Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland.
Soft Matter. 2017 Nov 8;13(43):7897-7906. doi: 10.1039/c7sm01690e.
An important parameter for the performance of nanomaterials is the degree by which the nanoparticles are dispersed in a matrix. Optical microscopy or scattering methods are useful to characterise the state of dispersion, but are not generally applicable to all materials. Electron microscopy methods are laborious in preparation and typically offer only quantitative information on a very local scale. In the present work we investigate how high frequency rheological measurements can be used for partially dispersed suspensions at intermediate to higher particle loadings, even for high viscous matrices. Although the contribution of the particles is particularly visible in the low frequency linear viscoelastic behaviour, a more direct relationship between rheological properties and degree of dispersion can be derived from the loss modulus in the high frequency limit. To this end, a home-built piezo shear rheometer is constructed to extend the frequency range typically accessible by commercial rotational rheometers. Measurements on spherical silica particles, with a varying degree of dispersion in low molecular weight PDMS, are used to demonstrate how high frequency rheometry can be used to quantify dispersion quality. The linear viscoelastic properties are compared to analytical scaling theories to demonstrate that a hydrodynamically dominated regime is reached. The dependence of the relative high frequency loss modulus on volume fraction is then compared to predictions of a hydrodynamic viscosity model for the derivation of a dispersion quality index. It is used to follow the evolution of the dispersion quality as a function of mixing time and consumed power.
纳米材料性能的一个重要参数是纳米颗粒在基体中分散的程度。光学显微镜或散射方法可用于表征分散状态，但通常不适用于所有材料。电子显微镜方法在制备方面很繁琐，通常只能提供非常局部范围内的定量信息。在本工作中，我们研究了高频流变测量如何可用于部分分散的悬浮液，即使是在中等至高颗粒负载下，甚至对于高粘性基体也是如此。虽然颗粒的贡献在低频线性粘弹性行为中特别明显，但可以从高频极限下的损耗模量推导出流变性质与分散程度之间更直接的关系。为此，构建了一台自制的压电剪切流变仪，以扩展商业旋转流变仪通常可达到的频率范围。使用具有不同分散程度的球形二氧化硅颗粒在低分子量 PDMS 中的实验，演示了如何使用高频流变测量来量化分散质量。将线性粘弹性质与分析缩放理论进行比较，以证明达到了以流体动力学为主导的区域。然后将相对高频损耗模量与用于推导分散质量指数的流体动力粘度模型的预测进行比较。它用于跟踪分散质量随混合时间和消耗功率的变化。
