(232o) Customizing Polyelectrolyte Complex Shape through Photodirected Assembly

Polyelectrolyte complexes (PECs) form when oppositely-charged polymers are mixed in water and are routinely used in diverse applications such as drug delivery, sensors and separation technologies. Despite their broad use, however, current fabrication techniques typically limit their geometries to planar, cylindrical or spherical structures. To prepare PECs with a broader range of shapes, we have developed two methods of directing PEC formation through photopatterning. In the first method, poly(allylamine) (PAH) was mixed with poly(styrene sulfonate) (PSS) and a photoacid generator (PAG) at a pH that greatly exceeded the effective pK_a of PAH and (since the PAH amine groups were mostly neutralized) prevented insoluble PEC formation. These precursor solutions were then photopatterned through photomasks, which caused localized photochemical ionization of PAH (due to the PAG-triggered pH reduction) and, consequently, formation of insoluble PAH/PSS PECs, whose size and shape reflected the photoirradiation pattern. Since this technique relied on the development of pH gradients, however, the resolution of this photopatterned PEC formation was limited by the diffusion of OH^- ions into the irradiated region (which, when irradiation zones with sub-millimeter dimensions were used, impeded the PAG-mediated pH change). To overcome this limitation, we have developed a second photodirected assembly method, where PAH was mixed with acrylic acid (AA) monomers and a photoinitiator, and where photopatterning instead initiated site-specific polymerization of AA into poly(acrylic acid) (PAA). This localized polyanion formation led to the assembly of photopatterned PAH/PAA PECs with finer structural features than those formed through a localized pH reduction. The photopatterned PECs were characterized by microscopy, spectroscopy and gravimetry and demonstrated ability to either alter their dimensions or dissolve in response to changes in their external environment. This ability to form environmentally responsive structures with customized shapes suggests photodirected PEC assembly as a potentially attractive route to stimulus-responsive soft device fabrication.