Tunable Thermoresponsiveness by Engineering the Co- and Self-Assembly of Oppositely Charged Surfactants inside Microgels: Reentrant Swelling Behavior and Thermoinduced Surfactant Release

Understanding the uptake into permeable microgels is a prerequisite to engineer more advanced applications devoted to the release of active molecules or even the transformation of substrates inside the microgels. Hence, the swelling behavior of cationic, viologen-containing microgels based on N - n -propylacrylamide (NNPAM) was characterized in absence and presence of a model guest, namely the anionic surfactant sodium dodecyl sulfate (SDS). The incorporation of viologen units into the microgels, which was achieved upon covalent attachment to copolymerized acrylic acid units (AAc), leads to increased microgel sizes, an increased volume phase transition temperature ( VPTT ) and to an incomplete thermal collapse compared to the pre-modified state of the parental NNPAM- co -AAc microgels. The addition of SDS to the viologen-modified microgels endows a control over the swelling properties: electrostatic complexation at low SDS concentrations leads to a shrinking and a decrease of the VPTT of the microgels, whereas high concentrations result in a charge inversion, a re-swelling and an increased VPTT . These characteristics were identified via dynamic light scattering (DLS), small angle x-ray scattering (SAXS), differential scanning calorimetry (DSC), electrophoretic measurements, NMR and isothermal titration calorimetry (ITC). All these methods suggest a surfactant decoration of the microgel network, which starts at the viologen units and eventually leads to the formation of SDS micelle-like aggregates inside the microgel below the critical micellization concentration ( cmc ). A two-step thermal collapse is observed at high SDS concentrations, which can be attributed to the thermo-induced breaking of these associative interactions and a release of SDS from the network.