Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
Soft Matter. 2019 Jan 30;15(5):1038-1052. doi: 10.1039/c8sm02014k.
We present an integrated experimental and quantitative theoretical study of the mechanics of self-crosslinked, slightly charged, repulsive pNIPAM microgel suspensions over a very wide range of concentrations (c) that span the fluid, glassy and putative "soft jammed" regimes. In the glassy regime we measure a linear elastic dynamic shear modulus over 3 decades which follows an apparent power law concentration dependence G' ∼ c5.64, a variation that appears distinct from prior studies of crosslinked ionic microgel suspensions. At very high concentrations there is a sharp crossover to a nearly linear growth of the modulus. To theoretically understand these observations, we formulate an approach to address all three regimes within a single conceptual Brownian dynamics framework. A minimalist single particle description is constructed that allows microgel size to vary with concentration due to steric de-swelling effects. Using a Hertzian repulsion interparticle potential and a suite of statistical mechanical theories, quantitative predictions under quiescent conditions of microgel collective structure, dynamic localization length, elastic modulus, and the structural relaxation time are made. Based on a constant inter-particle repulsion strength parameter which is determined by requiring the theory to reproduce the linear elastic shear modulus over the entire concentration regime, we demonstrate good agreement between theory and experiment. Testable predictions are then made. We also measured nonlinear rheological properties with a focus on the yield stress and strain. A theoretical analysis with no adjustable parameters predicts how the quiescent structural relaxation time changes under deformation, and how the yield stress and strain change as a function of concentration. Reasonable agreement with our observations is obtained. To the best of our knowledge, this is the first attempt to quantitatively understand structure, quiescent relaxation and shear elasticity, and nonlinear yielding of dense microgel suspensions using microscopic force based theoretical methods that include activated hopping processes. We expect our approach will be useful for other soft polymeric particle suspensions in the core-shell family.
我们呈现了一个综合的实验和定量理论研究，研究了在非常宽的浓度范围内（c）自交联、略带电荷、排斥性 pNIPAM 微凝胶悬浮液的力学性质，该范围跨越了流体、玻璃和所谓的“软堵塞”区域。在玻璃态区域，我们测量了一个跨越 3 个数量级的线性弹性动态剪切模量，其遵循明显的幂律浓度依赖性 G'∼c5.64，这种变化与先前交联离子微凝胶悬浮液的研究明显不同。在非常高的浓度下，模量会急剧增加到几乎线性增长。为了从理论上理解这些观察结果，我们提出了一种在单个概念布朗动力学框架内解决所有三个区域的方法。构建了一种最小的单粒子描述，允许由于空间去溶胀效应，微凝胶尺寸随浓度变化。使用赫兹排斥相互作用势和一套统计力学理论，在微凝胶集体结构、动态定位长度、弹性模量和结构弛豫时间的静态条件下进行了定量预测。基于一个常数的粒子间排斥强度参数，该参数是通过要求理论在整个浓度范围内再现线性弹性剪切模量来确定的，我们证明了理论与实验之间的良好一致性。然后提出了可测试的预测。我们还测量了非线性流变性质，重点是屈服应力和应变。一个没有可调参数的理论分析预测了在变形下静止结构弛豫时间如何变化，以及屈服应力和应变如何随浓度变化。我们的观察结果得到了合理的一致。据我们所知，这是首次尝试使用基于微观力的理论方法，包括激活跳跃过程，定量理解致密微凝胶悬浮液的结构、静止松弛和剪切弹性以及非线性屈服。我们期望我们的方法将对其他核壳家族的软聚合物粒子悬浮液有用。
