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 V04: Dynamics and Rheology of Polymers and Polyelectrolytes IIFocus Live
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Sponsoring Units: DPOLY DSOFT GSNP DBIO Chair: Thomas O'Connor, Sandia National Laboratories |
Thursday, March 18, 2021 3:00PM - 3:12PM Live |
V04.00001: Crossover from Rouse to Reptation Dynamics in Salt-Free Polyelectrolyte Complex Coacervates Boyuan Yu, Phillip Rauscher, Nicholas Jackson, Artem Rumyantsev, Juan De Pablo The dynamics and rheology of polyelectrolyte complex coacervates has attracted considerable interest due to their industrial application as viscosity modifiers. A central question is whether the classical Rouse and reptation models can be applied to these systems. By using molecular simulations, we directly demonstrate the crossover from Rouse to reptation dynamics in salt-free coacervates as a function of chain length. Our results show that this crossover shifts to short chain length with increasing electrostatics, which attributes to the formation of denser coacervates. To clarify the roles of Coulomb interactions and density, we further compare coacervates to those of neutral, semidilute solutions at the same density and chain conformations. A universal dynamical behavior is observed for both systems in the sub-diffusion and normal diffusion regimes, but the monomer relaxation time in coacervates is much longer and increases as Bjerrum length increases. Similar phenomenon called cage effect exists in glass-forming polymers, but the local dynamical slowdown in our system is caused by strong Coulomb attractions (ion pairing) between oppositely charged monomers. Our findings contribute to the fundamental understanding of coacervate dynamics and rheology in microscopic level. |
Thursday, March 18, 2021 3:12PM - 3:24PM Live |
V04.00002: Chain models for the simulation of polyelectrolytes in combination of flow and electric fields Shreyash Gulati, Angelo C. Setaro, Patrick T Underhill Combined Poiseuille flow and electric fields have been used to drive transverse migration of charged polyelectrolytes in channels with use in separation of DNA. But the migration is not monotonic in electric field strength. Previously a simple dumbbell model has been used to understand the mechanism for this non-monotonic trend. Migration can alter polymer conformations which then alters migration. The dumbbell model qualitatively captured some experimental behavior but may not be accurate for strong electric fields. We use chain models to overcome this problem. A combination of theoretical calculations and Brownian Dynamics simulations is used. Two different ways of calculating electrophoretic mobility are considered. Firstly we treat the polymer as having a single (average) mobility. Then we consider the individual springs as having unique mobilities. For average mobility, simulations are consistent with the mechanism found for the dumbbell model. Allowing springs to move with unique mobilities introduces additional mechanism for electric field to alter polymer conformations and migration. By using chain models, we expect to gain further insight into the interplay between migration and conformation for strong electric fields. |
Thursday, March 18, 2021 3:24PM - 3:36PM Live |
V04.00003: Fast Dynamics and its Role in Conductivity of Polymerized Ionic Liquids Vera bocharova, Anne-Caroline Genix, Naresh Osti, Eugene Mamontov, Alexei Sokolov
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Thursday, March 18, 2021 3:36PM - 3:48PM Live |
V04.00004: Shear Response of THF Swollen Ionic Polymer Melts: Molecular Dynamics Simulations Study Shalika Meedin, Manjula Senanayake, Supun Mohottalalage, Chathurika Kosgallana, Dvora Perahia, Gary Grest Ionizable groups determine the behavior of polymers through long range electrostatic interactions that drive assembly and affect the inherent dynamics of the macromolecules and their mechanical response. Here, using molecular dynamics simulations we probe the effects of ionic assemblies on the inherent dynamics and shear response of polystyrene sulfonate melts in the ionomer and polyelectrolyte regimes as small amounts of THF are added to modify the cohesion of the ionic assemblies. We find that the shear viscosity is higher for the polyelectrolyte regime compared with ionomer one, due to large continuous ionic assemblies in polyelectrolytes. With increasing shear rates, the assemblies break up in both regimes, reducing the shear viscosity. THF that resides predominantly along the ionic assemblies reduces only slightly the shear viscosity. Molecular understanding that underline the shear response will be discussed. |
Thursday, March 18, 2021 3:48PM - 4:00PM Live |
V04.00005: Dynamics of liquid coacervates: Higher charged density polyelectrolytes are in entangled semiflexible regime Christian Aponte-Rivera, Michael Rubinstein Solutions made from oppositely charged polyelectrolytes can phase separate to form a polymer-rich coacervate phase, important in biology and technological applications. In coacervates made from polyelectrolytes of high enough molecular weights, topological constraints known as entanglements alter the dynamic properties and can be due to either the polyanion, the polycation or both. In entangled charge density asymmetric coacervates, the high charge density polymer has an electrostatic persistence length that can be smaller or larger than the diameter of a confining tube by low charge density polymers. Here, we consider the case in which the electrostatic persistence length of the high charge density polymer is larger than the diameter of a confining tube by low charge density polymers. On length scales smaller than the tube diameter, high charge density polymers have a higher effective friction than in pure solutions, owing to a dynamic coupling between the polymers even in the unentangled regime. On length scales larger than the tube diameter, the high charge density polymer behaves similar to entangled semiflexible polymers. The effect of the higher effective friction and “semiflexible” dynamics on rheological properties and polymer diffusion will be discussed. |
Thursday, March 18, 2021 4:00PM - 4:12PM Live |
V04.00006: A critical Assessment of Linear Viscoelasticity and Time-Temperature-Salt and Other Superpositions in Polyelectrolyte Coacervates Ronald G Larson, Ying Liu, Huiling Li The validity of time-salt, time-pH, and time-hydration superpositions in the linear viscoelasticity of polyelectrolyte coacervates is assessed for a variety of polyelectrolytes. The comparison shows the frequent, but not universal, success of time-salt superposition, and master curves that are in many cases similar to those for neutral polymers, including Rouse and reptation theories. However, in some cases, solid-like, as opposed to fluid-like, response is observed at low frequencies, especially at low salt concentrations. Some coacervates seem to fit “sticky diffusion” theory reasonably well, wherein relaxation is controlled by the breakage rate of ion pairs; the dependence of the “sticker” lifetime on salt concentration has been explored but is not well understood as yet. It is also possible that local relaxation is not controlled by breakage of ion pairs, but by cooperative, “glassy,” relaxation of monomers, salt ions, and water molecules. A compilation and comparison of different data sets and suggested formulas for rheological time constants are presented, and some suggestions are given for future directions. |
Thursday, March 18, 2021 4:12PM - 4:24PM Live |
V04.00007: Molecular Basis for Elasticity and Viscoelasticity in Chitosan-Surfactant Hydrogels Suhas Gotla, Silvina Matysiak The carbohydrate polymer, chitosan, is a biocompatible material commonly used in biomedical applications. Chitosan's chemistry enables its switching from a stable, net-neutral, elastic hydrogel at basic pH, to a soluble polycationic state at acidic conditions. Experiments show that the addition of the anionic surfactant sodium dodecyl sulfate (SDS) to polycationic chitosan gives rise to electrostatically linked hydrogels displaying viscoelasticity. In this work, we employ multiscale molecular dynamics to identify the molecular phenomena that underpin these distinct mechanical behaviours. ~20nm-scale percolated hydrogels are first assembled with coarse grained molecular dynamics, and later back-mapped to atomic resolution. We subjected the atomistic structures to tension-compression cycles to study how the intermolecular interactions respond to mechanical stimuli. Direct chitosan-chitosan interactions were found to recover elastically with deformation. But, SDS-chitosan interactions in electrostatic gels increase in number dramatically, while simultaneously showing viscoelastic stress response. These results unify bulk mechanical properties with fine intermolecular phenomena, and promise to aid the engineering of customizable chitosan materials in the pH-counterion design space. |
Thursday, March 18, 2021 4:24PM - 4:36PM Live |
V04.00008: Nonlinear Elongation Flows Effects on Aggregation in Associating Polymer Melts Supun Mohottalalage, Manjula Senanayake, Thomas O'Connor, Gary Grest, Dvora Perahia Incorporating associating groups into polymer melts strongly effects both chain mobility and viscoelastic response. Associating groups form nanoscale clusters whose size depends on the competition of associating group interactions and chain conformational entropy. Here, we use molecular dynamics (MD) simulations of bead-spring chains to identify how associating groups alter the structure and dynamics of linear polymer melts in uniaxial elongation flows. Associating groups whose interaction strength is varied from 1-20 kBT are randomly included in the backbone of the chains. The cluster sizes of associating groups increase with increasing association strength, and a corresponding increase in an elongational stress overshoot and the steady-state viscosity. For weak association and small clusters, chains first elongate until tension is sufficient to break up the small associating clusters. As the association strength increases, chain elongation and cluster breakup occur simultaneously and are insensitive to strain rate, mimicking solid-like behavior. For association strength ≥ 10 kBT, clusters act like stiff inclusions and the system fails as voids are formed during elongation. |
Thursday, March 18, 2021 4:36PM - 4:48PM Live |
V04.00009: Determination of the Number-Average Molecular Weight of Polyelectrolytes Aijie Han, Shravan Uppala, Louis A Madsen, Ralph Colby We developed four methods using the chain dynamics in the semidilute unentangled regime to determine the number-average molecular weight (Mn) of polyelectrolytes. Our team is collaborating to measure the correlation length (ξ) using X-ray scattering, the specific viscosity (ηsp) and relaxation time (τ) using rheometry, and the diffusion coefficient (D) using NMR diffusometry. Combining the measured properties of polyelectrolyte solutions yields the number density of chains in solution which directly determines Mn. To test the four methods, we studied five cesium polystyrene sulfonate samples that have narrow molecular weight distribution. We found that all four methods work generally well for the highest molecular weight (N = 9150). However, lower molecular weight polyelectrolytes are limited to two methods because τ cannot be obtained from the shear rate dependence of viscosity. The method using ηsp systematically underestimates Mn compared with the manufacturer’s reported value, while the method using D overestimates Mn. The deviation from the expected Mn increases for shorter chains for both methods. This observation underlines the importance of the non-uniform stretching along the chain of charged polymers which disappears in the long chain limit. |
Thursday, March 18, 2021 4:48PM - 5:00PM Live |
V04.00010: Rheology and Pinching Dynamics of Polyelectrolyte Solutions Leidy N Jimenez, Carina Martinez, Jelena Dinic, Vivek Sharma Biological macromolecules like proteins, DNA and polysaccharides, and many industrial polymers, are classified together as polyelectrolytes for in solution, the repeat units in their backbone are decorated with disassociated, charge-bearing ionic groups, surrounded by counter-ions. In diverse applications like inkjet printing, sprayable cosmetics and insecticides, paints and coatings that involve the formation of fluid columns or sheets that undergo progressive thinning and pinch-off into drops, the dominant flow within the necking filament is extensional in nature. The extensional rheology response of the charged macromolecular solutions are not as well understood as that of their uncharged counterparts. Here focus on the characterization of capillary thinning and pinch-off dynamics, extensional rheology and printability of two model systems: sodium (polystyrene sulfonate) and poly(acrylic acid) by using dripping-onto-substrate (DoS) rheometry technique. Due to an interplay of hydrodynamics-induced and charged-induced stretching, both the measured extensional relaxation times and the extensional viscosity values show salt- and polymer concentration-dependent behavior that is not expected or anticipated from the typical shear rheology response. |
Thursday, March 18, 2021 5:00PM - 5:12PM Live |
V04.00011: Effect of counterion and solvent type on the scattering and rheology of polyelectrolyte solutions Anish Gulati, Carlos Lopez, Walter Richtering Polyelectrolytes are an important class of materials that play a role in many biological processes and find use in various industrial processes. Recently, the role of counter-ion solvation on the properties of polyelectrolyte solutions has received renewed attention. Here we use Small-Angle neutron Scattering (SANS) and rheology to study solutions of carboxymethyl cellulose (CMC) with different counterions of varying size and hydrophobicity in aqueous and organic media. The counterions used belong to two classes: alkaline and tetra-alkyl ammonium. |
Thursday, March 18, 2021 5:12PM - 5:48PM Live |
V04.00012: Polyelectrolyte solutions in complex macro- and micro-scale flows Invited Speaker: Cari Dutcher Studying the behavior of polymer and polyelectrolyte solutions under complex hydrodynamic flow conditions is of interest to a variety of processes, such as extrusion, coating, and flocculation. Often these flow fields include combinations of both shear and extension flows, requiring characterization of the shear and extensional rheology for optimized materials processing. Here, the behavior of polymer solutions in two flow fields will be discussed: Taylor-Couette flows and microfluidic flow-focusing. First, in Taylor-Couette flow, which is flow between two concentric, rotating cylinders, the complex solutions are subjected to a wide variation of hydrodynamic flow states. The addition of non-Newtonian polymer solutions increases the solution’s elasticity, which in turn can modify flow states that are typically dominated by inertial forces. In this study, a cationic polyacrylamide was used to modify the elasticity of the solution, and with varying concentrations of NaCl to alter the ionic strength of the solution. The coil conformation and relaxation times of charged polymers changes depending on the ionic strength, from a more rigid conformation at low ionic strengths to a more flexible conformation at high ionic strengths, resulting in different non-Newtonian responses to shear. The effect of polymer conformation as a result of varying solution ionic strength on TC flows with co- and counter-rotation of the cylinders will be discussed. Second, microfluidic geometries will be used to characterize the extensional properties of low molecular weight, low viscosity polymer and polyelectrolyte solutions. Specifically, filament stretching using a cross-slot microfluidic channel will be used to resolve extensional properties of polyelectrolyte solutions at varying NaCl concentrations. |
Thursday, March 18, 2021 5:48PM - 6:00PM Live |
V04.00013: Dynamic mechanical response of solid polyelectrolyte complexes at varying temperature, humidity and pH Suvesh Lalwani, Jodie L. Lutkenhaus The dynamic behavior of polyelectrolyte complexes (PECs) is of interest for applications ranging from health to personal care. Hydration is an important factor, but its effect on the dynamic of PECs is poorly described. Previously in our group, time-temperature (TTSP) and time-water superposition (TWSP) principles were applied to show equivalency between temperature and water for poly(acrylic acid) (PAA) – poly(allylamine hydrochloride) (PAH) PECs. However, information is lacking regarding the relaxation time of intrinsic ion pair in these PECs at different pH values and the role played by water. Here, we describe the dynamic mechanical behavior of PAA-PAH solid PECs at different pH values, hydration and temperature. The temperature shift factor followed an Arrhenius relation and the water shift factor, obtained after TWSP enabled the study of the relaxation time of intrinsic ion pairs at different water content. Differential scanning calorimetry experiments were performed to quantify the amount of “bound water” and show that bound water provides a major contribution to relaxation. |
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