Electrostatic Effects on the Internal Dynamics of Redox-Sensitive Microgel Systems

Microgels are flexible entities with a number of properties which can be tailored for a variety of applications. For redox-sensitive PNIPAM-based microgels involved in this study, the size and effective charge of microgels can be manipulated by electrochemical means. The electrochemical switching is implemented via interaction of redox-sensitive counterions (hexacyanoferrates: HCF) with oppositely charged (cationic) thermoresponsive microgels. Effects on the internal dynamics upon uptake of HCF and increased hydrophobicity with temperature are investigated with neutron spin echo spectroscopy. The polymer segmental dynamics is well described by the Zimm model. Unbalanced charges (in absence of HCF) apparently shorten the polymer length acting like confined discontinuity points (pinning). This effect vanishes in the presence of HCF. The ability of multivalent ferricyanides to bind several monovalent polymer charges at the same time produces an apparent secondary network. This effective bridging makes the dynamics slower analogous to an increase in cross-linker density. In support of this picture, an enhanced viscosity of the medium, where the polymer chains move, was obtained by the fitting