(527h) Liquids That Freeze When Mixed: Thermodynamics and Kinetics of Co-Crystallization of Polyoxacyclobutane (POCB) with Water

Polyoxacyclobutane (POCB) with repeat unit -[CH₂CH₂CH₂O-]_n is a rare class of polymer with a unique ability to form a co-crystalline hydrate when mixed with water. At low molecular weights, simply mixing liquid POCB with water at room temperature induces freezing. Yet, above the melting point of the hydrate, 37C, POCB−water mixtures can phase-separate into two coexisting liquid phases. This union of hydrate co-crystallization and LCST-type liquid-liquid equilibrium (LLE) yields a unique phase diagram. Unlike homopolymer crystallization kinetics, there is almost no knowledge of co-crystallization kinetics of a polymer with a small molecule. We report the first study of the cocrystallization kinetics of the POCB-water mixture.

Due to the usual phase diagram two crystallization pathways are possible when POCB/water mixtures are cooled. Mixtures with less than 25 wt% water form a homogeneous solution at high temperature, and hence hydrate crystallizes from a single-phase liquid mixture (Case 1). Mixtures high water content are in LLE at high temperature, and hence hydrate crystallizes from a two-phase liquid mixture (Case 2). In both cases, the rates of hydrate crystallization are measured using bulk dilatometry and optical microscopy across a range of mixture compositions and temperatures. Case 1 broadly resembles homopolymer crystallization in that the bulk kinetics of hydrate cocrystallization from a homogeneous solution can be well-fitted with the Avrami equation and the dependence of spherulite growth kinetics on temperature agrees with the Hoffman-Lauritzen model. One significant difference is that for mixtures with water content decreases far below the hydrate stoichiometry, spherulite velocity reduces during crystallization. This suggests that the excess POCB, which cannot crystallize, acts as an impurity, causing diffusion limitations to further crystallization. In Case 2, optical microscopy shows that hydrate crystallization occurs within the POCB-rich domains, and hydrate spherulites grow at constant growth velocity. In this cse, bulk crystallization studies are conducted with constant stirring to avoid gravitational sedimentation. 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 bulk kinetics of mixtures starting from homogeneous solution.