Structure Controlled Dynamics of Particles and Polymers in Aqueous Polyelectrolyte Solutions



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Complex fluids of particles and polymers can be found a wide range of industrial applications including enhanced oil recovery, drug delivery, and consumer goods. Polyelectrolytes are polymers with charged functional groups on their backbone. The charged groups allow polyelectrolytes to maintain high stability and good biocompatibility in aqueous solutions. The induced electrostatic repulsion, however, result in highly extended directed random walk conformations inside their correlation volumes. The size, strength, and recurrence of these charged groups between the monomers leads to conformations ranging from rigid rod to semiflexible chain. These unique electrostatically induced structural properties likely affect the transport of objects within their geometries as well as their chain relaxations in solutions. In this work, we investigated the dynamics of particles and polyelectrolyte chains in semidilute aqueous polyelectrolyte solutions. First, we explored the effect of chain conformation on the transport of spherical particles across an order of magnitude in size using fluorescence microscopy. We tuned the chain conformation by varying solution ionic strength. We found that large particles follow predictions according to bulk viscoelasticity. Smaller particles on the order of magnitude of chains, however, experience non-monotonic deviations from predictions. These deviations arise from the structural properties of polyelectrolytes in the form of confinements despite the absence of entanglements. We then studied the effects of chain flexibility on the dynamics of nanoparticles using dynamic x-ray scattering. The chain flexibility was altered by changing the degree of polymerization of the chains. We observed three unique behaviors at each chain size. In solutions with small chains, particles couple to the predictions according to the bulk viscoelasticity. Intermediate size polymer solutions showed signs of confinement effects with signature non-monotonic deviations from expectations. Solutions with largest chains, however, linearly deviated from Stokes-Einstein predictions at all concentrations. These observations confirmed the important role of chain flexibility on the dynamics of nanoparticles in solutions. Next, we studied the segmental dynamics of charged chains in solutions using neutron scattering techniques. We independently vary the geometry and electrostatics by changing polymer concentration and chain ionic strength. We found that the relaxations are slower than predictions with signatures of de Gennes’ narrowing phenomenon. Moreover, the structure factor of the chains tracks the observed dynamics. These observations suggested that we have a direct correlation between the structure and dynamics in these systems. Finally, we explore the structure, rheology, and dynamics of colloid-polymer suspensions using in situ x-ray scattering, rheo-XPCS, that allows rheological and scattering measurements simultaneously. We find that suspensions without and with small and dispersed depletants remain as fluids whereas particles in the presence of large depletants experience gelation. This observation suggests that suspensions with smaller size depletants require a stronger attraction potential compared to suspensions with larger ones.



Polymers Physics, Complex fluids, Soft Matter, Nanoparticle Transport


Portions of this document appear in: Slim, Ali H., Ryan Poling-Skutvik, and Jacinta C. Conrad. "Local confinement controls diffusive nanoparticle dynamics in semidilute polyelectrolyte solutions." Langmuir 36, no. 31 (2020): 9153-9159.