Abstract
The determination of the net charge and size of microgel particles as a function of their concentration, as well as the degree of association of ions to the microgel backbone, has been pursued in earlier studies mainly by scattering and rheology. These methods suffer from contributions due to inter-particle interactions that interfere with the characterization of single-particle properties. Here we introduce dielectric spectroscopy as an alternative experimental method to characterize microgel systems. The advantage of dielectric spectroscopy over other experimental methods is that the polarization due to mobile charges within a microgel particle is only weakly affected by inter-particle interactions. Apart from electrode polarization effects, experimental spectra on PNIPAM-co-AA [poly(N-isopropylacrylamide-co-acrylic acid)] ionic microgel particles suspended in de-ionized water exhibit three well-separated relaxation modes, which are due to the polarization of the mobile charges within the microgel particles, the diffuse double layer around the particles, and the polymer backbone. Expressions for the full frequency dependence of the electrode-polarization contribution to the measured dielectric response are derived, and a theory is proposed for the polarization resulting from the mobile charges within the microgel. Relaxation of the diffuse double layer is modeled within the realm of a cell model. The net charge and the size of the microgel particles are found to be strongly varying with concentration. A very small value of the diffusion coefficient of ions within the microgel is found, due to a large degree of chemical association of protons to the polymer backbone.
Highlights
D-40225 Dusseldorf, Germany e JARA-SOFT, 52425 Julich, Germany f Centro de Fısica de Materiales (CSIC-UPV/EHU), Materials Physics Center, 20018 San Sebastian, Spain g Universidad del Paıs Vasco (UPV/EHU), Departamento de Fısica de Materiales, 20080 San Sebastian, Spain h Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin, 14109 Berlin, Germany for example, in ref. 1–3)
The advantage of dielectric spectroscopy over other methods is that the dielectric response due to polarization of mobile ions within the microgel particles is only indirectly affected by inter-particle interactions through, for example, shrinkage, pH changes, and electric fields generated by surrounding particles
Dielectric spectroscopy measurements on PNIPAM-co-AA [poly(N-isopropylacrylamide-co-acrylic acid)] microgel suspensions in de-ionized water are quantitatively interpreted on the basis of an improved theory for electrode polarization and a new model for the polarization of the ionic microgel particles
Summary
Paper distribution in- and around the microgel particles, which may be validated by means of dielectric spectroscopy. The aim of the present paper is (i) to derive expressions for the polarization due to mobile charges within the microgel particles and for the full frequency dependence of the electrode-polarization contribution to dielectric spectra, (ii) to identify the various microgel relaxation modes that contribute to experimental spectra, and (iii) to extract the net charge and size of the particles as a function of their concentration, as well as the degree of dissociation/association of protons to the gel matrix. The concentration dependence of the net charge and size of the microgel particles is extracted from the experiments, using the theories for electrode polarization and for the amplitude and characteristic frequency of the mode corresponding to polarization due to mobile charges within the microgel
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