Co-reporter:Yue Zhao, Jianning Liu, Xiaoxiao Li, Yue Lu, and Shi-Qing Wang
Macromolecules March 14, 2017 Volume 50(Issue 5) pp:2024-2024
Publication Date(Web):February 20, 2017
DOI:10.1021/acs.macromol.6b02158
Tensile extension tests have been carried out to examine mechanical responses of polymer glasses under the influence of small molecule additives that plasticize the polymers by lowering their glass transition temperatures Tg and speeding up their α relaxation times. It is shown that incorporation of 4% triphenyl phosphate (TPP) into poly(methyl methacrylate) turns a brittle PMMA at 70 °C into a ductile glass, capable of significant cold drawing, as expected according to the conventional wisdom. However, despite the evidence that the same TPP also reduces polystyrene’s Tg and increases the segmental mobility, the ductile PS turns brittle at 70 °C upon addition of 4% TPP at the same low drawing speed. This counterintuitive behavior is also found in the pair of TPP and poly(styrene–acrylonitrile). Thus, the present observations require us to incorporate new ingredients into the classical Eyring-type interpretation of plastic flow in a glassy polymer.
Co-reporter:Panpan Lin, Jianning Liu, Zhichen Zhao, Zhen-Gang Wang, Shi-Qing Wang
Polymer 2017 Volume 124(Volume 124) pp:
Publication Date(Web):25 August 2017
DOI:10.1016/j.polymer.2017.07.041
•Melt stretching can store significant internal energy and cause conformational distortion at the bond level.•Transient rheological response to startup extension does not follow the time-temperature superposition.•Quenched melt stretched samples show retractive stress below Tg.We carry out simultaneous mechanical and IR-thermal-imaging based temperature measurements of SBR melts during uniaxial extension in order to delineate the nature of the observed mechanical responses. Using the first law of thermodynamics, we evaluate the enthalpy change h1 associated with the temperature rise in the extending melt, estimate the heat loss to the surrounding, and conclude that there is an appreciable non-thermal enthalpic buildup h2 = (w − h1 − q) during either adiabatic or isothermal extension. The monotonic increase of h2 with the stretching ratio λ until the onset of inhomogeneous extension or melt rupture reveals that fast melt extension is largely elastic even after yielding in presence of partial chain disentanglement. At high rates, the lock-up of chain entanglement produces such a high level of h2 that is rarely seen in extension of crosslinked rubbers. When melt extension is carried out under isothermal condition, we show that the time-temperature superposition principle (TTS) fails to predict the transient response of a SBR melt at a fixed effective rate involving three temperatures. The failure of the TTS suggests that the terminal chain dynamics show different temperature dependence from the local segmental dynamics that control the transient stress responses.Download high-res image (301KB)Download full-size image
Co-reporter:Shi-Qing Wang
Soft Matter (2005-Present) 2017 vol. 13(Issue 29) pp:5083-5083
Publication Date(Web):2017/07/26
DOI:10.1039/C7SM90111A
Correction for ‘Nonlinear rheology of entangled polymers at turning point’ by Shi-Qing Wang et al., Soft Matter, 2015, 11, 1454–1458.
Co-reporter:Shiwang Cheng;Yuyuan Lu;Gengxin Liu
Soft Matter (2005-Present) 2017 vol. 13(Issue 29) pp:5084-5084
Publication Date(Web):2017/07/26
DOI:10.1039/C7SM90110K
Correction for ‘Finite cohesion due to chain entanglement in polymer melts’ by Shiwang Cheng et al., Soft Matter, 2016, 12, 3340–3351.
Co-reporter:Sirui Ge, Xiangyang Zhu, Shi-Qing Wang
Polymer 2017 Volume 125(Volume 125) pp:
Publication Date(Web):8 September 2017
DOI:10.1016/j.polymer.2017.07.081
•Time-dependent “shear thinning” is demonstrated at a constant shear stress without strain localization.•Entanglement-disentanglement transition (EDT) is demonstrated to occur in absence of any edge failure.•EDT can also occur in stress-controlled oscillatory shear.We have carried out various rheometric experiments to probe the nonlinear rheological behavior of two well-entangled 1,4-polybutadiene solutions using both stress-controlled and rate-controlled startup shear. At a polymer concentration of 2% neither wall slip nor shear banding occurs, and there is no visible edge failure. At a moderate level of shear stress, the shear rate can take an induction time tind longer than the reptation time to rise toward an eventual steady-state value. The time tind for the entanglement-distanglement transition (EDT) shortens exponentially with increasing stress. The state of entanglement also changes in response to a sinusoidal stress σ0sinωt when its amplitude σ0 is beyond a threshold to produce a sufficiently high strain amplitude γ0 (>1). Moreover, the evolution of the state of chain entanglement during and after constant-stress creep has been studied using two different protocols. The first switches from creep to rate-controlled startup shear to determine the stress overshoot characteristic. In the second protocol, after creep to different stages, the sample is evaluated for its ability to undergo elastic recovery. We found that the overshoot response to the rate switching weakens and the ability to undergo elastic recoil deteriorates as the EDT progresses. These changes are clearly inherent because edge instability could not produce such effects. Thus, it is confirmed that an entanglement-disentanglement transition is a leading characteristic rheological response of these solutions.Download high-res image (295KB)Download full-size image
Co-reporter:Shiwang Cheng, Yuyuan Lu, Gengxin Liu and Shi-Qing Wang
Soft Matter 2016 vol. 12(Issue 14) pp:3340-3351
Publication Date(Web):22 Feb 2016
DOI:10.1039/C6SM00142D
Three different types of experiments, quiescent stress relaxation, delayed rate-switching during stress relaxation, and elastic recovery after step strain, are carried out in this work to elucidate the existence of a finite cohesion barrier against free chain retraction in entangled polymers. Our experiments show that there is little hastened stress relaxation from step-wise shear up to γ = 0.7 and step-wise extension up to the stretching ratio λ = 1.5 at any time before or after the Rouse time. In contrast, a noticeable stress drop stemming from the built-in barrier-free chain retraction is predicted using the GLaMM model. In other words, the experiment reveals a threshold magnitude of step-wise deformation below which the stress relaxation follows identical dynamics whereas the GLaMM or Doi–Edwards model indicates a monotonic acceleration of the stress relaxation dynamics as a function of the magnitude of the step-wise deformation. Furthermore, a sudden application of startup extension during different stages of stress relaxation after a step-wise extension, i.e. the delayed rate-switching experiment, shows that the geometric condensation of entanglement strands in the cross-sectional area survives beyond the reptation time τd that is over 100 times the Rouse time τR. Our results point to the existence of a cohesion barrier that can prevent free chain retraction upon moderate deformation in well-entangled polymer melts.
Co-reporter:Gengxin LiuShi-Qing Wang
Macromolecules 2016 Volume 49(Issue 24) pp:9647-9654
Publication Date(Web):December 9, 2016
DOI:10.1021/acs.macromol.6b02053
The present rheological study reveals for the first time that entangled polymer solutions made of linear polystyrene or poly(methyl methacrylate) can exhibit strain hardening due to non-Gaussian stretching during startup shear at sufficiently high rates and temperatures well above their overall glass transition temperatures: Tg,solute > Texp > Tg,solution. The solutions made of high-Tg polymers only show partial yielding in the sense that both shear and normal stresses grow monotonically in time until a point of rupture, signified by an emergent cusp in the stress vs strain curve and macroscopic breakup along a shear plane. The shear softening-to-hardening transition, which occurs as a function of the applied shear rate, happens at lower equivalent rate with decreasing temperature, violating the time–temperature superposition principle.
Co-reporter:Panpan Lin, Jianning Liu, Shi-Qing Wang
Polymer 2016 Volume 89() pp:143-153
Publication Date(Web):20 April 2016
DOI:10.1016/j.polymer.2016.02.051
•Simultaneous mechanical and thermal measurements of a glassy polymer reveal significant internal (potential) energy storage during large ductile extension.•The internal energy buildup is both intrasegmental and intersegmental in origin.•Strain hardening involves considerable elastic deformation, leading to much higher energy than that associated with rubbery elasticity.•The high intrasegmental stress is plausibly due to emergence of chain tension in load-bearing strands of the chain network.We carry out simultaneous mechanical and IR-thermal-imaging-based temperature measurements of tensile extension on untreated, milled (mechanically “rejuvenated”) and melt-stretched bisphenol A-polycarbonate (PC). The extension is found to cause significant buildup of both excess internal energy u2 and plastic dissipation. The magnitude of u2 is one to two orders of magnitude higher than the energy involved in rubbery elastic deformation. While the ratio of u2 to the mechanical work w decreases with increasing rate of extension for untreated PC, milled PC is found to be more dissipative at lower rates. Homogeneous extension of melt-stretched PC in the post-yield regime including strain hardening behavior reveals largely non-dissipative responses, emphasizing the plastic deformation of glassy polymer may not be fully dissipative. The experimental results clearly indicate that a significant component of stress can be intrasegmental leading to the observed buildup of internal energy by distortions of covalent bonds. The glassy polymer physics at the chain level complements the more familiar idea of inter-segmental dissipation as the dominant event during plastic deformation.
Co-reporter:Jianning Liu, Panpan Lin, Shiwang Cheng, Weiyu Wang, Jimmy W. Mays, and Shi-Qing Wang
ACS Macro Letters 2015 Volume 4(Issue 10) pp:1072
Publication Date(Web):September 9, 2015
DOI:10.1021/acsmacrolett.5b00442
Polystyrene of different molecular weights and their binary mixtures are studied in terms of their various mechanical responses to uniaxial compression at different temperatures. PS of Mw = 25 kg/mol is completely brittle until it is above its glass transition temperature Tg. In contrast, upon incorporation of a high molecular weight component, PS mixtures turn from barely ductile a few degrees below its Tg to ductile over 40° below Tg. In the upper limit, a PS of Mw = 319 kg/mol yields and undergoes plastic flow, even at T = −70 °C. The observed dependence of mechanical responses on molecular weight and molecular weight distribution can be adequately rationalized by the idea that yielding and plastic compression are caused by chain networking.
Co-reporter:Xiaoxiao Li and Shi-Qing Wang
ACS Macro Letters 2015 Volume 4(Issue 10) pp:1110
Publication Date(Web):September 16, 2015
DOI:10.1021/acsmacrolett.5b00554
We have carried out a series of tensile extension tests on the two most common polymer glasses to describe their generic mechanical responses as a function of deformation rate at different temperatures. The essentially defect-free polystyrene and poly(methyl methacrylate) both show remarkable re-entrant failure: being ductile at intermediate rates and showing diminishing toughness at both higher and lower rates. We draw phase diagrams to map out the relationship between brittle-like and yield-like states in terms of temperature, rate, and stress. A coherent understanding of the rich phenomenology requires us to describe in more detail the interplay between the chain network and the primary structure bonded by intersegmental van der Waals forces.
Co-reporter:Shi-Qing Wang
Soft Matter 2015 vol. 11(Issue 8) pp:1454-1458
Publication Date(Web):21 Jan 2015
DOI:10.1039/C4SM02664K
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.
Co-reporter:Shi-Qing Wang
Soft Matter 2015 vol. 11(Issue 8) pp:1646-1646
Publication Date(Web):29 Jan 2015
DOI:10.1039/C5SM90023A
Correction for ‘Nonlinear rheology of entangled polymers at turning point’ by Shi-Qing Wang et al., Soft Matter, 2015, DOI: 10.1039/c4sm02664k.
Co-reporter:Jianning Liu, Panpan Lin, Xiaoxiao Li, Shi-Qing Wang
Polymer 2015 Volume 81() pp:129-139
Publication Date(Web):16 December 2015
DOI:10.1016/j.polymer.2015.11.015
•Nonlinear stress relaxation behavior of polymer glasses studied experimentally.•Correlation between the strain rate and rate of stress relaxation is demonstrated.•Rate of the stress relaxation is found to be linearly proportional to the deformation rate for different polymer glasses.•Long lasting molecular mobility is plausibly due to chain tension in load-bearing strands (LBSs) of the chain network.We study nonlinear stress relaxation behavior of several polymer glasses during ductile large deformation of either extension or compression over a wide range of deformation rate at room temperature. In the pre-yield regime, the stress decline is logarithmically slow and essentially independent of the applied rate. The stress relaxation from the post-yield regime shows universal time rescaling, i.e., its time dependence is identical after multiplying the elapsing time with the deformational rate, for different polymer glasses undergoing either uniaxial extension or compression. After initial fast “plastic” relaxation, the stress decay returns to the same manner as that displayed by the pre-yield stress relaxation. After reviewing available modeling, we propose to interpret these results in terms of a recent picture for polymer glasses under large deformation that envision a chain networked embedded in a glassy matrix. Specifically we suggest that a) the enhanced molecular mobility during deformation is produced by load-bearing strands (LBSs) of the chain network, b) the initial high molecular mobility upon termination of external deformation survives thanks to the residual chain tension in LBSs, and c) the initial rapid “plastic relaxation” ceases when chain tension drops below a threshold that can no longer keep the segments (surrounding LBSs) activated.
Co-reporter:Panpan Lin, Shiwang Cheng, and Shi-Qing Wang
ACS Macro Letters 2014 Volume 3(Issue 8) pp:784
Publication Date(Web):July 24, 2014
DOI:10.1021/mz5004129
The origin of high mechanical stresses in large deformation of polymer glasses has been elusive because both plasticity and elasticity take place. In this work on the nature of the mechanical responses, we carry out uniaxial compression experiments to make simultaneous mechanical and thermal measurements of polycarbonate. Our results confirm that two factors contribute to the growing mechanical stress in the post-yield regime, which is known as “strain hardening”. Besides plastic deformation that is intersegmental in origin, chain tension as an intrasegmental component contributes considerably to the measured stress in post-yield. Such a conclusion modifies the previous consensus regarding the nature of strain hardening in mechanical deformation of polymer glasses.
Co-reporter:Yuyuan Lu, Lijia An, Shi-Qing Wang, and Zhen-Gang Wang
ACS Macro Letters 2014 Volume 3(Issue 6) pp:569
Publication Date(Web):May 30, 2014
DOI:10.1021/mz500260h
Using Brownian dynamics simulation, we determine the chain orientation and stretching and their connection to stress overshoot in an entangled polymer melt undergoing startup shear at rates lower than the reciprocal of the Rouse time yet higher than the reciprocal of the reptation time. In this rate regime, the prevailing tube theory attributes the stress overshoot to alignment of the primitive chain. In contrast, our results reveal that there is substantial chain stretching that persists well beyond the Rouse time, and it contributes significantly to the initial stress growth. In particular, stress overshoot is found to be primarily due to chain retraction after considerable stretching rather than chain orientation.
Co-reporter:Shiwang Cheng and Shi-Qing Wang
Macromolecules 2014 Volume 47(Issue 11) pp:3661-3671
Publication Date(Web):May 21, 2014
DOI:10.1021/ma500570w
This work delineates the molecular mechanism and various characteristics of a remarkable elastic yielding phenomenon observed in cold drawn ductile polymer glasses including poly(2,6-dimethyl-1,4-phenylene oxide) (PPE), bisphenol A–polycarbonate (PC), poly(ethylene terephthalate) (PET), and poly(methyl methacrylate) (PMMA). Elastic yielding occurs when a sizable retractive stress appears from a cold-drawn polymer glass (stored in stress-free state for days after unloading) during annealing above the storage temperature Tst that is still well below the glass transition temperature Tg. The induction time characterizing the emergence and buildup of the retractive stress to a level of σ* above 10 MPa and the magnitude of σ* both depends on Tst and the cold drawing temperature Tcd as well as the annealing temperature Tel-yield. It is asserted that there is significant chain tension in the load-bearing strands (LBSs) of the chain network, produced during cold drawing of these ductile polymer glasses. The chain tension is preserved by the vitrification during storage and can drive the glass to undergo localized yielding. Upon “thawing” the segments surrounding LBSs, the chain tension can transmit across the chain network to show up as macroscopic retractive stress.
Co-reporter:Hao Sun, Gengxin Liu, Konstantinos Ntetsikas, Apostolos Avgeropoulos, and Shi-Qing Wang
Macromolecules 2014 Volume 47(Issue 16) pp:5839-5850
Publication Date(Web):August 12, 2014
DOI:10.1021/ma500899s
This work studies the transient rheological responses of different entangled melts to startup deformation at unconventionally high rates, which can be accessed by working at sufficiently low temperature, e.g., at and below 130 °C for polystyrene. When the rate is as high as the second crossover frequency ωe observed from the small amplitude oscillatory shear (SAOS) data for storage and loss modulus G′ and G″, there is sizable viscous stress that dominates the initial mechanical response. It is shown based on the various polymer melts including PS, PMMA, SBR, PC, and PS mixtures that this viscous component of the stress cannot be neglected when characterizing such fast deformation in either extension or shear. This sizable frictional addition to the rubber elasticity component of the initial stress is founded to be preceded by solid-like deformation. The remarkable transient elasticity can be characterized by a modulus Ginterseg that grows well beyond the magnitude of the melt plateau modulus GN0 as the temperature lowers toward the glass transition temperature Tg. In the case of PS, Ginterseg reaches a level of 300 MPa at 110 °C and reduces to 2.0 MPa, i.e., 10GN0, at 130 °C. The initial elasticity is short-lived because the melts quickly transition to a state of viscous flow at a strain of just a few percent. The magnitude of the viscous stress at the solid-to-liquid transition defines a yield stress. This yield stress is found to scale with the applied rate in a power law, agreeing with the dependence of G″ on frequency ω from the SAOS data. Moreover, the intersegmental effect, i.e., the transient elasticity, is shown to also take place in unentangled and barely entangled PS melts as well as branched polyisoprene melts.
Co-reporter:Yuyuan Lu, Lijia An, Shi-Qing Wang, and Zhen-Gang Wang
Macromolecules 2014 Volume 47(Issue 15) pp:5432-5435
Publication Date(Web):July 18, 2014
DOI:10.1021/ma500131f
Co-reporter:Yuyuan Lu, Lijia An, Shi-Qing Wang, and Zhen-Gang Wang
ACS Macro Letters 2013 Volume 2(Issue 6) pp:561
Publication Date(Web):June 6, 2013
DOI:10.1021/mz400145m
Using Brownian dynamics simulation, we determine chain dimensions in an entangled polymer melt undergoing startup shear at a rate lower than the reciprocal of the Rouse time yet higher than the reciprocal reptation time. Here the tube model expects negligible chain stretching. In contrast, our simulation shows the deformed coil to conform closely to affine deformation. We find that the total number of entanglements decreases with increasing shear. Remarkably, up to many Rouse time, the decline in the number of initial entanglements is slower than that under the quiescent condition. These results point to fundamental deficiencies in the molecular picture of the tube model for startup shear.
Co-reporter:Gengxin Liu, Hongwei Ma, Hyojoon Lee, Hongde Xu, Shiwang Cheng, Hao Sun, Taihyun Chang, Roderic P. Quirk, Shi-Qing Wang
Polymer 2013 Volume 54(Issue 24) pp:6608-6616
Publication Date(Web):14 November 2013
DOI:10.1016/j.polymer.2013.10.007
We explored a new synthetic strategy for ultra-high molecular weight long-chain branched (LCB) polymers with equal spacing between adjacent branch points. This method can synthesize LCB polystyrene (LCB-PS) with total molecular weight of 4.9 million g/mole, 16 branches of 140 kg/mole and polydispersity index of 1.5. The introduction of multiple branch points with long side chains allows the LCB-PS to resist the elastic-driven decohesion. Even after a large step extension of stretching ratio λ = 7.4, the specimen would not undergo elastic breakup that occurs in linear PS even at λ = 2.7. These LCB-PSs are also extraordinarily more stretchable during startup uniaxial extension, with the maximum engineering stress emerging at stretching ratio λmax≈4Mbb/Me, where Mbb is the molecular weight of backbone and Me is the molecular weight between entanglements.
Co-reporter:Shiwang Cheng, Lilian Johnson, Shi-Qing Wang
Polymer 2013 Volume 54(Issue 13) pp:3363-3369
Publication Date(Web):7 June 2013
DOI:10.1016/j.polymer.2013.04.036
We apply a constant tensile load (i.e., fixed engineering stress) at different temperatures to examine the mechanical and structural responses of polycarbonate (PC) glass. Unlike the more common displacement-controlled extension, the tensile creep test allows the glass to develop structural failures on its time scales. At room temperature, surface crazes over time at sufficiently high tensile loads, and shear yielding as well as stable necking take place above an engineering stress of 48 MPa. However, none of these two types of strain localization occurs when the same PC is first melt-stretched and quenched back to room temperature. The suppression of crazing and necking by melt stretching can be anticipated based on the picture of a hybrid structure for polymer glasses under large deformation and emphasizes the geometric condensation as an effective way to enhance the chain network.
Co-reporter:Shi-Qing Wang, Yangyang Wang, Shiwang Cheng, Xin Li, Xiangyang Zhu, and Hao Sun
Macromolecules 2013 Volume 46(Issue 8) pp:
Publication Date(Web):April 12, 2013
DOI:10.1021/ma300398x
The present work discusses four types of new experiments that can improve the current theoretical description of nonlinear rheology of entangled polymers. First, a slowly imposed strain is found to result in nonmonotonic evolution of the state of chain entanglement during quiescent relaxation, consistent with the idea of chain disentanglement after step shear. Second, the stress relaxation upon a sizable step strain is found to be identical to that for small step strain, consistent with a molecular scenario that a strained entangled melt has an entropic barrier to resist chain retraction. Third, the ability of a step-strained polymer to undergo elastic recovery is found to be the same up to strain amplitude of unity, and a sample sheared for a period much longer than the Rouse time is shown to still undergo nearly full elastic recovery. Fourth, an entangled melt, stretched at a rate significantly lower than the Rouse relaxation rate, undergoes full elastic recovery until the point of tensile force maximum. We have discussed an alternative conceptual framework to describe these nonlinear responses of entangled polymers despite the possibility that the tube model might be further remedied to characterize the new rheometric measurements presented in this work.
Co-reporter:Hao Sun, Konstantinos Ntetsikas, Apostolos Avgeropoulos, and Shi-Qing Wang
Macromolecules 2013 Volume 46(Issue 10) pp:4151-4159
Publication Date(Web):May 2, 2013
DOI:10.1021/ma3025255
In rheological characterization of polymeric materials, the time–temperature superposition (TTS) principle allows us to acquire a wider spectrum of information on polymer dynamics. Although there are reports of the failure of TTS when both chain dynamics and local segmental dynamics are accessible at each of several temperatures, we have assumed that TTS would apply to describe the temperature dependence of transient responses of entangled melts to fast startup deformation. In this work we show that at the same effective rate (e.g., the same Rouse–Weissenberg number WiR equal to the product of the Hencky rate and the Rouse time) of uniaxial extension the nonlinear responses of several polymer melts are each different at different temperatures, pointing to an evident breakdown of the TTS. Specifically, we will show that for the same WiR well-entangled polymer melts rupture at relatively low temperatures, yet still 20–30 deg above the glass transition temperature Tg, but undergo necking-like failure at higher temperatures. Thus, at the same WiR, stress–strain curves are significantly different at different temperatures. Moreover, at the lower temperature, these polymer melts can reach an extreme level of extensibility at a critical WiR, which is completely unattainable at higher temperatures. These TTS violating phenomena present a serious challenge to our existing theoretical understanding of nonlinear rheology of entangled polymeric liquids.
Co-reporter:Gengxin Liu and Shi-Qing Wang
Macromolecules 2012 Volume 45(Issue 16) pp:6741-6747
Publication Date(Web):August 7, 2012
DOI:10.1021/ma3010026
Co-reporter:Gregory D. Zartman, Shiwang Cheng, Xin Li, Fei Lin, Matthew L. Becker, and Shi-Qing Wang
Macromolecules 2012 Volume 45(Issue 16) pp:6719-6732
Publication Date(Web):August 10, 2012
DOI:10.1021/ma300955h
This work takes a simple phenomenological approach to the questions of when, how, and why a brittle polymer glass turns ductile and vice versa. Perceiving a polymer glass as a hybrid, we recognize that both the primary structure formed by van der Waals forces (network 1) and chain network (i.e., the vitrified entanglement network) (network 2) must be accounted for in any discussion of the mechanical responses. To show the benefit of this viewpoint, we first carried out well-defined melt-stretching experiments on four common polymer glasses (PS, PMMA, SAN, and PC) in a systematic way either at a fixed Hencky strain rate to a given degree of stretching at several temperatures or at a given temperature to different levels of stretching using the same Hencky rate. Then we attempted to preserve the effect of melt-stretching on the chain network structure by rapid thermal quenching. Subsequent room-temperature tensile extension of these melt-stretched amorphous polymers reveals something universal: (a) along the direction of the melt-stretching, the brittle glasses (PS, PMMA, and SAN) all become completely ductile; (b) perpendicular to the melt-stretching direction, the ductile glass (PC) becomes brittle at room temperature. We suggest that the transformations (from brittle to ductile or ductile to brittle) arise from either geometric condensation or dilation of load bearing strands in the chain network due to the melt-stretching. Regarding a polymer glass as a structural hybrid, we also explored two other cases where the ductile PC becomes brittle at room temperature: (1) upon aging near the glass transition temperature; (2) when blended with PC of sufficiently low molecular weight. These results indicate that (i) the strengthening of the primary structure by aging can raise the failure stress σ* to a level too high for the chain network to sustain and (ii) the PC blend becomes brittle upon weakening the chain network by dilution with short chains.
Co-reporter:Hao Sun
Science China Chemistry 2012 Volume 55( Issue 5) pp:779-786
Publication Date(Web):2012 May
DOI:10.1007/s11426-012-4496-y
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.
Co-reporter:Yangyang Wang, Xin Li, Xiangyang Zhu, and Shi-Qing Wang
Macromolecules 2012 Volume 45(Issue 5) pp:2514-2521
Publication Date(Web):February 17, 2012
DOI:10.1021/ma2024842
Using four entangled 1,4-polybutadiene solutions as model systems, we carry out rate-switching startup shear and elastic recovery tests to probe the state of chain entanglement during startup continuous shear and after shear cessation, respectively. Specifically, we apply a second startup shear of higher rate either during the first startup shear or after cessation of the first shear. The nonmonotonic response (overshoot) to the startup shear is evaluated to elucidate the effect of any preceding shear on the entanglement structure. In a second type of experiments, at different strains during and after shear or relaxing for a certain amount of time after shear cessation we set the shear stress free to determine the amount of elastic recoil, which can also reveal the status of chain entanglement interactions. Consistent with the emerging theoretical interpretations, these multistep rheometric procedures allow us to learn more about (a) elastic yielding during relaxation after cessation of shear that is produced at a low rate, (b) existence of a critical strain amplitude for the elastic yielding, (c) nonmonotonic evolution of the chain entanglement upon startup shear, and (d) yielding of the entanglement network at the point of shear stress overshoot.
Co-reporter:Shi-Qing Wang, S. Ravindranath, and P. E. Boukany
Macromolecules 2011 Volume 44(Issue 2) pp:183-190
Publication Date(Web):January 18, 2011
DOI:10.1021/ma101223q
The recent particle-tracking velocimetric (PTV) observations revealed that well-entangled polymer solutions and melts tend to either exhibit wall slip or assume an inhomogeneous state of deformation and flow during nonlinear rheological measurements in simple-shear rheometric setups. Many material parameters and external conditions have been explored since 2006, and a new phenomenological picture has emerged. In this Perspective, we not only point out the challenges to perform reliable rheometric measurements but also discuss the relation between wall slip and internal (bulk) cohesive breakdown and summarize all available findings in terms of a phase diagram. This map specifies the conditions under which shear homogeneity, interfacial slip, and bulk shear inhomogeneity would prevail. The paper is closed by enumerating a number of unresolved questions for future studies.
Co-reporter:Yangyang Wang and Shi-Qing Wang
Macromolecules 2011 Volume 44(Issue 13) pp:5427-5435
Publication Date(Web):June 9, 2011
DOI:10.1021/ma200432q
The nonlinear responses of an entangled SBR melt and its solutions in rapid uniaxial extension have been studied by rheometric and rheo-optical measurements. During (startup) extension, the samples progressively lose entanglements. At relatively low rates, all entanglements are eventually lost, leading to yielding, nonuniform extension, and ductile specimen failure. At sufficiently high rates, some entanglements survive to allow non-Gaussian stretching and full chain extension leading to rupture-type sample breakdown when the failure mechanism switches from disentanglement, i.e., mutual chain sliding, to chain scission. The precursor to rupture, i.e., non-Gaussian stretching, is evidenced by the birefringence measurements that show the breakdown of the linear stress–optical relation. The corresponding critical stress is found to be proportional to the polymer concentration, in agreement with a scaling analysis based on an entanglement network picture. The onset of non-Gaussian stretching and rupture occurs at higher stretching ratios when the entanglement strands are made longer by dilution as achieved in the three solutions. In other words, both yielding and rupture occur at higher strains with increasing entanglement spacing, underscoring the legitimacy to represent entangled polymers in terms of an elastic network when depicting their predominant nonlinear responses to rapid uniaxial extension.
Co-reporter:Sham Ravindranath;M. Olechnowicz;V. S. Chavan
Rheologica Acta 2011 Volume 50( Issue 2) pp:97-105
Publication Date(Web):2011 February
DOI:10.1007/s00397-010-0507-0
This work aims to elucidate how molecular parameters dictate the occurrence of inhomogeneous cohesive failure during step strain and large amplitude oscillatory shear (LAOS) respectively in entangled polymer mixtures. Based on three well-entangled polybutadiene (PB) mixtures, we perform simultaneous rheometric and particle-tracking velocimetric (PTV) measurements to illustrate how the slip length controls the degree of shear banding. Specifically, the PB mixtures were prepared using the same parent polymer (Mw ∼ 106 g/mol) at 10 wt.% concentration in respective polybutadiene solvents (PBS) of three different molecular weights 1.5, 10, and 46 kg/mol. After step strain, the entangled PB mixture with PBS-1.5 K displayed interfacial failure whereas the PB mixture with PBS-10 K showed bulk failure, demonstrating the effectiveness of our strategy to suppress wall slip by controlling PBS’ molecular weight. Remarkably, the PBS-46K actually allows the elastic yielding to occur homogeneously so that no appreciable macroscopic motions were observed upon shear cessation. PBS is found to play a similar role in LAOS of these three PB mixtures. Finally, we demonstrate that in case of the slip-prone mixture based on PBS-1.5 K the interfacial failure could be drastically reduced by use of shearing plates with considerable surface roughness.
Co-reporter:Xin Li
Rheologica Acta 2010 Volume 49( Issue 1) pp:
Publication Date(Web):2010 January
DOI:10.1007/s00397-009-0389-1
The present work shows, based on an effective particle-tracking velocimetric (PTV) method, that a commercial diphenylmethylvinyl silicone gum displays nonlinear rheological responses to startup shear and large amplitude oscillatory shear (LAOS) in a homogeneous manner, in contrast to monodisperse melts. Using an effective cone-partitioned plate (CPP) setup along with PTV, rheological characterizations of shear thinning in continuous shear and strain softening in LAOS are carried out reliably without the inherent experimental complication associated with edge fracture. Conversely, based on the CPP, the rheological effects of edge fracture are also illustrated for both startup shear and LAOS.
Co-reporter:Xin Li
Rheologica Acta 2010 Volume 49( Issue 10) pp:985-991
Publication Date(Web):2010 October
DOI:10.1007/s00397-010-0465-6
This work examines the possibility that the previously observed elastic yielding, i.e., nonquiescent relaxation after a large step shear (Ravindranath and Wang, Macromolecules 40:8031–8039, 2007) is due to an intrinsic experimental difficulty technically known as edge fracture. By redesigning the rheometric apparatus to eliminate edge failure, we show by an example of a well-entangled polymer solution that elastic yielding still occurs in the absence of any edge failure. We are also able to confirm that shear banding during large amplitude oscillatory shear (Ravindranath and Wang, J Rheol 52:341–358, 2008a) is an inherent rheological characteristic related to internal yielding of the entanglement network.
Co-reporter:Xin Li;Xiaorong Wang
Rheologica Acta 2010 Volume 49( Issue 9) pp:971-977
Publication Date(Web):2010 September
DOI:10.1007/s00397-010-0474-5
The present work explores nonlinear rheological behavior of a strongly viscoelastic paste made of nano-sized polybutadiene particles. Apart from conventional rheometric measurements, particle-tracking velocimetric observations are carried out to determine the macroscopic state of deformation during startup shear and after step strain. Despite its highly nonlinear rheological characteristics, the system shows no sign of inhomogeneous response to large shear deformations in sharp contrast to well-entangled polymeric liquids made of linear chains. Apparently strongly nonlinear rheological behavior can occur in absence of inhomogeneous macroscopic deformation.
Co-reporter:Xin Li and Shi-Qing Wang
Macromolecules 2010 Volume 43(Issue 13) pp:5904-5908
Publication Date(Web):June 11, 2010
DOI:10.1021/ma100875v
We probe the nature of steady shear flow of entangled polymer solutions by superimposing either small amplitude oscillatory shear or small step strain and analyzing the resultant mechanic responses. Our results show that (a) polymer dynamics (in terms of stress relaxation) are accelerated relative to the quiescent dynamics in direct proportion to the underlying shear rate and (b) the steady shear is a viscous state where chains are displaced past one another on a time scale of the reciprocal rate, consistent with the idea of convective constraint release.
Co-reporter:Pouyan E. Boukany and Shi-Qing Wang
Macromolecules 2010 Volume 43(Issue 17) pp:6950-6952
Publication Date(Web):August 9, 2010
DOI:10.1021/ma101267b
Co-reporter:Yangyang Wang
Rheologica Acta 2010 Volume 49( Issue 11-12) pp:1179-1185
Publication Date(Web):2010 December
DOI:10.1007/s00397-010-0491-4
We have carried out uniaxial extension experiments on a monodisperse entangled melt to illustrate the origin of failure in the glass-like zone defined by Malkin and Petrie (J Rheol 41:1–25, 1997). The entangled melt was found to undergo a yield-to-rupture transition beyond a critical rate. We show that the onset of the “glass-like” zone defined by Malkin and Petrie is actually in the middle of the rubbery plateau where the mechanical response of entangled melts is not dictated by glassy chain dynamics. The rupture occurs plausibly through chain scission in the limit of finite chain extensibility.
Co-reporter:ShiQing Wang
Science China Chemistry 2010 Volume 53( Issue 1) pp:151-156
Publication Date(Web):2010 January
DOI:10.1007/s11426-010-0017-z
Polymer rheology has a long research tradition and holds an important position in the scientific community. It is supposed to be a valuable subject to the vast polymer industry. This article points out the exciting developments that are transforming our understanding of the world of polymer rheology. We discuss not only the emerging challenges in the area but also how China might wish to seize the moment, pick up the trend and take advantage of the potential economic benefits to the huge petro-chemical industry of China.
Co-reporter:Pouyan E. Boukany and Shi-Qing Wang
Soft Matter 2009 vol. 5(Issue 4) pp:780-789
Publication Date(Web):15 Dec 2008
DOI:10.1039/B804791J
In this study we have carried out a combination of rheometric and particle-tracking velocimetric (PTV) measurements to investigate nonlinear rheological behavior of three entangled DNA solutions (with ca. 150 entanglements per chain) and, in particular, to explore a transformation from slip-dominated steady-state flow to bulk shear inhomogeneity. In the stress plateau regime, an elastic recoil-like response occurs transiently at either interfaces or sample interior after stress overshoot during a startup shear. In both startup shear and creep mode, wall slip, bulk shear banding or a combination of both have been observed in both transient and steady states. The water-based solution shows massive wall slip allowing the bulk to remain in the Newtonian flow regime. Use of glycerol as a solvent can effectively reduce interfacial slip, permitting bulk shear banding to develop in both controlled-rate and controlled-stress modes. For the glycerol based solution, a sufficiently high Weissenberg number can attain in the rheometer where PTV observations reveal homogenous shear in steady state.
Co-reporter:Pouyan E. Boukany and Shi-Qing Wang
Macromolecules 2009 Volume 42(Issue 6) pp:2222-2228
Publication Date(Web):February 23, 2009
DOI:10.1021/ma802644r
We study yielding at interface between a solid wall and entangled polymer melts. At a average rate higher than the terminal relaxation rate, a combination of rheometric and particle-tracking velocimetric (PTV) measurements reveals apparent wall slip (or interfacial yielding) at a strain γiy, just beyond the stress overshoot. PTV observations show that the interfacial yielding occurs at a higher strain in the presence of a faster rate of shear. Shear cessation at a strain lower than γiy is observed to also result in interfacial failure, suggesting that the finite adhesion can be overcome in quiescence by the residual elastic retraction force. These two kinds of interfacial failure can be theoretically understood in terms of the intrachain elastic retraction forces overcoming the melt adhesion arising from interchain entanglement interactions between adsorbed and bulk chains.
Co-reporter:Pouyan E. Boukany, Shi-Qing Wang and Xiaorong Wang
Macromolecules 2009 Volume 42(Issue 16) pp:6261-6269
Publication Date(Web):July 24, 2009
DOI:10.1021/ma9004346
This work studies the most basic and important behavior of entangled linear polymer melts in sudden large shear deformations. In particular, melt elasticity resulting from the large step shear is extensively shown to produce cohesive breakdown. Unlike entangled solutions studied in Macromolecules 2007, 40, 8031, the residual elastic forces in sheared melts struggle quiescently for a significant induction period before bringing down the entanglement network. The induction time for the elastic yielding can be much longer than the longest Rouse relaxation time τR, making it difficult to associate this cohesive failure with a chain retraction process envisioned in the tube theory. The cohesive failure also occurs upon a step strain produced at rates too slow to produce chain stretching, again making it unreasonable to invoke the concept of chain retraction due to chain stretching.
Co-reporter:Shi-Qing Wang
Journal of Polymer Science Part B: Polymer Physics 2008 Volume 46( Issue 24) pp:2660-2665
Publication Date(Web):
DOI:10.1002/polb.21588
Co-reporter:Shi-Qing Wang
Macromolecular Materials and Engineering 2007 Volume 292(Issue 1) pp:15-22
Publication Date(Web):2 JAN 2007
DOI:10.1002/mame.200600351
This Feature Article describes a plausible picture of entangled polymer shear flow behavior, resulting from a decade of efforts in our laboratory to unravel the origins of a host of intricate and interconnected rheological phenomena. The first report of spurt flow by Bagley (1958) raised several key questions: Does the spurt flow reflect a constitutive failure as suggested by Vindogradov et al. (1972), Doi-Edwards (1979) and McLeish-Ball (1986)? How is it related to sharkskin, “melt fracture” and/or wall slip? The subject remained unsettled and “paradoxical” [Denn (1990)] until the 1990's when a first quantitative description of polymer slip was provided by Brochard and de Gennes (1992). The experimental elucidation [Rheol. Acta1998, 37, 415] of the interfacial origin of the spurt flow led to the conclusion that the stress maximum in the Doi-Edwards theory (1979) was a theoretical artefact and to a decade of efforts to derive a constitutive description of polymer flow by incorporating Marrucci's idea of convective constraint release (1996). Ongoing studies (2003–present) of constitutive flow and interfacial slip behavior of entangled polymer solutions have revealed something surprising: the entangled polybutadiene solutions appear to respond differently to rate-controlled vs. stress-controlled shear. The results [Macromolecules2004, 37, 9083] imply that there would not necessarily be a homogeneous shear field in a simple-shear apparatus in the rate-controlled mode. An effective particle tracking velocimetric (PTV) method was developed to allow determination of the velocity profile across the gap in various shear apparatuses including cone-plate and linear sliding-plate setups for startup shear, large amplitude oscillatory shear (LAOS) and step strain experiments. Our PTV observations show an initial linear velocity variation across the gap and a nonlinear velocity profile beyond the shear stress overshoot [Phys. Rev. Lett. 2006, 96, 016001], growth of shear banding in LAOS [Phys. Rev. Lett. 2006, 96, 196001] and elastic breakup of several entangled solutions after step strain [Phys. Rev. Lett. 2006, 97, 187801]. These results present great challenges to both the prevailing theoretical description of entangled polymer flow that is based on tube models and the conventional rheometric protocols used to experimentally determine constitutive flow behavior, and questions the validity to perform numerical simulations of polymer processing based on the available constitutive models.
Co-reporter:Shi-Qing Wang
Macromolecular Materials and Engineering 2007 Volume 292(Issue 1) pp:
Publication Date(Web):22 JAN 2007
DOI:10.1002/mame.200790000
Cover: Building a proper molecular-level description of polymer flow is a formidable task: How do entangled chains respond to shear deformation? Can the tube model depict elastic breakup? How to walk on this two-way road linking molecular structures to macroscopic behavior? Polymer is like elastic noodles. PTV (particle-tracking velocimetry) helps us detect their averaged flow responses. Further details can be found in the article by S.-Q. Wang on page 15.
Co-reporter:Shi-Qing Wang
Journal of Polymer Science Part B: Polymer Physics 2003 Volume 41(Issue 14) pp:1589-1604
Publication Date(Web):4 JUN 2003
DOI:10.1002/polb.10524
This review article scrutinizes and reanalyzes the extensively available literature data on the tracer and self chain diffusion coefficients Dtr and Ds along with the corresponding zero-shear viscosity η0 to show that Ds ∝ M−ν starts with ν > 2.0 and converges to the asymptotic scaling exhibited by Dtr ∝ M−2.0 as the molecular weight M increases beyond M/Me = 10–20, in contrast to the onset of the asymptotic scaling M3 for η0 taking place typically for M/Me ≫ 10–20. A coherent analysis of these observations leads to the suggestion that the observed crossover in Ds is due to the constraint release effect, which diminishes around M/Me = 10–20 and is negligible in measurements of Dtr when the matrix molecular weight P is much greater than M. The contour length fluctuation (CLF) effect, which is believed to cause the molecular weight scaling of η0 to deviate significantly from its limiting behavior of M3, has little direct influence on the chain diffusion. The absence of the CLF effect on Ds leads to a much stronger than linear dependence of the product η0Ds on M, which has been observed previously. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1589–1604, 2003
Co-reporter:Orin L. Hemminger, Pouyan E. Boukany, Shi-Qing Wang, L.J. Lee
Journal of Non-Newtonian Fluid Mechanics (December 2010) Volume 165(Issues 23–24) pp:1613-1624
Publication Date(Web):1 December 2010
DOI:10.1016/j.jnnfm.2010.08.009
The present work explores unusual flow behavior of entangled fluids in an abrupt contraction flow device. Fluorescent imaging was carried out on four different entangled DNA solutions with concentrations ranging from 0.1 to 1.0% (with a wide range of entanglements per chain Z = 7–55). For weakly entangled solutions (Z < 30), vortex flow was dominant at high flow rates. However, for well-entangled DNA solutions (Z ≥ 30), unusual time dependant shear banding was observed at the contraction entrance. Upon reducing the slip length by adding sucrose to the well-entangled DNA solution, vortex flow became dominant again. In vortex flow, most DNA chains remained coiled at the corner in regular recirculation. However, when jerky-shear-banding flow developed, significant stable stretching of DNA chains occurred at the center-line, with quasi-periodic switching between stretching and recoil at the corner.
Co-reporter:Shi-Qing Wang
Macromolecules () pp:
Publication Date(Web):February 29, 2008
DOI:10.1021/ma702332n
We have studied nonlinear flow behavior of entangled DNA solutions using particle tracking velocimetry in Couette and cone/plate geometries. At apparent shear rate γ̇app < 0.1 s−1, the velocity profile is linear across the gap. Beyond the terminal region with γ̇app < 40 s−1, the velocity profile becomes temporarily banded after the stress maximum and shows massive wall slip at long times with little shear banding in the bulk. At γ̇app > 40 s−1, the velocity profile progressively curves and becomes banded with a sharp interface after hundreds of strain units. In the steady state, the thickness of high-shear band increases linearly with the apparent shear rate. At γ̇app = 1000 s−1, the velocity profile returns to linearity.
Co-reporter:Sham Ravindranath ; Shi-Qing Wang ; Michael Olechnowicz ;Roderic P. Quirk
Macromolecules () pp:
Publication Date(Web):March 8, 2008
DOI:10.1021/ma7027352
Velocity profiles of six entangled polybutadiene solutions (PBD) have been determined during startup shear using a particle tracking velocimetric (PTV) technique, where the number Z of entanglements per chain in these solutions varies from 13 to 119, depending on the PBD molecular weight and solution concentration. Flow behavior of these solutions at various rates in the stress plateau region has been investigated in both parallel-disk and cone−plate cells. For the least entangled solution with Z = 13, homogeneous shear was observed under all flow conditions. The solution with Z = 27 displayed inhomogeneous shear after the stress maximum before returning to a linear velocity profile at long times. For solutions with Z ≥ 40, shear banding was observed in both transient and steady states for a range of shear rates in the stress plateau region. At sufficiently high rates, shear homogeneity returns in steady state for these solutions (Z ≥ 40) after initial banding.
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