Polymer Rheology Homepage

Fundational issues in rheology and flow^*

Currently polymer rheology is our main research focus. Several hundred billion pounds of polymers are annually used to make commercial plastic 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 homogenously, and shear thinning has been characterized in rheometric instruments under such an assumption.

Lately, we learned that well entangled polymers undergo cohesive breakdown upon startup deformation [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 more available under publications]. 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 emerging evidence, a book on the subject of Nonlinear Polymer Rheology is in the planning 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. In other words, we have not been able to convincingly rule out chain scission as a dominant event during yielding although there several pieces of indirect evidence do not support the notion that chain scission plays any essential role in ductile yielding and shear banding phenomena.

A summary of the recent progress has been discussed in an invited talk at APS-meeting 2008 -Window Media (WMV) or APS-meeting 2008 -Macromeida Flash (SWF) or APS-meeting 2008 -Quick Time (MOV). Additionally, here is a lecture (two formats: Nan Da-1 and Nan Da-2) 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.

In the following space, 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. For a quick overview, one can check out a standard seminar presentation in (PDF). All the intended movie presentations in this seminar are labeled such that one can go below to find the corresponding video clips used in the presentation.

We have recently opened the Pandora's box by developing an effective particle-tracking velocimetric (PTV) method that allows the flow field to be determined in real time. Below we describe our PTV observations in movie format, in each category, of flow behavior of entangled solutions and melts, along with flow birefringence and direct visualization of extensional flow behavior of melts.

To make sure these movies play on your computer, you may need to download first. Explorer appears to be the preferred browser since Firefox appears to have difficulty in opening some of these recorded files.

A. Yielding in startup shear

B. Large amplitude oscillatory shear

C. Flow birefringence

D. Large step shear

E. Continuous shear

F. PTV observations of other complex fluids

G. Wall slip during and after shear

H. Elastic breakup in simple shear of melts

I. Yield in melt extension

* supported by a small exploratory research grant (DMR-0603951) and standard grant (DMR-0821697) from the National Science Foundation.