Frka-Petesic Bruno, Kamita Gen, Guidetti Giulia, Vignolini Silvia
Melville laboratory for polymer Synthesis, Chemistry dept., University of Cambridge.
Phys Rev Mater. 2019 Apr 17;3(4). doi: 10.1103/PhysRevMaterials.3.045601. eCollection 2019 Apr.
Cellulose nanocrystals (CNCs) are bio-sourced chiral nanorods that can form stable colloidal suspensions able to spontaneously assemble above a critical concentration into a cholesteric liquid crystal, with a cholesteric pitch usually in the micron range. When these suspensions are dried on a substrate, solid films with a pitch of the order of few hundreds of nanometers can be produced, leading to intense reflection in the visible range. However, the resulting cholesteric nanostructure is usually not homogeneous within a sample and comports important variations of the cholesteric domain orientation and pitch, which affect the photonic properties. In this work, we first propose a model accounting for the formation of the photonic structure from the vertical compression of the cholesteric suspension upon solvent evaporation, starting at the onset of the kinetic arrest of the drying suspension and ending when solvent evaporation is complete. From that assumption, various structural features of the films can be derived, such as the variation of the cholesteric pitch with the domain tilt, the orientation distribution density of the final cholesteric domains and the distortion of the helix from the unperturbed cholesteric case. The angular-resolved optical response of such films is then derived, including the iridescence and the generation of higher order reflection bands, and a simulation of the angular optical response is provided, including its tailoring under external magnetic fields. Second, we conducted an experimental investigation of CNC films covering a structural and optical analysis of the films. The macroscopic appearance of the films is discussed and complemented with angular-resolved optical spectroscopy, optical and electron microscopy, and our quantitative analysis shows an excellent agreement with the proposed model. This allows us to access the precise composition and the pitch of the suspension when it transited into a kinetically arrested phase directly from the optical analysis of the film. This work highlights the key role that the anisotropic compression of the kinetically arrested state plays in the formation of CNC films and is relevant to the broader case of structure formation in cast dispersions and colloidal self-assembly upon solvent evaporation.
纤维素纳米晶体（CNCs）是生物源手性纳米棒，可形成稳定的胶体悬浮液，在临界浓度以上能够自发组装成胆甾相液晶，其胆甾相螺距通常在微米范围内。当这些悬浮液在基板上干燥时，可制备出螺距在几百纳米量级的固体薄膜，从而在可见光范围内产生强烈反射。然而，所得的胆甾相纳米结构在样品内通常不均匀，胆甾相畴的取向和螺距存在重要变化，这会影响光子特性。在这项工作中，我们首先提出一个模型，该模型考虑了在溶剂蒸发时胆甾相悬浮液垂直压缩形成光子结构的过程，从干燥悬浮液的动力学停滞开始，到溶剂蒸发完成结束。基于该假设，可以推导出薄膜的各种结构特征，例如胆甾相螺距随畴倾斜的变化、最终胆甾相畴的取向分布密度以及与未受扰动的胆甾相情况相比螺旋的扭曲。然后推导了此类薄膜的角分辨光学响应，包括虹彩和高阶反射带的产生，并提供了角光学响应的模拟，包括其在外部磁场下的调控。其次，我们对CNC薄膜进行了实验研究，涵盖了薄膜的结构和光学分析。讨论了薄膜的宏观外观，并辅以角分辨光谱、光学和电子显微镜，我们的定量分析表明与所提出的模型具有极好的一致性。这使我们能够直接从薄膜的光学分析中获取悬浮液转变为动力学停滞相时的精确组成和螺距。这项工作突出了动力学停滞状态的各向异性压缩在CNC薄膜形成中所起的关键作用，并且与铸型分散体中的结构形成以及溶剂蒸发时的胶体自组装这一更广泛的情况相关。
