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Abstract Tailor‐made auxiliaries are generally less stereochemically discriminating of crystals grown from the melt than crystals grown from solution. However, it is possible to make supramolecular inferences using well‐chosen additives in the manner taught to us by Lahav and Leiserowitz. Spherulites manifesting needle‐like growth and small angle branching grow frequently from supercooled melts under high crystallization driving forces, along one principal crystallographic direction. If this direction is polar, it is important to establish its absolute sense as a basis for understanding growth at the interface between crystals and melt. This assignment, for polycrystalline ensembles, is beyond the capabilities of X‐ray crystallography. Two examples of this discrimination are described with tailor‐made additives, one for theβform of resorcinol and another for form I ofL‐malic acid. Assigning the absolute sense of a polar axis with molecular additives is a problem that resembles both the science and style of previous experiments from the Weizmann Institute. 

Two new metastable polymorphs of the tuberculosis drug isoniazid, considered monomorphic for sixty years, were discovered using melt crystallization and nanoscale confinement. The two new forms are readily distinguished from the known form by optical microscopy, Raman spectroscopy and X-ray powder diffraction. A single crystal structure was obtained for one of the new polymorphs. 

Flufenamic acid (FFA) is a highly polymorphic compound, with nine forms to date. When melt crystallization was performed under nanoscale confinement in controlled pore glass (CPG), the formation of the extremely unstable FFA form VIII was favored. Under confinement, form VIII was sufficiently stable to allow the measurement of its melting point, which decreased with decreasing pore size in accord with the Gibbs−Thomson relationship, enabling determination of the otherwise elusive melting point of the bulk form. Moreover, the transformation pathways among the various polymorphs depended on pore size, proceeding as form VIII → form II → form I for nanocrystals embedded in 30−50-nm diameter pores, and form VIII → form IV → form III in 100−200 nm pores. In contrast, form VIII converts directly to form III in the bulk. Whereas previous reports have demonstrated that nanoconfinement can alter (thermodynamic) polymorph stability rankings, these results illustrate that nanoscale confinement can arrest and alter phase transformations kinetics such that otherwise hidden pathways can be observed.