Glycine Polymorphism in Nanoscale Crystallization Chambers

Crystallization of glycine by evaporation of aqueous solutions in nanometer-scale channels of controlled-pore glass (CPG) powders and porous polystyrene-poly(dimethyl acrylamide) (p-PS-PDMA) monoliths, the latter prepared by etching polylactide (PLA) from aligned PS-PDMA-PLA triblock copolymers, preferentially results in exclusive formation of the β polymorph, which is not observed during crystallization in bulk form under identical conditions. X-ray diffraction (XRD) reveals that the dimensions of the embedded crystals are commensurate with the pore diameter of the matrix. β-Glycine persists for at least one year in CPG and p-PS-PDMA with pore diameters less than 24 nm, but it transforms slowly to α-glycine over several days when confined within 55 nm CPG. Moreover, variable temperature XRD reveals that β-glycine nanocrystals embedded within CPG are stable at temperatures at which bulk β-glycine ordinarily transforms to the α form. XRD and differential scanning calorimetry (DSC) reveal the melting of glycine nanocrystals within CPG below the temperature at which bulk glycine melts with concomitant decomposition. The melting point depression conforms to the Gibb-Thompson equation, with the melting points decreasing with decreasing pore size. This behavior permits an estimation of the melting temperature of bulk β-glycine, which cannot be measured directly owing to its metastable nature. Collectively, these results demonstrate size-dependent polymorphism for glycine and the ability to examine certain thermal properties under nanoscale confinement that cannot be obtained in bulk form. The observation of β-glycine at nanometer-scale dimensions suggests that glycine crystallization likely involves formation of β nuclei followed by their transformation to the other more stable forms as crystal size increases, in accord with Ostwald’s rule of stages.