A sol–gel synthesis using modified poly(methyl methacrylate) (PMMA) bearing trimethoxysilane groups and tetraethoxysilane (TEOS) allowed immobilization of very high loadings of ionic liquid [BMIm][NTf2] within nanocomposite PMMA-silica membranes. The ionic liquid (IL) provided both flexibility and ionic conductivity, while the silica nanofiller covalently bonded to the polymer chains maintained mechanical strength, as shown by a DMA and tensile test. Complex impedance spectroscopy evidenced a critical threshold in ionic conductivity around 76%, which was ascribed to an unusual transition from “polymer-in-salt” to “quasi-liquid” electrolyte behaviour. Outstanding IL loadings as high as 90 wt% were reached, resulting in the same ionic conductivity as the pure IL.
