Enantiotropic Transition Research Articles

The effect of a compatible liquid crystal polymer on homogeneous reinforcement of mesogenic and nonmesogenic rigid-rod monomers

Discrete amount of main-chain Liquid Crystal Polymers (LCP) could mechanically reinforce the mesophase of rigid-rod monomers to form a supramolecular homogeneous composite for Liquid Crystal Display (LCD) materials. Moreover, a homogeneously dispersed LCP may induce an enantiotropic transition in monotropic or non-mesogenic rigid-rod compounds. In this work, we studied the blends of a flexible main-chain nematic LCP with an enantiotropic nematic, a monotropic nematic, and two non-mesogenic rigid-rod compounds as model systems. The results indicate that homogenous reinforcement of thermotropic LCP in these monomers is a valid concept and could lead to improvement of mesogenic stability, orienational and mechanical properties of rigid-rod materials for various applications.

Enantiotropic inconstancy, crystalline solid solutions and co-crystal in the salicylic acid-anthranilic acid system.

The phase boundaries and thermodynamic properties of crystal phases in the salicylic acid (SA) - anthranilic acid (AA) system have been determined experimentally. The complete binary T-X diagram reveals a total of four crystalline phases, including a co-crystal and three crystalline solid solutions. The two eutectics were determined through triplicate DSC analyses at 33 compositions. By adding a liquid solvent and generating a ternary phase diagram, a methodology is introduced to determine the solid-state miscibility limits of the solid solutions at 20 and 55 °C. The crystalline solid solutions exhibit substantial differences in physical properties relative to the pure components, including solubility enhancements that are relevant for chemical processing and material properties. The thermodynamic relationships of the three polymorphs of AA have been resolved showing an enantiotropic transition temperature of 50-55 °C between Form I and III of pure AA. However, as a result of the solid solutions with SA, the enantiotropic transition temperature was suppressed by around 30 °C at the eutectoid. In addition, a co-existence envelope is formed, wherein the two AA polymorph solid solutions exist in equilibrium with one another over a wide range of temperatures and compositions.

Homogeneous reinforcement of liquid crystal monomers by compatible mesogenic polymers

The thermal and mechanical properties of Liquid Crystal Monomers (LCM) could be reinforced by a discrete amount of Liquid Crystal Polymers (LCP) with compatible mesogenic moieties to establish a supramolecular homogeneous composite for improvement of LCM materials in display devices. Moreover, a homogeneously dispersed LCP may induce an enantiotropic transition in monotropic LCM compounds. In this work, we studied the blends of a flexible nematic LCP with two LCM compounds as model systems. The results indicate that homogenous reinforcement of LCM with LCP is a valid concept and could lead to improvements in mesogenic, thermal, orientational and mechanical properties of LCM materials for application in LCD devices.

Thermoanalytical studies of a cytotoxic derivative of carbamazepine: iminostilbene

Iminostilbene is an organic compound, being a precursor of some active pharmaceutical ingredients including well-known anti-epileptic drug carbamazepine as well as substance exhibiting toxic properties and posing threat to natural environment itself. Although its derivatives are commonly studied, there is a lack of thorough investigation of that compound itself, especially concerning its properties at temperatures close to the melting point. Herein, thermodynamic and structural properties of iminostilbene as well as its vibrational and dynamic properties were studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), powder X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). DSC measurements suggests that in the vicinity of melting point, some enantiotropic transition takes place. TGA measurements reveal that strong sublimation of examined substance is observed at temperatures even tens of degrees lower than melting point. Finally, all measurements indicate that the applied heat treatment induces an irreversible phase transition in the studied sample.

A simultaneous X-ray diffraction–differential scanning calorimetry study into the phase transitions of mefenamic acid

In this study, the polymorphic transitions of mefenamic acid (MA) were studied by synchrotron X-ray powder diffraction combined with differential scanning calorimetry (XRD-DSC). The initial material was found to be phase-pure form I which, when heated, produces two endotherms that can be observed by DSC at 162.72 and 219.55°C. The former was found to correspond to a solid–solid enantiotropic transition from form I to a mixture of forms II and III. The latter is the melting point of form II. As form I is heated, significantly greater unit-cell expansion is seen in the a direction than in b and c, which can be explained by the stronger intermolecular interactions in the bc plane. Refinements of the reported MA structures against the patterns collected during heating revealed that at 175°C there exists a mixture of forms I, II and III, whereas only forms II and III remain at 205°C. However, reflections are observed at both temperatures which cannot be fitted with the known forms of MA. It is hypothesized that a new form of MA is produced upon heating. The stability of MA after the enantiotropic transition temperature is II > III > I, which differs from the previously reported II > I > III.

Origins of the Relative Stabilities of Anhydrous and Hydrated d-Mannitol Crystals.

d-Mannitol, a common bulking agent used in the pharmaceutical and food industries, is known to crystallize in three anhydrous polymorphs (α, β, and δ) and a hemihydrate form. In this work, solid-state density functional theory simulations and experimental terahertz spectroscopy were used to evaluate the stabilities, thermodynamic relationships, and spectral signatures of these various solids. The thermodynamic analyses of the anhydrous polymorphs identified enantiotropic transitions for β-δ at approximately room temperature and α-δ near the α melting point, but showed α and β to be monotropically related. Computational refinement of the d-mannitol hemihydrate crystal structure and comparison of its Gibbs free energy to the anhydrous forms showed the hemihydrate to be stable only at low temperatures (<253 K), in agreement with experiment, and reveals its conversion to the δ polymorph upon dehydration. Quantification of the intramolecular and intermolecular energies within these solids highlights the competition that exists between molecular conformation and cohesion energies and how a balance of these forces dictates the observed behaviors.

Solubility and Crystal Nucleation in Organic Solvents of Two Polymorphs of Curcumin

Two crystal polymorphs of 1,7-bis-(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione (curcumin) have been obtained by crystallization from ethanol (EtOH) solution. The polymorphs have been characterized by differential scanning calorimetry, infrared spectroscopy, and X-ray powder diffraction and shown to be the previously described forms I and III. The solubility of both polymorphs in EtOH and of one polymorph in ethyl acetate (EA) has been measured between 10°C and 50°C with a gravimetric method. Primary nucleation of curcumin from EtOH solution has been investigated in 520 constant temperature crystallization experiments in sealed, magnetically stirred vials under different conditions of supersaturation, temperature, and agitation rate. By a thermodynamic analysis of the melting data and solubility of form I, the solid-state activity is estimated from 10°C up to the melting point. The solubility is lower in EtOH than in EA, and in both solvents, a positive deviation from Raoult's law is observed. Form I has lower solubility than form III and is accordingly thermodynamically more stable over the investigated temperature interval. Extrapolation of solubility regression models indicates that there should be a low-temperature enantiotropic transition point, below which form I will be metastable. By slurry conversion experiments, it is established that this temperature is below -30°C. All nucleation experiments resulted in the stable form I. The induction time is observed to decrease with increasing agitation rate up to a certain point, and then increase with further increasing agitation rate; a trend previously observed for other compounds. By correlating the induction time data obtained at different supersaturation and temperature, the interfacial energy of form I in EtOH is estimated to be 3.0 mJ/m(2) .

Dielectric and Optical Studies in Smectic C of A Novel Hydrogen Bonded Liquid Crystal Homologous Series

A novel hydrogen bonded liquid crystalline series comprising of p-n alkyloxy benzoic acids is isolated. This series include seven complexes in which hydrogen bond is formed between undecyloxy benzoic acid and various homologues of benzoic acid varying from pentyl to dodecyloxy carbon number. All the homologues of the present series show rich mesomorphism exhibiting orthogonal and tilted phases. The formation of the hydrogen bond is confirmed by Fourier transform infrared spectroscopy studies. Transition temperatures and corresponding enthalpy values along with enantiotropic and monotropic transitions are elucidated from differential scanning calorimetry thermograms. Polarizing optical microscopy revealed various phases in the form of textures. An interesting observation is the identification of a new smectic ordering labeled as smectic X which is sandwiched between traditional nematic and convectional smectic C phases. Dielectric relaxations in smectic C phase of three homologues have been analyzed and the activation energy is calculated from respective Arrhenius plots.

Relative Stabilities of the Five Polymorphs of Sulfathiazole

The relative stabilities of the five polymorphs of sulfathiazole have been investigated using solution-based and solid-state methods. In the lower temperature range, the stability order is proposed to be FI < FV < FIV < FII < FIII. FI and FV were identified as the least stable polymorphs below 50 °C using a combination of solubility measurements and isothermal suspension equilibration, with FII, FIII, and FIV displaying very similar stabilities. Between 30 and 50 °C, the stability order was established as FIV < FII < FIII. At 10 °C, FII is still more stable than FIV, but it was not possible to place FIII in relation to these two forms. Above 100 °C, the results from DSC and high-temperature XRD measurements suggest that the stability order changes completely as a result of several enantiotropic transitions. In this upper temperature range, FII and FIII are the least stable forms, with FII being less stable than FIII. The stability order among the remaining three forms is FI < FV < FIV initially, but this ...

Physical transformations of the active pharmaceutical ingredient BN83495: enantiotropic and monotropic relationships. Access to several polymorphic forms by using various solvation–desolvation processes

BN83495 is an API in development, used in the treatment of hormone dependent cancers. This molecule exhibits an interesting polymorphic landscape consisting of three polymorphic forms (I, II, and III), three 〈1:1〉 DMSO solvate polymorphs, and one 1,4-dioxane hemi-solvate. Among the three DMSO solvates, one can only be obtained in the presence of an inorganic impurity: the letovicite. Between the two polymorphic forms I and III, an enantiotropic transition has been highlighted. This solid–solid transition (Form I → Form III) seems to be controlled by the mosaicity of the particles, where both polymorphic form domains cohabit and this could remain unchanged for months. The reverse transition has not been detected in the solid state; slurrying in a solvent appears necessary to return to Form I. Each of the three polymorphs can be obtained by soaking in cold water the appropriate solvated compound. The crystal structure of each phase was determined from single crystal X-ray diffraction, except for Form II (the monotropic form under normal pressure). The analyses of the starting and final crystal structures involved in a desolvation process led to the proposal of mechanisms with and without transmission of structural information from the mother phase (i.e. the initial solvate) to the daughter phase. The presence of direct hydrogen bonds between BN83495 molecules in DMSO-I and 1,4-dioxane hemi-solvate seems to ensure the structural filiations during the desolvation mechanisms leading respectively to Form III and Form II.

Liquid-crystalline oxovanadium(IV) complexes accessed from bidentate [N, O] donor salicylaldimine Schiff-base ligands

A series of bis[4-(n-alkoxy)-N-(4′-R-phenyl)salicylideneiminato]oxovanadium(IV) complexes (n = 6, 10, 14, 16, 18 and R = C3H7) were prepared and their mesogenic properties were investigated. The mesomorphic behaviors of the compounds were studied by polarized optical microscopy and differential scanning calorimetry. Ligands display SmA/SmC and unexpected nematic mesophases. The complexes bearing longer alkoxy carbon chain (n = 10, 14, 16, and 18) showed both monotropic or enantiotropic transitions with smectic A and high ordered smectic E phases. However, the complex with shorter carbon chain length (n = 6) showed monotropic transition with an unprecedented nematic (N) phase. A density functional theory study was carried out using DMol3 at BLYP/DNP level to obtain a stable optimized structure. A square-pyramidal geometry for the vanadyl complexes has been suggested. A νV=O stretching value of ∼970 cm−1 corroborated absence of any V = O ··· V = O interactions. Cyclic voltammetry revealed a quasireversible one-electron response at 0.61 V for the VO(IV)–VO(V) redox couple. Variable temperature magnetic susceptibility measurements of the vanadyl complexes suggested absence of any exchange interactions among the vanadyl spin centers.

Synthesis, mesomorphism and luminescence properties of palladium(II) and platinum(II) complexes with dimeric Schiff base liquid crystals

Abstract A series of ortho -metallated Pd and Pt complexes containing dimeric liquid crystals Schiff base as cyclometallated ligands and N -benzoyl thiourea derivatives as co-ligands were prepared and investigated for their liquid crystalline properties. Their structures were assigned based on elemental analysis, IR and 1 H NMR spectroscopy while the mesogenic properties were investigated by DSC and polarising optical microscopy. The complexes show either monotropic or enantiotropic transitions with nematic and smectic A phases being displayed, with the mesomorphic behaviour strongly related to the type of N -benzoyl thiourea as well as the metal center used. The structure of a palladium(II) complex has been solved by X-ray diffraction. The platinum(II) complexes show photoluminescence properties both in solution and in solid state at room temperature, with the emission band centered around 600 nm. These are the first examples of metallomesogens based on Schiff base cyclometallated ligands that display luminescence properties.

Description of the even-odd alternation of the clearing temperatures over homologous series of mesogens by second-order differential equations

It is shown for liquid crystals of various chemical classes that the nematic-isotropic transition temperatures in homologous series of compounds undergoing enantiotropic transitions are described by secondorder inhomogeneous recurrent equations with constant coefficients: T(n + 2) = aT(n + 1) + bT(n) + c (n is the number of alkyl carbon atoms). Solutions of the corresponding differential equations as T(n) functions allow reproduction of the even-odd alternation of the clearing temperatures.

Photoimaging through in-Situ Photopolymerization of Heterobifunctional Mesogenic Compounds in Liquid Crystalline State

A series of heterobifunctional mesogenic biphenyl esters having two different polymerizable groups, i.e., acryl and diacetylene groups, were synthesized and their thermal behaviors and polymerization investigated. All compounds showed enantiotropic transitions. Under POM, highly birefringent focal-conic fan textures appeared on heating and cooling from the isotropic melt. Compounds 6−8 having a butyl spacer between a biphenyl and a diacetylene group exhibited LC phases even at room temperature. The X-ray diffractograms of compounds 6−8 showed a set of reflections in the small-angle region. They consisted of more than three sharp diffraction peaks with d spacings in the ratio of 1:1/2:1/3, showing that the compounds had well-defined smectic A structures. For the photoimaging a mixture of 6 and a photoinitiator (2,2-dimethoxy-2-phenylacetophenone, 4 wt %) was cast on a glass plate and sheared with a cover glass at room temperature to result in an LC monodomain. The acryl group was then selectively polymeriz...

Reversibility of Enantiotropically Related Polymorphic Transformations from a Practical Viewpoint: Thermal Analysis of Kinetically Reversible/Irreversible Polymorphic Transformations

Although enantiotropically related polymorphic transitions are thermodynamically reversible with temperature and pressure, they may be observed as either reversible or irreversible in practical investigations. This behavior may be explained simply by a difference in the energy barrier for the transition. However, this difference may have a significant effect on strategy in formulation development. This article summarizes the characteristics of enantiotropic transitions with reference to the literature. Also provided are careful investigations of polymorphic transitions by thermal analysis, which is the most promising method to investigate transition behavior.

Measuring Free-Energy Difference between Crystal Polymorphs through Eutectic Melting

We describe a method to measure the free-energy difference, DeltaG, between crystal polymorphs from their calorimetric data of eutectic melting with a common additive. The use of different additives yields DeltaG as a function of temperature. The method is suitable for crystals that chemically decompose or physically transform before melting. It applies to not only true polymorphs but also pairs of racemate and conglomerate of resolvable enantiomers. We illustrate the method with the polymorphs of glycine, d-mannitol, and tazofelone and report a new value (123 degrees C) for the enantiotropic transition temperature of alpha and gamma glycine. We show how different additives (including a liquid additive, water) can be used for different compounds. The DeltaG data thus obtained are important for structure-stability studies and controlling crystallization in polymorphic systems.

Mesomorphism and Glass Formation of Phthalocyanine Metal Complexes with Bulky Substituents

This paper deals with the synthesis of phthalocyanine complexes, containing four oligo(oxyethylene)-, oxybenzene-, esther of p-oxybenzoic acid or p-oxyazobenzene side chains terminated by a bulky trityl substituent. Their purity was established by column chromatography and by satisfactory elemental analysis. The structures of resultant phthalocyanines were characterized by nuclear magnetic resonance (NMR), fast atom bombardment mass spectroscopy (FAB-MS), ultraviolet-visible (UV-VIS) and infrared (IR) spectroscopy and their phase transitions were studied by polarizing optical microscopy. Some of these complexes exhibit enantiotropic mesomorphic properties and transition into glass state on cooling. The glass appearance is influenced not only by side chains’ structure, but also by the nature of metal.

Supramolecular discotic liquid crystals from wedge‐shaped diacetylenes and their polymerization

AbstractWedge‐shaped diacetylenic compounds were synthesized by the amidation reaction between the linear diacetylenic side groups and 5‐acetamidoisophthalic acid. The compounds showed enantiotropic transitions. In the liquid‐crystalline states, they were dimerized via hydrogen bonding of acetamide groups, forming disklike supramolecules and had rectangular columnar phases. Polymerization was carried out by UV irradiation and thermal annealing at their mesophase temperatures, giving polydiacetylenes. After polymerization, the compounds became dark red, but their textures were still birefringent. The IR and ultraviolet–visible study demonstrated that the polymerization proceeded topochemically by 1,4‐addition. X‐ray analysis showed that the UV‐polymerized samples maintained rectangular columnar phases. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1881–1891, 2003

X-ray diffraction/Calorimetry coupling

Polymorphism of trilaurin mixed with 4% of cholesterol was studied with a setup coupling calorimetry and phase characterisation by in-situ X-ray diffraction (Microcalix). Four polymorphic forms were identified. Monotropic and enantiotropic transitions were identified from the reconstruction of Gibbs free energy diagram which allows the control of trilaurin polymorphism.

Thermodynamic properties of a series of semi-rigid polyesters

The thermodynamic properties of a series of poly[oxy(2,2′ dialkylpropane-1,3-diyl) carboxybisphenyl- 4,4′-dicarbonyl] have been investigated by P-V-T-measurements as a function of the length of one side-chain at the 2′-position.The pressure-and temperature dependence of the specific volume was recorded from room temperature to 300 °C and from ambient pressure to 200 MPa. Depending on the chemical constitution glass-transitions, enantiotropic and monotropic mesophase transitions were identified. A pressure-induced transition in the glass was observed in one case and unusual behaviour of the volume for a first order mesophase transition in another case.