Crystallization kinetics of polyoxacyclobutane and water to form polymer hydrate

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 of hydrate starting from POCB/water mixtures in LLE. Crystallization is conducted in “puddles” of the POCB-rich liquid phase in equilibrium with the water-rich phase which is nearly pure water. Hydrate crystallizes by spherulite growth and, similar to most homopolymers, hydrate spherulites are found to grow at constant velocity. Since hydrate formation consumes the water dissolved into the puddles, continued hydrate growth requires water to be continuously replenished by diffusion from the water-rich phase. The dependence of spherulite growth velocity on temperature agrees with the Hoffman-Lauritzen model. Increasing molecular weight of the POCB slows hydrate growth, partly because the solubility of water in the POCB decreases as MW increases. At all molecular weights and temperatures studied, POCB hydrate crystallization is orders of magnitude faster than POCB homopolymer crystallization, i.e. as long as water is available, the POCB hydrate crystals will always outcompete POCB homopolymer crystals.