Microscopic Softening Mechanisms of an Ionic Liquid Additive in an Electrically Conductive Carbon-Silicone Composite

Abstract The microstructural changes caused by the addition of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to polydimethylsiloxane (PDMS) elastomer composites filled with carbon black (CB) are analyzed to explain the electrical, mechanical, rheological, and optical properties of IL-containing precursors and composites. Swelling experiments and optical analysis indicate a limited solubility of the IL in the PDMS matrix that reduces the cross-linking density of PDMS both globally and locally, which reduces the Young's moduli of the composites. A rheological analysis of the precursor mixture shows that the IL reduces the strength of carbon–carbon and carbon–PDMS interactions, thus lowering the filler–matrix coupling and increasing the elongation at break. Electromechanical testing reveals a combination of reversible and irreversible piezoresistive responses that is consistent with the presence of IL at microscopic carbon–carbon interfaces, where it enables re-established electrical connections after stress release but reduces the absolute conductivity.