Stimuli-Responsive Polyelectrolytes: Thermosensitive Zwitterionic Polymers and Charged Shape-Changing Star Molecular Bottlebrushes

This dissertation work investigated two classes of stimuli-responsive polyelectrolytes: thermosensitive zwitterionic poly(sulfobetaine methacrylate)s (PSBMAs) and charged shape-changing star molecular bottlebrushes (SMBs). While zwitterionic polymers are explored for numerous applications, their structure-solution behavior relationship was poorly understood. The first part of this dissertation focused on the effects of N-substituents of PSBMAs on their behavior in water. A series of PSBMAs were synthesized, with systematically changed N-substituents, including symmetric N-n-alkyl substituents of various lengths and asymmetric N-substituents comprising one methyl and either one cyclohexyl, phenyl or 2-hydroxyethyl group. The behavior of PSBMAs with symmetric N-substituents in water varied from upper critical solution temperature (UCST) to water-soluble, lower critical solution temperature (LCST), and water-insoluble with increasing N-n-alkyl length due to two competing effects: weakening of zwitterionic attractive interactions and increasing of hydrophobic effect. For PSBMAs with asymmetric N-substituents, while substituting the N-n-hexyl group with cyclohexyl diminished the hydrophobic effect, making the LCST disappear, introducing an N-phenyl or N-2-hydroxyethyl substituent enhanced zwitterionic interactions, producing UCST-type thermoresponsive polymers. A collaborative study showed that LCST-type thermoresponsive PSBMA-based polymers were effective kinetic hydrate inhibitors for gas and oil transportation. The second part of this dissertation focused on star-globule shape transitions of three-arm SMBs composed of heterografted poly(ethylene oxide) (PEO) and either protonated or iodomethane-quaternized poly(N,N-dialkylaminoethyl methacrylate) side chains in aqueous solution. Super and moderate chaotropic anions (CAs) were shown to induce star-globule shape changes of protonated SMBs, caused by the ion pairing of CAs with ammonium cations and chaotropic effect. While super CA (e.g., [Fe(CN)₆]^3-)-collapsed SMBs remained in the globular state in acidic water upon heating to 70 °C, moderate CA (e.g., ClO₄^-)-collapsed brushes exhibited globule-to-star shape transitions. In contrast, weak chaotropic I^- and kosmotropic SO₄^2- had only small effects on the conformations of protonated SMBs in the studied salt concentration and temperature ranges. For SMBs with permanently quaternary ammonium groups in water, the size transitions induced by super and moderate CAs occurred at salt concentrations much lower than those of unquaternized SMBs. These findings not only improve our understanding of the behavior of polyelectrolytes but also provide guidelines for design of stimuli-responsive polyelectrolytes for potential applications.