Article pubs.acs.org/cm
Perpendicularly Aligned, Anion Conducting Nanochannels in Block Copolymer Electrolyte Films Christopher G. Arges,†,‡ Yu Kambe,†,‡ Hyo Seon Suh,†,‡ Leonidas E. Ocola,§,⊥ and Paul F. Nealey†,‡ †
Institute for Molecular Engineering, University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States Materials Science Division and §Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States ⊥ MRSEC, Chicago Materials Research Center, University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States ‡
S Supporting Information *
ABSTRACT: Connecting structure and morphology to bulk transport properties, such as ionic conductivity, in nanostructured polymer electrolyte materials is a difficult proposition because of the challenge to precisely and accurately control order and the orientation of the ionic domains in such polymeric films. In this work, poly(styrene-block-2-vinylpyridine) (PSbP2VP) block copolymers were assembled perpendicularly to a substrate surface over large areas through chemical surface modification at the substrate and utilizing a versatile solvent vapor annealing (SVA) technique. After block copolymer assembly, a novel chemical vapor infiltration reaction (CVIR) technique selectively converted the 2vinylpyridine block to 2-vinyl n-methylpyridinium (NMP+ X−) groups, which are anion charge carriers. The prepared block copolymer electrolytes maintained their orientation and ordered nanostructure upon the selective introduction of ion moieties into the P2VP block and post ion-exchange to other counterion forms (X− = chloride, hydroxide, etc.). The prepared block copolymer electrolyte films demonstrated high chloride ion conductivities, 45 mS cm−1 at 20 °C in deionized water, the highest chloride ion conductivity for anion conducting polymer electrolyte films. Additionally, straight-line lamellae of block copolymer electrolytes were realized using chemoepitaxy and density multiplication. The devised scheme allowed for precise and accurate control of orientation of ionic domains in nanostructured polymer electrolyte films and enables a platform for future studies that examines the relationship between polymer electrolyte structure and ion transport.
■
INTRODUCTION Polymer electrolytes are found at the heart of many electrochemical processes because they facilitate ion transport, while being electron insulating, and serve as a separator between the two electrodes. These properties enable electrochemical cells, such as batteries, fuel cells, electrolyzers,1 solar fuel generators,2 dye-sensitized solar cells,3 and supercapacitors,4 to harness energy from chemicals or convert stored energy into chemicals. In addition, they play a pivotal role in water treatment technologies such as electrodialysis, membrane capacitive deionization, and electrodeionization.5−7 They are also being explored as emerging materials for nanoionics used in information processing and storage.8 Comprehensive reviews exist on block copolymer electrolytes (BCEs), and it is widely recognized that the phase separation between ionic and nonionic blocks spurs higher ionic conductivity at lower fixed ion concentrations over their random copolymer counterparts.9−12 Lowering the ionic loading is desirable because fewer fixed ion carriers minimizes detriment to film mechanical properties, and it makes it easier to attain a robust, desired © XXXX American Chemical Society
morphological structure. However, the phase separation between the ionic and nonionic blocks is isotropic, and thus selective ion transport in one direction is difficult to realize. A few researchers have oriented the ionic blocks in BCEs using external force methods (mechanical, electrical, or magnetic), track etches, holographic polymerization, or supramolecular assembly.13−21 However, the process described herein differed because it relied solely upon the control of self-assembly through the tuning of the surface energy and utilization of chemical patterns. In this work, the self-assembly (SA) and directed selfassembly (DSA) processes were adopted to achieve perpendicular alignment of poly(styrene-block-2-vinylpyridine/nmethyl pyrdinium) (PSbP2VP/NMP+) in sub-50 nm thin films.22−24 This is the first report, to the best of our knowledge, utilizing the processes of chemically assisted SA and DSA of Received: November 15, 2015 Revised: December 17, 2015
A
DOI: 10.1021/acs.chemmater.5b04452 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Preparation of Self-Assembled Samples. Silicon wafers without interdigitated electrodes were cleaned with piranha solution (70:30 volume mixture of concentrated sulfuric acid with 35 wt % of hydrogen peroxide) for 30 min at 130 °C. The polished silica wafers and the interdigitated electrode substrates were then exposed to oxygen plasma (PO2 = 100 mTorr) in a reactive ion etcher (RIE-2000 from South Bay Technologies) for 12 s at 50 W to introduce more surface functional groups (e.g., silanol and SiOx) for the grafting of polymer brushes. The SA of PSbP2VP diblock copolymer samples was achieved by first preparing a 1 wt % solution of the polymer brush PSrP2VPrPHEMA containing 61% styrene (by weight) in toluene and spin coating the polymer solution on the oxygenated wafer substrates and interdigitated substrates at 4000 rpm for 45 s. The polymer brush was then grafted at 250 °C for 5 min in a nitrogen filled glovebox (
