Nanostructured Stable Radical Polymer Thin Films: A Battery Electrode Material

Abstract

Stable radical polymers have attracted attention in the past decade as battery electrode materials due to their fast redox activity, enabled by a persistent radical moiety in the pendant group of the polymer. While early studies focused on the electrochemical performance of these materials, recent studies have aimed at improving their inherent conductivity. Block copolymer (BCP) architecture enables placement of this functionality in specific geometries, to generate nanostructures. In particular we study diblock copolymers comprising of blocks of poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA) and poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA). The thin film BCP is phase segregated into cylindrical and lamellar morphologies by controlling solvent vapor annealing (SVA) and the block ratios. Preferential wetting of the silicon substrate by the PTMA block inhibits long-range order in these films. To overcome this challenge, neutral underlayers are designed to generate a non-preferential substrate. The order is improved in both, the lamellae and cylinder forming BCPs, confirmed by GISAXS data. AFM images of the thin films delineate a hexagonal close-packed cylindrical morphology and fingerprint-like lamellar morphology. To further induce a unidirectional order, the lamellar BCPs are graphoepitaxially self-assembled, parallel and perpendicular to the trench direction. Gold sidewall based chemically heterogeneous graphoepitaxy is also explored to enable polymer morphology-conductivity relationship studies. After attaining control over the desired morphologies by SVA and graphoepitaxy, selective etching of PTFEMA block is performed using DUV and E-Beam exposure, aimed at improving the electrolyte diffusion and uptake. DUV exposure on homopolymer PTMA crosslinks it by ring-opening of the TEMPO moiety, as confirmed by FT-IR spectroscopy. DUV exposure on the BCPs degrade the PTFEMA which is corroborated by XPS data and AFM images, but at the expense of the stable radical functionality in PTMA. Furthermore, E-Beam exposure etches the PTFEMA as confirmed by AFM images. Thus, selective etching of the PTFEMA block from the BCP is demonstrated, and a trade-off required to create the nanostructured and crosslinked PTMA is elucidated.