Relaxation Behavior by Time-Salt and Time-Temperature Superpositions of Polyelectrolyte Complexes from Coacervate to Precipitate

TL;DR

This study investigates the relaxation dynamics of polyelectrolyte complexes across the transition from coacervate to precipitate, revealing how salt concentration and temperature influence their viscoelastic behavior and superposition properties.

Contribution

It demonstrates the application of time-temperature superposition to polyelectrolyte complexes and explores how salt concentration affects their relaxation and transition to gel-like states.

Findings

Complexes exhibit time-temperature superposition at all salt concentrations.

The superposition range shifts with salt concentration, moving to higher frequencies as salt decreases.

The sticky-Rouse model effectively describes relaxation behavior across conditions.

Abstract

Complexation between anionic and cationic polyelectrolytes results in solid-like precipitates or liquid-like coacervate depending on the added salt in the aqueous medium. However, the boundary between these polymer-rich phases is quite broad and the associated changes in the polymer relaxation in the complexes across the transition regime are poorly understood. In this work, the relaxation dynamics of complexes across this transition is probed over a wide timescale by measuring viscoelastic spectra and zero-shear viscosities at varying temperatures and salt concentrations for two different salt types. We find that the complexes exhibit time-temperature superposition (TTS) at all salt concentrations, while the range of overlapped-frequencies for time-temperature-salt superposition (TTSS) strongly depends on the salt concentration (Cs) and gradually shifts to higher frequencies as Cs is decreased. The sticky-Rouse model describes the relaxation behavior at all Cs. However, collective relaxation of polyelectrolyte complexes gradually approaches a rubbery regime and eventually exhibits a gel-like response as Cs is decreased and limits the validity of TTSS.