Liquids that freeze when mixed: Thermodynamics and kinetics of co-crystallization of polyoxacyclobutane with water

Polyoxacyclobutane (POCB) with structure –[CH2-CH2-CH2-O]_n- has the remarkable ability to cocrystallize with water to form a hydrate. At low molecular weights, simply mixing liquid POCB with water at room temperatures induces freezing. Yet, above the melting point of the hydrate, 37C, and POCB/water mixtures phase-separate into two co-existing liquid phases. This combination of hydrate cocrystallization and LCST-type liquid-liquid equilibrium (LLE) yields a unique of phase diagram. We examine the kinetics of isothermal cocrystallization in mixtures with less than 25 wt% POCB which are homogeneous prior to hydrate formation. The bulk kinetics of hydrate cocrystallization can be well-fitted with the Avrami equation, with the exponent decreasing as temperature increases. The dependence of spherulite growth kinetics on temperature is generally in agreement with the Hoffman-Lauritzen model. Bulk kinetics and spherulite velocities both slow down as the water content decreases far below the hydrate stoichiometry. This suggests that the excess POCB, which cannot crystallize, acts as an impurity.