We report the rheological and structural properties of a suspension comprising poly(ethylene oxide)-polystyrene-poly(ethylene oxide) core-shell micellar nanoparticles dispersed in 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid. A liquid to soft solid transition was observed at a copolymer concentration of 10 wt % above which an elastic soft material was formed, which was composed of non-ordered jammed core-shell micellar nanoparticles. In the soft solid state, a significant reduction in the size of the nanoparticles, approaching hard sphere behavior, was observed by small-angle X-ray scattering which is attributed to compression of the soft poly(ethylene oxide) coronas. The nonvolatile ionic liquid-based glassy soft solid formed exhibited remarkable thermal stability with a melting temperature of 141 °C at 20 wt % copolymer, which renders it suitable for applications involving elevated temperatures and/or reduced pressure where water-based formulations are inappropriate.
Keywords: block copolymer nanoparticle; ionic liquid; jamming; rheological properties; soft colloidal glass; thermal stability.