纤维素纳米纤丝在剪切和拉伸半稀释分散体系中的动态表征
Dynamic characterization of cellulose nanofibrils in sheared and extended semi-dilute dispersions
摘要：
通过分散的纤维素纳米纤维（CNF）的受控组装制成的新材料有可能发展成为当今一些最高性能材料的生物基竞争对手。这些新型纤维素材料的性能取决于如何容易地用流体动力控制CNF排列，流体动力总是与不同的过程竞争，该过程将系统驱动到各向同性，称为旋转扩散。在这项工作中，我们提出了一个流动停止实验，使用偏光显微镜（POM）研究旋转扩散的CNF分散体在过程相关的流量和浓度。这是结合小角X射线散射（SAXS）实验来分析的真实取向分布函数（ODF）的流动原纤。研究发现，CNF的旋转扩散过程发生在多个时间尺度上，其中最快的尺度似乎取决于停止前的分散体的变形历史。同时，旋转扩散依赖于初始ODF的假设不成立，因为相同的分布可以导致不同的扩散时间尺度。旋转扩散被发现是更快的剪切为主的流动相比，纯拉伸流。此外，该实验装置可用于快速表征流动CNF的动态特性，从而有助于确定分散体的质量及其在材料加工中的可用性。
英文摘要：
New materials made through controlled assembly of dispersed cellulose nanofibrils (CNF) has the potential to develop into biobased competitors to some of the highest performing materials today. The performance of these new cellulose materials depends on how easily CNF alignment can be controlled with hydrodynamic forces, which are always in competition with a different process driving the system towards isotropy, called rotary diffusion. In this work, we present a flow-stop experiment using polarized optical microscopy (POM) to study the rotary diffusion of CNF dispersions in process relevant flows and concentrations. This is combined with small angle X-ray scattering (SAXS) experiments to analyze the true orientation distribution function (ODF) of the flowing fibrils. It is found that the rotary diffusion process of CNF occurs at multiple time scales, where the fastest scale seems to be dependent on the deformation history of the dispersion before the stop. At the same time, the hypothesis that rotary diffusion is dependent on the initial ODF does not hold as the same distribution can result in different diffusion time scales. The rotary diffusion is found to be faster in flows dominated by shear compared to pure extensional flows. Furthermore, the experimental setup can be used to quickly characterize the dynamic properties of flowing CNF and thus aid in determining the quality of the dispersion and its usability in material processes.
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