3D-printable ionic liquid formulations for filler-free, transparent ion-conducting soft materials

This work presents a family of photocurable ionic liquids (pILs) based on a polymerizable methacrylate anion and readily available organic cations, designed for vat photopolymerization (VP) of soft ion-conducting networks. The pILs were synthesized by straightforward ion exchange, providing liquid salts that are fully compatible with commercial acrylate resins and enable filler-free 3D printing of transparent ionic polymers. By systematically mapping the resin compositions as a function of the total acrylate content and the fraction of crosslinking species, a clear printability window was identified, allowing prediction of the maximum printable ionic liquid concentration from the molecular weight of the chosen cation. Mechanical properties were shown to be governed primarily by network architecture, with imidazolium-based systems displaying linear correlations between stiffness and crosslinking density, while tetraalkylammonium cations introduced exponential softening due to their plasticizing effect. In contrast, ionic conductivity was largely dictated by cation size, hydrophobicity, and charge shielding, enabling conductivity values in the 10–6–10–4 S/cm range without the need for conductive fillers. These results demonstrate a chemically simple and generalizable approach to obtain printable, filler-free ion-conducting materials, opening opportunities for applications in antistatic coatings, electrostatic discharge protection, flexible and stretchable sensors, and low-voltage energy harvesting devices.