(619b) pH-Mediated Control over Polyelectrolyte Complex Shape and Internal Structure

When oppositely charged polyelectrolytes are mixed in water they form polyelectrolyte complexes (PECs), whose size and morphology typically reflect their formation kinetics. This process has an electrostatic driving force and (when weak polyelectrolytes are used) depends strongly on pH. Using poly(styrene sulphonate) (PSS) as the polyanion and poly(allylamine) (PAH) as the weak (pH-sensitive) polycation, we have exploited this sensitivity to generate PECs with custom-designed shapes and internal structures. PAH and PSS were mixed at high pH (far above the pKa of PAH), where the PAH amine groups were largely uncharged and the PAH/PSS mixtures formed single-phase solutions. PECs were then prepared from these solutions by reducing the pH, thus ionizing the PAH and triggering complexation. This was achieved through two strategies. The first strategy, which was used to form PEC beads with solid internal structures, relied on the dropwise addition of PAH/PSS mixtures to acid baths. The diffusion of acid into the droplets led to rapid PAH ionization and, consequently, conversion of the PAH/PSS solution droplets into PEC beads, which (unlike the solvent-filled PEC capsules prepared through traditional methods) had solid internal morphologies. Furthermore, to extend PEC formation to more-intricate structural motifs, the pH-sensitivity of PAH was exploited to form custom-designed shapes through photopatterning. In this second strategy, a photoacid generator (PAG), which lowered the pH when exposed to UV light, was added to the PAH/PSS mixtures. The localized protonation of PAH amine groups then led to the formation of PECs, whose custom-designed shapes reflected the photoirradiation patterns. These distinct PEC shapes and (in the case of the beads) internal morphologies suggest pH-triggered PEC formation as an effective approach to directing PEC structure.