Majewski Pawel W, Gopinadhan Manesh, Osuji Chinedum O
Department of Chemistry, University of Warsaw, 02098 Warsaw, Poland.
Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06511, USA.
Polymers (Basel). 2019 May 15;11(5):887. doi: 10.3390/polym11050887.
The transport properties of block copolymer-derived polymer electrolyte membranes (PEMs) are sensitive to microstructural disorder originating in the randomly oriented microdomains produced during uncontrolled self-assembly by microphase separation. This microstructural disorder can negatively impact performance due to the presence of conductivity-impeding grain boundaries and the resulting tortuosity of transport pathways. We use magnetic fields to control the orientational order of Li-doped lamellar polyethylene oxide (PEO) microdomains in a liquid crystalline diblock copolymer over large length scales (>3 mm). Microdomain alignment results in an increase in the conductivity of the membrane, but the improvement relative to non-aligned samples is modest, and limited to roughly 50% in the best cases. This limited increase is in stark contrast to the order of magnitude improvement observed for magnetically aligned cylindrical microdomains of PEO. Further, the temperature dependence of the conductivity of lamellar microdomains is seemingly insensitive to the order-disorder phase transition, again in marked contrast to the behavior of cylinder-forming materials. The data are confronted with theoretical predictions of the microstructural model developed by Sax and Ottino. The disparity between the conductivity enhancements obtained by domain alignment of cylindrical and lamellar systems is rationalized in terms of the comparative ease of percolation due to the intersection of randomly oriented lamellar domains (2D sheets) versus the quasi-1D cylindrical domains. These results have important implications for the development of methods to maximize PEM conductivity in electrochemical devices, including batteries.
嵌段共聚物衍生的聚合物电解质膜（PEMs）的传输特性对微观结构无序很敏感，这种无序源于微相分离过程中不受控制的自组装所产生的随机取向微区。由于存在阻碍导电的晶界以及由此产生的传输路径曲折度，这种微观结构无序会对性能产生负面影响。我们利用磁场在大长度尺度（>3毫米）上控制液晶双嵌段共聚物中锂掺杂层状聚环氧乙烷（PEO）微区的取向有序性。微区排列导致膜的电导率增加，但相对于未排列的样品，改善程度适中，在最佳情况下限制在约50%。这种有限的增加与PEO磁性排列的圆柱形微区所观察到的数量级改善形成鲜明对比。此外，层状微区电导率的温度依赖性似乎对有序-无序相变不敏感，这再次与形成圆柱状材料的行为形成显著对比。这些数据与萨克斯和奥蒂诺开发的微观结构模型的理论预测进行了对比。圆柱形和层状系统通过微区排列获得的电导率增强之间的差异，根据随机取向的层状微区（二维片）与准一维圆柱形微区相交时渗流的相对难易程度进行了合理化解释。这些结果对开发使包括电池在内的电化学装置中PEM电导率最大化的方法具有重要意义。
