Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z20: Invited Session: Physics of Entanglements |
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Sponsoring Units: DPOLY Chair: Scott Milner, Pennsylvania State University Room: Ballroom B |
Friday, March 6, 2015 11:15AM - 11:51AM |
Z20.00001: Shear banding in time dependent flows of polymers and wormlike micelles Invited Speaker: Suzanne Fielding We study theoretically the formation of shear bands in time-dependent flows of polymeric fluids and wormlike micellar surfactant solutions, focussing in particular on the commonly studied experimental protocols of step shear stress and shear startup. For each protocol, we perform a linear stability analysis to provide a fluid-universal criterion (with some caveats in the case of shear startup) for the onset of shear banding (following Moorcroft and Fielding Phys. Rev. Lett. 2013). In each case this criterion depends only on the shape of the experimentally measured rheological response function for that protocol, independent of the constitutive properties of the material in question. In this way our criteria in fact concern all complex fluids and not just the polymeric ones of interest here. (See Fielding Rep. Prog. Phys. 2014 for a study of these effects in a broad class of soft glassy materials including dense emulsions, microgels and dense colloids.) An important prediction is that pronounced shear banding can arise transiently in each of these time-dependent protocols, even in fluids for which the underlying constitutive curve of the material (stress as a function of strain-rate) is monotonic and a steadily flowing state is accordingly unbanded. Further details can be found in Moorcroft and Fielding J. Rheol. 2014. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:27PM |
Z20.00002: How polymer entanglement responds to fast large deformation: are we there yet? Invited Speaker: Shi-Qing Wang Nearly all polymeric materials are of high molecular weight and therefore entangled in their liquid state. Significant melt elasticity arises from the transient networking due to chain entanglement. All rheological behavior stems from how the entanglement responds to external deformation of various forms. Unfortunately, the concept of entanglement still remains theoretically elusive to describe. On other hand, modeling the evolution of chain entanglement is the key to answering the core questions in polymer rheology: a) where chain deformation comes from? b) when affine-like elastic molecular deformation ceases? In other words, yielding at both macroscopic (which is obviously taking place, e.g., signified by the stress overshoot response to startup shear) and molecular levels (through chain disentanglement) is an essential ingredient of any theoretical description of nonlinear polymer rheology. Macroscopic observations are valuable to afford useful insights, but it is the molecular dynamics simulations that are expected to address the foundational issues. This presentation will attempt to make a coherent discussion of what is known and where we are going from here. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 1:03PM |
Z20.00003: From molecules to non-linear rheology of highly branched, entangled polymers: getting your priorities right Invited Speaker: Daniel Read The tube model for polymer dynamics offers the promise of predicting the flow properties of entangled polymeric liquids. Given a knowledge of the sizes and shapes of the polymers, the tube model suggests dynamical rules for the relaxation of stress carried by the molecules. Over the last couple of decades these rules have been, for the most part, established through experiments and simulations on liquids containing molecules of well-defined size and shape. For prediction of small amplitude flows, these rules are now codified in computer algorithms such as Larson's hierarchical model (http://www.engin.umich.edu/dept/che/research/larson/) and our own ``BoB'' model (http://sourceforge.net/projects/bob-rheology). As a result, it is now possible to make meaningful predictions for flow properties of industrial polymeric resins with distributions of randomly branched structures. Since real polymers are subjected to large deformations in realistic processing, we have recently extended the above work to prediction of the large-deformation response of branched polymers. This talk will describe the extra physics that applies in the non-linear flow regime, and how this has been implemented in our model: the central message is that one needs to know three quantities for every strand in the resin: 1) an orientation relaxation time, (2) a stretch relaxation time, and (3) a limiting value for the chain stretch. The latter is often discussed in terms of a topological quantity known as ``priority.'' Motivated by recent experiments on well defined ``comb'' molecules, we discuss some shortcomings in our current prediction of the ``priority'' and how this may be improved upon. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:39PM |
Z20.00004: Entangled linear, branched and hyperbranched polymers in shear flow Invited Speaker: Dimitris Vlassopoulos Despite substantial progress in understanding the dynamics of long flexible polymers, several outstanding challenges remain. We address some of them. We discuss the response of well-characterized linear and branched (stars, combs, H) polymers to simple shear flow. The start-up stress behavior at high shear rates exceeding the inverse Rouse time, where according to tube-model theories polymer chains are oriented (and eventually stretched), is considered. We identify conditions under which combs are considered as effective diluted linear chains. We address the failure of stress-optical and Cox-Merz rules and the role of branching. Relaxation upon flow cessation is analyzed and a connection to convected constraint release is suggested. We apply the ``probe rheology'' approach to branched polymers diluted in polymeric matrix. Careful choice of matrix molar mass allows controlling constraint release effects. The shear response of asymmetric linear chain mixtures is also discussed in the context of recent studies in uniaxial extension suggesting enhancement of extensional viscosity. Entanglement-like effects are observed in dendronized polymers with branches below the entanglement limit, which interpenetrate to reduce inherent density heterogeneity. \\[4pt] Collaboration with S. Costanzo, F. Snijkers, H. Lentzakis, L. G. Leal (Santa Barbara), R. H. Colby (Penn State), N. Hadjichristidis (KAUST), A. D. Schlter (Zurich) and support from EU (Supolen, ESMI) and GGSRT (Aristeia-Rings) are acknowledged. [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 2:15PM |
Z20.00005: Polymer twists: entanglement and packing ansatz Invited Speaker: Jian Qin Polymer motions in dense liquids (molten plastics) are severely constrained by surrounding chains, due to the fact that chains cannot cut through each other. Effectively, polymers may be considered as being confined inside a tube-like region. The tube diameter is the key parameter needed by the modern molecular theory for polymer rheology. But a molecular understanding of the tube diameter is missing. We summarize our recent attempts at estimating the tube diameter from simulated topologically equilibrated ring polymers, which have well--defined topological states and are free from the complication caused by chain end relaxation dynamics. We consider two non--invasive methods for estimating the tube diameter, one based on the extent of bead position spreading over an ensemble of short dynamic trajectories, and another based on statistics of topologically distinct states collected with the help of a generalized knot invariant polynomial. For simulated polymer melts, we get a tube diameter value that agrees with values obtained by more heuristic methods. We then present results on the effects of chain stretching and neutral solvent dilution on the tube diameter, and examine the three possible variants of the Lin-Noolandi packing arguments for the tube diameter, which all yield the same prediction for unperturbed polymer melts, but each gives different prediction when applied to stretched and diluted systems. The analyses are in favor of a binary view of polymer entanglement. [Preview Abstract] |
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