Exploring the Impact of Zwitterions in Discrete Charge Arrangements of Stimuli-Responsive Polyelectrolyte Complexes

The effects of introducing zwitterionic 2-methacryloyloxyethyl­phosphorylcholine (MPC) into synthetic aqueous coacervates are described. A series of polyampholytes and polyelectrolytes composed of methacrylic acid and N-(3-aminopropyl)­methacrylamide hydrochloride with increasing amounts of MPC were prepared by free radical copolymerization. The compositional drift of charged monomers in polyampholytes was minimized by controlling the degree of ionization of the methacrylic acid, and the rate of incorporation MPC was monitored by 1H NMR. Macroscopic phase separation of a series of charge-balanced complexes with different degrees of charge separation was examined by both potentiometric and thermal turbidimetry as well as UV–vis spectroscopy to explore pH-, salt-, and temperature-responsive properties. MPC incorporation was found to affect the stimuli-responsiveness of all complexes, particularly in single-component charge-balanced polyampholytes. The intramolecular charge compensation in fully charge-balanced polyampholytes, compared to pairs of partially balanced polyampholytes or polyelectrolytes, is known to reduce the driving force for phase separation. Additionally, incorporation of MPC reduces the net complex charge density and disrupts localized charge runs, which weaken the associative interactions involved in complexation and destabilize the resulting coacervates. This allows for multifaceted tuning of the stimuli-responsive and physical properties of the derived complexes, which are dependent on both charge density and charge arrangement. The different coacervate compositions offer different degrees of stability and resistance to various stimuli, making them suitable models for, inter alia, intrinsically disordered proteins and underwater adhesives.