Manipulating Crystal Growth and Polymorphism by Confinement in Nanoscale Crystallization Chambers. Hamilton, B., D., Ha, J., Hillmyer, M., A., & Ward, M., D. ACCOUNTS OF CHEMICAL RESEARCH, 45(3):414-423, AMER CHEMICAL SOC, 3, 2012.
abstract   bibtex   
The phase behaviors of crystalline solids embedded within nanoporous matrices have been studied for decades. Classic nucleation theory conjectures that phase stability is determined by the balance between an unfavorable surface free energy and a stabilizing volume free energy. The size constraint imposed by nanometer-scale pores during crystallization results in large ratios of surface area to volume, which are reflected in crystal properties. For example, melting points and enthalpies of fusion of nanoscale crystals can 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 this tipping point, 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. In the food, pharmaceutical, explosive, and dye technological sectors, this understanding and control over polymorphism is critical both for function and for regulatory compliance. This Account reviews recent studies of the polymorphic and thermotropic properties of crystalline materials embedded in the nanometer-scale pores of porous glass powders and porous block-polymer-derived plastic monoliths. 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 materials using conventional methods. Well-aligned cylindrical pores of the polymer monoliths also allow determination and manipulation of nanocrystal orientation. In these systems, the constraints imposed by the pore walls result In a competition between crystal nuclei that favors those with the fastest growth direction aligned with the pore axis. Collectively, the examples described in this Account 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.
@article{
 title = {Manipulating Crystal Growth and Polymorphism by Confinement in Nanoscale Crystallization Chambers},
 type = {article},
 year = {2012},
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 pages = {414-423},
 volume = {45},
 month = {3},
 publisher = {AMER CHEMICAL SOC},
 city = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA},
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 abstract = {The phase behaviors of crystalline solids embedded within nanoporous
matrices have been studied for decades. Classic nucleation theory
conjectures that phase stability is determined by the balance between an
unfavorable surface free energy and a stabilizing volume free energy.
The size constraint imposed by nanometer-scale pores during
crystallization results in large ratios of surface area to volume, which
are reflected in crystal properties. For example, melting points and
enthalpies of fusion of nanoscale crystals can 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 this tipping point, 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. In the food, pharmaceutical, explosive, and dye
technological sectors, this understanding and control over polymorphism
is critical both for function and for regulatory compliance.
This Account reviews recent studies of the polymorphic and thermotropic
properties of crystalline materials embedded in the nanometer-scale
pores of porous glass powders and porous block-polymer-derived plastic
monoliths. 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 materials using conventional methods.
Well-aligned cylindrical pores of the polymer monoliths also allow
determination and manipulation of nanocrystal orientation. In these
systems, the constraints imposed by the pore walls result In a
competition between crystal nuclei that favors those with the fastest
growth direction aligned with the pore axis.
Collectively, the examples described in this Account 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.},
 bibtype = {article},
 author = {Hamilton, Benjamin D and Ha, Jeong-Myeong and Hillmyer, Marc A and Ward, Michael D},
 journal = {ACCOUNTS OF CHEMICAL RESEARCH},
 number = {3}
}

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