Polymer Physics: Rheology and Glasses Group
Ph.D. (Physics), University of Chicago (1987)
Fellow of the American Physical Society (1997)
Fellow of American Association for the Advancement of Science (2014)
Member of American Physical Society, Society of Rheology
Physics and engineering of polymeric and other structured materials. Experimental and theoretical foundations of polymer rheology and processing:
Our current research focuses on two major subjects in polymer science: rheology of polymeric liquids and mechanics of glassy polymers. Several hundred billion pounds of polymers are annually consumed 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. After the processing, they solidify for their end use, and many turn glassy. Thus, it is essential for us to understand
1. Mechanics of polymer glasses
Recently we have come to realize how we should think about
It turns out that a crucial clue comes from the observation that all non-polymeric organic glasses are brittle. There arises the natural question of why polymeric glasses can be ductile at all? Being polymeric is the key. We have formulated a molecular model for yielding, crazing and brittle-ductile transition. In the zeroth-order picture, polymer glasses are structural hybrids, made of a primary structure due to
2. Polymer Rheology
Many of these polymers, such as polyethylene and polybutadiene, are
Given the rapidly accumulating evidence, a new book titled Nonlinear Polymer Rheology will soon be published by Wiley in 2017. Click TOC for its outline. It discusses in depth where "elasticity" originates from to produce viscoelasticity in entangled polymers, what causes the elasticity to subdue, and how we should think about the response of polymer entanglement to large deformation.
Much of the growing experimental evidence has been captured in the form of video clips. In particular, the seven real-time movies below illustrate a) sharkskin formation (extrusion of polybutadiene) followed by wall slip at higher pressure; b) particle-tracking velocimetric (PTV) observations (Macromol. Mater. Engr. 2007, 292, 15) of c) startup shear on an entangled PBD solution (Macromolecules 2008, 41, 2663); d) elastic yielding after a sudden stretching due to residual elastic forces, e) an animated movie based on 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 (Macromolecules 2009, 42, 6261); g) strain localization at the die entry during extrusion of a monodisperse 1,4-polybutadiene of 200 Kg/mol (J. Rheol. 2013, 57, 349); h) PTV evidence of shear yielding during startup uniaxial extension to initialize (unstable) necking. (J. Rheol. 2013, 57, 223).
Below are a group of movies showing the nonlinear rheological phenomena in different categories. They are available for viewing and downloading.
- Yielding in startup shear
- Large amplitude oscillatory shear
- Flow birefringence
- Large step shear
- Continuous shear
- PTV observations of other complex fluids
- Wall slip during and after shear
- Elastic breakup in simple shear of melts
- Yield in melt extension
- Shear strain localization at die entry during extrusion
Recently, the discoveries such as shear banding upon startup shear have been called into question. Since the phenomena of shear banding upon startup shear and non-quiescent relaxation after stepwise shear have resulted in new understanding, the validity of these PTV observations needs to be scrutinized. Shear banding does not occur for (a) moderately entangled polymer solutions that cannot undergo measurable wall slip and (b) well entangled polymers at low rates where wall slip may dominate (cf. Macromolecules 2011, 44, 183). When shear banding take place, edge effects may also be present. However, edge instability is not the cause. For example, it is indicated in this three-min presentation that shear banding can be absent when edge effects are stronger. On the contrary, J. Rheol. 52, 957 (2008) and Rheol. Acta. 49, 985 (2010) already showed the shear strain localization in absence of edge effects. We have reached the point of no return and cannot be like the three Japanese monkeys who wish to see nothing, to hear nothing and to say nothing. The Pandora’s Box is open. So let us face it instead of asking who opened it. Regarding the skepticism [J. Rheol. 57, 1411 (2013)], we wrote a Letter to JOR, J. Rheol. 58, 1059 (2014), to which a remarkable response was also published at J. Rheol. 58, 1071 (2014). Since JOR is unable to publish the replies to the Response (JOR, p1071), we outline here the key issues to make sure the reader is fully informed. This rejected manuscript is published herein.