Biomedical subjects

Jeong-Myeong Ha

Publications and source records attributed to Jeong-Myeong Ha.

2 recordsLinked to original sources

PubMed2005

Thermotropic properties of organic nanocrystals embedded in ultrasmall crystallization chambers.

Crystallization of organic compounds in nanometer-scale channels of controlled pore glass (CPG) and porous polystyrene (p-PS), the latter prepared by etching of the polylactide (PLA) component of shear-aligned PS-PLA diblock copolymers, produces crystals with dimensions that reflect the size constraints imposed by the channels. The nanoscopic dimensions of the organic crystals embedded in the channels result in a substantial melting point depression compared with the bulk, as demonstrated here for 2,2,3,3,4,4-hexafluoro-1,5-pentanediol (HFPD) and (R)-(+)-3-methyladipic acid (R-MAA). The melting points decreased with decreasing channel diameter, consistent with the increasing surface-area-to-volume ratio of the crystals. Furthermore, at these length scales the latent heat of melting decreased with decreasing crystal size. The melting point depression for both HFPD and R-MAA was greater in p-PS than in CPG, which can only be explained by interactions of the nanocrystals and their corresponding melts with the channel walls. Collectively, these discoveries reveal that simplified descriptions used in previous investigations of embedded crystals, which were limited to porous glass matrixes and ignored the influence of the channel walls, do not capture all the factors affecting the thermotropic properties of the embedded nanocrystals.

Journal Article↗

PubMed2004

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.

Journal Article↗