Browsing by Subject "Polymorphism"

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    The influence of nanoscale size confinement on the phase behavior of molecular organic crystals.
    (2009-06) Hamilton, Benjamin Dale
    This thesis details the evolution of the crystallization of molecular organic compounds under nanoconfinement. Within the confines of nanoporous matrices, crystals are limited to sizes comparable to their critical sizes, where their unfavorable surface energy outweights their favorable volume energy. The central contribution of this thesis is the crystallization of glycine within nanoporous matrices. Namely, 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 beta 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. Beta 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 alpha glycine over several days when confined within 55 nm CPG. Moreover, variable temperature XRD reveals that beta glycine nanocrystals embedded within CPG are stable at temperatures at which bulk beta glycine ordinarily transforms to the alpha form in the bulk. 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 beta 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 beta glycine at nanometer-scale dimensions suggests that glycine crystallization likely involves formation of beta nuclei followed by their transformation to the other more stable forms as crystal size increases, in accord with Ostwald's rule of stages. When embedded in p-PS-PDMA, the nanocrystals also adopt preferred orientations with their fast-growth axes aligned parallel with the pore direction. When grown from aqueous solutions alone, the nanocrystals were oriented with their [010] and [0-10] axes, the native fast growth directions of the (+) and (-) enantiomorphs of beta glycine, respectively, aligned parallel with the pore direction. In contrast, crystallization in the presence of racemic mixtures of chiral auxiliaries known to inhibit growth along the [010] and [0-10] directions of the enantiomorphs produced beta glycine nanocrystals with their <001> axes nearly parallel to the pore direction. Enantiopure auxiliaries that inhibit crystallization along the native fast growth direction of only one of the enantiomorphs allow the other enantiomorph to grow with the <010> axis parallel to the cylinder. Collectively, these results demonstrate that crystal growth occurs such that the fast-growing direction, which can be altered by adding chiral auxiliaries, is parallel to the pore direction. This behavior can be attributed to a competition between differently aligned crystals due to critical size effects, the minimization of the surface energy of specific crystal planes, and a more effective reduction of the excess free energy associated with supersaturated conditions when the crystal grows with its fast-growth axis unimpeded by pore walls. These observations suggest that the beta glycine nanocrystals form by homogeneous nucleation, with minimal influence of the pore walls on orientation. Collectively, these results suggest new routes for controlling crystallization outcomes and new studies on the exploration of crystal properties on the nanometer length scale.
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    Minnesota's Red-tailed Hawks: Probabilistic Origins of B.j. abieticola and Dark-morph Migrants
    (2022-08) Pesano, Alexandra
    The Red-tailed Hawk (Buteo jamaicensis) is one of North and Central America’s most common, polymorphic raptor species, with an extensive geographic range divided into 12 putative subspecies ranges. In North America, plumage polymorphism occurs along a clinal gradient with dark-morph individuals becoming less prevalent east of the Rocky Mountains. Polymorphism and other plumage traits can be used to identify individuals to a subspecies, but high levels of intergradation and individual variation can complicate identification. Duluth, Minnesota, USA is a migratory hotspot well known for phenotypically diverse Red-tailed Hawks, including B.j. abieticola and dark-morphs plumages. Due to atypical plumage traits of B.j. abieticola and dark-morphs, subspecific origins of Minnesota’s migratory individuals are not always resolved. Genetic data was collected from Duluth’s migratory Red-tailed Hawk population and known subspecies populations, B.j. calurus and B.j. borealis, to determine the probabilistic subspecific origins of B.j. abieticola and dark-morph migrants. Twelve microsatellite markers were used to analyze and compare genetic diversity and population structure within and among breeding populations and the migratory individuals. Bayesian statistics were also performed to determine probabilistic assignments of migratory individuals to putative subspecies. Supplemental spatial data was also collected from two presumed adult dark-morph B.j. abieticola. Pairwise FST revealed the B.j. abieticola and dark-morph migrants were both more genetically similar to B.j. borealis than B.j. calurus. Population assignment probabilities supported that these migratory individuals were more closely related to B.j. borealis than B.j. calurus. Furthermore, preliminary satellite transmitter data from one presumed adult dark-morph B.j. abieticola migrant revealed the individual spent at least one summer east of the Rocky Mountains. These findings suggest Minnesota’s B.j. abieticola and dark-morph migrants have a higher probability of originating from B.j. borealis, a Red-tailed Hawk subspecies historically known to only present light-morph plumage, than B.j. calurus.
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    Recognition and assembly at multiple length-scales.
    (2010-05) Olmsted, Brian Keith
    Many molecular materials capable of crystallizing into an ordered solid state may assume multiple packing arrangements. This behavior is called polymorphism and is common among organic molecules such as pharmaceuticals and dyes. Controlling the nucleation of specific polymorphic crystals is not well understood, but is tantamount to the development and manufacture of new industrial products. One phenomena that has been observed to influence crystal orientation, growth rate, and morphology is epitaxy. Epitaxy refers to a condition by which a crystalline substrate presents a similar two-dimensional lattice to a crystalline plane of a nucleating species, resulting in a condition that lowers the energy barrier to nucleation and results in a preferential orientation of crystal growth on the substrate. Therefore, epitaxial nucleation may provide routes to selectively nucleate polymorphs and attain control over otherwise unpredictable crystallization events. The literature provides several examples of epitaxial relationships between a substrate and a crystal overlayer in fields involving inorganic crystals as well as organic crystals, and because epitaxy relies on geometric comparisons between lattice parameters, computational prediction of epitaxy is an active area of research. Our laboratories have developed software; named GRACE, to attempt to predict epitaxial relationships and this software has been used to verify epitaxy reported in the literature. One particularly useful feature of GRACE is its ability to handle a library of substrates and screen them against a corresponding database of crystal structures available as candidate crystal overlayers. In this capacity GRACE allows large libraries of substrates and crystals to be reduced to an experimentally manageable size, whereby combinatorial crystallizations can be tested for selective nucleation arising from epitaxial interfaces. This research also focuses on other aspects of nucleation that are not yet fully understood. Epitaxial interfaces are by definition, abrupt. However, a specialized class of crystals involving a domain that completely overgrows a core crystal by epitaxial mechanisms has revealed a zone of intermixing spanning close to a micron. In situ Atomic Force Microscopy (AFM) reveals the mechanisms for these observations and provides insight into how epitaxial interfaces behave mechanistically. Notably, it was revealed that process conditions between phases of growth in the formation of core-shroud heterocrystals may yield controllable interfacial thicknesses between crystalline domains, It was also discovered that the propensity for abrupt, epitaxial interfaces may be limited by the thermodynamic behavior of specific crystal interfaces under conditions of near-equilibrium. Although the use of in situ AFM is excellent for the study of crystal growth, the mass-transfer limitations at crystallizing interfaces inside an (AFM) fluid cell are not directly discernable and the assumption is typically made that conditions in the bulk solution are the same inside the cell. By implementing computational fluid dynamic (CFD) simulations for flow and mass transport, in situ AFM was studied to determine how the different conditions at the crystal surface are in comparison to the bulk solution outside the cell. The geometry of the internal volume of the AFM fluid cell imparts specific fluid flow and mass transport limitations on the environment directly at the area of investigation for crystal growth and in some cases may have significant ramifications for the appropriate correlation of bulk solution variables to crystal growth variables. The results of the CFD calculations indicate that differences are significant, though usually minor and these results may prove useful for future fluid cell design. Finally, photolithographic techniques were employed to produce millions of micron-sized particles with shapes mimicking molecular contours and other crystallographically significant contours to study how symmetry and packing originates at the micron length-scale. Although much is known about assembly at the molecular level for symmetry and packing, the assembly of anisotropic particles at longer length scales, which involve different interactive forces, has not been studied. This work concludes by performing preliminary work in elucidating the general behavior towards symmetry and packing in two-dimensions of micron-sized particles by using gravitational gradients and dielectrophoresis.