Grafted Radical Polymer Brush for Surface-driven Switching of Chiral Nematic Liquid Crystals
Grafted Radical Polymer Brush for Surface-driven Switching of Chiral Nematic Liquid Crystals
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用于手性向列液晶表面驱动切换的接枝自由基聚合物刷
DOI：
10.1038/pj.2017.43
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
K. Oyaizu
中科院分区：
文献类型：
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作者：
K. Sato;T. Yamasaki;H. Nishide;K. Oyaizu
氧化还原活性聚合物作为下一代软且环境友好的电化学装置（包括二次电池、电致变色电池和存储器）中的候选组分已经受到相当大的关注。1-3特别是，具有明确结构的聚合物已显示出独特的化学和物理性质。例如，表面接枝的刷状聚合物基于其精确控制的结构提供快速的长距离电荷传输、增强的机械强度和更长的氧化还原循环。4-6已经发现瓶刷聚合物可用作氧化还原液流电池的活性材料，其特征在于固有刚度和在电解质中的高溶解度。此外，刷子已被应用于新的设备和系统，如高灵敏度的生物传感器，表面张力的电化学切换，以及胶束的形成/变形。8-10在这项研究中，我们提出了一种新的技术来切换的取向的手性和手性液晶电解质，同时进行的氧化还原反应的表面接枝聚合物刷（图1a）。手性手性液晶相的特征在于自组装分子的螺旋结构，显示出有趣的光学性质，包括具有波长选择性的光反射和圆偏振。螺旋结构应用于光学滤波器、显示器、激光器甚至超材料。11-14对于手性双折射相，对准的控制比对于正常双折射相更具挑战性。利用液晶分子的各向异性电磁特性，通过施加外部磁场或电场，可以容易地切换液晶相的取向。[15]然而，当施加电磁场时，手性双螺旋相的螺旋结构通常会被破坏（即胆甾相转变，图1b）。15本研究集中于双折射液晶的“命令表面”16切换以保持原始螺旋结构。在氧化还原位点的中性状态下，由于聚合物电极和液晶之间的货车范德华力而获得平面配向。然而，在氧化还原活性部分的氧化/还原之后，由于液晶的偶极矩与在带电聚合物电极附近形成的双层中的电场之间的静电相互作用，可以观察到垂面配向。17，18虽然先前报道的可切换液晶仅限于手性相，但这项研究表明，即使是手性手性相的排列也可以使用氧化还原活性聚合物刷来控制。与传统方法相比，即使施加外部电压，手性相的螺旋结构也得以保持，因为电场仅存在于电极附近，这是由于电解质离子和双电层的存在。此外，诱导电化学反应和切换所需的电压（2 V）显著低于常规方法的电压（101-2 V），这有利于低功率器件。
Redox-active polymers have received considerable attention as candidate components in the next generation of soft and environmentally friendly electrochemical devices, including secondary batteries, electrochromic cells and memory. 1–3 In particular, polymers having a well-defined architecture have been shown to exhibit unique chemical and physical properties. For instance, surface-grafted brush polymers provide fast long-distance charge transport, enhanced mechanical strength and longer redox cycles based on their precisely controlled architectures. 4–6 Bottlebrush polymers, which are characterized by intrinsic stiffness and high solubility in electrolytes, have been found to be useful as active materials for redox flow cells. 7 Further, the brushes have been applied to new devices and systems, such as highly sensitive biosensors, electrochemical switching of surface tension, and formation/deformation of micelles. 8–10 In this study, we propose a new technique to switch the orientation of nematic and chiral nematic liquid crystal electrolytes, which proceeds simultaneously with the redox reactions of the surface-grafted polymer brushes (Figure 1a). A chiral nematic liquid crystal phase characterized by a helical structure of self-assembled molecules displays interesting optical properties, including light reflection with wavelength selectivity and circular polarization. 11–14 The helical structures are applied to optical filters, displays, lasers and even metamaterials. 11–14 Control of alignment is more challenging for chiral nematic phases than for normal nematic phases. Orientation of the nematic phases is easily switched by application of an external magnetic or electric field, utilizing the anisotropic electromagnetic properties of the liquid crystal molecules. 15 However, the helical structures of the chiral nematic phases are normally broken when an electromagnetic field is applied (that is, the cholestericnematic transition, Figure 1b). 15 The present study focuses on ‘command surface’16 switching of nematic liquid crystals to maintain the original helical structure. In the neutral state of the redox sites, planar alignment is obtained due to the van der Waals force between the polymer electrode and the liquid crystal. However, after oxidation/reduction of the redox-active moieties, homeotropic alignment can be observed because of the electrostatic interaction between the dipole moment of the liquid crystal and the electric field in the double layer, which is formed in the vicinity of the electrically charged polymer electrode. 17, 18 Although previously reported switchable liquid crystals have been limited to only nematic phases, this study revealed that even the alignment of chiral nematic phases could be controlled using redox-active polymer brushes. In contrast to the conventional method, the helical structures of the chiral phase were maintained even when an external voltage was applied, because the electric field existed only in the vicinity of the electrodes due to the presence of electrolyte ions and the electrical double layers. Furthermore, the required voltage (2 V) to induce the electrochemical reaction and switching was substantially lower than that of the conventional method (101–2 V), which is favorable for low-power devices.