Ion Transport in Polymerized Ionic Liquid Block Copolymers: Insights for High Polymerized Ionic Liquid Content

Block copolymers (BCPs) containing polymerized ionic liquids (PILs) have garnered considerable interest in ion-conducting applications due to their tunable physicochemical properties and intrinsic ion transport capabilities. Previous studies primarily focused on BCPs with low PIL content (<25 mol %), and higher PIL contents are less explored. In this study, the thermal properties, self-assembly, and ion transport of poly( N -isopropylacrylamide)- b -poly(ionic liquid) (PNIPAM- b -PIL) BCPs were investigated across a broad PIL content ranging from 3.2 to 70.4 mol %. The neutral PNIPAM block length was also probed. Weak microphase separation of PIL BCPs was confirmed by a PIL composition-dependent glass transition temperature in differential scanning calorimetry (DSC) and the absence of long-range periodic structures in small-angle X-ray scattering. At fixed PNIPAM block length, the ionic conductivity increases with PIL content, with BCPs containing more than 50 PIL mol % exhibiting larger conductivity than the corresponding homopolymer at high temperature due to enhanced phase separation and nanoconfinement effects of the ionic domains. PNIPAM- b -PILs with similar PIL compositions but significantly longer PNIPAM blocks show a 5-fold conductivity enhancement. Notably, PNIPAM- b -PILs with comparable PIL compositions exhibit ionic conductivity (at 413 K) trends consistent with those of previously reported systems, despite variations in monomer structures. Higher PIL content can further improve ionic conductivity due to stronger phase separation and ordering of the ionic domains, highlighting the critical role of high PIL incorporation on ion transport.