Protic Ionic Liquids for Intrinsically Stretchable Conductive Polymers

Inspired by the classic hard−soft acid−base theory and intrigued by a theoretical prediction of spontaneous ion exchange between poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and hard-cation−soft-anion ionic liquid (IL), we treat PEDOT:PSS with a new IL composed of a protic (i.e., extremely hard) cation (3-methylimidazolium, p-MIM⁺) and an extremely soft anion (tetracyanoborate, TCB⁻). In fact, this protic IL (p-MIM:TCB) accomplishes the same levels of ion-exchange-mediated PEDOT−PSS separation, PEDOT-rich nanofibril formation, and electrical conductivity enhancement (∼2500 S/cm) as its aprotic counterpart (EMIM:TCB with 1-ethyl-3-methylimidazolium), the best IL used for this purpose so far. Furthermore, p-MIM:TCB significantly outperforms EMIM:TCB in terms of improving the stretchability (i.e., the highest tensile strain) of the PEDOT:PSS thin film. This enhancement is a result of the aromatic and protic cation p-MIM⁺, which acts as a molecular adhesive holding the exchanged ion pairs (PEDOT⁺:TCB⁻-p-MIM⁺:PSS⁻) via ionic intercalation (at the surface of TCB⁻decorated PEDOT⁺ clusters) and hydrogen bonding (to PSS⁻), in which washing p-MIM⁺ out of the film degrades the stretchability while keeping the morphology. Our results offer molecular-level insight into the morphological, electrical, and mechanical properties of PEDOT:PSS and a molecular-interaction-based enhancement strategy that can be used for intrinsically stretchable conductive polymers.