Coote Jonathan P, Adotey Samuel K J, Sangoro Joshua R, Stein Gila E
Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States.
Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, Ohio 43210, United States.
ACS Polym Au. 2023 Apr 5;3(4):331-343. doi: 10.1021/acspolymersau.2c00068. eCollection 2023 Aug 9.
The ionic conductivity in lamellar block copolymer electrolytes is often anisotropic, where the in-plane conductivity exceeds the through-plane conductivity by up to an order of magnitude. In a prior work, we showed significant anisotropy in the ionic conductivity of a lamellar block copolymer based on polystyrene (PS) and a polymer ionic liquid (PIL), and we proposed that the through-film ionic conductivity was depressed by layering of lamellar domains near the electrode surface. In the present work, we first tested that conclusion by measuring the through-plane ionic conductivity of two model PIL-based systems having controlled interfacial profiles using impedance spectroscopy. The measurements were not sensitive to changes in interfacial composition or structure, so anisotropy in the ionic conductivity of PS--PIL materials must arise from an in-plane enhancement rather than a through-plane depression. We then examined the origin of this in-plane enhancement with a series of PS--PIL materials, a P(S-r-IL) copolymer, and a PIL homopolymer, where impedance spectra were acquired with a top-contact electrode configuration. These studies show that enhanced in-plane ionic conductivities are correlated with the formation of an IL-rich wetting layer at the free surface, which presumably provides a low-resistance path for ion transport between the electrodes. Importantly, the enhanced in-plane ionic conductivities in these PS--PIL materials are consistent with simple geometric arguments based on properties of the PIL, while the through-plane values are an order of magnitude lower. Consequently, it is critical to understand how surface and bulk effects contribute to impedance spectroscopy measurements when developing structure-conductivity relations in this class of materials.
层状嵌段共聚物电解质中的离子电导率通常是各向异性的，其中面内电导率比面外电导率高出一个数量级。在之前的一项工作中，我们展示了基于聚苯乙烯（PS）和聚合物离子液体（PIL）的层状嵌段共聚物在离子电导率方面存在显著的各向异性，并且我们提出电极表面附近层状域的分层会抑制跨膜离子电导率。在本工作中，我们首先通过使用阻抗谱测量两个具有可控界面轮廓的基于PIL的模型体系的面外离子电导率来检验这一结论。这些测量对界面组成或结构的变化不敏感，因此PS - PIL材料离子电导率的各向异性必定源于面内增强而非面外抑制。然后我们用一系列PS - PIL材料、一种P（S - r - IL）共聚物和一种PIL均聚物研究了这种面内增强的起源，其中阻抗谱是通过顶部接触电极配置获得的。这些研究表明，面内离子电导率的增强与在自由表面形成富IL的润湿层相关，这大概为电极之间的离子传输提供了一条低电阻路径。重要的是，这些PS - PIL材料中增强的面内离子电导率与基于PIL性质的简单几何论证一致，而面外值则低一个数量级。因此，在研究这类材料的结构 - 电导率关系时，理解表面和本体效应如何影响阻抗谱测量至关重要。
