Physics and engineering of polymeric and other structured materials. Experimental and theoretical foundations of polymer rheology and processing: phenomenology of linear and nonlinear viscoelastic processes; flow instabilities and processing phenomena including wall slip and melt fracture. Molecular foundation for mechanics of polymeric glasses: deformation, yielding, brittle-ductile transition and fracture.
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 rheological behavior of polymer liquids under large and rapid deformation, and to understand mechanical behavior including yielding and failure of these polymers in their solid state.
Recently we have come to realize how we should think about mechanical behavior of glassy polymers. It has been a very difficult, outstanding challenge to gain some molecular level understanding of yielding and failure behavior of polymer glasses under large deformation. For many polymer glasses with Tg well above room temperature, why are some brittle and others ductile? If it is a matter of entanglement density, why do these polymers turn from ductile to brittle upon cooling to a lower temperature, and vice versa? Ultimately, can we answer why polystyrene is the most brittle (typically still brittle at 80 ℃) and bisphenol A polycarbonate is most ductile (known to be ductile at -110 ℃) among all known polymer glasses?
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 short-ranged inter-segmental attractions (causing vitrification) and long-ranged chain networking that can be extended to allow buildup of chain tension, plausibly corresponding to bond length and angle deviations from the equilibrium values. A polymer glass gains global plasticity and is ductile at a given temperature if its chain network can activate the glassy state globally. Conversely, if the chain network breaks down before it "finishes its job", a polymer glass is going to undergo brittle fracture. Here the chain network is envisioned in terms of junctions formed by at least two pairs of hairpins from different chains.
Many of these polymers, such as polyethylene and polybutadiene, are well entangled liquids above melting and glass transition temperatures. The research in the past decade has informed us that well entangled polymers, as highly viscoelastic liquids, behave as transient solids and undergo cohesive breakdown upon either startup deformation or large step strain or large amplitude oscillatory [Phys. Rev. Lett. 96, 016001, 196001, 97, 187801 (2006), 99, 237801 (2007), Macromol. Mater. Engr. 292, 15 (2007), J. Chem. Phys. 127,064903 (2007)]. The localized yielding phenomenon leads to strain localization such as shear banding and non-quiescent relaxation. A new worldview has emerged, in which we have developed a unified treatment of deformation and flow in both shear and extension. About sixty papers have been published on this subject in the past decade from our lab, and a list of these publications is provided here.
A one hour seminar is available at both YouTube and Tudou, summarizing what we know now after six years of intense studies (between 2006 and 2012), which was given at the University of Akron on March 2, 2012. Moreover, a recent publication discussed the construction as well as structure of the tube model and the emergence of an alternative conceptual framework. To help us understand the difference between the new worldview and standard paradigm due to the tube model, we have made a short (9 min) presentation to highlight the need to address the issue of causality: a) where affine deformation comes from and b) when chain deformation ceases to increase. For the latest presentation that discusses more critically about the current status of our understanding of polymer rheology, see the invited talk at the APS 2015 March meeting.
Given the rapidly accumulating evidence, a new book titled Physics of Nonlinear Polymer Rheology is on schedule to be published in 2015. Click TOC for the book 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.
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.