The Effects of Non-Electrostatic Intermolecular Interactions on the Phase Behavior of pH-Sensitive Polyelectrolyte Complexes

Polyelectrolyte complexes (PECs) offer enormous material tunability and desirable functionalities, and consequently have found broad utility in biomedical and materials industries. Poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) are one of the most commonly used pairings to form PECs. However, various aspects of the phase behavior of PAA-PAH complexes have not been sufficiently quantified. We present a comprehensive experimental study depicting the binodal phase boundaries for the PAA-PAH complexes prepared in acidic, neutral and basic conditions using thermogravimetric analysis, turbidimetry and optical microscopy. In neutral and basic conditions, phase behaviors of the complexes were largely similar to each other and followed general expectations of PEC phase behavior, except for unusually high salt resistance with stable complexes observed up to 4 M NaCl concentrations. In acidic conditions, a remarkably different phase behavior of the PAA-PAH complexes was observed. The polymer content in the complex phase increased initially followed by an expected decrease as salt was added to the complexes. This behavior may result from a combination of associative phase separation of PAA and PAH chains, influenced by electrostatic interactions, and segregative phase separation which can be ascribed to the influence of a combination of the hydrophobic interactions of the aliphatic polymer backbone and the interpolymer hydrogen bonding of un- ionized acrylic monomer units. Our systematic investigations detailing these discrepancies in the PAA-PAH phase behavior are expected to clarify the inconsistencies among the reports in the literature and inform the materials design strategies for practical use of the PAA-PAH complexes and multilayer assemblies.