Yang Man, Liu Chunyan, Zhao Kongshuang
College of Chemistry, Beijing Normal University, Beijing 100875, China.
Phys Chem Chem Phys. 2017 Jun 14;19(23):15433-15443. doi: 10.1039/c7cp01378g.
The dielectric behavior of thermo-sensitive poly-(N-isopropylacrylamide) (PNIPAM) microgel with three different concentrations was investigated for the frequencies ranging from 40 Hz to 110 MHz as a function of temperature from 10 to 60 °C. Two remarkable and temperature-dependent relaxation processes were observed. The slow relaxation originates from the segmental motion over the whole temperature range. The fast relaxation is due to the fluctuation of counterions below the lower critical solution temperature (LCST) and the interfacial polarization above the LCST. It was concluded from the temperature-dependent dielectric parameters that the microgel concentration will not influence the LCST but affects the phase behavior of the microgel suspension: the dense system experienced a colloidal crystal-to-liquid transition and volume phase transition, while the dilute system only underwent a volume phase transition. Based on the interfacial polarization theory, the electrical parameters for the constituent phases (permittivity, conductivity, and volume fraction of the microgel (ε, κ, ϕ) and the conductivity of water κ) and the water content in the microgel (f) were calculated using Hanai's equation. In addition, the thermodynamics parameters of the two relaxations were calculated from the Eyring equation. The electrical and thermodynamic parameters indicate that the microgel concentration influences the volume, charge density, thickness of the electric double layer, and degrees of freedom of the segments of the microgel, thereby resulting in the differences in collapse dynamics.
研究了三种不同浓度的热敏聚（N-异丙基丙烯酰胺）（PNIPAM）微凝胶在40 Hz至110 MHz频率范围内，温度从10℃到60℃变化时的介电行为。观察到两个显著的且与温度相关的弛豫过程。慢弛豫源于整个温度范围内的链段运动。快弛豫是由于在较低临界溶液温度（LCST）以下抗衡离子的波动以及在LCST以上的界面极化。从与温度相关的介电参数得出结论，微凝胶浓度不会影响LCST，但会影响微凝胶悬浮液的相行为：浓体系经历了胶体晶体到液体的转变和体积相变，而稀体系仅经历了体积相变。基于界面极化理论，使用花井方程计算了组成相的电学参数（微凝胶的介电常数、电导率和体积分数（ε、κ、ϕ）以及水的电导率κ）和微凝胶中的含水量（f）。此外，根据艾林方程计算了两种弛豫的热力学参数。电学和热力学参数表明，微凝胶浓度影响微凝胶的体积、电荷密度、双电层厚度和链段的自由度，从而导致塌陷动力学的差异。
