Polymorph selectivity under nanoscopic confinement.

Polymorph selectivity has been achieved during crystallization of anthranilic acid (AA) and 5-methyl-2-[(2-nitrophyenyl)amino]-3-thiophenecarbonitrile (ROY), both considered benchmarks of polymorphic behavior, within nanoporous glass beads and polymer monoliths. Whereas polymorph III of AA crystallizes from the melt on nonporous glass beads or within larger pores, the metastable polymorph II crystallizes in pores with diameters <23 nm, with the selectivity toward this form increasing with decreasing pore size. Of the six ROY polymorphs characterized by single-crystal X-ray diffraction, the yellow form (Y) crystallizes during evaporation of pyridine solutions imbibed by the 30-nm cylindrical pores of porous polycyclohexylethylene (p-PCHE) monoliths. Although both R and ON grow from the melt on the external surfaces of PCHE, only the red form (R) crystallizes in the pores. Amorphous ROY also forms in p-PCHE pores during evaporation from pyridine solutions, subsequently crystallizing to the R nanocrystals upon heating. Although heterogeneous nucleation on the pore walls may play a role, these observations suggest that nucleation and polymorph selectivity is governed by critical size constraints imposed by the ultrasmall pores. The ability to achieve polymorph selectivity in both glass and polymer matrices suggests wide-ranging compatibility with various organic crystalline solids, promising a new approach to controlling polymorphism and searching for unknown polymorphs.

Ordered nanoporous polymers from polystyrene-polylactide block copolymers.

Nanoporous polystyrene monoliths were prepared from polystyrene-polylactide (PS-PLA) block copolymers that form hexagonally packed nanocylinders of PLA in a PS matrix. A morphology diagram was developed to determine the range in composition and molecular weight over which this morphology existed. Macroscopic alignment of these materials gave anisotropic monoliths that were subjected to mild degradation conditions leading to the chemical etching of the PLA. The resulting nanoporous monoliths consisted of a polystyrene matrix containing hexagonally close-packed, oriented, and continuous nanoscopic channels (pore size was tunable through synthesis or blending) lined with chemically accessible hydroxyl functional groups. Both the precursors and the porous materials were analyzed moleculary (size-exclusion chromatography and proton nuclear magnetic resonance spectroscopy) and structurally (small-angle X-ray scattering, scanning electron microscopy, and differential scanning calorimetry). In addition, the surface area and pore size distribution of the nanoporous monoliths were characterized (N2 adsorption measurements). These nanoporous materials have remarkable potential as hosts for nanomaterial synthesis, size-selective catalyst supports, and advanced separations.