Thermally Responsive Hyperbranched Poly(ionic liquid)s: Assembly and Phase Transformations

A library of linear and branched amphiphilic poly(ionic liquid)s based on hydrophobic cores and peripheral thermally sensitive shells was synthesized and studied with regard to their ability to form stimuli-responsive, organized assemblies in aqueous media. The thermally responsive derivatives of poly(ionic liquid)s were synthesized by neutralizing 32 terminal carboxyl groups of functionalized polyester cores by amine-terminated poly(N-isopropylacrylamide)s (PNIPAM) (50% and 100%). We observed that these hyperbranched poly(ionic liquid)s possessed a narrow low critical solution transition (LCST) window with LCST for hyperbranched compounds being consistently lower than that for linear PNIPAM containing counterparts. We found that the poly(ionic liquid)s form spherical micellar assemblies with diverse morphologies, such as micelles and their aggregates, depending on the terminal compositions with reduced sizes for hyperbranched poly(ionic liquid)s. Increasing temperature above LCST promoted formation of network-like aggregates, large vesicles, and spherical micelles. Moreover, all PNIPAM-terminated compounds exhibited distinct unimolecular prolate nanodomain morphology in contrast to common spherical domains of initial cores. We proposed a multilength scale organized morphology to describe the thermoresponsive poly(ionic liquid)s micellar assemblies and discussed their morphological transformations during phase transitions associated with changes in hydrophobic-hydrophilic balance of poly(ionic liquid)s with distinct hydrophobic cores and variable peripheral shells.