Upper critical solution temperature behavior of a strong-weak polyelectrolyte complex in acid medium

Phase separation is entropically favored when oppositely charged polyelectrolytes are mixed, releasing small counterions to form intrinsic ion pairs. This results in the formation of a supernatant phase and a dense, solid or liquid coacervate phase containing polyelectrolyte complexes (PECs). Understanding PECs is valuable for advancing drug delivery systems, underwater adhesives, and multi-compartment cells. PEC phase boundaries have most commonly been explored with respect to salt and polymer concentration, pH, and mixing ratio. More recently, the effects of temperature on the phase transitions of PECs have gained interest but a gap in knowledge still remains. In this study, using turbidity and conductivity measurements, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy and optical microscopy (OM), we identify the phase boundaries of poly(diallyldimethyl ammonium)/poly(acrylic acid) (PDADMA/PAA) PECs at pH 3. We observe reversible temperature-induced phase transitions showing upper critical solution temperature (UCST) behavior at different PDADMA:PAA mixing ratios and ionic strengths. These results provide more understanding of salt, water, and temperature contributions to phase transitions.