title: Temperature- and pH-responsive micelles with collapsible poly(N-isopropylacrylamide) headgroups.
abstract: We have studied the micelle formation and phase behavior of a series of temperature- and pH-responsive surfactants prepared by controlled radical (RAFT) polymerization. These C12NIPAMm surfactants consist of a dodecyl tail, a poly(N-isopropylacrylamide) (polyNIPAM) headgroup with average degrees of polymerization of between 7 and 96, and an ionizable carboxylate group. In the un-ionized state, these surfactants phase separate on warming toward a lower critical solution temperature (LCST), which decreases as the length of the NIPAM group is decreased. This is in agreement with the behavior of conventional nonionic poly(ethylene oxide)-based surfactants but is very different from that of polyNIPAM oligomer solutions. Small angle neutron scattering (SANS) shows that these surfactants self-assemble into micelles consisting of a nearly spherical hydrophobic core surrounded by a "hairy" polyNIPAM shell far below their LCST. Upon warming, the micelles undergo a sphere-to-rod transition induced by the collapse of the polyNIPAM shell, causing a reduction in the headgroup area. In the un-ionized state the demixing follows at the LCST, but a single charge on the free polymer end completely suppresses phase separation, allowing micelles to undergo a shape change but remain dissolved.
authors.affiliation: The School of Chemistry, Building F11, The University of Sydney , Sydney, NSW 2006, Australia.
authors.forename: Paul A
authors.initials: PA
authors.lastname: FitzGerald
authors.forename: Sushen
authors.initials: S
authors.lastname: Gupta
authors.forename: Kathleen
authors.initials: K
authors.lastname: Wood
authors.forename: Sébastien
authors.initials: S
authors.lastname: Perrier
authors.forename: Gregory G
authors.initials: GG
authors.lastname: Warr