Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session E51: Polymer Physics Prize SymposiumInvited
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Sponsoring Units: DPOLY Chair: Sharon Glotzer, University of Michigan Room: BCEC 253A |
Tuesday, March 5, 2019 8:00AM - 8:36AM |
E51.00001: Polymer Physics Prize Talk: Getting the kinks out: extensional flow in polymer solutions, melts, and glasses Invited Speaker: Ronald Larson Using molecular dynamics and Brownian dynamics, and “kink dynamics” simulations, polymer chains under extensional flow at high stress and strains are found to unravel through folded states, wherein highly stretched strands connected by kinks, or fold points, bear most of the stress. This process occurs universally at high stresses in polymer solutions, melts, and glasses, and is responsible for the strain hardening seen in extensional flows in all these states, regardless of whether the polymer is entangled or not. We show that experimental data for stress and local orientation can be explained semi-quantitatively or even quantitatively with models that capture these dynamics. |
Tuesday, March 5, 2019 8:36AM - 9:12AM |
E51.00002: Accessing the Unexplored Regions of the Glassy State to Test Paradigms of the Glass Transition Invited Speaker: Gregory McKenna A major challenge in glass physics is the determination of the temperature dependence of the relaxation times in the equilibrium state well below the laboratory glass temperature Tg. This is because the times required to achieve equilibrium in this regime become of geological/astronomical scale [1,2]. To finesse this problem, we are using materials with extremely low fictive temperature Tf relative to Tg, hence unlocking an unexplored region of the glassy state to investigation. First, we measured the viscoelastic response of a 20 million year old amber with Tf ∼ 43.6 K below Tg. The relaxation times deviated strongly from the expected VFT or WLF-behaviors turning towards an Arrhenius-response, albeit with a high activation energy. Though convincing as evidence that the dynamics of the glass do not diverge at a finite temperature, often linked to the Kauzmann [3] temperature TK, the amber work is complicated because the natural origins of amber make reproducing the experiments difficult. We also built on the ultra-stable glasses exploited by Ediger and co-workers [4] and made an amorphous Teflon material withTf ∼55 K below Tg and close to the putative TK. Made only in microgram quantities, we needed the TTU bubble inflation [5] method to measure the creep response in the range between Tf and Tg, expanding the amber work to TK. The observed relaxation times deviate from the extrapolated WLF-line challenging the view that there is an "ideal" glass transition as posited by multiple theories and commonly considered an important aspect of glass-formation and glassy behavior. |
Tuesday, March 5, 2019 9:12AM - 9:48AM |
E51.00003: Edge fracture in polymeric fluids Invited Speaker: Suzanne Fielding We study theoretically edge fracture in sheared polymeric fluids, using linear stability analysis and nonlinear simulations. We derive an exact analytical expression for the onset of edge fracture in terms of the shear-rate derivative of the second normal stress difference, the shear-rate derivative of the shear stress, the jump in shear stress across the interface between the fluid and the outside air, the surface tension of that interface, and the rheometer gap size. We provide a full mechanistic understanding of the edge fracture instability, and validate this against our simulations. These findings also suggest a possible route to mitigating edge fracture, potentially allowing experimentalists to achieve and accurately measure flows stronger than hitherto. We then consider the interaction of edge fracture with the fluid bulk. For fluids with a rather flat (but still monotonically increasing) bulk constitutive curve of shear stress as a function of shear rate, we show that edge fracture can cause a pronounced apparent shear banding that invades the fluid bulk to a distance of many gap widths in from the sample edge. To paraphrase this first scenario: "edge fracture causes (apparent) shear banding". For fluids that have a non-monotonic constitutive curve and therefore show bulk shear banding (even in the absence of any edge instabilities), we show that the jump in the second normal stress difference between the shear bands causes strong edge fracture at the fluid-air interface, consistent with the earlier intuition of Skorski and Olsmted. To paraphrase this second scenario: "shear banding causes edge fracture". |
Tuesday, March 5, 2019 9:48AM - 10:24AM |
E51.00004: Statistical Field Theory of Inhomogeneous Polarizable Soft Matter Invited Speaker: Glenn Fredrickson Standard approaches to modeling the electrostatic properties of inhomogeneous soft matter systems involves either neglecting dielectric contrast entirely, or imposing an ad-hoc dielectric constitutive law that is not consistent with pairwise van der Waals (VDW) interactions included elsewhere in the model. We recently developed a framework for building statistical field theories from coarse-grained particle models where the force centers can optionally carry monopole charges, dipoles, and/or classical Drude oscillators that confer polarizability. The resulting polarizable field theories self-consistently embed dielectric constitutive laws, VDW interactions, and a rich variety of charge and structure correlation physics. This talk will report on recent analytical results from loop expansions and numerical results from complex Langevin simulations that address: 1) the VDW contribution to the Flory interaction parameter in polymer blends and block copolymers, 2) the dielectric decrement or increment on adding salt to a polar or polarizable solvent, and 3) the electric-field induced shift in the critical temperature of a binary dielectric fluid mixture. |
Tuesday, March 5, 2019 10:24AM - 11:00AM |
E51.00005: Fast but Inaccurate or Slow but Accurate: The Dilemma of Tubes and Slip Links Invited Speaker: Sachin Shanbhag The tube model for entangled polymers projects a complex multi-body problem of interacting chains onto a mean-field representation. This popular framework has been enormously successful at predicting the rheology of model (and even some industrial) polymers. However, its success cloaks several fundamental problems that become obvious when it is subjected to strong tests. One such example is binary blends with well-separated relaxation times. In such cases, slip link models which retain the multi-body character of the original problem offer a more accurate and robust alternative. However, slip link models are computationally demanding. Here, I resuscitate a super-fast slip link model that Ron Larson and I first introduced over 15 years ago. With a few small modifications, it predicts the linear rheology of binary blends with remarkable accuracy and speed. |
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