Polymer Dynamics, Interfaces and Rheology Group

Department of Polymer Science - University of Akron

Shi-Qing Wang

Ph.D. (Physics), University of Chicago (1987).

Fellow of the American Physical Society (1997)

Member of American Physical Society, Society of Rheology and Materials Research Society.

Research Interests

Physics and engineering of polymeric and other structured materials: experimental and theoretical foundations of rheology; phenomenology of linear and nonlinear viscoelastic processes; dynamic, yielding and fracture behaviors; flow instabilities and processing phenomena including wall slip and melt fracture. The four real-time movies below illustrate (a) sharkskin formation (extrusion of polybutadiene) followed by wall slip at higher pressure, (b) particle-tracking velocimetric (PTV) observations of (c) startup shear on an entangled PBD solution, (d) yielding after a sudden stretching due to residual elastic forces. (e) The animated movie uses a rubber band to elucidate the processes of yielding during or elastic yielding after sudden startup deformation (in the example of stretching). (f) PTV revelation of elastic yielding after a SBR melt experienced 7 shear strain units. PTV method has been described in some detail in Macromol. Mater. Eng, 292, 15 (2007) A recent 30 min webseminar (made possible by Malvern Instruments) gives a brief account of the emerging physics in nonlinear rheology at brainshark.com/malvern, where 13 movies were concluded as attachments.

In addition, a recent lecture at Kavli Institute in Beijing discussed the difficulty facing the current theoretical description of nonlinear polymer rheology.

(a)	(b)
(c)	(d)
(e)	(f)

Current Activities

Currently polymer rheology is our main research focus. Several hundred billion pounds of polymers are annually used to make commercial plastic (milk bottles) and rubber (e.g., auto tires) products for worldwide consumption. Before they turn into their final forms, most are brought to their liquid states for processing. Thus, it is essential to understand the polymer flow behavior. Many of these polymers, such as polyethylene and polybutadiene, are well entangled. Their apparent viscosities decrease during processing flow. This basic phenomenon of shear thinning has been recognized for decades although its origin has remained elusive. In the past eight decades [M. Reiner, J. Rheol. 1, 5 (1929)], polymer flow has been perceived to usually take place homogeneously, and shear thinning has been characterized in rheometric instruments under such an assumption. 

Lately, we learned that well entangled polymers undergo cohesive breakdown upon either startup deformation or large step strain or large amplitude oscillatory shear[Phys. Rev. Lett. 96, 016001, 196001, 97, 187801 (2006), 99, 237801 (2007), Macromol. Mater. Engr. 292, 15 (2007), J. Chem. Phys. 127, 064903 (2007) and a sub-list of our publications on the subject of nonlinear polymer rheology]. The localized yielding phenomenon leads to subsequent inhomogeneous flow. The emerging new understanding has allowed us to make specific predictions of unexpected behavior that would appear counterintuitive to the conventional views. Two such phenomena are arrested wall slip and filament failure after sudden extension, listed below as G and I. The new theoretical picture has also allowed us to unify the description of deformation and flow in shear and extension. A short list of publications on Nonlinear Polymer rheology is provided here.

Given the rapidly accumulating evidence, a new textbook on the subject of Nonlinear Polymer Rheology is in the preparation stage. Click TOC for the book outline (TOC in Chinese). A one-credit course is being taught every year based on the TOC and is available for viewing through stream-video upon request. The rapidly accumulating results suggest that entangled polymers yield to fast large deformation as if they are breakable solids of finite cohesive strength. The animated movie above (also available as Movie 18 under I. Yield in melt extension) uses a rubber band to elucidate the process of yielding during sudden startup deformation (in the example of stretching). Whether the initial shear banding would surrender to homogeneous shear at long time remains a conceptually and academically interesting question. Nevertheless, the metastability of shear banding appears to be very strong even in solutions. This longevity of shear banding is elusive and also makes shear banding an important phenomenon to reckon with in practice. There is little doubt that yielding must take place when the applied rate is significantly higher than the polymer relaxation rate: Disintegration of the entanglement network begins when each chain can no longer take on further deformation and mutual molecular sliding occurs as shown in the illustrative movie Elastic Yielding - the chain (rubber band in the movie) retracts as disentanglement sequentially occurs via chain ends where the fingers represent inter-chain interactions, acting to cause affine deformation initially. We are currently also actively pursuing any possible direct evidence for chain scission that could occur in rupture like material failure.

A summary of the recent progress has been discussed in an invited talk at APS-meeting 2008. Additionally, here is a lecture given in Nanjing University in December 2007 (in Chinese), where the opening missing video is on the sharkskin melt fracture. The three key movies presented in the lecture can be viewed below under A to C by downloading Movie 2, Movie 3’ and Movie 6.

We summarize the growing experimental evidence in the form of movie video clips that captured the essence of various phenomena in entangled polymer solutions and melts, in each category listed below.

1. Yielding in startup shear
2. Large amplitude oscillatory shear
3. Flow birefringence
4. Large step shear
5. Continuous shear
6. PTV observations of other complex fluids
7. Wall slip during and after shear
8. Elastic breakup in simple shear of melts
9. Yield in melt extension

Footnote:

In response to our statement “The transition from elastic deformation to flow must occur upon startup shear or extension. It occurs essentially at the force maximum”, one reviewer of our recent manuscript (2011) asserted ‘Polymer melts are always in a liquid state and therefore flows. It may be viscoelastic flow but they are not elastic (in) nature.’ In 2008, another reviewer indicated ‘The imposed flow (constant strain rate) is such that flow must take place right from the start of the experiment.' Such responses may be popular in the community.

This disagreement highlights the difference in how to understand responses of entangled polymers to large deformation. In our understanding, entangled polymers cannot be regarded as liquids when suddenly subjected to external deformation. Upon sudden deformation with Wi = tau/t1 >> 1 where the time t1 to take 100 % deformation, i.e., the reciprocal rate, is much shorter than the relaxation time tau, such systems would essentially suffer elastic deformation until the system breaks apart to allow irrecoverable deformation, i.e., flow. The evidence for the elastic response before the force maximum also comes from the fact that the mechanical stress arises from zero monotonically upon startup deformation, and there is no evidence of flow that would produce a finite stress at t= 0+ according to the Newtonian law for liquids.