College of Chemistry, Beijing Normal University, Beijing 100875, China.
College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210046, China.
Soft Matter. 2017 Apr 5;13(14):2663-2676. doi: 10.1039/c7sm00144d.
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 (ε, κ, ϕ); the conductivity of the medium water (κ); the water content in the PNIPAM microgel (f)) 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.
我们研究了在 10-60°C 的宽温度范围内，频率范围为 40 Hz 至 110 MHz 的密集浓度下的温敏聚（N-异丙基丙烯酰胺）（PNIPAM）微凝胶悬浮液的介电行为。通过成功地从原始数据中去除电极极化效应，观察到两个显著且依赖于温度的弛豫过程。这两个相变过程（即胶态晶体到液体的转变），以及体积相转变，都通过弛豫参数来检测。基于微凝胶悬浮液的三种物理状态，详细讨论了弛豫机制。慢弛豫源于整个温度范围内的链段运动和离子沿聚合物链的运动。发现当体系处于胶态晶体和液体状态时，链段运动与侧链和氢键网络协同，而在相分离状态（温度高于下临界溶解温度（LCST））时，协同相互作用消失。快弛豫是由于在 LCST 以下的反离子波动和在 LCST 以上的界面极化。基于描述常规粒子分散体介电模型的界面极化理论，使用 Hanai 方程计算了组成相（微凝胶的介电常数、电导率和体积分数（ε、κ、ϕ）、介质水的电导率（κ）、PNIPAM 微凝胶中的含水量（f））的电特性随温度的变化。含水量接近使用光散射得到的结果，表明常规粒子分散体的介电模型也适用于软非典型胶体分散体。
