Second Newtonian region of polymer solutions

聚合物溶液是一类应用广泛的基本流变学体系。它们的粘度通常强烈地依赖于剪切载荷：在低剪切速率下，在第一牛顿分支中，聚合物溶液具有恒定的粘度。超过这一分支，粘度就会大大降低。最后，在非常高的剪切率下，另一个恒定粘度的区域，第二牛顿分支是可望的。除了剪切速率，粘度在很大程度上取决于聚合物的类型和结构、分子量及其分布、聚合物浓度和溶剂。在第一牛顿分支中，聚合物溶液的不同区域特别取决于浓度和相对分子质量。虽然第一牛顿分支和切变变薄的开始已经被很好地研究了，但对于第二牛顿分支来说却不是这样。在这里，缺乏关于聚合物的粘度、法向应力差和取向的详细知识，它们对上述参数的依赖性，以及关于第二牛顿分支开始的研究。然而，对这一分支的适当了解不仅是对聚合物解决方案的基本理解所必需的，而且在许多工业应用中也是必要的。缺乏对第二牛顿分支的了解的根本原因是缺乏足够的测量技术：剪切速率范围通常是商业流变仪覆盖的参数范围之外的1-2个小数量级。此外，还需要检测到非常低的粘度。我们已经将旋转流变仪的几何精度提高了大约30到100倍。这使得测量可以在相当窄的间隙进行，从而能够达到必要的高剪切率和精度。此外，据作者所知，改进的流变仪是唯一一台能够检测聚合物溶液中的法向力并在高达105 S-1的剪切速率下进行流变光学研究的仪器。它的目的是进一步提高项目内的精度。该项目的中心目标是描述第二牛顿分支的定义良好的以及与工业相关的聚合物解决方案。在测量粘度的同时，要检测由于溶液粘弹性和聚合物取向而产生的剪切法向力。后者是为了研究第二牛顿分支的微观结构是否发生了变化。主要的问题是弄清楚在第二牛顿分支中是否存在与第一牛顿分支中类似的不同的作为聚合物浓度函数的区域。此外，还应阐明相对分子质量、聚合物结构和溶剂变化的影响。为了避免剪切引起的降解，这项研究计划用半柔性聚合物进行。

Polymer solutions are a class of fundamental rheological systems with countless applications. Their viscosity typically depends strongly on the shear load: At low shear rates, in the 1st Newtonian branch, the polymer solutions have a constant viscosity. Beyond this branch, the viscosity diminishes considerably. Finally, at very high shear rates another regime of constant viscosity, the 2nd Newtonian branch is expected. Apart from shear rate, the viscosity depends substantially on the type of polymer and its structure, the molecular weight and its distribution, the polymer concentration, and on the solvent. In the 1st Newtonian branch, different regimes of polymer solutions can be distinguished depending particularly on concentration and on molecular weight. While the 1st Newtonian branch and the onset of shear thinning are well studied, this is not the case for the 2nd Newtonian branch. Here, detailed knowledge about viscosity, normal-stress differences and the orientation of the polymers, their dependencies on the above-mentioned parameters as well as studies about the onset of the 2nd Newtonian branch are lacking. Proper knowledge about this branch, however, is not only necessary for a fundamental understanding of polymer solutions but is also needed in a number of industrial applications. Essential reason for the lack of knowledge about the 2nd Newtonian branch is the deficiency of adequate measuring technique: The shear-rate range is typically 1-2 decimal orders beyond the parameter range covered by commercial rheometers. Besides, very low viscosities need to be detected. We have improved the geometrical precision of rotational rheometers by about a factor of 30 to 100. This permits measurements at considerably narrower gaps that enable reaching the necessary high shear rates and precision. Furthermore, to the best of the authors knowledge, the improved rheometer is the only one that allows for detecting normal forces in polymer solutions and for carrying out rheo-optical studies at shear rates up to 105 s-1. It is aimed for further enhancing the precision within the project. Central aim of the project is to characterize the 2nd Newtonian branch of well-defined as well as industrially relevant polymer solutions. Simultaneously with the viscosity measurements, shear-induced normal forces due to the solution viscoelasticity and the orientation of the polymers are to be detected. The latter is to study whether the microstructure changes in the 2nd Newtonian branch. The main issue is to clarify if there are different regimes as a function of polymer concentration in the 2nd Newtonian branch similar to those in the 1st Newtonian branch. Furthermore, the impact of molecular weight, changes in polymer structure and solvent should be elucidated. To avoid shear-induced degradation, the study is planned to be carried out with semi-flexible polymers.