# Cosolvent Control of Lower and Upper Critical Solution Behavior in Polyelectrolyte Complexes

## TL;DR

This paper shows how adding different cosolvents to polyelectrolyte mixtures can control their phase separation behavior, leading to new insights into how these systems work.

## Contribution

The study introduces a new method using cosolvents to manipulate both LCST and UCST behaviors in polyelectrolyte complexes.

## Key findings

- Adding cosolvents like ethylene glycol or N-methyl formamide shifts the LCST and introduces a UCST in the system.
- The UCST behavior involves the segregation of polycation into the supernatant from the polyanion-rich phase.
- Electrostatic correlations may not be the main driver of phase behavior in cosolvated coacervate systems.

## Abstract

We report that polar
cosolvent–water mixtures
offer a unique
approach to controlling the liquid–liquid phase separation
(LLPS) of polyelectrolyte complex solutions formed from degree of
polymerization-matched mixtures of strong and weak polyelectrolytesrespectively,
quaternary poly­(N,N-dimethylaminoethyl methacrylate
chloride) (qPDMAEMA) and sodium poly­(acrylate) (PA). As observed in
prior work, associative LLPS in water exhibits an upper-critical salt
concentration with stoichiometric complexes and lower-critical solution
temperature (LCST) behavior, where electrostatic correlations are
believed to drive phase behavior. However, upon addition of a miscible
cosolvent prior to mixing the individual polyelectrolytes at room
temperature, we observe a shift in the LCST and the appearance of
an upper-critical solution temperature (UCST). This new UCST feature
corresponds to a segregative LLPS, whereby the polycation partitions
out of the polyanion-rich dense phase and into the supernatant. This
behavior arises with cosolvents that decrease (e.g., ethylene glycol)
or increase (e.g., N-methyl formamide) the average
solvent dielectric constant, suggesting that electrostatic correlations
may not primarily control the phase behavior for cosolvated coacervate
systems. A conceptual 3D phase surface summarizing these observations
for the cosolvated system suggests that two distinct surfaces with
critical lines appear on the polymer–salt–temperature
phase diagram.