Liquids That Freeze When Mixed: Co-Crystallization and Liquid-Liquid Equilibrium in Polyoxacyclobutane-Water Mixtures

We show that liquid polyoxacyclobutane –[CH₂-CH₂-CH₂-O]_n- when mixed with water at room temperature, precipitates solid co-crystals of the polymer and water. Such co-crystals are formally known as a clathrate hydrate. Hydrate co-crystals can also be formed by simply exposing the liquid polymer to saturated humidity. Outside of metal alloys, this is a rare example of an co-crystal whose melting point exceeds that of the pure species, and the only known example of non-reacting liquids that combine to form a solid co-crystal at room temperature. At high temperatures, the same polymer-water mixtures phase separate into two co-existing liquid phases. This combination of co-crystal hydrate formation and LCST-type liquid-liquid equilibrium (LLE) gives rise to an unusual, possibly unique, type of phase diagram. We examine the effects of polymer molecular weight on the phase behavior and show that at molecular weights exceeding ~2000 g/mol, nearly the entire composition-temperature space is split between regions of solid-liquid equilibrium and liquid-liquid equilibrium. Furthermore, this unusual phase diagram produces distinct crystallization pathways depending on whether the mixture is single-phase or two-phase prior to crystallization