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

Area of Science:

• Polymer Science
• Physical Chemistry
• Materials Science

Background:

• Polyelectrolyte complex solutions exhibit associative liquid-liquid phase separation (LLPS) driven by electrostatic correlations.
• In water, this LLPS shows upper-critical salt concentration and lower-critical solution temperature (LCST) behavior.
• Controlling LLPS is crucial for applications in materials and biomaterials.

Purpose of the Study:

• To investigate the effect of polar cosolvents on the LLPS of polyelectrolyte complex solutions.
• To explore the emergence of new phase boundaries and critical phenomena.
• To understand the role of solvent properties and electrostatic interactions in cosolvated systems.

Main Methods:

• Preparation of polyelectrolyte complex solutions using degree of polymerization-matched quaternary poly(N,N-dimethylaminoethyl methacrylate chloride) (qPDMAEMA) and sodium poly(acrylate) (PA).
• Addition of miscible polar cosolvents (e.g., ethylene glycol, N-methyl formamide) to aqueous solutions.
• Observation and analysis of liquid-liquid phase separation behavior across varying temperatures and salt concentrations.

Main Results:

• Addition of polar cosolvents significantly alters the phase behavior of polyelectrolyte complexes.
• A shift in the lower-critical solution temperature (LCST) and the appearance of an upper-critical solution temperature (UCST) were observed.
• The new UCST corresponds to a segregative LLPS, where the polycation separates from the polyanion-rich phase.
• This behavior was observed with cosolvents that alter the dielectric constant, suggesting non-electrostatic drivers for phase separation.

Conclusions:

• Polar cosolvents provide a versatile method for tuning the phase behavior of polyelectrolyte complexes.
• The emergence of UCST indicates a transition from associative to segregative LLPS under cosolvation.
• Electrostatic correlations may not be the sole determinant of phase behavior in cosolvated polyelectrolyte systems.
• A conceptual 3D phase diagram reveals complex phase surfaces with distinct critical lines in cosolvated systems.