Frustration-mediated structural-state selection tunes ionic conductivity in confined polymer--ionic liquid materials

<title>Abstract</title> <p> Structural order and ionic transport are usually regarded as competing requirements in solid ionic materials, because crystallization suppresses the molecular motions that enable ion conduction. While confinement is known to alter the phase behavior of ionic liquids, it remains unclear whether competing ordering tendencies in host polymers and confined ionic liquids merely generate unusual ordered phases or instead select the structural state that governs transport. Here, we identify two distinct structural and transport responses in confined polymer–ionic liquid materials that are consistent with frustration-mediated state selection. By combining crystalline poly(m-benzamide) (pMBA) or amorphous poly(m-oxybenzoate) (pMOB) with 1-butyl-3-methylimidazolium ionic liquids containing either PF ₆ ⁻ or FSI ⁻ , we systematically vary the polymer host and ionic-liquid anion across four composite systems. Structural analysis reveals two distinct modes through which frustration is accommodated. In pMBA-PF ₆ , pMOB-PF ₆ and pMOB–FSI, thermally persistent ordered diffraction features are observed. In contrast, pMBA–FSI exhibits strongly suppressed diffraction features attributable to long-range polymer and ionic ordering, yielding a thermally robust predominantly amorphous state. This predominantly amorphous state exhibits the highest ionic conductivity in the series, 3.7 × 10 ^− 4 S cm ^− 1 at 30°C, despite retaining less ionic liquid than pMOB–FSI, the composite with the highest retained ionic-liquid content. These results establish structural-state selection as a governing variable for ionic transport in confined polymer–ionic liquid materials. We propose that competing polymer- and ionic-liquid-derived ordering tendencies are accommodated through two distinct pathways: the formation of thermally persistent ordered ionic domains or the suppression of long-range order into a frustrated amorphous state. In the present series, the latter pathway is uniquely realized in pMBA–FSI and yields the highest ionic conductivity despite its intermediate ionic-liquid content, establishing frustration-mediated structural-state selection as a design principle for controlling the order–mobility trade-off in confined ionic materials. </p>