(716j) Characterization of Thermoresponsive Polyelectrolyte Complex Micelles

Mixing oppositely charged polyelectrolytes in solution results in the formation of polyelectrolyte complexes that phase separate from solution into liquid (coacervate) or solid (precipitate) forms. This phenomenon can be confined to the nanoscale by using block copolyelectrolytes, which contain a neutral block linked to a charged block, thus forming polyelectrolyte complex micelles with a polyelectrolyte core and neutral polymer corona. This work explores using a thermosensitive polymer, poly(N-isopropyl acrylamide) (pNIPAM) as the neutral block. pNIPAM has a lower critical solubility temperature (LCST), above which a hydrophilic to hydrophobic transition occurs. As the corona segments become less soluble, a core-corona inversion could occur that could facilitate the release of molecules in the polyelectrolyte core leading to potential applications in controlled release drug delivery. We are characterizing micelles formed using a diblock copolymer of pNIPAM-b-polyacrylic acid mixed with (1) polylysine (2) polyethylene glycol-b-polylysine or (3) pNIPAM-b-polylysine in order to investigate the role of the corona in micelle stability and morphology before and after the temperature transition. Characterization is performed using light and small angle x-ray scattering for the different systems as function of parameters known to influence polyelectrolyte complexes (polymer concentration, charge ratio, pH, salt) and as function of temperature. Preliminary results show micelle aggregation after the LCST has been crossed, but it remains unclear if core-corona inversion has been achieved or if core and corona segments are mixed post transition, which is the subject of ongoing experiments. Time permitting an additional thermosensitive system will be discussed using elastin-like-polypeptides as the corona forming segment. Elastin-like-polypeptides also exhibit LCST behavior, but have more tunable transition temperatures through modification of the peptide sequence.