(662f) Manipulating Crystal Growth and Polymorphism by Confinement In Nanoscale Crystallization Chambers

The phase behaviors of crystalline solids embedded in nanoporous matrices have been studied for decades. Classic nucleation theory conjectures that phase stability is determined by the balance between a unfavorable surface free energy and stabilizing volume free energy. The size constraint imposed by nanometer-scale pores during crystallization results in large surface-to-volume ratios, which are reflected in crystal properties – for example, melting points and enthalpies of fusion – that differ drastically from their bulk-scale counterparts. Moreover, confinement within nanoscale pores can dramatically influence crystallization pathways and crystal polymorphism, particularly when the pore dimensions are comparable to the critical size of an emerging nucleus, at which the surface and volume free energies are in delicate balance and polymorph stability rankings may differ from bulk. Recent investigations have demonstrated that confined crystallization can be used to screen for and control polymorphism, which can impact food, pharmaceutical, explosive, and dye technological sectors, for which polymorph identity is critical for regulatory compliance and function.

We review our recent studies of the polymorphic and thermotropic properties of crystalline materials embedded in the nanometer-scale pores of porous glass powders and polymer monoliths, the latter generated from selective chemical etching of one component from shear-aligned diblock or triblock samples. The embedded nanocrystals exhibit an array of phase behaviors, including the selective formation of metastable amorphous and crystalline phases, thermodynamic stabilization of normally metastable phases, size-dependent polymorphism, formation of new polymorphs, and shifts of thermotropic relationships between polymorphs. Size confinement also permits the measurement of thermotropic properties that cannot be measured in bulk form using conventional methods. The well-aligned cylindrical pores of the polymer monoliths also allow determination and manipulation of nanocrystal orientation, wherein the constraints imposed by the pore walls result in a Darwinistic competition between nuclei that favors alignment of the fastest growth direction with the pore axis. Collectively, the examples described provide substantial insight into crystallization at a size scale that is difficult to realize by other means. Moreover, the behaviors resulting from nanoscopic confinement are remarkably consistent for a wide range of compounds, suggesting a reliable approach to studying the phase behaviors of compounds at the nanoscale. Newly emerging classes of porous materials promise expanded explorations of crystal growth under confinement and new routes to controlling crystallization outcomes.

Authors B.D. Hamilton and J.-M. Ha participated in this work while at the University of Minnesota Department of Chemical Engineering and Materials Science.