Relaxations and phase transitions during the collapse of a dense PNIPAM microgel suspension-thorough insight using dielectric spectroscopy
Relaxations and phase transitions during the collapse of a dense PNIPAM microgel suspension-thorough insight using dielectric spectroscopy
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致密 PNIPAM 微凝胶悬浮液塌陷过程中的弛豫和相变 - 使用介电谱进行全面了解
DOI：
10.1039/c7sm00144d
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发表时间：
2017
期刊：
影响因子：
3.4
通讯作者：
Zhou Jianfeng
中科院分区：
文献类型：
--
作者：
Yang Man;Liu Chunyan;Lian Yiwei;Zhao Kongshuang;Zhu Dan;Zhou Jianfeng
在10-60 °C宽温窗内，研究了高浓度温敏聚N-异丙基丙烯酰胺（PNIPAM）微凝胶悬浮液在40 ~ 110 MHz频率范围内的介电行为。通过成功地从原始数据中去除电极极化效应，观察到两个显着的和温度相关的弛豫过程。两个相变过程，即，胶体晶体到液体的转变，这还没有被检测到的介电光谱之前，以及体积相变，被检测到的弛豫参数。基于微凝胶悬浮液的三种物理状态，详细讨论了其弛豫机理。在整个温度范围内，慢弛豫起源于沿聚合物链的链段运动和沿着的逆运动。结果表明，当体系处于胶体晶体和液体状态时，链段运动与侧链和氢键网络协同作用，而在相分离状态（高于低临界溶解温度（LCST））时，协同作用消失。这种快速弛豫是由于LCST以下的反离子涨落和LCST以上的界面极化引起的。基于描述常规颗粒分散体介电模型的界面极化理论，利用Hanai方程计算了PNIPAM微凝胶各组成相的电性能（微凝胶的介电常数、电导率和体积分数（εp，κp，ε）;介质水的电导率（κa）;微凝胶中水含量（fw））与温度的关系。水含量接近使用光散射获得的结果，表明常规颗粒分散体的介电模型也适用于软非典型胶体分散体。
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.