Dielectric properties of thermo-reversible hydrogels: the case of a dextran copolymer grafted with poly(N-isopropylacrylamide)

We investigated the dielectric properties of aqueous solutions of a grafted copolymer, consisting of a polysaccharide, Dextran, grafted with a thermo-sensitive polymer, poly(N-isopropylacrylamide), [pNIPAAM], over broad temperature and frequency ranges. The graft copolymers, prepared by atom-transfer radical polymerization [ATRP], form temperature-responsive materials that represent a class of self-assembled structures in water of great interest because of their potential use as drug delivery formulations and in diverse biotechnological applications. In these systems, in the dilute regime and below the lower critical solution temperature, relaxation modes corresponding to two different length-scales have been observed and analyzed in terms of ion fluctuation dielectric models specifically developed to describe the dielectric relaxation in highly charged polyion aqueous solutions. Regardless of whether the ions were produced by the ionization of the polymer chain, as in polyelectrolyte solutions, or not, as in the present case, they represent a probe at a microscopic level that is expected to reveal the structural characteristics of the system at different scales. We have identified a characteristic length associated with the size of the polymer coil in the dilute regime and a length due to fixed cross-links, where ions are partially localized by the local profile of the Coulombic field. These lengths are in reasonable agreement with analogous lengths derived from structural information and from the hydrodynamic radius of the polymer coils, measured by means of a dynamic light-scattering technique.