Ion Transport in Microphase Separating Polymer Thin Films
Authors: Kambe Yu , Arges Christopher George , Dong Ban , Czaplewski David A. , Patel Shrayesh N. , Nealey Paul F. 
Published: February 2020

Microphase separating block copolymers (BCPs) are attractive candidates as electrolytes and ion exchange membranes due to their ability to simultaneously optimize for two or more orthogonal material properties. For an example, BCP electrolytes have been synthesized with one mechanically stable domain and one ion conducting domain to overcome the inverse relationship of the two properties. These films can exhibit both high structural stability and high ionic conductivity at the bulk scale.1,2 However, the formation of nanoscale architectures results in the creation of domain interfaces, grain boundaries, and tortuous ion conduction pathways. In such a film, ions can experience transport pathways that are non-parallel to the applied electric field. To date, this structure transport relationship has been investigated in thick films with bicontinuous structures as well as thick films with nanoscale domains aligned across macroscopic scales.3–5 However, assumptions are made about structure when correlating to the transport. This is due to the difficulties of characterizing all possible nanoscale pathways across macroscales (e.g. 20 nm domains across a 1 cm2 x 1mm membrane).6 In this work, the structure - ion transport relationship was investigated by characterizing all possible ion transport pathways from one electrode to the other by assembling thin film morphologies on top of interdigitated electrodes.

The poly(styrene)-block-poly(2vinyl pyridine) (PSbP2VP) model BCP thin film (ca. 40nm) was assembled into a variety of morphologies on top of interdigitated electrodes. Trench topographies were formed on top of the IDEs to confine the BCPs. Neutrality conditions of the trench walls and the substrate were controlled by grafting monohydroxy terminated brushes at different stages of device fabrication. The confined BCP was microphase separated using solvent vapor annealing. Following orientation, the P2VP domain was converted into n-methyl 2-pyrilidinium iodide converting the domain into an anion conductor.7 Due to the perpendicular orientation of the PS and P2VP domains to the substrate and free surface, simple top down metrology could be used to characterize all possible ion conduction pathways from one electrode to the other. Visual analysis software was used to quantify the pathways that were connected from one electrode to the other. Molecular dynamics simulations of the lamellae cross section were used to understand water and ion distributions near the PS/P2VP interface. The ion transport behavior was studied under different humidity, temperature, and degrees of functionalization conditions.

With path information alone, the conductivity values of different morphologies could be predicted within experimental error. When the ion conduction pathways were aligned parallel to the applied electric field, the resulting conductivity approached 74% of the theoretical maximum conductivity predicted using the Sax Ottino model.8 The remaining 26% difference in the conductivity could be rectified by the reduced water content at the interface of the hydrophilic P2VP/NMP+ I- domain and the hydrophobic PS domain. With the knowledge of the structure and the dimensions of the interface, the ion conductivity of the parallel oriented film could be predicted within experimental error. Surprisingly, the conductivity of the isotropic fingerprint lamellae morphology could also be predicted within error by assuming: 1) that the resistance of the total conduction path is linearly related to the tortuosity of the conduction path and 2) that only the domains that are connected from one electrode to the other contribute to conductivity. As demonstrated, the control of morphology and the ability to quantify structure and ion transport is envisioned as an attractive analytical platform to understand the role of heterogenous interfaces of microphase separating systems on ion transport behavior.

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