Relaxations and phase transitions during the collapse of a dense PNIPAM microgel suspension—thorough insight using dielectric spectroscopy
The dielectric behavior of a thermo-sensitive poly-(N-isopropylacrylamide) (PNIPAM) microgel suspension with a dense concentration was investigated over the frequency range of 40 Hz to 110 MHz in a wide temperature window of 10–60 °C. By successfully removing the electrode polarization effect from the original data, two remarkable and temperature-dependent relaxation processes were observed. Both of the two-phase transition processes, i.e., the colloidal crystal-to-liquid transition, which has not yet been detected by dielectric spectroscopy before, as well as the volume phase transition, were detected by the relaxation parameters. Based on the three physical states of the microgel suspension, the relaxation mechanisms are discussed in detail. The slow relaxation originates from the segmental motion and the counterion motion along the polymer chain over the whole temperature range. It was found that when the system is in the colloidal crystal and liquid state, the segmental motion is cooperative with side chain and hydrogen bonding networks, while in the phase separation state (at temperatures above the lower critical solution temperature (LCST)), the cooperative interaction disappears. The fast relaxation is due to the fluctuation of counterions below the LCST and the interfacial polarization above the LCST. Based on interfacial polarization theory, which describes the dielectric model of a conventional particle dispersion, the temperature dependence of the electrical properties for the constituent phases (the permittivity, conductivity and volume fraction of the microgel (εp, κp, ϕ); the conductivity of the medium water (κa); the water content in the PNIPAM microgel (fw)) were calculated using the Hanai equation. The water content is close to the result obtained using light scattering, indicating that the dielectric model for a conventional particle dispersion is also applicable to a soft atypical colloidal dispersion.