Wang Zhongyang, Wang Chaoqiuyu, Sun Yangyang, Wang Kai, Strzalka Joseph W, Patel Shrayesh N, Nealey Paul F, Ober Christopher K, Escobedo Fernando A
Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois60637, United States.
Department of Materials Science and Engineering, Cornell University, Ithaca, New York14853, United States.
ACS Nano. 2022 Dec 27;16(12):20714-20729. doi: 10.1021/acsnano.2c07789. Epub 2022 Dec 7.
Leveraging the self-assembling behavior of liquid crystals designed for controlling ion transport is of both fundamental and technological significance. Here, we have designed and prepared a liquid crystal that contains 2,5-bis(thien-2-yl)thieno[3,2-]thiophene (BTTT) as mesogenic core and conjugated segment and symmetric tetra(ethylene oxide) (EO4) as polar side chains for ion-conducting regions. Driven by the crystallization of the BTTT cores, BTTT/dEO4 exhibits well-ordered smectic phases below 71.5 °C as confirmed by differential scanning calorimetry, polarized optical microscopy, temperature-dependent wide-angle X-ray scattering, and grazing incidence wide-angle X-ray scattering (GIWAXS). We adopted a combination of experimental GIWAXS and molecular dynamics (MD) simulations to better understand the molecular packing of BTTT/dEO4 films, particularly when loaded with the ion-conducting salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Ionic conduction of BTTT/dEO4 is realized by the addition of LiTFSI, with the material able to maintain smectic phases up to = [Li]/[EO] = 0.1. The highest ionic conductivity of 8 × 10 S/cm was attained at an intermedium salt concentration of = 0.05. It was also found that ion conduction in BTTT/dEO4 is enhanced by forming a smectic layered structure with irregular interfaces between the BTTT and EO4 layers and by the lateral film expansion upon salt addition. This can be explained by the enhancement of the misalignment and configurational entropy of the side chains, which increase their local mobility and that of the solvated ions. Our molecular design thus illustrates how, beyond the favorable energetic interactions that drive the assembly of ion solvating domains, modulation of entropic effects can also be favorably harnessed to improve ion conduction.
利用为控制离子传输而设计的液晶的自组装行为具有重要的基础和技术意义。在此，我们设计并制备了一种液晶，其包含2,5-双（噻吩-2-基）噻吩并[3,2-]噻吩（BTTT）作为介晶核心和共轭链段，以及对称四（环氧乙烷）（EO4）作为离子传导区域的极性侧链。由BTTT核心的结晶驱动，BTTT/dEO4在71.5℃以下呈现出有序的近晶相，这通过差示扫描量热法、偏光显微镜、温度依赖的广角X射线散射和掠入射广角X射线散射（GIWAXS）得以证实。我们采用实验GIWAXS和分子动力学（MD）模拟相结合的方法，以更好地理解BTTT/dEO4薄膜的分子堆积，特别是在负载离子传导盐双（三氟甲磺酰）亚胺锂（LiTFSI）时。通过添加LiTFSI实现了BTTT/dEO4的离子传导，该材料在高达[Li]/[EO] = 0.1时仍能保持近晶相。在中间盐浓度为 = 0.05时，获得了最高离子电导率8×10 S/cm。还发现，BTTT/dEO4中的离子传导通过在BTTT和EO4层之间形成具有不规则界面的近晶层状结构以及盐添加时薄膜的横向膨胀而增强。这可以通过侧链的错位和构型熵的增加来解释，这增加了它们的局部流动性以及溶剂化离子的局部流动性。因此，我们的分子设计说明了如何在驱动离子溶剂化域组装的有利能量相互作用之外，还可以有利地利用熵效应的调节来改善离子传导。
