Crystallization near glass transition: transition from diffusion-controlled to diffusionless crystal growth studied with seven polymorphs

A remarkable property of certain glass-forming liquids is that a fast mode of crystal growth is activated near the glass transition temperature Tg and continues in the glassy state. This growth mode, termed GC (glass-crystal), is so fast that it is not limited by molecular diffusion in the bulk liquid. We have studied the GC mode by growing seven polymorphs from the liquid of ROY, currently the top system for the number of coexisting polymorphs of known structures. Some polymorphs did not show GC growth, while others did, with the latter having higher density and more isotropic molecular packing. The polymorphs not showing GC growth grew as compact spherulites at all temperatures; their growth rates near Tg decreased smoothly with falling temperature. The polymorphs showing GC growth changed growth morphologies with temperature, from faceted single crystals near the melting points, to fiber-like crystals near Tg, and to compact spherulites in the GC mode; in the GC mode, they grew at rates 3-4 orders of magnitude faster with activation energies 2-fold smaller than the polymorphs not showing GC growth. The GC mode had rates and activation energies similar to those of a polymorphic transformation observed near Tg. The GC mode was disrupted by the onset of the liquid's structural relaxation but could persist well above Tg (up to 1.15 Tg) in the form of fast-growing fibers. We consider various explanations for the GC mode and suggest that it is solid-state transformation enabled by local molecular motions native to the glassy state and disrupted by the liquid's structural relaxation (the alpha process).     

The crystal polymorphs of metazachlor

Five crystal polymorphs of the herbicide metazachlor (MTZC) were characterized by means of hot stage microscopy, differential scanning calorimetry, IR- and Raman spectroscopy as well as X-ray powder diffractometry. Modification (mod.) I, II and III° can be crystallized from solvents and the melt, respectively, whereas the unstable mod. IV and V crystallize exclusively from the super-cooled melt. Based on the results of thermal analysis and solvent mediated transformation studies, the thermodynamic relationships among the polymorphic phases of metazachlor were evaluated and displayed in a semi-schematic energy/temperature-diagram. At room temperature, mod. III° (T _fus =76°C, Δ_fus H _III =26.6 kJ mol^-1) is the thermodynamically stable form, followed by mod. II (T _fus =80°C, Δ_fus H _II =23.0 kJ mol^-1) and mod. I (T _fus =83°C, Δ_fus H _II=19.7 kJ mol^-1). These forms are enantiotropically related showing thermodynamic transition points at ~55°C (T _{trs, III/II}), ~60°C (T _{trs, III/I}) and ~63°C (T _{trs, II/I}). Thus mod. I is the thermodynamically stable form above 63°C, mod. III° below 55°C and mod. II in a small window between these temperatures. Mod. IV (T _fus =72-74°C, Δ_fus H _II =18.7 kJ mol^-1) and mod. V (T _fus =65°C) are monotropically related to each other as well as to all other forms. The metastable mod. I and II show a high kinetic stability. They crystallize from solvents, and thus these forms can be present in commercial samples. Since metazachlor is used as an aqueous suspension, the use of the metastable forms is not advisable because of a potential transformation to mod. III°. This may result in problematic formulations, due to caking and aggregation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Decomposition kinetics of the AlH3 polymorphs

Aluminum hydride polymorphs (alpha-AlH3, beta-AlH3, and gamma-AlH3) were prepared by organometallic synthesis. Hydrogen capacities approaching 10 wt % at desorption temperatures less than 100 degrees C have been demonstrated with freshly prepared AlH3. The temperature-dependent rate constants were determined by measuring the isothermal hydrogen evolution between 60 degrees C and 140 degrees C. Fractional decomposition curves showed good fits using both the second and third-order Avrami-Erofeyev equations, indicating that the decomposition kinetics are controlled by nucleation and growth of the aluminum phase in two and three dimensions. The large activation energies measured for the AlH3 polymorphs suggest that the decomposition occurs via an activated complex mechanism with complexes consisting of approximately nine AlH3 molecules (1-2 unit cells for alpha-AlH3).     

Deformation and failure kinetics of iPP polymorphs

In this study, the mechanical performance of the different polymorphs of isotactic polypropylene, typically present in iPP crystallized under industrial processing conditions, is assessed. Different preparation strategies were used to obtain samples consisting of almost solely α, β, or γ crystals. X‐Ray measurements were used to validate that the desired phase was obtained. The intrinsic true stress ‐ true strain response of all individual phases was measured in uniaxial compression at several strain rates (deformation kinetics). Moreover, measurements were performed over a wide temperature range, covering the window in between the glass transition and the melting temperature. The relation between obtained yield stress and the strain rate is described with a modification of the Ree‐Eyring model. Differences and similarities in the deformation kinetics of the different phases are presented and discussed. Furthermore, the presence of three deformation processes, acting in parallel, is revealed. The Ree‐Eyring equation enables lifetime prediction for given thermal and mechanical conditions. These predictions were experimentally validated using constant load tests in uniaxial compression. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 729–747     

From nonpolar to ferroelectric crystal structure: the temperature-tuned growth of two guanidinium ethoxysulfonate polymorphs

Guanidinium ethoxysulfonate, [C(NH2)3]+[C2H5O-SO3]-, was synthesized, and two polymorphs, both stable at normal conditions, were grown from an aqueous solution by only a slight change in the crystallization temperature. The nonpolar polymorph I is built of hydrogen-bonded bilayers, while the ferroelectric polymorph II consists of single-layers. The diversity in the crystals' architecture and properties originates from the excessive number of proton-acceptor sites. At 298 K, the structure of polymorph I is orthorhombic, space group Pbam, formed of supramolecular hydrogen-bonded sheets. Within such a sheet, the ethoxysulfonate anions assume alternately cis and trans conformations, both disordered at room temperature and at 150 K. The anisotropy of the crystal structure is mirrored by a strong anisotropy of its thermal expansion. Upon cooling at 120 K, the crystal undergoes a first-order order-disorder phase transition. The structure of polymorph II is also reinforced by the two-dimensional network of NH...O hydrogen bonds, but the supramolecular motif formed is different from that of polymorph I. The H-bonded strongly corrugated sheets are stacked, forming a densely packed single-layer structure. All the anions assume the same trans conformation. At 298 K, they are disordered between the two sites related by the mirror symmetry plane. The onset of ordering of the anions coincides with the Curie point at TC = 211 K, at which the dielectric constant exceeds 4000. The continuous paraelectric-ferroelectric phase transition is associated with the symmetry change Pnma --> Pna21. Despite the apparent order-disorder character of the transition, the transition mechanism also involves a substantial displacement of the ions and a rearrangement of the H-bonded network.     

Two crystal polymorphs of a flavonoid from Melicope ellyrana

The crystal structure determinations of two crystalline components of the hexane extract of the fruit of the indigenous Australian tree Melicope ellyrana have shown them to be polymorphs of the same compound, namely the flavonoid 4′,5-di­hydroxy-3,3′,8-tri­methoxy-7-(3-methyl­but-2-enyl­oxy)­flavone [systematic name: 5-hydroxy-2-(4-hydroxy-3-methoxy­phenyl)-3,8-di­methoxy-7-(3-methyl­but-2-enyl­oxy)-4H-1-benzo­pyran-4-one], C₂₃H₂₄O₈. The two polymorphs, one monoclinic (polymorph A) and the other triclinic (polymorph B), show significant conformational differences, particularly in the enyl­oxy side chain, while only one (polymorph A) shows intermolecular hydrogen bonding.     

Transitions among five polymorphs of chlorpropamide near the melting point

Five polymorphs of chlorpropamide (α, β, δ, γ, and ε) were investigated near the melting point by using DSC. Structure of samples was tested by X-ray powder diffraction. Four first polymorphs were found to transform into ε-polymorph, which melts at T _m=128°C, Δ_m H=24 kJ mol⁻¹. Enthalpy of the polymorph transitions ranges from +3 kJ mol⁻¹ for α→ε to −0.8 kJ mol⁻¹ for β→ε. Structure of three first polymorphs was published elsewhere, and the structure of δ-polymorph is published for the first time. XRPD patterns for all polymorphs are reported, together with the atomic coordinates for the δ-polymorph.     

Isoconfigurational molecular dynamics study of the kinetics of ice crystal growth

Spontaneous self-assembling, such as formation of molecular crystals, is a fascinating topic for investigation. Ability to initiate and control such transformations promises numerous benefits, but our knowledge of underlying mechanisms of such processes is rather limited. The process of freezing of water is an excellent testing ground for such studies. In this paper we report the results of a systematic molecular dynamics study of ice growth at three different temperatures below the melting point initiated from a number of initial interface structures within the isoconfigurational ensemble. It is shown that a specific structure at a growing ice-water interface is able to affect the growth process over a time scale of 1-2 ns. This structural effect can be characterized in terms of relative growth propensities. On the basis of the differences in the shape between isoconfigurational rate distributions and the rate distribution typical of the specific temperature several different kinds of relative growth propensities have been identified. The initial interfacial configurations employed in this work have been assigned using the proposed classification and possible mechanisms of propensity realization have been suggested for selected cases. Results reported in this paper clearly indicate that local structure effects can have significant impact on tendency for a particular ice surface to grow (or melt). The structural effect on ordering propensities is, most probably, a more universal behaviour and might be expected to be seen in other similar problems such as, for example, protein folding.     

The kinetics of crystal growth and dissolution of strontium oxalate monohydrate

The crystal growth and dissolution of strontium oxalate monohydrate have been studied at 25°C by following the changes in electrical conductivity of supersaturated and subsaturated aqueous solutions seeded with aged crystals of the monohydrate salt. Both processes follow a kinetic equation in which the rate of reaction is proportional to the concentration of the lattice ions. The critical supersaturation or threshold level for homogeneous nucleation and growth of strontium oxalate is considerably lower than that for many other sparingly soluble salts. Sodium pyrophosphate shows a strong inhibiting effect on the rate of crystal growth whereas its influence on the dissolution process is negligible. The thermodynamic association constant for the formation of strontium monooxalate complexes in solution has been determined conductimetrically; the value is 510 l mole⁻¹.     

Tailoring crystal polymorphs of organic semiconductors towards high-performance field-effect transistors

As a quite ubiquitous phenomenon, crystal polymorph is one of the key issues in the field of organic semiconductors. This review gives a brief summary to the advances on polymorph control of thin film and single crystal of representative organic semiconductors towards high-performance field-effect transistors. Particularly, the relationship between crystal polymporh and charge transport behaviour has been discussed to shed light on the rational preparation of outstanding organic semiconducting materials with desired crystal polymorph.     

Selective nucleation and discovery of organic polymorphs through epitaxy with single crystal substrates.

Crystallization of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (1), previously found to produce six conformational polymorphs from solution, on single-crystal pimelic acid (PA) substrates results in selective and oriented growth of the metastable "YN" (yellow needle) polymorph on the (101)(PA) faces of the substrate. Though the freshly cleaved substrate crystals expose (101)(PA) and (111)(PA) faces, which are both decorated with [101](PA) ledges that could serve as nucleation sites, crystal growth of YN occurs on only (101)(PA). Goniometry measurements performed with an atomic force microscope reveal that the (001)(YN) plane contacts (101)(PA) with a crystal orientation [100](YN)//[010](PA) and [010](YN)//[101](PA). A geometric lattice analysis using a newly developed program dubbed GRACE (geometric real-space analysis of crystal epitaxy) indicates that this interfacial configuration arises from optimal two-dimensional epitaxy and that among the six polymorphs of 1, only the YN polymorph, in the observed orientation, achieves reasonable epitaxial match to (101)(PA). The geometric analysis also reveals that none of the polymorphs, including YN, can achieve comparable epitaxial match with (111)(PA), consistent with the absence of nucleation on this crystal face. In contrast, sublimation of 1 on cleaved succinic acid (SA) substrates, which expose large (010)(SA) faces decorated with steps along [101](SA), affords growth of several polymorphs, each with multiple orientations, as well as oriented crystals of a new metastable polymorph on the (010)(SA) surfaces. The lack of polymorphic selectivity on (010)(SA) can be explained by the geometric lattice analysis, which reveals low-grade epitaxial matches between (010)(SA) and several polymorphs of 1 but no inherent selectivity toward a single polymorph. These observations demonstrate the sensitivity of crystal nucleation to substrate surface structure, the potential of crystalline substrates for selective nucleation and discovery of polymorphs, and the utility of geometric lattice modeling for screening of substrate libraries for controlling polymorphism.     

Crystallization kinetics of thermosensitive colloids probed by transmission spectroscopy

The kinetics of crystallization of poly-N-isopropylacrylamide (PNIPAM) particles has been investigated using the UV-visible transmission spectroscopy. Since the particle size decreases with the increase in temperature, microgel dispersions of different volume fractions have been obtained by varying the temperature of a single sample. It is found that the rates of the change in crystallinity, the average crystallite size, and the number density of crystallites at the most rapid stage over a certain time interval at various temperatures can be described by the power-law relations. At 19 degrees C, the PNIPAM system behaves as a hard sphere system under microgravity. The hard sphere theory based on Monte Carlo simulation has been used as a reference point to compare with conventional hard spheres, soft spheres, and PNIPAM spheres.     

The kinetics of homogeneous and two-step nucleation during protein crystal growth from solution

Many experimental reports for the kinetics of crystal nucleation and growth, from an isothermal solution, point to a sigmoidal-like behavior for the process. Here we consider three different nucleation models from the literature and show that all lead to sigmoidal or sigmoidal-like behavior for the kinetics of nucleation. A two-step nucleation process is known to occur within certain supersaturated protein solutions, and it is demonstrated in this report how the sigmoidal law yields kinetic information for the two-step and homogeneous nucleation modes. We propose here that two-step solute-rich associates form in the solution around seed nuclei that are already present at or near the point in time when the solution is prepared. Using this hypothesis, we are able to model the time-dependent volume of the two-step phase per unit volume of solution and show that this compares well with reported experimental data. A kinetic model is given for the proposed process, which leads to a sigmoidal rate law. Additionally, a relation between the initial and final nuclei densities and the induction time is derived. As a result of this study, the conclusion is that two-step activity increases with increasing initial supersaturation or increasing salt concentration.     

Crystallization kinetics of linear polyethylene: The maximum in crystal growth rate-temperature dependence

This rapid communication reports a summary of the key findings of crystallization kinetics studies of unfractionated high density (linear) polyethylene at extremely large supercoolings. We report, for the first time, the maximum in crystal growth rate-crystallization temperature data for linear polyethylene, which has been sought by many researchers since the 1950s. The maximum growth rate was found to occur in the range of 70-75 °C with two separate methods. The kinetics studies were performed using a newly developed quench-crystallization technique based on depolarized reflection light microscopy that is capable of achieving enormously higher quench rates than existing methods. Typical onset crystallization temperatures accessed with this technique range from 40 to 90 °C. Bulk growth rates of crystals were obtained as the reciprocal of crystallization half times measured from the change in the depolarized light intensity upon direct crystallization from the melt. Separately, radial growth rates of spherulites were measured over a wide range of supercoolings. Secondary nucleation analysis of the crystal growth rates resulted in single linear fits extending into deep regime III, suggesting no change in mechanism of formation of the crystals at the largest supercoolings. The deeply quenched films, crystallized at temperatures below the maximum, contain non-impinged spherulites, capable of further crystallization.     

Effect of self-seed crystal structure on growth of polymorphs in poly(butylene 2,6-naphthalate): A cross-nucleation study

Cross-nucleation between different crystal polymorphs is a particular, self-seed assisted type of heterogeneous nucleation, where a fast-growing polymorph nucleates at a pre-existing crystal surface of another polymorph. Here, we present a study on cross-nucleation between different crystalline phases of poly(butylene 2,6-naphthalate) (PBN), employing hotstage polarized-light optical microscopy and temperature-resolved wide-angle X-ray scattering as analysis tools. PBN forms α-crystals at relatively low temperature and β′-crystals at rather high temperature, with cross-nucleation experiments designed such to first obtain few α- or β′-seed crystals (mother phase) which then are transferred to higher or lower temperature, respectively, to monitor the continuation of the crystallization process and possible growth of the other polymorph. In case of cooling β′-crystals to lower temperature where typically α-crystals form in the non-seeded isotropic melt, β′-crystals nucleate growth of α-crystals, following many examples of cross-nucleation in the literature. In contrast, if low-temperature-generated α-crystals are heated to a temperature where β′-crystals form in a non-seeded melt, the cross-nucleation efficacy is reduced as, beside growth of cross-nucleated β′-crystals, also growth of the mother phase is observed. This unexpected result demonstrates the importance of the structure of the nucleating substrate and the interplay between kinetic and thermodynamic aspects of crystal growth.     

Surfactant-doped reverse-mode polymer-dispersed liquid crystal display with enhanced properties

In this article, a nonionic surfactant Brij30 has been used to vertical align the negative anisotropic liquid crystal. The surfactant is further used in reverse-mode polymer-dispersed liquid crystal display to enhance the electro-optical properties. This work shows an enhancement of contrast ratio simply with the addition of surfactant. In addition, contact angle and surface free energies have been determined to judge the role of surfactant in control of surface orientation of liquid crystal molecules.     

Hexagonal single crystal growth of WO3 nanorods along a [110] axis with enhanced adsorption capacity

Hexagonal single crystal WO(3) nanorods with dominant (001) and (1 ?10) facets were synthesized, which exhibited high adsorption capacities for organic dyes.     

Label-free imaging of cell attachment with photonic crystal enhanced microscopy

We introduce photonic crystal enhanced microscopy (PCEM) as a label-free biosensor imaging technique capable of measuring cell surface attachment and attachment modulation. The approach uses a photonic crystal optical resonator surface incorporated into conventional microplate wells and a microscope-based detection instrument that measures shifts in the resonant coupling conditions caused by localized changes in dielectric permittivity at the cell-sensor interface. Four model systems are demonstrated for studying cancer cells, primary cardiac muscle cells, and stem cells. First, HepG2/C3 hepatic carcinoma cells were cultured and observed via PCEM in order to characterize cell adhesion in the context of growth and locomotion. Second, Panc-1 pancreatic cancer cells were used to verify that cell attachment density decreases in response to staurosporine, a drug that induces apoptosis. Third, we used PCEM to confirm the influence of integrin-mediated signaling on primary neonatal cardiomyocyte growth and development. Rounded cardiomyocytes consistently showed decreased cell attachment density as recorded via PCEM, while spreading cells exhibited greater attachment strength as well as increased contractility. Finally, PCEM was used to monitor the morphological changes and extracellular matrix remodeling of porcine adipose-derived stem cells subjected to a forced differentiation protocol. Each of these experiments yielded information regarding cell attachment density without the use of potentially cytotoxic labels, enabling study of the same cells for up to several days.     

Origin of the nonadhesive properties of fibrinogen matrices probed by force spectroscopy

The deposition of a multilayered fibrinogen matrix on various surfaces results in a dramatic reduction of integrin-mediated cell adhesion and outside-in signaling in platelets and leukocytes. The conversion of a highly adhesive, low-density fibrinogen substrate to the nonadhesive high-density fibrinogen matrix occurs within a very narrow range of fibrinogen coating concentrations. The molecular events responsible for this transition are not well understood. Herein, single-cell and molecular force spectroscopy were used to determine the early steps in the formation of nonadhesive fibrinogen substrates. We show that the adsorption of fibrinogen in the form of a molecular bilayer coincides with a several-fold reduction in the adhesion forces generated between the AFM tip and the substrate as well as between a cell and the substrate. The subsequent deposition of new layers at higher coating concentrations of fibrinogen results in a small additional decrease in adhesion forces. The poorly adhesive fibrinogen bilayer is more extensible under an applied tensile force than is the surface-bound fibrinogen monolayer. Following chemical cross-linking, the stabilized bilayer displays the mechanical and adhesive properties characteristic of a more adhesive fibrinogen monolayer. We propose that a greater compliance of the bi- and multilayer fibrinogen matrices has its origin in the interaction between the molecules forming the adjacent layers. Understanding the mechanical properties of nonadhesive fibrinogen matrices should be of importance in the therapeutic control of pathological thrombosis and in biomaterials science.