In Situ Electron Microscopy of Polyethylene Glycol Crystallizing in an Ionic Liquid

In this study, the nonvolatile room temperature ionic liquid (IL) 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM][ETSO]) was chosen as a suitable solvent to study the thermally induced solution crystallization and gelation of polyethylene glycol (PEG) by electron microscopy and electron diffraction. For both phenomena, key features occur at the nanoscale. PEG-IL samples were prepared as thin freestanding or supported liquid films on TEM grids, with crystallization observed upon cooling of heated films to room temperature. In free-standing films, crystals predominantly resided at the liquid surface, and in supported films, on the solid substrate, varied by use of different support films on TEM grids. Crystalline morphologies were observed in the form of rods (width <100nm), fibers, spherulites (80-500 nm diameter), compact faceted single crystals (<100 nm), and interconnected networks. Electron diffraction patterns on the rod- and fiber-like crystals reveal single crystal order at length scales greater than one micron. Electron microscopy and electron diffraction were also performed on fiber-like PEG crystals grown from mixtures of [EMIM][ETSO4] with the less polar ethyltributylphosphonium diethyl phosphate ([P2444] [DEP]).