利用介电松弛谱研究浓胶体分散体系的挤阻突变

When the volume fraction of the colloid particles increases as high as a certain value in a colloidal dispersion, such physical properties as modulus and viscosity undergo a sudden change because of the jamming of the particles, which is similar to the phase transition. Because of the difficulties in controlling the critical condition and the lack of effective characterization on non-equilibrium or concentrated dispersions, we are confronted with the conundrum in the experimental studies of jamming transition and its inconsistance with the theoretical work. We plan to use the thermally sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels which swell with the decreasing temperatures below the LCST of PNIPAM, controlling the volume fraction and their packing state of the colloidal dispersions by adjusting the temperaures,to achieve the critical condition of jamming precisely and repeatedly. The dielectric relaxation spectroscopy, which is featured in in-situ, noninvasive and broadband, will be used to study the microscopic state, interactions and their statistics of the dispersion, which are correlated with the structural dynamics and macroscopic properties.The dielectric spectrometer will be further combined with the rheometer, so that the electrical and mechanical properties can be recorded synchronously, and the corresponding relaxations of jamming or jamming transition under shear stress can be detected. We will study the jamming transition and the jamming state of colloidal dispersions systematically and thoroughly, with their dependence on volume fraction, packing state, applied field, hierarchy structures and structural dynamics, so that the microscopic or mesoscopic condensed structures and dynamics of the polymers at inner and outer cooperativity and their relations with the macroscopic properties can be profoundly understood.

当胶体分散系中粒子的体积分数高达至一定值时，粒子间的挤阻可导致体系的某些物理性质(如模量、粘度)发生类似相变的突变。由于实际临界状态的难以控制和在非平衡态及浓分散体系研究中有效表征方法的缺乏，目前挤阻突变的实验研究面临困难且与理论之间产生脱节。我们计划利用聚异丙基丙烯酰胺微凝胶随温度降低而溶胀的特性，通过温度变化控制分散系的体积分数和堆集状态，精确逼近和重现挤阻突变，并利用介电谱仪实时、无侵、宽频的特点，研究挤阻过程中微观状态、相互作用及其统计对结构动力学及宏观物理性能的影响，进而将介电谱仪和流变仪集成联用，不仅可同步测定体系的电、力性能，而且可以研究在剪切场中与挤阻突变相关的松弛行为，全面系统地探讨挤阻过程对体积分数、外场作用和各层次结构及其动力学的依赖性。深入研究挤阻过程，将对我们深刻理解高聚物在各种内外因协同作用下的微观及介观凝聚态结构、动力学特性与宏观物性之间的关联具有重大意义。

胶体分散系中粒子的统计行为和体系的某些物理性质(如电学、力学)是有着紧密关联的，这种关联性既和粒子及介质本身的性质有关、也和粒子-介质界面结构有关，因为界面结构直接影响粒子动力学、粒子之间的相互作用乃至于浓分散系中粒子的聚集行为。由于实际临界状态的难以控制和在非平衡态及浓分散体系研究中有效表征方法的缺乏，关于表征粒子结构和动力学的实验研究面临困难且与理论之间产生脱节。.该项目研究了热敏性聚异丙基丙烯酰胺（PNIPAM）微凝胶在水介质分散体系中的介电和导电响应，完成了经典作图、函数分析以及对数导数法对介电松弛谱、电容率、导电率温度谱的分析，结合频谱、温谱解析结果，对链密度、荷电量、浓度不同的体系进行了详尽的对比分析，得出了PNIPAM水分散液介电响应源于微凝胶外层荷电量影响下的对离子云极化。衍生电容率虚部频谱上给出的极化过程与微凝胶的体积相变相对应，而电导率（阻抗）温谱中的拐点可以对应液-固转变（挤阻突变或溶胶-凝胶转变）。.我们完成了对介电谱仪的-100~250 oC范围惰性氛围温控设备的搭建，以及通过与粘度仪的结合联用实现了剪切流场下介电检测，研究了w/o微乳液以及PNIPAM水分散液在不同转速剪切场下的介电行为，利用界面极化的弛豫时间和强度的变化以及电导率对温度的导数函数表征了粒子的聚集尺度及其在剪切场下的减小，解释了分散液的触变性（剪切变稀）行为。.该项目的立项和实施对于研究者言是一个重要的学习过程，我们通过胶体水分散体系的介电响应的分析，将动态电学测量原理、不同厂家和型号的仪器异同、分析方法（包括实验和数据分析）、乃至胶体分散系本身的结构复杂性做了一一梳理，对我们深刻理解高聚物在各种内外因协同作用下的微观及介观凝聚态结构、动力学特性与宏观物性之间的关联具有重大意义。