Alexander G. Shtukenberg

Growth actuated bending and twisting of single crystals.

Crystals of a variety of substances including elements, minerals, simple salts, organic molecular crystals, and high polymers forgo long-range translational order by twisting and bending as they grow. These deviations have been observed in crystals ranging in size from nanometers to centimeters. How and why so many materials choose dramatic non-crystallographic distortions is analyzed, with an emphasis on crystal chemistries that give rise to stresses operating either on surfaces of crystallites or within the bulk.

Reversible Twisting during Helical Hippuric Acid Crystal Growth

Crystals grow in the minds eye by the addition of small units to a monolith each part of which is in fixed translational relation to every other part. Here, it is shown that growth can induce reversible twisting and untwisting of macroscopic crystals of hippuric acid (N-benzoylglycine, C(9)H(9)NO(3)) on the scale of radians. Crystals growing in undercooled melts of hippuric acid twist about the axis of elongation. At the same time the twisting is undone by new elastic stresses that build up as the crystal thickens. The dynamic interplay of twisting and untwisting ultimately fixes the crystal morphology. A correspondence between the optical properties of hippuric acid single crystals and twisted needles measured with a Mueller matrix microscope is established. The measured crystalloptical properties are in complete accord with the optical indicatrix rotating helically along the axis of elongation at the growth front, or counter-rotating so as to unwind earlier growth. The reversible morphological changes captured here in situ are likely also found in banded spherulites of high molecular weight polymers, optically modulated chalcedony minerals, elements, proteins, and other molecular crystals for which there is evidence of helical twisting. The analysis of such systems is usually predicated on the relationship of crystalline helical pitch to a power law exponent. However, in the absence of previous considerations of untwisting, the mechanistic content of such relationships is questionable.

Twisted aspirin crystals

Banded spherulites of aspirin have been crystallized from the melt in the presence of salicylic acid either generated from aspirin decomposition or added deliberately (2.6-35.9 mol %). Scanning electron microscopy, X-ray diffraction analysis, and optical polarimetry show that the spherulites are composed of helicoidal crystallites twisted along the <010> growth directions. Mueller matrix imaging reveals radial oscillations in not only linear birefringence, but also circular birefringence, whose origin is explained through slight (∼1.3°) but systematic splaying of individual lamellae in the film. Strain associated with the replacement of aspirin molecules by salicylic acid molecules in the crystal structure is computed to be large enough to work as the driving force for the twisting of crystallites.

Twisted Mannitol Crystals Establish Homologous Growth Mechanisms for High-Polymer and Small-Molecule Ring-Banded Spherulites

D-Mannitol belongs to a large and growing family of crystals with helical morphologies (Yu, L. J. Am. Chem. Soc.2003, 125, 6380). Two polymorphs of D-mannitol, α and δ, when grown in the presence of additives such as poly(vinylpyrrolidone) (PVP) or D-sorbitol, form ring-banded spherulites composed of handed helical fibrils, where the helix axes correspond to the radial growth directions. The two polymorphs form helices with opposite senses in the presence of PVP but the same sense in the presence of D-sorbitol. The characteristic dimensions of the fibrils, including thickness, aspect ratio, and pitch, were determined by scanning probe and electron microscopies. These values must form the basis of any theory that presupposes what forces give rise to crystal twisting, a problem that has been broached but unsettled in the literature of polymer crystallization. The interdependence of the rhythmic variations of both linear and circular birefringence, as determined by Mueller matrix microscopy, informs the cooperative organization of mannitol fibers. The microstructure of mannitol ring-banded spherulites compares favorably to that of high polymers and is evaluated within the context of current theories of crystal twisting.

Illusory spirals and loops in crystal growth

Significance Molecular mechanisms of crystal growth from solution remain ill-defined. Scanning probe microscopies have begun to illustrate what was before insightful theory. The in situ observations described here for hexagonal l-cystine crystals, which are known to form kidney stones, demonstrate that crystals with certain symmetries can exhibit unusual structural and growth behaviors that produce unexpected and deceptive morphological features. Such features can appear to violate a classic theory of crystal growth enshrined more than 60 y ago and could lead to incorrect conclusions about growth mechanisms. The theory of dislocation-controlled crystal growth identifies a continuous spiral step with an emergent lattice displacement on a crystal surface; a mechanistic corollary is that closely spaced, oppositely winding spirals merge to form concentric loops. In situ atomic force microscopy of step propagation on pathological l-cystine crystals did indeed show spirals and islands with step heights of one lattice displacement. We show by analysis of the rates of growth of smaller steps only one molecule high that the major morphological spirals and loops are actually consequences of the bunching of the smaller steps. The morphology of the bunched steps actually inverts the predictions of the theory: Spirals arise from pairs of dislocations, loops from single dislocations. Only through numerical simulation of the growth is it revealed how normal growth of anisotropic layers of molecules within the highly symmetrical crystals can conspire to create features in apparent violation of the classic theory.

Crystallization kinetics in binary solid solution–aqueous solution systems

Phenomenological description of crystallization kinetics in a binary system “aqueous solution–solid solution” is given. This approach involves consideration of the true equilibrium and possible quasi-equilibrium states. It gives description of the crystallization kinetics close to equilibrium and features of the growth process occurring at high driving forces and growth rates. Special attention is paid to interactions between an aqueous solution and a seed crystal placed into this solution. In particular, this includes analysis of pure chemical interactions as well as effect of misfit strain. The theory developed predicts the growth/dissolution rate of a mixed crystal based on the knowledge of phase diagram and growth kinetics of the end members of the series. The key point of the paper is discussion of supersaturation concept in such aqueous solution–solid solution systems.

Dyeing crystals since 2000

Progress in the supramolecular chemistry of dyeing crystals in the 21st century is reviewed.

Circular Birefringence of Banded Spherulites

Crystal optical properties of banded spherulites of 21 different compounds--molecular crystals, polymers, and minerals--with helically twisted fibers were analyzed with Mueller matrix polarimetry. The well-established radial oscillations in linear birefringence of many polycrystalline ensembles is accompanied by oscillations in circular birefringence that cannot be explained by the natural optical activity of corresponding compounds, some of which are centrosymmetric in the crystalline state. The circular birefringence is shown to be a consequence of misoriented, overlapping anisotropic lamellae, a kind of optical activity associated with the mesoscale stereochemistry of the refracting components. Lamellae splay as a consequence of space constraints related to simultaneous twisting of anisometric lamellae. This mechanism is supported by quantitative simulations of circular birefringence arising from crystallite twisting and splaying under confinement.

Manipulating crystallization with molecular additives

Given the importance of organic crystals in a wide range of industrial applications, the chemistry, biology, materials science, and chemical engineering communities have focused considerable attention on developing methods to control crystal structure, size, shape, and orientation. Tailored additives have been used to control crystallization to great effect, presumably by selectively binding to particular crystallographic surfaces and sites. However, substantial knowledge gaps still exist in the fundamental mechanisms that govern the formation and growth of organic crystals in both the absence and presence of additives. In this review, we highlight research discoveries that reveal the role of additives, either introduced by design or present adventitiously, on various stages of formation and growth of organic crystals, including nucleation, dislocation spiral growth mechanisms, growth inhibition, and nonclassical crystal morphologies. The insights from these investigations and others of their kind are likely to guide the development of innovative methods to manipulate crystallization for a wide range of materials and applications.

Relationship between tribology and optics in thin films of mechanically oriented nanocrystals.

Many crystalline dyes, when rubbed unidirectionally with cotton on glass slides, can be organized as thin films of highly aligned nanocrystals. Commonly, the linear birefringence and linear dichroism of these films resemble the optical properties of single crystals, indicating precisely oriented particles. Of 186 colored compounds, 122 showed sharp extinction and 50 were distinctly linearly dichroic. Of the latter 50 compounds, 88% were more optically dense when linearly polarized light was aligned with the rubbing axis. The mechanical properties of crystals that underlie the nonstatistical correlation between tribological processes and the direction of electron oscillations in absorption bands are discussed. The features that give rise to the orientation of dye crystallites naturally extend to colorless molecular crystals.