Segregative phase separation of strong polyelectrolyte complexes at high salt and high polymer concentrations

Area of Science:

• Polymer Science
• Physical Chemistry
• Materials Science

Background:

• Polyelectrolyte complexes and coacervates (PECs) phase behavior is understood at low salt.
• Behavior of PECs at high salt concentrations, above the binodal, is not well understood.
• Investigating PECs in high salt and polymer concentration regimes is crucial for fundamental understanding.

Purpose of the Study:

• To investigate the phase behavior of stoichiometric poly(styrene sulfonate)/poly(diallyldimethylammonium) (PSS/PDADMA) mixtures at high salt and polymer concentrations.
• To understand the molecular-scale factors driving phase separation in PECs under these conditions.

Main Methods:

• Preparation of PSS/PDADMA PECs by direct mixing with water and KBr salt.
• Observation of phase separation at salt concentrations approximately 1 M above the binodal.
• Characterization using thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR).

Main Results:

• Phase separation was observed at high salt concentrations.
• Both separated phases contained significant polymer amounts.
• The polymer-rich phase was enriched in PSS, while the polymer-poor phase was enriched in PDADMA.
• At highest concentrations, the polymer-rich phase contained both PSS and PDADMA.

Conclusions:

• High salt concentrations promote "salting out" of the hydrophobic PSS polyelectrolyte.
• This salting-out effect is consistent with observations in weak polyelectrolyte systems.
• High salt concentrations can also induce phase separation of partially neutralized PECs.
• Studying PECs in the high concentration limit offers insights into phase separation mechanisms.