Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session P64: Revealing the Microscopic Dynamics Driving Nonlinear Polymer FlowsInvited Live
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Sponsoring Units: DPOLY Chair: Emanuela Del Gado, Georgetown University |
Wednesday, March 17, 2021 3:00PM - 3:36PM Live |
P64.00001: Threading-Unthreading Transition of Linear-Ring Polymer Blends in Extensional Flow Invited Speaker: Qian Huang Adding small amounts of entangled ring polymers to entangled linear matrices yields unusual rheological properties. For example, it has been established that the zero-shear-rate viscosity of the blends is increased compared to their linear counterparts, due to linear-ring threading. In this work, we show that the linear-ring blends exhibit a stress overshoot in uniaxial extensional flow, and this overshoot is driven by a transient threading-unthreading transition of rings embedded within the linear entanglement network. |
Wednesday, March 17, 2021 3:36PM - 4:12PM Live |
P64.00002: Simulating the Far-From-Equilibrium Dynamics and Elongational Rheology of Architecturally Diverse Polymer Melts Invited Speaker: Thomas O'Connor Many industrial processes elongate polymer liquids at rates much faster than the molecular chain's characteristic relaxation times. These nonlinear elongation flows can strongly deform microscopic polymer conformations and drive dynamic transitions that produce large changes in polymer viscosity. Understanding how flow depends upon and drives such changes in polymer microstructure is essential for improving established and emerging fabrication methods like fiber spinning and 3D printing, and diversifying the applications of upcyclable chemistries. However, most microscopic understanding of these nonlinear flows has been drawn from indirect techniques that infer molecular dynamics from macroscopic rheology. This has begun to change with the recent development of new experimental and numerical simulation techniques that allow researchers to control, sustain, and microscopically probe polymer dynamics during strong elongational flows. Here, I’ll present molecular simulations for linear, star, and ring polymer melts and blends deformed in uniaxial elongational flow. In all three cases, coarse-grained molecular dynamics simulations reproduce the nonlinear rheology observed in extensional flow experiments and reveal the polymer dynamics and emergent chain topologies that underly each liquid's far-from-equilibrium behavior. |
Wednesday, March 17, 2021 4:12PM - 4:48PM Live |
P64.00003: Polymer Scission in Contraction Flows Invited Speaker: Peter Olmsted Polymer scission has been studied for many years! It has numerous practical implications in areas such as drug injection, spraying and printing, and oilfield harvesting. When injected through a contraction flow, high molecular weight polymer solutions exhibit a sharp increase of apparent viscosity due to chain stretching during fluid extension. This stretching can induce polymer scission, which then decreases the extensional viscosity. We revisit this old problem using specially-designed microfluidic hyperbolic contraction flows. We study the pressure-flux relation for high molecular weight polymer solutions passing through the contraction, and find that the ratio of the pressure drop to that of the (Newtonian) solvent has a maximum due to the competition between polymer extension and degradation (scission). From the dependence of the pressure maximum on flow rate and molecular weights we can quantify the decrease in equivalent molecular weight due scission in the contraction |
Wednesday, March 17, 2021 4:48PM - 5:24PM Live |
P64.00004: Self-Healing Recovery and Dynamics of Associating Polymers under Uniaxial Extension Invited Speaker: Nicolas Alvarez Associating polymers are a desirable class of materials with intrinsic abilities to self-heal without the need for additional components or added stimulus. While experimental and theoretical measurements of recovery exist in the literature, quantifying recovery in terms of fundamental rheological parameters is often difficult. In this work we use filament stretching uniaxial extension to probe the recovery of unentangled and entangled hydrogen bonding polymers. Using a novel methodology, we confirm the role of process timescales such as the time before and after recovery on both the transient rheology and ultimate recovery of associating polymers. Furthermore, we show the importance of architecture and molecular timescales of the network on recovery. We find that while strongly associating groups provide improved mechanical strength, they often delay the process of recovery. Additionally, we show that the presence of entanglements has a complex role on recovery, introducing t he flow timescale as an additional parameter. With this work, we develop a framework that is both useful for evaluation of self-healing soft materials and for design of novel self-healing polymers. |
Wednesday, March 17, 2021 5:24PM - 6:00PM Live |
P64.00005: Evidence of Flow-Induced Crystallization in Material Extrusion Additive Manufacturing Invited Speaker: Anthony Kotula Material extrusion additive manufacturing processes force molten polymer through a printer nozzle at high shear rates prior to cooling and crystallization. This can lead to flow-induced crystallization in common polymer processing techniques, but the magnitude of this effect is unknown for additive manufacturing. Here, we will show the effect of the material extrusion on the morphology of poly(caprolactone) and poly(lactic acid), two common semicrystalline polymers used in material extrusion. The talk will encompass materials characterization (rheology and crystallization kinetics) relevant to printing, process line temperature and crystallinity measurements, and polarized optical microscopy techniques to characterize the final microstructure, which we compare with continuum modeling. When crystallization kinetics are fast compared to the timescale of the print process, the crystallization rate near the extrudate surface can be tracked in situ using Raman spectroscopy. Even when the as-printed part has negligible crystallinity, a post-print annealing process reveals spherulitic domains with sizes that dramatically decrease near the weld line. Our results show that residual chain stretch from the extrusion and deposition process enhances the nucleation rate in the weld regions between extruded layers. |
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