Many members of the large family of 1,3-bis(X-phenyl)ureas (PUs) crystallize in C(4)R12(6) motifs formed from intermolecular hydrogen bonding between urea amide and carbonyl groups.  Needle-like morphologies are frequently exhibited in these systems.  Previous work in our laboratory has shown that improved crystal quality and/or polymorph control can be accomplished by heterogeneous nucleation on functionalized siloxane-glass substrates.  Slow evaporation crystallization of several PUs on siloxane substrates revealed unusual circular arrays of crystals under some solvent conditions.  Fast drop-cast crystallization onto siloxane substrates resulted in the formation of fibers with similar curvature from the same select solvents.  These observations suggest that the substrates stabilize fast growing and flexible 1-dimensional aggregates of PU molecules, which eventually thicken and fracture.  This same growth mechanism has been observed in other molecular crystal systems.

Patterned crystallization on unpatterned substrates

252nd National Meeting (2016)

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