Thin-Film Self-Assembly of Block Copolymers Containing an Azobenzene-Based Liquid Crystalline Polymer and a Poly(ionic liquid)

Abstract

By combining reversible addition–fragmentation chain transfer (RAFT) with postfunctionalization and ion exchange, we synthesized a series of block copolymers (BCPs) containing an azobenzene-based liquid crystalline (LC) polymer (PAzo) and an imidazolium-containing poly­(ionic liquid) (PIL, PVB­(TFSI)) with the volume fraction of PIL (f PIL) values ranging from 15.9% to 69.9%. The samples obtained self-assemble into hexagonally packed cylinders (HEX), lamellae (LAM), and the mixed phase with coexisting HEX and LAM. The thin-film self-assembly of the samples PAzo101-b-PVB­(TFSI)22 and PAzo101-b-PVB­(TFSI)67 was studied systematically. We investigated the assembled structures of the thin films with various initial thicknesses after thermal annealing (145 °C for 12 h) or mixed solvent vapor annealing with tetrahydrofuran and n-hexane. Thin films with large-scale uniaxial PIL nanocylinders were obtained, which will greatly broaden the application of IL-based BCPs. Inverse phases were also observed for the thin films with thicknesses less than ∼120 nm. The different mechanisms of the formation of inverse nanostructures formed in the thinner films under the thermal annealing and mixed solvent vapor annealing conditions were also elucidated.

Thickness-Dependent Photo-Aligned Thin-Film Morphologies of a Block Copolymer Containing an Azobenzene-Based Liquid Crystalline Polymer and a Poly(ionic liquid)

“…Poly­(ionic liquid)­s (PILs), a type of polymers with an electrolyte group on each monomeric unit, possess high charge density while maintaining low glass transition temperatures and are expected to have application perspectives in electrochemical devices owing to the combination of the excellent electrochemical performance of ionic liquids (ILs) and the processability of polymers. − However, electrochemical devices require thin films with both high mechanical strength and high ionic conductivity, which prompts the development of a new class of materials referred to as IL-based block copolymers (BCPs). − …”

“…BCPs composed of two or more incompatible segments can obtain excellent properties of each segment orthogonally by self-assembling to form various ordered nanostructures, including spheres, hexagonally packed cylinders (HEX), lamellae (LAM), the gyroid network phase (GYR), and the Fddd network phase (O 70 ). − However, many properties of BCPs, including the modulus and conductivity, are usually affected by the long-range order of nanostructures. − Fortunately, thin-film assembly of BCPs provides a fascinating approach for preparing thin films with long-range ordered nanostructures. In the past decades, various strategies, including magnetic field, electric field, − graphoepitaxy, − solvent vapor annealing, ,, and supramolecular cooperative motions (SMCMs), ,− have been explored to control the orientation of nanostructures in BCP thin films. In addition, surface modification of the substrate can also manipulate the order of nanostructures (randomly parallel or uniformly perpendicular to the substrate) in the thin films because the differences in the affinity of the two segments of the BCP with respect to the substrate may vary when the surface chemistry of the substrate is different. − …”

“…The PIL segments can dissociate freely moving counterions as carriers, and thus there is no need for doping of lithium salt. In addition, most of the neutral-PIL BCPs reported in the literature still maintain the ordered microphase-separated structures at high temperatures, ,,− which endows the IL-based BCPs a promising application in the field of high-temperature ionic conduction.…”

“…Only a few works have achieved uniaxial conducting channels in IL-based BCP films. ,,, Ocola et al attained perpendicular alignment of ion-conducting paths in the thin films of poly­(styrene- block -2-vinylpyridine/nmethyl pyrdinium) (PS- b -P2VP/NMP + I – ) by adopting the chemical-assisted directed self-assembly method . Nealey et al obtained uniaxial ion-conducting paths in the films of lamellae-forming PS- b -P2VP/NMP + I – to investigate the relationship between structure and electrochemical properties .…”

“…In our previous work, we obtained uniaxially aligned ionic domains in IL-based BCP films by thermal annealing or mixed solvent vapor annealing . However, the uniaxially oriented structure only appears in films with relatively large thicknesses.…”