Polymorph Selection via Sublimation onto Siloxane Templates

Diphenylurea (PU) crystallizes via slow evaporation from a range of solvents as phase pure alpha-PU or in a concomitant mixture with the less stable beta-PU phase. Sublimation onto glass also yields alpha-PU in high phase purity. In contrast, sublimation onto siloxane-coated glass templates with various terminal groups (e.g., isocyanate, acetate, bromine) resulted in primarily beta-PU. Scanning electron microscopy showed that the individual crystals on siloxanes exhibit a range of different morphologies (e.g., plates, needles, hollow tubes, spirals) which appear to be influenced by the terminal siloxane functionality. Under extended sublimation times (>5 h), mixtures of beta and alpha become apparent on most siloxanes. Grazing incidence X-ray diffraction studies showed that the beta:alpha ratio changes as a function of distance from the siloxane, with more alpha-PU appearing at higher angles. Diffraction line intensities also indicate strong preferred orientations for beta-PU crystallites within the polycrystalline layer with the fastest growth direction (b-axis) parallel to the template, but more varied orientations for alpha-PU. This suggests alpha-PU may nucleate elsewhere in the sublimation apparatus rather than on the beta-coated siloxane. This study demonstrates the utility of template directed sublimation methods as a way to engineer metastable phases but also illustrates some of the challenges associated with achieving high phase purity on templates with limited surface area.