Deep eutectic silsesquioxane hybrids with quaternary ammonium/urea derivatives: synthesis and physicochemical and ion-conductive properties

Abstract

We report the synthesis of deep eutectic silsesquioxane hybrids (DE-SQs) by simple mixing of quaternary-ammonium-containing SQ and urea derivatives. Cationic SQ, which was prepared by the hydrolytic condensation of a triethoxysilane precursor derived from 2-(dimethylamino)ethyl acrylate, followed by a quaternization reaction with methyl iodide, was used as a quaternary-ammonium-containing SQ component. Cationic SQ reacted with urea at a 1:2 M ratio at 80 °C for 48 h to yield a viscous DE-SQ (2Urea) liquid with a low glass transition temperature (Tg = −11 °C). Urea derivatives—1,3-dimethylurea (DMU) and 1,3-dimethylthiourea (DMTU)—were additionally used as hydrogen bond donors to form low-Tg DE-SQs. The thermal, physical, and ion-conductive properties of the DE-SQ family of organic–inorganic hybrids were investigated and characterized, and the influences of the nature of the urea derivative and their feed ratios on DE-SQ formation were evaluated. Among the DE-SQs developed in this study, DE-SQ (2Urea) and DE-SQ (2DMTU) achieved the highest ionic conductivity, with DE-SQ (2Urea) exhibiting 2.35 × 10−6 and 6.63 × 10−4 S cm−1 at 25 and 75 °C, respectively, under anhydrous conditions. This is the first report on the synthesis of DE-SQs by simple mixing of two solids, wherein the resulting compounds exhibit low Tg, thermal stability, and characteristic ionic conductivity. The ability to incorporate unique DE units into the SQ structure facilitates the development of advanced organic–inorganic hybrid materials possessing a wide range of functions and applications.