Influence of Small Ions on Composition and Viscoelasticity of Polyelectrolyte Complexes
- Author(s): Syed, Vaqar Mustafa Shah
- Advisor(s): Srivastava, Samanvaya
- et al.
The addition of salt speeds up chain relaxation dynamics in polyelectrolyte complexes (PECs), and time-salt superposition (TSS) approaches to describe the linear viscoelastic response of PECs are well-established. However, TSS is carried out at fixed initial polyelectrolyte concentrations, and varying the initial polyelectrolyte concentration results in distinct TSS master curves. In this thesis, we show that accounting for the small ions that accompany the oppositely charged polyelectrolyte chains (designated as accompanying counterions) enables assimilation of these distinct TSS master curves into a single universal master curve. This approach, that we christen as time-ionic strength superposition (TISS), enables a unified description of the PEC viscoelastic response in terms of the solution ionic strength, that accounts for both the accompanying counterions and the added ions, and underlines the dynamic similarities between PECs and semi-dilute polymer solutions. The sticky electrostatic associations among the oppositely charged chains, however, contribute additional relaxation modes in the PECs. We demonstrate that the timescales of these additional relaxation modes are described quantitatively by a modified sticky Rouse model that accounts for the influence of solution ionic strength on the electrostatic screening and chain friction. We then investigate the effect of the cationic valency of the added salt on the composition and rheology of the PECs. A stronger screening of electrostatic interactions is observed with increasing cation valency, leading to higher concentrations of polyelectrolytes in the supernatant phase and lower concentrations in the complex phase. In addition, electrostatic bridging of the polyanion chains by the multivalent cation alters the chain relaxation process in PECs by competing with the electrostatic interactions between the oppositely charged chains, resulting in non-monotonic variations of PEC moduli with increasing ionic strength of the solution.