Liquids that Freeze when Mixed: Homogeneous Cocrystallization Kinetics of Polyoxacyclobutane–Water Hydrate

We report on the kinetics of crystallization of polyoxacyclobutane (POCB) and water to form a cocrystal hydrate, the first paper to explore the kinetics of crystallization of a polymer with a small molecule. POCB has unusual cocrystallization behavior when mixed with water, which could be exploited for a variety of applications including improving the nonvolatile memory storage capabilities of carbon nanotube devices, water purification, and biomedical applications such as drug delivery. The rates of hydrate crystallization of a series of mixtures containing up to 24 wt % water were measured using both a bulk, volumetric and a localized, spherulite growth approach. At these compositions, all mixtures were single-phase homogeneous liquids prior to hydrate crystallization, and all mixtures were POCB-rich as compared to the cocrystal stoichiometry. The time dependence of crystallization kinetics is well described by the Avrami equation. Reducing temperature, i.e., increasing undercooling, increases the Avrami exponents and increases spherulite growth velocities consistent with the Hoffmann–Lauritzen model. Reducing water content reduces spherulite growth velocities. The velocities also reduce with time during spherulite growth reminiscent of impurity effects in crystallization. Talc was found to accelerate nucleation. Broadly, the cocrystallization process starting from the homogeneous mixtures of the components resembles homopolymer crystallization, but with the complexity that when the mixture composition deviates from the cocrystal stoichiometry, the excess species cannot crystallize fully.