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

Abstract

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. File list (2) download file view on ChemRxiv Lu_PAAPAH_Phasebehavior_V2.pdf (3.98 MiB) download file view on ChemRxiv SI_Lu_PAAPAH_Phasebehavior_submitted.pdf (1.33 MiB)

Looping-in complexation and ion partitioning in nonstoichiometric polyelectrolyte mixtures

“…Second, in strongly asymmetric mixtures, salting-out and salting-in behavior may both be present, resulting in a looping-in or reentrant demixing transition. Third, any molecular factors that cause an abundance of counterions to accumulate in the coacervate phase, such as a severe mismatch in charge density, may cause the looping-in behavior, which may rationalize the effect of pH reported recently (48).…”

“…Biological PECs may consist of many types of charged biopolymers, and the impact of asymmetry is likely prominent. A thorough investigation of multiple types of heterogeneity, such as in PE flexibility ( 49 ), charge density ( 48 ), or molecular weights, may help to further our understanding of how seemingly nonspecific cellular LLPS regulates biological functions with spatial and temporal precision.…”

Polyelectrolyte Complex Coacervation across a Broad Range of Charge Densities

“…This behavior is well in line with that for previously described PECs stabilized by non-covalent interactions other than electrostatics. 35 , 74 In comparison, the analogous coacervate formed with poly(Am ox )/poly(Sulf) was characterized by a lower coacervate density, with C P ∼ 37 wt % that diminished with increasing [NaCl] and was fully dissolved at [NaCl] = 1.0 M ( Table S3 ). The narrowing of the binodal phase envelope and decrease of C s,cr with polycation oxidation are consistent with observations by Lou et al 34 for PECs that were not otherwise stabilized by nonelectrostatic interactions.…”

Deciphering the Role of π-Interactions in Polyelectrolyte Complexes Using Rationally Designed Peptides

“…Although increased charge density has been shown to increase the amount of complex formation and stability against salt [ 68 , 69 ], the combination of electrostatic, hydrophobic, and π-interactions results in higher stability against salt as shown with both p(kF) + p(eF) and p(k(fl)F) + p(e(fl)F) sequence pairs that have high turbidity values even at 4 M of added salt. Li et al [ 70 ], also observed a turbidity plateau at high salt concentrations, indicative of high salt resistance, for polyacrylic acid and polyallylamine hydrochloride complexes. They attributed this salt stability to non-electrostatic interactions such as hydrophobicity and hydrogen bonding.…”