Polymer co-crystallization from LLE: Crystallization kinetics of POCB hydrate from two-phase mixtures of POCB and water

Abstract

Polyoxacyclobutane (POCB, –[CH2CH2CH2O−]n) has the rare ability to co-crystallize with water to form a hydrate. Above the melting point of the hydrate (∼37 °C), mixtures exist in liquid-liquid equilibrium (LLE) between a POCB-rich and a water-rich liquid phase over a wide range of POCB:water ratios. Such phase behavior, which combines liquid-liquid equilibrium (LLE) and co-crystallization of a polymer with a small molecule, appears to be unique to POCB-water mixtures. Here, we examine the kinetics of isothermal hydrate co-crystallization when mixtures that are initially in liquid-liquid equilibrium (LLE) are cooled to below the hydrate melting point. Hydrate co-crystallization requires both POCB and water, and hence, it is coupled with transport of species between the liquid phases. Optical microscopy shows that hydrate crystallization occurs within the POCB-rich domains, and hydrate spherulites grow at constant speed. The temperature-dependence of the speed of hydrate growth front is consistent with the Hoffman Lauritzen model for homopolymer crystallization. Dilatometric measurements of bulk co-crystallization kinetics are conducted with constant stirring to avoid gravitational separation of the liquid phases. The temperature-dependence of bulk crystallization kinetics of these stirred mixtures in LLE is found to be far weaker than that of spherulitic growth kinetics, or of the quiescent bulk crystallization kinetics of mixtures starting from homogeneous conditions.