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 L15: Fantastic Polyelectrolytes and How They Behave in CoacervatesInvited Live
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Sponsoring Units: DPOLY Chair: Michael Rubinstein, Duke University Scott Danielsen, Duke University |
Wednesday, March 17, 2021 8:00AM - 8:36AM Live |
L15.00001: Structure and Dynamics of Polyelectrolyte Solutions and Coacervates Invited Speaker: Ralph Colby The shear rate dependence of viscosity is used to evaluate the concentration dependences of specific viscosity, relaxation time and terminal modulus for several polyelectrolytes in three solvents: deionized water, ethylene glycol and glycerol. Small-angle X-ray scattering determines the correlation length of these polyelectrolyte solutions from a peak in their scattering function, which changes as expected with solvent dielectric constant. In semidilute unentangled solutions, dynamics are described by the Rouse model and combined with the correlation length in different ways to determine the number density of chains, which allows calculation of the number-average molecular weight using a robust method that is insensitive to the presence of salt. |
Wednesday, March 17, 2021 8:36AM - 9:12AM Live |
L15.00002: Phase Behavior and Viscoelasticity of Polyelectrolyte Coacervates Above the Binodal Invited Speaker: Jennifer Laaser Preparing coacervate samples at compositions above the binodal allows the polymer and salt concentrations in these materials to be varied independently. Here, we exploit this approach to investigate the dynamics of polystyrenesulfonate (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC) coacervates with salt concentrations between 1.2 and 2 M and volume fractions of polymer between 0.1 and 0.25. Small-amplitude oscillatory shear rheology reveals that the relaxation times scale significantly more strongly with polymer volume fraction than has been previously assumed, highlighting the need to account for both salt and polymer-dependent contributions to coacervate dynamics. Additionally, we identify a second critical salt concentration above which the coacervates phase-separate again at high salt concentrations. Thermogravimetric analysis reveals significant differences in the tie line slope in this high-salt phase window compared to those seen in the low-salt regime. These results demonstrate that intentionally moving above the binodal is a powerful approach to understanding the composition dependence of complex coacervates and offers exciting new opportunities for understanding the physics of these systems. |
Wednesday, March 17, 2021 9:12AM - 9:48AM Live |
L15.00003: Entanglements between Polyelectrolytes in Solutions Invited Speaker: Carlos Lopez How entanglements between ionic polymers are formed remains an open question in polymer science. For some 20 years, it has been known that theoretical predictions for polyelectrolyte entanglement in solution are at odds with experimental evidence, but remarkably little progress has been made on this topic. Here we present a systematic study of the diffusion and rheological properties of polyelectrolytes in solution, and use these results to shed light on the dynamics of polyelectrolyte solutions. |
Wednesday, March 17, 2021 9:48AM - 10:24AM Live |
L15.00004: Structure and rheology of polyelectrolyte complex coacervates Invited Speaker: Amanda Marciel Polyelectrolyte complexes are highly tunable materials that span from low-viscosity liquids (coacervates) to high-modulus solids with high water content, making them attractive as surface coating, membrane purification and bioadhesive materials. However, most of their properties and their effects with salt, pH, polymer ratio and temperature have only been qualitatively described. Here, we present an investigation of the structure and chain conformations, and rheological properties of polyelectrolyte complex (PEC) coacervates comprising biomimetic model polyelectrolytes. Systematic studies using small-angle X-ray scattering (SAXS) of the structure and chain behavior in liquid PEC coacervates revealed a physical description of these materials as strongly screened semidilute solutions of polyelectrolytes comprising oppositely charged chains. At the same time, solid PECs were found to be composed of hydrogen-bonding driven stiff ladder-like structures with large correlation lengths. While the liquid complexes behaved akin to semidilute polyelectrolyte solutions upon addition of salt, the solids were largely unaffected by it. Terminal relaxations of the chains in PEC coacervates were explored by rheology measurements. Excellent superposition of the dynamic moduli data was achieved by a time-salt superposition. |
Wednesday, March 17, 2021 10:24AM - 11:00AM Live |
L15.00005: Fantastic Viscoelastic Saloplastic Invited Speaker: Joseph Schlenoff Spontaneous liquid-liquid phase separation, into a polyelectrolyte complex, occurs when aqueous solutions of oppositely-charged polyelectrolytes are mixed. In the absence of salt, stoichiometric mixtures of these polyelectrolyte complexes contain well-paired repeat units Pol+ and Pol-. Salt water “dopes” and plasticizes the complex, yielding a broad set of materials properties from glassy to rubbery to viscous fluid to single-phase solutions. This “saloplasticity” is reversed by rinsing the complex in water to extract the salt. This talk will illustrate the concept of saloplasticity and will show how the “sticky associations” between Pol+ and Pol-, which control the viscoelastic behavior of complexes, may be reversibly regulated with salt. An unusual scaling of viscosity, η, with molecular weight, M is observed in the sticky reptation regime: η ~ M5 . |
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