Phase behavior and hydrate crystallization kinetics in polyoxacyclobutane−water mixtures: Effects of molecular weight

Abstract

Polyoxacyclobutane, -[(CH₂)₃-O-]_n has the unusual ability to co-crystallize with water to form a hydrate. But above the hydrate melting temperature, POCB/water mixtures phase separate into a liquid-liquid equilibrium state. Thus, the POCB-water system appears to be the only example of a polymer co-crystallizing with a solvent with which it is immiscible in the amorphous state. We examine the dependence of phase behavior and hydrate crystallization kinetics on the POCB molecular weight. In the molecular weight range from ∼1 kg/mol to ∼12 kg/mol, the hydrate melting temperature is found to be nearly insensitive to molecular weight. Above the hydrate melting temperature, the mixtures are in liquid-liquid equilibrium where the water-rich phase is nearly pure at all molecular weights, whereas the fraction of water in the POCB-rich phase decreases with increasing molecular weight. We also examine isothermal hydrate spherulite growth, which occurs exclusively within the POCB-rich phase. The spherulite growth velocity is found to decrease with increasing molecular weight regardless of whether samples are compared at fixed temperature or fixed undercooling. The spherulite growth velocity of the POCB hydrate is orders of magnitude higher than of pure POCB at the same temperature indicating that, as long as sufficient water is available, hydrate co-crystallization will always outcompete POCB homopolymer crystallization.