Reconfiguration of Langmuir Monolayers of Thermo-Responsive Branched Ionic Polymers with LCST Transition

Thermo-responsive ionic polymers have the ability to form adaptive and switchable morphologies, which may offer enhanced control in energy storage and catalytic applications. Current thermoresponsive polymers are composed of covalently attached thermoresponsive moieties, restricting their mobility and global dynamic response. Here, we report the synthesis and assembly at the water-air interface of symmetric and asymmetric amphiphilic thermo-responsive branched polymers with weakly ionically bound arms of amine terminated poly(N-isopropylacrylamide) (PNIPAM) macro-cations. As we observed, symmetric branched polymers formed multimolecular nanosized micellar assemblies, whereas corresponding asymmetric polymers formed large, interconnected worm-like aggregates. Dramatic changes in localized and large-scale chemical composition confirmed the reversible adsorption and desorption of the mobile PNIPAM macro-cations below and above the low critical solution temperature (LCST) and their non-uniform redistribution within polymer monolayer. Increasing the temperature above LCST led to the formation of large interconnected micellar aggregates because of the micelle-centered aggregation of the hydrophobized PNIPAM macro-cationic terminal chains in the aqueous subphase. Overall, this work provides insights into the dynamic nature of the chemical composition of branched ionic polymers with weakly ionically bound thermo-responsive terminal chains and its effect on both morphology and local/surface chemistry of monolayers at LCST transition.