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.