title: Charge segregation in weakly ionized microgels.
abstract: We investigate microgels synthesized from N-isopropylacrylamide (NIPAM) copolymerized with a large mol% of acrylic acid, finding that when the acid groups are partially ionized at high temperatures, competition between ion-induced swelling and hydrophobic deswelling of poly(NIPAM) chains results in microphase separation. In cross-linked microgels, this manifests as a dramatic decrease in the ratio between the radius of gyration and the hydrodynamic radius to ∼0.2, indicating that almost all the mass of the microgel is concentrated near the particle center. We also observe a concurrent decrease of the polymer network length scale via small-angle neutron scattering, confirming the presence of a dense, deswollen core surrounded by a diffuse, charged periphery. We compare these results to those obtained for a system of charged ultralow-cross-linked microgels; the form factor shows a distinct peak at high q when the temperature exceeds a threshold value. We successfully fit the form factor to theory developed to describe scattering from weakly charged gels in poor solvents, and we tie this behavior to charge segregation in the case of the cross-linked microgels.
authors.affiliation: School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0430, USA.
authors.forename: John S
authors.initials: JS
authors.lastname: Hyatt
authors.affiliation: Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.
authors.forename: Alison M
authors.initials: AM
authors.lastname: Douglas
authors.affiliation: Biology and Soft Matter Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: Chris
authors.initials: C
authors.lastname: Stanley
authors.affiliation: Biology and Soft Matter Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: Changwoo
authors.initials: C
authors.lastname: Do
authors.affiliation: Department of Biomedical Engineering, University of Virginia, Charlottesville, Virginia 22908, USA.
authors.forename: Thomas H
authors.initials: TH
authors.lastname: Barker
authors.affiliation: School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0430, USA.
authors.forename: Alberto
authors.initials: A
authors.lastname: Fernández-Nieves