Crystal Polymorphism Research Articles

Phase Transformation of a Dual Phase Al–Fe Alloy Prepared by Mechanical Alloying

Abstract A metastable Al90Fe10 (at.%) alloy, composed of a supersaturated solid solution of face-centred cubic (fcc) Al(Fe) and an amorphous phase matrix, was prepared by mechanical alloying of a mixture of Al and Fe elemental blends. The thermally induced microstructural evolution in the dual phase alloy was characterized by X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, and magnetothermal analysis. On continuous heating two stages of solid state phase transformation occurred: (i) a polymorphous crystallization of the amorphous phase to a metastable crystalline Al6Fe that structurally stabilizes over a temperature range of about 200 K, and (ii) a eutectoid decomposition of the crystalline Al6Fe into the equilibrium phases of Al13Fe4 and fcc Al(Fe). Thermodynamic and kinetic analyses of the observed phase transformation processes were given. The formation of the dual phase alloy in the as-milled state and the phase transitions can be illustrated by a hypothetical free energy diagram, implying the dual phase structure facilitates the polymorphous crystallization of Al6Fe.

Isotropic liquid state of triacylglycerols

Over the years, several models of triacylglycerol (TAG) molecular conformations and bulk arrangements in isotropic liquid state have been proposed and are still up for debate. Such organization has a major impact on nucleation and crystal formation. The current research aims to further explore this debate by implementing experimental methods, such as X-ray diffraction and small-angle X-ray scattering coupled with a computational method, such as molecular dynamic simulation. These techniques were used to study tristearin and triolein as models for saturated and unsaturated TAGs, respectively. Four different conformations were suggested for the two TAGs, and the results showed conformation abundancy in the order: trident (Tr) > chair (Ch) > propeller (Pr) > tuning-fork (Tf). The existence of clusters was demonstrated for both TAGs, each of which exhibited a heterogeneous distribution of conformations. The preferability to find a specific pair of conformations next to each other was analyzed and, surprisingly, it was found that Tf will preferably pair only with Tr although Tf is the preferable conformation in most crystal polymorphs. High general conversion rates from any conformation to another, and high specific conversion rates from and to the Tf conformation were calculated. It is proposed that the high conversion rates observed enable the crystallization process, despite the low proportion of Tf molecules, which are in the suitable conformation for the crystal state. The conversion rates in triolein were lower compared with tristearin. This observation was ascribed to the lower simulated temperatures and the higher rigidity of the fatty acid chain induced by the double bond. Overall, the results confirm the formation of specific structures in liquid state, which combine all previously suggested models and further expand the knowledge using experimental and computational tools.

Exploring the role of crystal habit in the Ostwald rule of stages.

The crystallization of calcium carbonate is shown to be dictated by the Ostwald rule of stages (ORS), for high relative initial supersaturations (), under sweet (carbon dioxide saturated) and anoxic (oxygen depleted) solution conditions. Rhombohedral calcite crystals emerge after the sequential crystallization and dissolution of the metastable polymorphs: vaterite (snowflake-shaped) and aragonite (needle-shaped). However, the presence of certain cations, which can form trigonal carbonates (e.g. Fe2+ and Ni2+), in concentrations as low as 1.5 mM, triggers the emergence of calcite crystals, with a star-shaped crystal habit, first. These star-shaped crystals dissolve to yield needle-shaped aragonite crystals, which in turn dissolve to give the rhombohedral calcite crystals. The star-shaped crystals, formed at high SCaCO3, possess higher surface free energy (therefore higher apparent solubility) than their rhombohedral counterparts. This sequence of dissolution and recrystallization demonstrates that the ORS does not only drive the crystal towards its thermodynamically most stable polymorph but also towards its most stable crystal habit.

Physical Properties and Polymorphism of Acrylic Acid-Grafted Poly(1,4-butylene adipate-co-terephthalate)/Organically Modified Layered Double Hydroxide Nanocomposites.

Novel and biodegradable acrylic acid-grafted poly(1,4-butylene adipate-co-terephthalate)/organically modified layered double hydroxide (g-PBAT/m-LDH) nanocomposites were synthesized through the polycondensation and transesterification process, with the covalent linkages between the polymer and the inorganic materials. X-ray diffraction and transmission electron microscopy were used to characterize the structure and morphology of the g-PBAT/m-LDH nanocomposites. The experimental results show that the m-LDH was exfoliated and widely distributed in the g-PBAT matrix. The addition of m-LDH into the g-PBAT extensively improved the storage modulus at −90 °C, when compared to that of the pure g-PBAT matrix. The effects of the minor comonomer of the butylene terephthalate (BT) unit and the addition of m-LDH on the crystallization behavior, and the polymorphic crystals of the g-PBAT at numerous crystallization temperatures, were examined, using a differential scanning calorimeter (DSC). The data indicate that the minor comonomer of the BT unit into g-PBAT can significantly change the starting formation temperatures of the α-form and ꞵ-form crystals, while a change in the starting formation temperatures of the α-form and ꞵ-form crystals using the addition of m-LDH into g-PBAT is not evident.

Retarded Crystallization and Promoted Phase Transition of Freeze-Dried Polybutene-1: Direct Evidence for the Critical Role of Chain Entanglement

Polymorphism and crystal transition are common phenomena of semicrystalline polymers. These two behaviors are known to be controlled by the nucleation and chain mobility of polymers, both of which are constrained by the chain entanglement at the molecular level. However, the role of chain entanglement in polymorphic crystallization and crystal phase transition of polymers has not been well understood. Herein, we use isotactic polybutene-1 (PB-1) as a model polymorphic polymer and present the crucial role of chain entanglement in the polymorphic crystallization kinetics and solid-solid phase transition. A series of less-entangled PB-1 with different entanglement degrees were successfully prepared by freeze-drying the polymer dilute solution. Compared to the bulk sample and re-entangled one, chain disentangling of PB-1 suppressed the crystallization kinetics of form II but significantly increased the phase transition rate and final transition degree from form II to form I. The disentangling-promoted II-I phase transition originated from the reduced nucleation barrier and enhanced chain mobility. This work would advance the in-depth understanding on the formation and transition mechanisms of polymorphic polymer crystals at the molecular level.

Conformational Selectivity and Evolution Affected by the Desolvation Process

The inconsistency between solution chemistry and the crystal structure indicates the potential conformational evolution and rearrangement during the nucleation process. However, the mechanism of how the conformation is selected to form the corresponding polymorph is still not clear due to the complicated crystallization environment. In this work, conformation and polymorph selectivity of three anhydrous forms of gabapentin were investigated. Quantum chemical computation was used to analyze gabapentin conformations in the solution state and explore their transformation pathway. Besides, visual molecular dynamics was adopted to calculate the structural difference of the conformers in solution and crystal states, which was closely related to conformational selectivity. Moreover, the effects of solvent types and the supersaturation on solute–solvent interactions were investigated by Fourier transform infrared spectra and molecular dynamics simulations. Combined with the outcome of rapid cooling experiments, it was found that the solute–solvent interaction may affect the difficulty of desolvation so as to affect the conformational selectivity before nucleation. A special evaporation experiment was designed to obtain the unstable polymorph of gabapentin. Finally, the possible mechanism of selective polymorph crystallization was proposed and analyzed.

RELATIVE LENGTH AS A CLASSIFICATION PARAMETER OF THE CRYSTALLIZATION MODE OF AMORPHOUS FILMS

The possibility of using of the relative length δ0 as a parameter, determining the polymorphous crystallization mode of amorphous films, was considered. Following polymorphous crystallization modes have been identified based on the structural and morphological characteristics. Layer polymorphous crystallization mode, describes the nucleation and growth of a single-crystal layer in the field of the electron-beam impact, for which δ0 is about several thousand (3000…5000). Island polymorphous crystallization mode, describes the nucleation and growth of a polycrystalline layer, for which δ0 is about several hundred (100…1100). Dendrite polymorphous crystallization mode, describes the nucleation and growth of dendrite, for which δ0 is about several thousand (~ 4000).

Crystal polymorphism and crystalline-state photochromism of a rhodium dithionite complex with n-methoxypropyl moieties

The crystalline-state photochromism of 1MPro in the β-crystal was successfully disclosed by time-dependent X-ray diffraction experiments.

Pyridyl substitution at the 2,6-positions of anthracene toward crystal polymorphs with distinct optical characteristics

Via pyridyl substitution at the 2,6-positions of anthracene, we achieved two packing polymorphs with different crystal morphologies and optical characteristics.

Luminescent polymorphic crystals: mechanoresponsive and multicolor-emissive properties

Polymorphic organic crystals that can switch their photophysical properties in response to mechanical stimuli are highlighted.

Synthesis and transformation of calcium carbonate polymorphs with chiral purine nucleotides

Crystallization of CaCO3 polymorphs is controlled using the chiral purine nucleotides adenosine triphosphate (ATP) and guanosine triphosphate (GTP). The effects of ATP and GTP on the transformation of calcite into vaterite are investigated.

Crystal growth and structure of a high temperature polymorph of Sr2TiO4 with tetrahedral Ti-coordination, and transition to the Ruddlesden–Popper tetragonal phase

We have grown single crystals of a new polymorph of Sr2TiO4. It contains titanium in an unusual tetrahedral coordination and transforms to the Ruddlesden–Popper structure with an interesting orientational relationship.

D–π–A-type fluorinated tolanes with a diphenylamino group: crystal polymorphism formation and photophysical behavior

Ph2N-Substituted fluorinated tolanes produced two polymorphisms, which display a distinct photoluminescence behavior; one polymorphism showed green PL and the other exhibited yellow PL.

New patterns of twist-bend liquid crystal phase behaviour: the synthesis and characterisation of the 1-(4-cyanobiphenyl-4'-yl)-10-(4-alkylaniline-benzylidene-4'-oxy)decanes (CB10O·m).

The synthesis and characterisation of the 1-(4-cyanobiphenyl-4'-yl)-10-(4-alkylanilinebenzylidene-4'-oxy)decanes (CB10O·m) are reported. This series shows a rich liquid crystal polymorphism including twist-bend nematic and smectic phases. All the homologues reported exhibit an enantiotropic conventional nematic phase. For the homologues with m ≤ 10, the local packing in the nematic phases and the layer spacing in the smectic phases indicates an intercalated arrangement of the molecules. An intercalated smectic CA phase is observed if m/11 ≈ 0.5. Either side of this condition, the twist-bend nematic phase is observed, a novel pattern of behaviour for a series on increasing a terminal chain length. For longer chain lengths, m = 12, 14, 16 and 18, two twist-bend smectic C (SmCTB) phases are observed, and the packing of the molecules is now of a bilayer-type. The higher temperature variant is termed SmCTB-SH in which SH (single helix) refers to the presence of a short, distorted clock-type helix. In the lower temperature SmCTB-DH phase, an additional longer helix is superimposed on the short one, and DH denotes double helix.

Impact of free energy of polymers on polymorphism of polymer-grafted nanoparticles.

Colloidal crystals have gathered wide attention as a model material for optical applications because of their feasibility in controlling the propagation of light by their crystal structure and lattice spacing as well as the simplicity of their fabrication. However, due to the simple interaction between colloids, the colloidal crystal structures that can be formed are limited. It is also difficult to adjust the lattice spacing. Furthermore, colloidal crystals are fragile compared to other crystals. In this study, we focused on polymer-grafted nanoparticles (PGNP) as a possible solution to these unresolved issues. We expected that PGNPs, composed of two distinct layers (the hard core of a nanoparticle and the soft corona of grafted polymers on the surface), will demonstrate similar behaviors as star polymers and hard spheres. We also predicted that PGNPs may exhibit polymorphism because the interaction between PGNPs strongly depends upon their grafting density and the length of the grafted polymer chains. Moreover, we expected that crystals made from PGNPs will be structurally tough due to the entanglement of grafted polymers. From exploration of crystal polymorphs of PGNPs by molecular dynamics simulations, we found face-centered cubic (FCC)/hexagonal close-packed (HCP) and body-centered cubic (BCC) crystals, depending on the length of the grafted polymer chains. When the chains were short, PGNPs behaved like hard spheres and crystals were arranged in FCC/HCP structure, much like the phase transition observed in an Alder transition. When the chains were long enough, the increase in the free energy of grafted polymers was no longer negligible and crystals were arranged in BCC structure, which has a lower density than FCC/HCP. When the chains were not too short or long, FCC/HCP structures were first observed when the volume fraction of system was small, but a phase transition occurred when the system was further compressed and the crystals arranged themselves in a BCC structure. These results most likely have laid strong foundations for future simulations and experimental studies of PGNP crystals.

Tricolor fluorescence switching in the three crystal polymorphs of tetraphenylethylene modified fluorenone AIEgen

We employ the molecular design strategy of introducing multiple weak interactions and twisted molecular structure into the same molecular system to synthesize a new AIEgen, which exhibited stimuli-responsive crystalline tricolor fluorescent switching.

Zwitterionic iodonium species afford halogen bond-based porous organic frameworks.

Porous architectures characterized by parallel channels arranged in honeycomb or rectangular patterns are identified in two polymorphic crystals of a zwitterionic 4-(aryliodonio)-benzenesulfonate. The channels are filled with disordered water molecules which can be reversibly removed on heating. Consistent with the remarkable strength and directionality of the halogen bonds (XBs) driving the crystal packing formation, the porous structure is stable and fully preserved on almost quantitative removal and readsorption of water. The porous systems described here are the first reported cases of one-component 3D organic frameworks whose assembly is driven by XB only (XOFs). These systems are a proof of concept for the ability of zwitterionic aryliodonium tectons in affording robust one-component 3D XOFs. The high directionality and strength of the XBs formed by these zwitterions and the geometrical constraints resulting from the tendency of their hypervalent iodine atoms to act as bidentate XB donors might be key factors in determining this ability.

Tuning polymorphs of precipitated calcium carbonate from discarded eggshells: effects of polyelectrolyte and salt concentration.

Biowaste eggshells are a valuable source of calcium carbonate suitable for various applications. In this study, spherical vaterite and calcite calcium carbonate polymorphs have been synthesised from discarded eggshells by the precipitation technique at ambient temperature. The influence of initial salt concentration with different polyelectrolytes such as ethylene glycol (EG), polyethylene glycol (PEG, 600 and 6000), and poly(sodium 4-styrenesulfonate) (PSS) at various w/v% concentrations on the polymorph crystal formation of precipitated calcium carbonate (PCC) particles was studied. The results indicated that PCC crystals with spherical, star-shaped and yarn shaped morphologies can be obtained based on the concentration of calcium ions and the presence of different polyelectrolyte solution. At low salt molar concentration, PEG-6000 and PSS polyelectrolytes were found to promote the formation of spherical vaterite calcium carbonate particles with particle mean diameters of 5.05 μm and 2.17 μm, respectively. Furthermore, silver nanoparticles were also loaded into the PCC particles in situ, and the surface area significantly increased from 2.2813 m2 g−1 in untreated ground eggshells to 30.4632 m2 g−1 in PCC particles in the presence of PSS and silver colloid solution. The EDS mapping revealed the average wt% of silver atoms loaded in PCC particles in the presence of PSS polyelectrolyte was lower (1.44 wt%) than in the presence of PEG-6000 (4.27 wt%) due to the silver encapsulation possibility during the core–shell formation, as confirmed by SEM images. The silver nanoparticle-loaded PCC particles in this study can be incorporated into the polymer matrix and employed for antimicrobial food packaging or wound dressing application.

Otolith Analyses Highlight Morpho-Functional Differences of Three Species of Mullet (Mugilidae) from Transitional Water

Otoliths are used in taxonomy and ichthyology as they can provide a wide range of information about specimens. They are an essential tool to monitor the most sensitive species for a sustainable exploitation level. Despite the increasing use of sagittae in research, their inter- and intra-specific variability and eco-functionality are still poorly explored. This paper aims to investigate the inter- and intra-specific variability of Mugilidae sagittae using morphological and morphometrical analysis, as well as scanning electron microscopy and shape analysis. The sagittae of 74 specimens belonging to three different Mugilidae species, collected from a coastal lagoon, were analyzed to give an accurate description of their morphology, morphometry, shape and crystalline habits. The results highlighted the intra- and inter-specific variability of sagittae, showing morphometrical differences among species and slight differences between left and right sagittae in C. labrosus individuals. Moreover, SEM images showed a peculiar crystal organization, with several different crystal habits and polymorphs. This study provides an accurate description of sagittae in the studied species, deepening the knowledge on inter- and intra-specific variations and crystal habits and providing data which will be useful for future studies on otoliths. With this data, it will be possible to improve conservation and exploitation sustainability in sensitive habitats.

GABA-Controlled Synthesis of the Metastable Polymorphic Form and Crystallization Behavior with a Chiral Malic Acid

GABA-Controlled Synthesis of the Metastable Polymorphic Form and Crystallization Behavior with a Chiral Malic Acid