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(3,4-ethylenedioxythiophene):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.