, a reasonable mechanical response shows up in the glassy state. Specifically, we show a younger glass prepared with a faster quenching rate shows glassy responses only when the imposed deformation rate is proportionally higher.

Bulletin of the American Physical Society, Mar 17, 2009

How do entangled polymeric liquids flow? SHAM SUNDAR RAVIN-DRANATH, SHI-QING WANG, University of ... more How do entangled polymeric liquids flow? SHAM SUNDAR RAVIN-DRANATH, SHI-QING WANG, University of Akron -This work focused on investigating fundamental questions in polymer dynamics such as how entangled polymeric liquids respond to fast external deformation. By developing an effective particle tracking velocimetric (PTV) method, along with conventional rheometric measurements, new insights can be gained into the phenomenology of entangled polymers in presence of startup shear, step strain and large amplitude oscillatory shear (LAOS). During startup shear of well entangled systems, the shear field becomes inhomogeneous after the stress overshoot for a range of applied shear rates beyond the Newtonian region [1]. The emergence of shear banding after stress overshoot helped us to identify the stress overshoot as indicating yielding, whose characteristics obey some scaling laws. In step shear, contrary to the conventional perception that entangled polymers would undergo quiescent relaxation, the PTV observations reveal macroscopic motions after shear cessation [2]. The recoil-like macroscopic motions appears to reflect an elastic breakdown of the entanglement network due to sufficient build-up of retractive forces. LAOS experiments also demonstrate that entangled polymers cannot sustain a high magnitude of fast deformation without undergoing cohesive failure [3].

Bulletin of the American Physical Society, Mar 23, 2005

In this talk, we discuss the latest results from our experimental studies of flow behavior of ent... more In this talk, we discuss the latest results from our experimental studies of flow behavior of entangled polymers, in the context of the prevailing physical picture [1] prior to this work [2]. The model entangled polymers under study were 1,4polybutadiene melts and their solutions. Flow behavior of these PBD samples was examined under various experimental conditions where shear flow was imposed by applying either a constant torque or a constant velocity on one of the two surfaces in a cone-plate shear cell (commonly known as controlled-stress or controlled-rate measurements respectively), and small or large step-strain was applied by a sudden displacement of one of the two surfaces in the same cell. The flow responses were found to be drastically different under these different conditions. When the applied shear stress was of a comparable magnitude to the elastic plateau modulus of the entangled solutions, a sharp yield-like constitutive transition was observed, revealing a discontinuous relationship between the shear rate and the shear stress, which was not anticipated according to the understanding prior to these experimental results. Such a discontinuity does not manifest itself in controlled-rate experiment and therefore has not been seen outside our lab. The implications of these results will be discussed to project our future efforts and activities.

Bulletin of the American Physical Society, Nov 21, 2005

From interfacial slip to bulk flow: surprises in non-Newtonian flow of polymers? SHI-QING WANG, U... more From interfacial slip to bulk flow: surprises in non-Newtonian flow of polymers? SHI-QING WANG, University of Akron -All types of flow involve interfacial contact between the confining solid boundary and the fluid. Thus, understanding the nature of the hydrodynamic boundary condition (HBC) is crucial to a realistic description of the fluid mechanics of both simple and structured liquids including polymeric liquids. Polymeric liquids are uniquely capable of violating no-slip HBC on large length scales because of a high level of chain entanglement and not because "the molecular scale over which slip might occur is large enough to result in macroscopic effects" [1]. This dynamic structure of chain entanglement appears to make polymeric liquids behave similarly to other yield-stress fluids such as foams, gels, dense suspensions and glassy colloids. In other words, the "structure" is breakable by external stress to display yield-like flow. This unanticipated feature of polymeric liquids has begun to produce several big surprises. In this presentation, we will describe these surprising results revealed by a combination of mechanical and optical measurements. Specifically, we show, using a newly developed particle tracking velocimetry, that not only the well-known protocol of controlled-rate seems problematic as a reliable way to delineate the nature of polymer flow, but also questionable is the popular apparatus of cone-plate shear cell in terms of its ability to generate uniform simple shear. Guided by these experimental results we are pursuing the fundamental questions of (a) how to revise the prevailing molecularly based theoretical understanding of entangled polymers, (b) whether the widely used coneplate flow cell is suitable for exploring flow behavior of various non-Newtonian fluids, (c) what options and choices of experimental apparatus we should equip in any lab working on non-Newtonian flow. [1] M.M. Denn, abstract of APS 2005 March Meeting.

Bulletin of the American Physical Society, Mar 17, 2010

Characterize cohesive failure in simple shear of entangled polymer solutions on micron scales 1 P... more Characterize cohesive failure in simple shear of entangled polymer solutions on micron scales 1 POUYAN BOUKANY, OSU, SHAM RAVIN-DRANATH, SHI-QING WANG, University of Akron, JAMES LEE, OSU -We apply laser confocal microscopy in conjunction with a commercial rheometer to study the yielding behavior of entangled polymer solutions on length scales as low as 40 microns. The goal is to illustrate how the sample thickness dictates the overall mechanical response to sudden deformation and why the same behavior observed at conventional gap sizes (ca. 1 mm) must show up under proper conditions. Contrary to the claim of one recent study from Cornell, we show that for well-entangled polymeric liquids cohesive breakup of the entanglement network does occur for gap distance as low as 40 microns. In other words, interfacial slip alone is insufficient to take up the imposed displacement of the sample boundary. 1 Sponsored by an NSF grant (DMR-0821697).

Bulletin of the American Physical Society, Mar 7, 2007

The American Physical Society Disentanglement behavior of DNA and wormlike micellar solutions as ... more The American Physical Society Disentanglement behavior of DNA and wormlike micellar solutions as probed with particle-tracking velocimetry. POUYAN BOUKANY, SHI-QING WANG, University of Akron -We study an ideal entanglement network to test a number of emerging ideas about how topological entanglement reorganizes in presence of shear flow. Aqueous DNA solutions and wormlike micellar solutions can be highly entangled at very low concentrations and thus very soft yet sluggish. A particle tracking velocimetric method, which was developed recently in our lab [1], was applied to determine the velocity profile of these solutions in simple shear under several flow conditions including large step strain, large amplitude oscillatory shear, startup continuous shear and creep. It is shown [2] that all of the nonlinear viscoelastic flow behavior is associated with development of inhomogeneous shear when nucleation of chain disentanglement takes place in reaction to imposed shear deformation.

Bulletin of the American Physical Society, Mar 16, 2016

How plasticizer makes a ductile polymer glass brittle? 1 YUE ZHAO, XIAOXIAO LI, SHI-QING WANG, Un... more How plasticizer makes a ductile polymer glass brittle? 1 YUE ZHAO, XIAOXIAO LI, SHI-QING WANG, Univ of Akron -During uniaxial extension, a polymer glass of high molecular weight is ductile at high temperatures (still below Tg) and turns brittle when the temperature is sufficiently lowered. Incorporation of small-molecular additives to polymer glasses can speed up segmental relaxation considerably. The effect of such plasticization should be to make the polymers more ductile. We examined the effect of blending a few weight percent of either triphenyl phosphate (TPP) or a mineral oil to a commercial-grade PS and PMMA. Our Instron tests show that the plasticized PS is less ductile. Specifically, at 70 oC, the original PS is ductile at an extensional rate of 0.02 s-1 [1] whereas the PS with 4 wt. % TPP turns brittle. Mechanical spectroscopic measurements show that the alpha relaxation time is shortened by more than two orders of magnitude with 4 wt. % TPP. On the other hand, such anomalous behavior did not occur in PMMA. We need to go beyond the conventional description to rationalize these results.

Bulletin of the American Physical Society, Feb 27, 2012

Exploring the role of long-chain branching in large deformation of entangled melts GENGXIN LIU,

Bulletin of the American Physical Society, Mar 15, 2017

Impact of melt-deformation on molecular structure and mechanical behavior of glassy polymers 1 JI... more Impact of melt-deformation on molecular structure and mechanical behavior of glassy polymers 1 JIANNING LIU, XIAOXIAO LI, ZHICHEN ZHAO, SHI-QING WANG, Department of Polymer Science, University of Akron -This work studies effects of melt deformation such as extension and compression on mechanical behavior of glassy polymers. Depending on how the entanglement network is altered during melt deformation, mechanical properties of polystyrene and poly(methyl methacrylate) are changed at temperatures below Tg. Conversely, the observed mechanical behavior below Tg reveals how molecular structures at segmental levels have undergone distortion due to melt stretching or shear. This research expands well beyond our previous investigations that have demonstrated how and why melt-stretched PS and PMMA turns ductile at room temperature.... 1 and why a cold-drawn ductile polymer glass produces significant retractive stress upon annealing above the cold-drawing temperature.. 2 .1.Wang, S.-Q.; Cheng, S.; Lin, P.; Li, X. A phenomenological molecular model for yielding and brittle-ductile transition of polymer glasses.

Bulletin of the American Physical Society, Mar 17, 2006

Large step shear has been a popular way to interrogate nonlinear viscoelastic responses of polyme... more Large step shear has been a popular way to interrogate nonlinear viscoelastic responses of polymeric materials. In absence of any severe interfacial failure, the experimental data [1] were found to agree with the Doi-Edwards model of entangled chains. A separate set of experimental studies [2-4] produced strain-softening and showed disagreement with the D-E model. We have successfully prevented interfacial breakdown for the first time to show that the strain-softening is an interfacial artifact [5] and that the stress relaxation behavior of entangled melts and solutions can be reliably depicted experimentally and accounted for within the D-E model.

Bulletin of the American Physical Society, Mar 5, 2007

Opening the Pandora's box to understand flow behavior of polymeric fluids. SHAM RAVINDRANATH, Uni... more Opening the Pandora's box to understand flow behavior of polymeric fluids. SHAM RAVINDRANATH, University of Akron, POUYAN BOUKANY, U, YANGYANG WANG, SHI-QING WANG, University of Akron -Structure-property relationship has been explored for decades in the context of flow behavior of entangled polymeric liquids. For a long time, it has been assumed that the structure of an entangled polymer, i.e., the entanglement network would experience smooth changes during flow. Using an effective particle-tracking velocimetric (PTV) method recently developed in our lab [1], we found that the nonlinear flow dynamics are associated with an elastic breakdown of the fluid structure. This cohesive failure does not necessarily occur homogeneously in a macroscopic-scale experiment, making it ambiguous to interpret traditional rheological measurements. The presentation complies a whole set of PTV observations to elucidate the physical origin of nonlinear flow phenomena in complex fluids such as polymers. [1] Phys.

Materials Science and Engineering: A, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Soft matter, Jan 28, 2015

Thanks to extensive observations of strain localization upon startup or after stepwise shear, a c... more Thanks to extensive observations of strain localization upon startup or after stepwise shear, a conceptual framework for nonlinear rheology of entangled polymers appears to have emerged that has led to discovery of many new phenomena, which were not previously predicted by the standard tube model. On the other hand, the published theoretical and experimental attempts to test the limits of the tube model have largely demonstrated that the most experimental data appear consistent with the tube-model based theoretical calculations. Therefore, the field of nonlinear rheology of entangled polymers is at a turning point and is thus a rather crucial area in which further examinations are needed. In particular, more molecular dynamics simulations are needed to delineate the detailed molecular mechanisms for the various nonlinear rheological phenomena.

Applied Physics Letters, 2010

We have investigated the anisotropic transport behavior of InAs/AlSb heterostructures grown on a ... more We have investigated the anisotropic transport behavior of InAs/AlSb heterostructures grown on a (001) InP substrate. An electrical analysis showed anisotropic sheet resistance Rsh and electron mobility μn in the two dimensional electron gas (2DEG). Hall measurements demonstrated an enhanced anisotropy in μn when cooled from room temperature to 2 K. High electron mobility transistors exhibited 27% higher maximum drain current IDS and 23% higher peak transconductance gm when oriented along the [1-10] direction. The anisotropic transport behavior in the 2DEG was correlated with an asymmetric dislocation pattern observed in the surface morphology and by cross-sectional microscopy analysis of the InAs/AlSb heterostructure.

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.

Two inorganic-organic hybrid polymers, Al(OH) 3 -polyacrylamide (Al-PAM) and Fe(OH) 3 -polyacryla... more Two inorganic-organic hybrid polymers, Al(OH) 3 -polyacrylamide (Al-PAM) and Fe(OH) 3 -polyacrylamide (Fe-PAM) were synthesized and used in flocculating model tailings (5 wt% kaolin suspensions) and laboratory extraction tailings. For comparison, a commercial anionic flocculant, partially hydrolyzed polyacrylamide or Magnafloc 1011 (MF 1011), was also examined. Moreover, a fundamental understanding on flocculation dynamics of model and laboratory extraction tailings by different polymers as well as adsorption kinetics of these polymers on different surfaces was established using focused beam reflectance measurement (FBRM) and quartz crystal microbalance with dissipation (QCM-D), respectively.

Bulletin of the American Physical Society, Mar 6, 2007

A new molecular theory beyond tube model to describe cohesive breakdown in nonlinear flow of enta... more A new molecular theory beyond tube model to describe cohesive breakdown in nonlinear flow of entangled polymers. SHI-QING WANG, University of Akron -When an entangled polymer is subjected to shear or extensional flow at a rate of deformation (RD) much greater than its dominant relaxation rate (dRR), it may not flow homogenously all the way to the limiting strain of (RD/dRR) before it suffers cohesive failure. What keeps the chains entangled is an essential question to answer before an appropriate theory of polymer flow can be established. Unlike the tube model that assumes presence of an infinitely high energy barrier preventing escape of chain entanglement, our theory [1] recognizes a finite barrier height given by kT(M/Me) for a polymer whose number of entanglements per chain is (M/Me). A second essential ingredient is to realize that a sufficiently high level of elastic force can be generated per chain by the externally imposed flow. This elastic force can overcome the entanglement (cohesive) force as a rate-activation process, leading to the onset condition for the cohesive breakup either during flow or upon cessation of flow. Flow produces frictional inter-chain interactions among all entangling chains. These interactions also resist constitutive disintegration, delaying the onset of cohesive collapse to a larger strain. A higher level of cohesive strength results from the very flow deformation that could eventually produce enough internal (elastic) forces to destroy the cohesive structure made of chain entanglement.

Bulletin of the American Physical Society, Mar 13, 2017

The Kremer-Grest bead-spring model has been the standard model in molecular dynamics simulation o... more The Kremer-Grest bead-spring model has been the standard model in molecular dynamics simulation of polymer glasses. However, due to current computational limitations in accessing relevant time scales in polymer glasses in a reasonable amount of CPU time, simulation of mechanical response of polymer glasses in molecular dynamic simulations requires a much higher quenching rate and deformation rate than used in experiments. Despite several orders of magnitude difference in time scale between simulation and experiment, previous studies have shown that simulations can produce meaningful results that can be directly compared with experimental results. In this work we show that by tuning the quenching rate and deformation rate relative to the segmental relaxation times, a reasonable mechanical response shows up in the glassy state. Specifically, we show a younger glass prepared with a faster quenching rate shows glassy responses only when the imposed deformation rate is proportionally higher.

Bulletin of the American Physical Society, Mar 17, 2009

How do entangled polymeric liquids flow? SHAM SUNDAR RAVIN-DRANATH, SHI-QING WANG, University of ... more How do entangled polymeric liquids flow? SHAM SUNDAR RAVIN-DRANATH, SHI-QING WANG, University of Akron -This work focused on investigating fundamental questions in polymer dynamics such as how entangled polymeric liquids respond to fast external deformation. By developing an effective particle tracking velocimetric (PTV) method, along with conventional rheometric measurements, new insights can be gained into the phenomenology of entangled polymers in presence of startup shear, step strain and large amplitude oscillatory shear (LAOS). During startup shear of well entangled systems, the shear field becomes inhomogeneous after the stress overshoot for a range of applied shear rates beyond the Newtonian region [1]. The emergence of shear banding after stress overshoot helped us to identify the stress overshoot as indicating yielding, whose characteristics obey some scaling laws. In step shear, contrary to the conventional perception that entangled polymers would undergo quiescent relaxation, the PTV observations reveal macroscopic motions after shear cessation [2]. The recoil-like macroscopic motions appears to reflect an elastic breakdown of the entanglement network due to sufficient build-up of retractive forces. LAOS experiments also demonstrate that entangled polymers cannot sustain a high magnitude of fast deformation without undergoing cohesive failure [3].

Bulletin of the American Physical Society, Mar 23, 2005

In this talk, we discuss the latest results from our experimental studies of flow behavior of ent... more In this talk, we discuss the latest results from our experimental studies of flow behavior of entangled polymers, in the context of the prevailing physical picture [1] prior to this work [2]. The model entangled polymers under study were 1,4polybutadiene melts and their solutions. Flow behavior of these PBD samples was examined under various experimental conditions where shear flow was imposed by applying either a constant torque or a constant velocity on one of the two surfaces in a cone-plate shear cell (commonly known as controlled-stress or controlled-rate measurements respectively), and small or large step-strain was applied by a sudden displacement of one of the two surfaces in the same cell. The flow responses were found to be drastically different under these different conditions. When the applied shear stress was of a comparable magnitude to the elastic plateau modulus of the entangled solutions, a sharp yield-like constitutive transition was observed, revealing a discontinuous relationship between the shear rate and the shear stress, which was not anticipated according to the understanding prior to these experimental results. Such a discontinuity does not manifest itself in controlled-rate experiment and therefore has not been seen outside our lab. The implications of these results will be discussed to project our future efforts and activities.

Bulletin of the American Physical Society, Nov 21, 2005

From interfacial slip to bulk flow: surprises in non-Newtonian flow of polymers? SHI-QING WANG, U... more From interfacial slip to bulk flow: surprises in non-Newtonian flow of polymers? SHI-QING WANG, University of Akron -All types of flow involve interfacial contact between the confining solid boundary and the fluid. Thus, understanding the nature of the hydrodynamic boundary condition (HBC) is crucial to a realistic description of the fluid mechanics of both simple and structured liquids including polymeric liquids. Polymeric liquids are uniquely capable of violating no-slip HBC on large length scales because of a high level of chain entanglement and not because "the molecular scale over which slip might occur is large enough to result in macroscopic effects" [1]. This dynamic structure of chain entanglement appears to make polymeric liquids behave similarly to other yield-stress fluids such as foams, gels, dense suspensions and glassy colloids. In other words, the "structure" is breakable by external stress to display yield-like flow. This unanticipated feature of polymeric liquids has begun to produce several big surprises. In this presentation, we will describe these surprising results revealed by a combination of mechanical and optical measurements. Specifically, we show, using a newly developed particle tracking velocimetry, that not only the well-known protocol of controlled-rate seems problematic as a reliable way to delineate the nature of polymer flow, but also questionable is the popular apparatus of cone-plate shear cell in terms of its ability to generate uniform simple shear. Guided by these experimental results we are pursuing the fundamental questions of (a) how to revise the prevailing molecularly based theoretical understanding of entangled polymers, (b) whether the widely used coneplate flow cell is suitable for exploring flow behavior of various non-Newtonian fluids, (c) what options and choices of experimental apparatus we should equip in any lab working on non-Newtonian flow. [1] M.M. Denn, abstract of APS 2005 March Meeting.

Bulletin of the American Physical Society, Mar 17, 2010

Characterize cohesive failure in simple shear of entangled polymer solutions on micron scales 1 P... more Characterize cohesive failure in simple shear of entangled polymer solutions on micron scales 1 POUYAN BOUKANY, OSU, SHAM RAVIN-DRANATH, SHI-QING WANG, University of Akron, JAMES LEE, OSU -We apply laser confocal microscopy in conjunction with a commercial rheometer to study the yielding behavior of entangled polymer solutions on length scales as low as 40 microns. The goal is to illustrate how the sample thickness dictates the overall mechanical response to sudden deformation and why the same behavior observed at conventional gap sizes (ca. 1 mm) must show up under proper conditions. Contrary to the claim of one recent study from Cornell, we show that for well-entangled polymeric liquids cohesive breakup of the entanglement network does occur for gap distance as low as 40 microns. In other words, interfacial slip alone is insufficient to take up the imposed displacement of the sample boundary. 1 Sponsored by an NSF grant (DMR-0821697).

Bulletin of the American Physical Society, Mar 7, 2007

The American Physical Society Disentanglement behavior of DNA and wormlike micellar solutions as ... more The American Physical Society Disentanglement behavior of DNA and wormlike micellar solutions as probed with particle-tracking velocimetry. POUYAN BOUKANY, SHI-QING WANG, University of Akron -We study an ideal entanglement network to test a number of emerging ideas about how topological entanglement reorganizes in presence of shear flow. Aqueous DNA solutions and wormlike micellar solutions can be highly entangled at very low concentrations and thus very soft yet sluggish. A particle tracking velocimetric method, which was developed recently in our lab [1], was applied to determine the velocity profile of these solutions in simple shear under several flow conditions including large step strain, large amplitude oscillatory shear, startup continuous shear and creep. It is shown [2] that all of the nonlinear viscoelastic flow behavior is associated with development of inhomogeneous shear when nucleation of chain disentanglement takes place in reaction to imposed shear deformation.

Bulletin of the American Physical Society, Mar 16, 2016

How plasticizer makes a ductile polymer glass brittle? 1 YUE ZHAO, XIAOXIAO LI, SHI-QING WANG, Un... more How plasticizer makes a ductile polymer glass brittle? 1 YUE ZHAO, XIAOXIAO LI, SHI-QING WANG, Univ of Akron -During uniaxial extension, a polymer glass of high molecular weight is ductile at high temperatures (still below Tg) and turns brittle when the temperature is sufficiently lowered. Incorporation of small-molecular additives to polymer glasses can speed up segmental relaxation considerably. The effect of such plasticization should be to make the polymers more ductile. We examined the effect of blending a few weight percent of either triphenyl phosphate (TPP) or a mineral oil to a commercial-grade PS and PMMA. Our Instron tests show that the plasticized PS is less ductile. Specifically, at 70 oC, the original PS is ductile at an extensional rate of 0.02 s-1 [1] whereas the PS with 4 wt. % TPP turns brittle. Mechanical spectroscopic measurements show that the alpha relaxation time is shortened by more than two orders of magnitude with 4 wt. % TPP. On the other hand, such anomalous behavior did not occur in PMMA. We need to go beyond the conventional description to rationalize these results.

Bulletin of the American Physical Society, Feb 27, 2012

Exploring the role of long-chain branching in large deformation of entangled melts GENGXIN LIU,

Bulletin of the American Physical Society, Mar 15, 2017

Impact of melt-deformation on molecular structure and mechanical behavior of glassy polymers 1 JI... more Impact of melt-deformation on molecular structure and mechanical behavior of glassy polymers 1 JIANNING LIU, XIAOXIAO LI, ZHICHEN ZHAO, SHI-QING WANG, Department of Polymer Science, University of Akron -This work studies effects of melt deformation such as extension and compression on mechanical behavior of glassy polymers. Depending on how the entanglement network is altered during melt deformation, mechanical properties of polystyrene and poly(methyl methacrylate) are changed at temperatures below Tg. Conversely, the observed mechanical behavior below Tg reveals how molecular structures at segmental levels have undergone distortion due to melt stretching or shear. This research expands well beyond our previous investigations that have demonstrated how and why melt-stretched PS and PMMA turns ductile at room temperature.... 1 and why a cold-drawn ductile polymer glass produces significant retractive stress upon annealing above the cold-drawing temperature.. 2 .1.Wang, S.-Q.; Cheng, S.; Lin, P.; Li, X. A phenomenological molecular model for yielding and brittle-ductile transition of polymer glasses.

Bulletin of the American Physical Society, Mar 17, 2006

Large step shear has been a popular way to interrogate nonlinear viscoelastic responses of polyme... more Large step shear has been a popular way to interrogate nonlinear viscoelastic responses of polymeric materials. In absence of any severe interfacial failure, the experimental data [1] were found to agree with the Doi-Edwards model of entangled chains. A separate set of experimental studies [2-4] produced strain-softening and showed disagreement with the D-E model. We have successfully prevented interfacial breakdown for the first time to show that the strain-softening is an interfacial artifact [5] and that the stress relaxation behavior of entangled melts and solutions can be reliably depicted experimentally and accounted for within the D-E model.

Bulletin of the American Physical Society, Mar 5, 2007

Opening the Pandora's box to understand flow behavior of polymeric fluids. SHAM RAVINDRANATH, Uni... more Opening the Pandora's box to understand flow behavior of polymeric fluids. SHAM RAVINDRANATH, University of Akron, POUYAN BOUKANY, U, YANGYANG WANG, SHI-QING WANG, University of Akron -Structure-property relationship has been explored for decades in the context of flow behavior of entangled polymeric liquids. For a long time, it has been assumed that the structure of an entangled polymer, i.e., the entanglement network would experience smooth changes during flow. Using an effective particle-tracking velocimetric (PTV) method recently developed in our lab [1], we found that the nonlinear flow dynamics are associated with an elastic breakdown of the fluid structure. This cohesive failure does not necessarily occur homogeneously in a macroscopic-scale experiment, making it ambiguous to interpret traditional rheological measurements. The presentation complies a whole set of PTV observations to elucidate the physical origin of nonlinear flow phenomena in complex fluids such as polymers. [1] Phys.

Materials Science and Engineering: A, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Soft matter, Jan 28, 2015

Thanks to extensive observations of strain localization upon startup or after stepwise shear, a c... more Thanks to extensive observations of strain localization upon startup or after stepwise shear, a conceptual framework for nonlinear rheology of entangled polymers appears to have emerged that has led to discovery of many new phenomena, which were not previously predicted by the standard tube model. On the other hand, the published theoretical and experimental attempts to test the limits of the tube model have largely demonstrated that the most experimental data appear consistent with the tube-model based theoretical calculations. Therefore, the field of nonlinear rheology of entangled polymers is at a turning point and is thus a rather crucial area in which further examinations are needed. In particular, more molecular dynamics simulations are needed to delineate the detailed molecular mechanisms for the various nonlinear rheological phenomena.

Applied Physics Letters, 2010

We have investigated the anisotropic transport behavior of InAs/AlSb heterostructures grown on a ... more We have investigated the anisotropic transport behavior of InAs/AlSb heterostructures grown on a (001) InP substrate. An electrical analysis showed anisotropic sheet resistance Rsh and electron mobility μn in the two dimensional electron gas (2DEG). Hall measurements demonstrated an enhanced anisotropy in μn when cooled from room temperature to 2 K. High electron mobility transistors exhibited 27% higher maximum drain current IDS and 23% higher peak transconductance gm when oriented along the [1-10] direction. The anisotropic transport behavior in the 2DEG was correlated with an asymmetric dislocation pattern observed in the surface morphology and by cross-sectional microscopy analysis of the InAs/AlSb heterostructure.

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.