Angular optical response of cellulose nanocrystal films explained by the distortion of the arrested suspension upon drying
Angular optical response of cellulose nanocrystal films explained by the distortion of the arrested suspension upon drying
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DOI：
10.1103/physrevmaterials.3.045601
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发表时间：
2019-04-17
影响因子：
3.4
通讯作者：
Vignolini, Silvia
中科院分区：
文献类型：
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作者：
Frka-Petesic, Bruno;Kamita, Gen;Vignolini, Silvia
纤维素纳米晶体（CNC）是生物来源的手性纳米棒，其可以形成稳定的胶体悬浮液，所述胶体悬浮液能够在临界浓度以上自发地组装成具有通常在微米范围内的纳米节距的纳米液晶。当这些悬浮液在基底上干燥时，可以产生具有几百纳米量级的间距的固体膜，导致在可见光范围内的强烈反射。然而，所得到的纳米结构通常是不均匀的样品内，并comports重要的变化，影响的光子特性的纳米结构域的取向和间距。在这项工作中，我们首先提出了一个模型占的光子结构的形成从垂直压缩的悬浮液溶剂蒸发后，开始在发病的动力学逮捕的干燥悬浮液和溶剂蒸发完成时结束。从这一假设出发，可以得到薄膜的各种结构特征，如畴间距随畴倾斜的变化、最终畴的取向分布密度以及未扰动畴的螺旋畸变等。角分辨的光学响应，这样的薄膜，然后推导出，包括虹彩和高阶反射带的产生，并提供了一个模拟的角光学响应，包括其定制下的外部磁场。其次，我们进行了实验研究的CNC电影覆盖的结构和光学分析的电影。薄膜的宏观外观进行了讨论，并补充角分辨光谱，光学和电子显微镜，我们的定量分析表明，与所提出的模型非常吻合。这使我们能够直接从膜的光学分析中获得悬浮液转变为动力学停滞相时的精确组成和螺距。这项工作突出的关键作用，各向异性压缩的动力学逮捕状态在形成CNC薄膜，并有关更广泛的情况下，在铸造分散体和胶体自组装溶剂蒸发后的结构形成。
Cellulose nanocrystals (CNCs) are biosourced 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.