A thermodynamically consistent model for the thixotropic behavior of concentrated star polymer suspensions
A thermodynamically consistent model for the thixotropic behavior of concentrated star polymer suspensions
复制标题
DOI：
10.1016/j.jnnfm.2007.10.016
复制
发表时间：
2008-06-01
影响因子：
3.1
通讯作者：
Vlassopoulos, D.
中科院分区：
文献类型：
--
作者：
Beris, A. N.;Stiakakis, E.;Vlassopoulos, D.
集中的多臂星星聚合物的解决方案在这项工作中使用的原型，定义明确，系统，清楚地表现出粘塑性和触变性。在这里使用的一个一致的方法来开发一个连续体的流变模型，这些系统是新的触变系统，并具有相当大的优势，比以前假设的模型：基本的流变模型从一开始就构建在一个实质性的目标和流动类型独立的形式，在对比以前的连续体模型的触变性，通常只适合于剪切流。这也符合非平衡态热力学。该模型是基于一个可变的非仿射参数的Johnson-Segalman粘弹性本构方程。非仿射参数被认为是系统中颗粒之间的结构和聚集体的代表：随着变形速率的增加，这些聚集体被认为被破坏得越来越多，从而导致组成多臂聚合物颗粒的均匀粘弹性变形的不连续性。这在所提出的模型中通过非仿射参数值从零（其被用来表示原始结构（其中变形被假设为完全仿射））到高速率下的最大值（其表示粘弹性颗粒的浓缩悬浮液）的一致增加来从现象上描述。同时在非仿射参数，这是由一个经验动力学方程所描述的那些变化，该系统的特征松弛时间也被假定为显着变化：从无穷大，在静态平衡，到一个最小值表征的残余粘弹性的完全打破了介质。通过假设弛豫时间与非仿射参数成反比来建模这种行为。除了这种粘弹性贡献的应力，一个纯粹的粘性贡献被认为是由一个标准的幂律表达的应变率建模。该模型已被应用到瞬态和稳态简单剪切流。结果显示出良好的半定量协议与模型多臂星星聚合物进行的实验。(C)2007 Elsevier B. V.保留所有权利。
Concentrated multiarm star polymer solutions are used in this work as prototype, well defined, systems that clearly exhibit viscoplasticity and thixotropy. A thermodynamically consistent approach is used here to develop a continuum theological model for these systems that is novel for thixotropic systems and has considerable advantages over previously postulated models: The basic theological model is constructed from the beginning in a materially objective and flow type-independent form, in contrast to previous continuum models for thixotropy that typically are suitable only for shear flows. It is also consistent with non-equilibrium thermodynamics. The model is based on a Johnson-Segalman viscoelastic constitutive equation with a variable non-affine parameter. The non-affine parameter is considered representative of the structure and the aggregations between the particles in the system: as the deformation rate increases, those aggregations are assumed to be destroyed more and more, thus leading to discontinuities in the uniform viscoelastic deformation of the constituent multiarm polymer particles. This is described phenomenologically in the proposed model by a consistent increase in the non-affine parameter value from zero, which was taken to represent the virgin structure (where the deformation was assumed to be perfectly affine), to a maximum value at high rates, representing a concentrated suspension of viscoelastic particles. Simultaneously to those changes in the non-affine parameter, which are described by an empirical kinetic equation, the characteristic relaxation time of the system is also assumed to change dramatically: from infinity, at static equilibrium, to a minimum value characterizing the residual viscoelasticity of the fully broken-down medium. This behavior is modeled by assuming the relaxation time to be inversely proportional to the non-affine parameter. In addition to this viscoelastic contribution to the stress, a purely viscous contribution is considered modeled by a standard power law expression of the rate of strain. The model has been applied to transient and steady simple shear flows. The results show good semi-quantitative agreement with the experiments performed with the model multiarm star polymers. (C) 2007 Elsevier B.V. All rights reserved.