Directional crystallization of dioxane in the presence of PVDF producing porous membranes

Melt crystallization methods offer high purity and energy-efficient purification for many chemicals. The solubility of solutes in a crystal phase is extremely low, but they can be engulfed by crystal phases. Therefore, the control of crystal morphology and size is critical in melt crystallization. In this work, we investigated the crystallization behavior of dioxane in the presence of a polymer solute under a controlled temperature gradient. By controlling the movement of a sample toward a liquid nitrogen reservoir, the nucleation and growth of crystals were regulated to produce uniformly distributed cylindrical crystals. Upon cooling, two crystallization steps were observed. While the first crystallization step mainly involved solidification of the bulk solution, the second crystallization at a lower temperature was accompanied by a further reduction in transparency and micro-cracking. This two-step crystallization behavior was used to make defect-free polymer structures with a unique cylindrical morphology of through-thickness pores, regardless of the dioxane removal method. Such understanding of directional crystallization will facilitate the future development of melt crystallization routes for dioxane and novel preparation methods for porous polymeric materials.

► The crystallization of polymer/dioxane solutions was controlled under a temperature gradient.
► The controlled melt crystallization produced uniformly distributed cylindrical crystals.
► Upon cooling, the first crystallization mainly involved solidification of the bulk solution.
► The subsequent second crystallization was accompanied by micro-cracking.