Thermoreversible gelation of an ionic liquid by crystallization of a dissolved polymer.

Poly(ethylene glycol) (PEG) dissolves in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtSO(4)] above approximately 60 degrees C, the neat polymer's melting temperature, and if polymer concentration and molecular weight are high enough, the solution transforms into a semitransparent gel when cooled. The modulus, reaching 100 KPa or higher, is strongly affected by PEG concentration, and self-supporting materials are made even from solutions somewhat below coil overlap. Via differential scanning calorimetry (DSC), rheology, and optical microscopy, thermoreversible solidification is traced to kinetically frustrated polymer crystallization, an established mechanism for many pairings of crystallizable polymer with aqueous or organic solvent. Optical microscopy reveals nucleation and growth of PEG crystals with a largest dimension of tens to hundreds of micrometers. Crystalline chain packing in gels is identical to that of neat PEG, and degrees of crystallization are similar. Simple preparation, nontoxicity, and vanishing volatility suggest unique new gel applications.