Low-temperature Crystalline Phase Research Articles

Coupling various in-situ methods to reveal the peculiar semi-crystalline morphology of melt-processed fluorinated copolymers

Poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-co-TrFE)) copolymer and poly(vinylidene fluoride-ter-trifluoroethylene-ter-chlorotrifluoroethylene) P(VDF-ter-TrFE-ter-CTFE) terpolymer thin films are usually produced by solvent-casting followed by annealing to enhance their electroactive properties in electronic devices. In this study, we investigate the effect of a different thermal treatment, namely crystallization from the melt, on the crystalline structure and morphology of P(VDF-ter-TrFE-ter-CTFE) terpolymers with varying CTFE content (ranging from 0 to 8.3 mol %) using a combination of different in-situ techniques. First, in-situ Wide-Angle X-Ray Scattering (WAXS) shows the different crystal-crystal transitions during cooling for all the samples. For the terpolymers, two sharp Bragg peaks ((110) and (200) family of planes) of the low temperature crystalline phase can be detected. Their sharpness reflects a high lateral extension of the crystalline domains. Moreover, coupled morphological studies by in-situ Small-Angle X-Ray Scattering (SAXS) and Atomic Force Microscopy (AFM) show that the crystallization starts with the formation of very extended semi-crystalline domains with a sub-micrometric thickness that contributes to the SAXS signal. For significant amounts of CTFE, crystallization is spread out, leading to the appearance of increasingly finer lamellar stacks during cooling, filling the spaces left unoccupied during crystallization at higher temperatures. This contrasts with the very rapid crystallization observed in the polymer with 0 mol % of CTFE. Finally, the Curie transition is clearly visible in the AFM mechanical images, providing an original method for detecting the structural transitions occurring in these polymers.

Nature of Thermal Hysteresis of Thermoelectric Properties in Ag2TexS1-x Compounds.

Ag2TexS1-x usually undergo various phase structures upon heating or cooling processes; however, the correlation between the heat treatment, the phase structure, and the physical properties is still a controversy. Herein, three different phases are realized for Ag2TexS1-x (0.35 ≤ x ≤ 0.65) samples during the heat treatment, including the low-temperature crystalline phase, amorphous phase, and high-temperature cubic phase. The metastable amorphous phase is an intermediate phase formed during transition from the high-temperature cubic phase to the low-temperature crystalline phase upon cooling via a solid-state conversion rather than the conventional liquid quenching process. The relative content of these three phases is highly sensitive to the heat treatment process. This as-formed low-temperature crystalline phase, amorphous phase, and high-temperature cubic phase convert into the low-temperature crystalline phase and high-temperature cubic phase through long-time dwelling at the temperature below or above the transition temperature around 567 K, respectively. The status of the low-temperature crystalline phase, amorphous phase, and high-temperature cubic phase significantly affects the thermoelectric properties, resulting in the thermal hysteresis of thermoelectric properties. Below the phase transition temperature (TM), the electrical conductivity of the amorphous phase surpasses that of the low-temperature crystalline phase, which shows a growth of 112% for the Ag2Te0.60S0.40 sample annealed at 823 K in comparison with that of the sample annealed at 473 K. For Ag2Te0.50S0.50 samples annealed at 473 K, the maximum ZT value reaches 1.02 at 623 K during the initial test, while the maximum ZT value is improved to 1.34 at 523 K in the second-round test.

Stereochemistry-dependent thermotropic liquid crystalline phases of monosaccharide-based amphiphiles.

Conformational rigidity controls the bulk self-assembly and liquid crystallinity from amphiphilic block molecules to copolymers. The effects of block stereochemistry on the self-assembly have, however, been less explored. Here, we have investigated amphiphilic block molecules involving eight open-chain monosaccharide-based polyol units possessing different stereochemistries, derived from D-glucose, D-galactose, L-arabinose, D-mannose and L-rhamnose (allylated monosaccharides t-Glc*, e-Glc*, t-Gal*, e-Gal*, t-Ara*, e-Ara*, t-Man*, and t-Rha*), end-functionalized with repulsive tetradecyl alkyl chain blocks to form well-defined amphiphiles with block molecule structures. All compounds studied showed low temperature crystalline phases due to polyol crystallization, and smectic (lamellar) and isotropic phases upon heating in bulk. Hexagonal cylindrical phase was additionally observed for the composition involving t-Man*. Cubic phases were observed for e-Glc*, e-Gal*, e-Ara*, and t-Rha* derived compounds. Therein, the rich array of WAXS-reflections suggested that the crystalline polyol domains are not ultra-confined in spheres as in classic cubic phases but instead show network-like phase continuity, which is rare in bulk liquid crystals. Importantly, the transition temperatures of the self-assemblies were observed to depend strongly on the polyol stereochemistry. The findings underpin that the stereochemistry in carbohydrate-based assemblies involves complexity, which is an important parameter to be considered in material design when developing self-assemblies for different functions.

Low-Temperature Infrared Spectra and Ultraviolet-Induced Rotamerization of 5-Chlorosalicylaldehyde.

5-Chlorosalicylaldehyde (abbreviated as 5CSA) is an important chemical used in the synthesis of fragrances, dyes, and pharmaceuticals. In this investigation, 5CSA isolated in solid N2, at 10 K, and in its neat amorphous and crystalline phases, at 50 and 190 K, respectively, were investigated by infrared spectroscopy and DFT(B3LYP)/6-311++G(d,p) calculations. The systematic theoretical analysis of the 5CSA conformational landscape showed that the compound exhibits four different conformers, which were structurally characterized in detail. In the as-deposited low-temperature matrices of 5CSA, only the most stable conformer, the intramolecularly hydrogen-bonded form I, was found. The same was observed in the case of the investigated low-temperature amorphous and crystalline phases of 5CSA. Conformer I was successfully converted into a higher-energy conformer(II), where both aldehyde and hydroxyl groups are rotated by 180° relative to their position in the initial conformer, through narrowband ultraviolet (UV) (λ = 308 nm) in situ irradiation of the as-deposited N2 matrix of 5CSA. The infrared spectra of both matrix-isolated conformers, as well as those of the neat amorphous and crystalline phases of 5CSA, were assigned and interpreted in comparative terms, allowing us to elucidate structurally and vibrationally relevant effects of the main intra- and intermolecular interactions operating in the different studied phases. Very interestingly, the observed UV-induced I → II rotamerization was found to take place in an exclusive basis, with no other photochemical processes being observed to occur upon UV irradiation, under the experimental conditions used in the present investigation.

Nanometer Confinement Induces Nematic Order in 1-Dodecanol.

The phase state and molecular dynamics of 1-dodecanol are studied in the bulk and under nanometer confinement within self-ordered nanoporous alumina templates. A rotator phase in the bulk is absent under confinement. A nematic liquid crystalline phase is formed instead in pores with diameters from 400 down to 25 nm. Results are based on the changes in temperature-dependence of dielectric permittivity and X-ray diffraction. The phase diagram under confinement is explored, and the limits of the nematic-to-isotropic and crystalline-to-nematic phase transitions are identified. The phase diagram allows for a direct transition from the liquid to the low-temperature crystalline phase in pores with a diameter below 20 nm. Furthermore, we report on the dielectric fingerprint of the rotator phase and the molecular dynamics in bulk 1-dodecanol.

Mean-field phase diagram and spin-glass phase of the dipolar kagome Ising antiferromagnet

We derive the equilibrium phase diagram of the classical dipolar Ising antiferromagnet at the mean-field level on a geometry that mimics the two dimensional Kagome lattice. Our mean-field treatment is based on the combination of the cluster variational Bethe-Peierls formalism and the cavity method, developed in the context of the glass transition, and is complementary to the Monte Carlo simulations realized in [Phys. Rev. B 98, 144439 (2018)]. Our results confirm the nature of the low temperature crystalline phase which is reached through a weakly first-order phase transition. Moreover, they allow us to interpret the dynamical slowing down observed in the work of Hamp & al. as a remnant of a spin glass transition taking place at the mean-field level (and expected to be avoided in 2 dimensions).

Morphological Evolution of Tetrachlorinated Perylene Bisimides with Lengthy Alkyl Substituent Polycrystalline Thin Films during Reversible Phase Transitions.

The phase behavior and related morphological evolution of thin films of lengthy alkyl substituted core-tetrachlorinated perylene bisimide (C18-4ClPBI), a large π-conjugated molecule, have been studied. It is found that the C18-4ClPBI can exist in two different crystalline phases depending on temperature, which transform reversibly in heating and cooling processes. The X-ray diffraction results demonstrate that the two crystalline forms of C18-4ClPBI exhibit a similar packing geometry but with different unit cell dimensions. It is confirmed that the low-temperature phase is packed more compactly than its high-temperature counterpart. During high-temperature to low-temperature crystalline phase transition, nonbirefringent protrusions were observed, which disassembled in the reverse crystalline phase transition process during heating. The exact formation mechanism of the protrusions is not clear at the moment. Nevertheless, their influence on the transfer characteristics of the polycrystalline C18-4ClPBI thin film has been clearly illustrated.

Preparation, identification, and low-temperature infrared spectra of two elusive crystalline nitrile ices

Infrared (IR) spectra of the alkyl nitrile ices CH3CN and CH3CH2CN are relevant to the study of the low-temperature chemistry of objects within and beyond the Solar System, but the thermodynamically favored low-temperature crystalline phases of these compounds have not been presented and identified in the planetary-sciences literature. Moreover, there is a large variation in the published IR spectra of these two icy solids, including spectra that are used for the analyses of spacecraft data from the Voyager and Cassini missions. Here the IR spectra of the low-temperature crystalline forms of CH3CN and CH3CH2CN, which are the thermodynamically stable phases at Titan temperatures, are presented for the first time with all samples being made by vapor-phase deposition. Conditions are described for producing these ice phases for both compounds, and new measurements are reported of ice density and refractive index, quantities needed to compute IR absorption coefficients, band strengths, optical constants, and, ultimately, nitrile abundances in extraterrestrial environments. Comparisons are drawn between CH3CN and CH3CH2CN, an earlier prediction on their similar crystallization behaviors is verified, and previous work is examined in light of these new results, including a counter-intuitive observation in which the low-temperature phase of an ice is made by heating a high-temperature phase. Applications and extensions are described.

Structure and Dynamics of the Crystalline Stable Phase of 2-Chlorothiophene

The dynamics of a simple and rigid molecule (2-chlorothiophene) has been studied by means of broadband dielectric spectroscopy (BDS) within the low-temperature stable crystalline phase, whose struc...

Phase behaviour and dynamics of the liquid crystal 4-ethyl-4′-octylazoxybenzene (4EOB)

ABSTRACTThe results of Broadband Dielectric Spectroscopy and Differential Scanning Calorimetry of the 4-ethyl-4′-octylazoxybenzene (4EOB) liquid crystal are presented. Numerical analysis of the dielectric spectra results points to complex dynamics of the liquid crystal molecules in isotropic liquid, nematic and conformationally disordered crystalline phases. Two well-separated maxima on the imaginary part of electric permittivity were detected and described in liquid-like and crystalline phases. A third relaxation process, hidden in the conductivity region, describing conformational disorder of alkyl chains has been found in some plastic crystal phases. Temperature dependences of mean relaxation times characterising conformational disorder, flip-flop motions and rotation around long axis are of the Arrhenius type in liquid-like phases and conformationally disordered crystalline phases, and of the Vogel–Fulcher–Tammann type in the low-temperature crystalline phases for relaxation processes around short and long axis.

Superionic conduction and interfacial properties of the low temperature phase Li7P2S8Br0.5I0.5

Sulfide-based solid electrolytes have attracted much attention for their potential application in high energy and power bulk-type all-solid-state lithium batteries, due to their excellent transport properties and favorable mechanical properties. The Li10GeP2S12-type materials with extremely high ionic conductivity have been demonstrated in all-solid-state cathodes, however they cannot be deployed directly in common anodes due to the interfacial instability. An electrolyte stable with anodes is urgently needed for construction of all-solid-state batteries. In this work, we explore the phase and transport properties of mechanically-milled Li7P2S8Br0.5I0.5, with emphasis on the favorable kinetics at Li metal anode. The low temperature crystalline phase Li7P2S8Br0.5I0.5 (LT-LPSBI) has exceptionally high ionic conductivity of 4.7 mS/cm at room temperature. Moreover, the LT-LPSBI supports long-term Li cycling at 0.5 mA cm−2 with low and stable interfacial resistance (5 Ω cm2). Transport phenomenon of Li/LT-LPSBI interface and cycling behavior of Li symmetric cells are studied and discussed in detail.

Thermodynamic Properties of Molecular Crystals Calculated within the Quasi-Harmonic Approximation.

A computational study of the possibilities of contemporary theoretical chemistry as regards calculated thermodynamic properties for molecular crystals from first-principles is presented. The study is performed for a testing set of 22 low-temperature crystalline phases whose properties such as densities of phonon states, isobaric heat capacities, and densities are computed as functions of temperature within the quasi-harmonic approximation. Electronic structure and lattice dynamics are treated by plane-wave based calculations with optPBE-vdW functional. Comparison of calculated results with reliable critically assessed experimental data is especially emphasized.

Non-monotonic temperature response of polymer mediated interactions.

In a recent publication, Feng et al. [Feng et al., Nat. Mater., 2015, 14, 61] reported a very interesting re-entrant solidification behaviour of colloidal particles in an aqueous solution containing polyethylene oxide (PEO). In this system, a crystalline colloidal phase, which is present at low temperatures, melts to a homogeneous fluid upon increasing the temperature. Further raising the temperature, however, eventually gives rise to a flocculated colloidal phase. Feng et al. proposed that the low-temperature crystalline phase is caused by polymer depletion while, at higher temperature, an increased attraction between polymers and particles leads to bridging attractions, and colloidal flocculation. The intermediate temperature regime sees the colloidal interactions dominated by charge repulsion, giving rise to a fluid phase. In the model by Feng et al., polymers are treated as hard spheres, which interact with the colloids via a phenomenological, temperature dependent potential. In this work, we develop a more detailed polymer density functional treatment, based on a model for aqueous PEO solutions that was originally developed by Karlström [Karlström, J. Phys. Chem., 1985, 89, 4962] for bulk solutions. In this model, monomers are assumed to be in either of two classes of states, labelled A and B, where B is more solvophobic than A. On the other hand, the degeneracy of the B states exceed that of A, causing the population of solvophobic monomers to increase with temperature. If the colloidal particles are also solvophobic, then this model displays the same qualitative temperature response as was observed by Feng et al. That is, at low temperatures, A type monomers predominate and one observes depletion interactions, whereas polymer bridging dominates at higher temperatures, due to the attraction between B-type monomers and the colloidal surface. Interestingly, the intermediate temperature regime is characterized by a polymer mediated interaction between colloids which is repulsive. That is, according to our model the homogeneous fluid phase would form even in the absence of repulsive electrostatic interactions. We emphasize that our model does not rely upon any temperature-dependent interactions. Furthermore, all possible polymer configurations are accounted for, subject to a mean-field Boltzmann weight. Finally, we show that interactions between colloids, mediated by neutral non-grafted polymers, generally follow the trend: attraction ⇒ repulsion (or vanishing interaction) ⇒ attraction as the surface affinity toward monomers proceeds from repulsive to attractive.

Protic and aprotic anionic oligomeric ionic liquids

We report on synthesis of linear and hyperbranched protic and aprotic anionic oligomeric ionic liquids (OILs). α,ω-Dicarboxy- and α,ω-disulfooligo(ethylene oxide)s, α-carboxy- and α-sulfooligo(ethylene oxide monomethyl ether)s, and di[(α-carboxyoligo(ethylene oxide monomethyl ether)] were synthesized using reaction of oligo(ethylene oxide diol) (MW 1000) and its monomethyl ether (MW 750) with phthalic-, 2-sulfobenzoic anhydride and pyromellitic dianhydride. Di- and mono-substituted anionic OILs were prepared by neutralizing these compounds with N-methylimidazole. Aprotic anionic OILs were synthesized by reaction of sodium salts of the prepared oligomeric di- and monoacids with 1,3-dimethyl imidazolium iodide. Hyperbranched protic and aprotic anionic OILs were prepared in a similar manner. The structure, thermal stability and ionic conductivity of the synthesized compounds in the range of 20–120 °C in anhydrous conditions is governed by the molecular architecture of the oligomeric chains and the type of the cation/anion moieties. OILs under study are amorphous at room temperature but some protic and aprotic linear-chain OILs prone to form a low melting temperature crystalline phase. The ionic conductivity of the synthesized OILs can be varied in broad range reaching 10−3 S/cm value at temperatures over 100 °C under anhydrous conditions.

Polymorphism of 2-Adamantanone

The polymorphism of 2-adamantanone (C10H14O) has been investigated by means of X-ray diffraction and high-pressure thermal analysis. The intricate behavior of the low-temperature crystalline phases has been disentangled. The stable phase has been found to be orthorhombic (Cmc21, Z = 4), fully ordered, with lattice parameters a = 6.8884(18) Å, b = 10.830(3) Å, c = 10.658(3) Å, and V/Z = 198.8(1) Å3. The metastable phase was determined to be monoclinic (P21/c, Z = 4) with lattice parameters a = 6.5920(17) Å, b = 11.118(3) Å, c = 12.589(3) Å, β= 118.869(11) o, and V/Z = 202.0(1) Å3. The pressure–temperature phase diagram irrefutably shows the stability relation between both phases and, accordingly, the long-time unknown polymorphic behavior is now revealed and gives coherent physical explanation of the literature published so far.

Terahertz Vibrational Modes of the Rigid Crystal Phase of Succinonitrile

Succinonitrile (N ≡ C-CH2-CH2-C ≡ N), an orientationally disordered molecular plastic crystal at room temperature, exhibits rich phase behavior including a solid-solid phase transition at 238 K. In cooling through this phase transition, the high-temperature rotational disorder of the plastic crystal phase is frozen out, forming a rigid crystal that is both spatially and orientationally ordered. Using temperature-dependent terahertz time-domain spectroscopy, we characterize the vibrational modes of this low-temperature crystalline phase for frequencies from 0.3 to 2.7 THz and temperatures ranging from 20 to 220 K. Vibrational modes are observed at 1.122 and 2.33 THz at 90 K. These modes are assigned by solid-state density functional theory simulations, corresponding respectively to the translation and rotation of the molecules along and about their crystallographic c-axis. In addition, we observe a suppression of the phonon modes as the concentration of dopants, in this case a lithium salt (LiTFSI), increases, indicating the importance of doping-induced disorder in these ionic conductors.

Interaction of charge carriers with lattice and molecular phonons in crystalline pentacene

The computational protocol we have developed for the calculation of local (Holstein) and non-local (Peierls) carrier-phonon coupling in molecular organic semiconductors is applied to both the low temperature and high temperature bulk crystalline phases of pentacene. The electronic structure is calculated by the semimpirical INDO/S (Intermediate Neglect of Differential Overlap with Spectroscopic parametrization) method. In the phonon description, the rigid molecule approximation is removed, allowing mixing of low-frequency intra-molecular modes with inter-molecular (lattice) phonons. A clear distinction remains between the low-frequency phonons, which essentially modulate the transfer integral from a molecule to another (Peierls coupling), and the high-frequency intra-molecular phonons, which modulate the on-site energy (Holstein coupling). The results of calculation agree well with the values extracted from experiment. The comparison with similar calculations made for rubrene allows us to discuss the implications for the current models of mobility.

Conformational Space and Vibrational Spectra of Methyl 4-Chloro-5-phenyl-1,3-oxazole-2-carboxylate

Methyl 4-chloro-5-phenyl-1,3-oxazole-2-carboxylate (MCPOC) has been synthesized and isolated in cryogenic matrices (argon and xenon). FTIR spectroscopy studies on the matrix isolated compound, supported by DFT(B3LYP)/6-311++G(d,p) calculations, allow for the identification of two low-energy conformers (I and II) of the molecule, which differ from each other in the orientation of the ester group relative to the oxazole ring. In both these conformers, the ester moiety is in the s-cis configuration (O horizontal lineC-O-CH(3) dihedral: 0 degrees ). Conformer II is ca. 3.0 kJ mol(-1) higher in energy than form I in the gas phase. Two additional higher energy conformers, III and IV, with relative energies of ca. 30 and 45 kJ mol(-1), respectively, were predicted to exist by the calculations, corresponding to structures where the ester group is in an approximately s-trans arrangement. Annealing of the compound isolated in xenon at 60 K led to aggregation and simultaneous reduction of the population of I compared to that of the more polar conformer II. These results suggest the inversion of the order of stability of the two conformers in that matrix, eventually accompanied by a higher trend of conformer I to aggregate. Full assignment of the observed infrared bands to the two experimentally accessible conformers was carried out for the matrix isolated monomeric species. In addition, the infrared spectra of the neat compound in the low-temperature (10 K) amorphous and crystalline phases, as well as the infrared and Raman spectra of the crystal at room temperature were also obtained and assigned.

Effects of Water on Aggregation and Stability of Monoglycerides in Hydrophobic Solutions

We apply a set of different techniques to analyze the physical properties and phase transitions of monoglyceride (MG)-oil-water ternary systems. The effect of MGs on water absorption in food-grade hazelnut oil and in pure hydrocarbon oil (decane) is reported. Comparison between decane and hazelnut oil backgrounds indicates that the effect of water absorption is significant and universal in different MG ternary systems. Adding small amounts of cosurfactant (stearic acid) is necessary to stabilize the MGs in oil-water combinations by enhancing the swelling capacity of lamellar layers; as a result, the structures become sensitive to the pH of the aqueous phase used. The dramatic changes on increasing the aqueous content are recorded by the calorimetry. In samples with small quantities of water, the phase behavior is almost independent of the pH. Once the proportion of water increases, the effect of pH is prominent. At low pH, the solubility of MG in water is limited, and the ternary system retains key features of the oil-dominated environment, such as the sequence of two transitions on cooling, with the low-temperature sub-alpha crystalline phase. At high pH and a sufficient amount of water, the MG layers remain properly swollen, and the crystalline phase disappears from the phase diagram. We spend considerable effort identifying the inverse lamellar phase of MGs in an oil-dominated environment with the so-called alpha-gel phase that is well-established in water-dominated systems, and distinguishing "demixing" from water and from oil. The rheology is examined in different fluid and gel phases; the storage modulus generally decreased on increasing the water proportion, but a gel-like response is found in the high-temperature lamellar phase over a wide range of water dilution. We then focus on aging phenomena in the inverse lamellar (or alpha-gel) phase and show that the rearranging of hydrogen bonds is slowed down and disrupted by the presence of water, giving the lamellar gel longer life times.

Low Temperature Infrared Spectroscopy Study of Pyrazinamide: From the Isolated Monomer to the Stable Low Temperature Crystalline Phase

A structural and spectroscopic analysis of the anti-tuberculosis drug pyrazinamide (PZA) was carried out. The PZA molecule was predicted theoretically to possess two conformers differing by internal rotation around the C-C( horizontal lineO) bond, with the E conformer (C(s) symmetry point group; N-C-C horizontal lineO dihedral: 180 degrees ) being ca. 30 kJ mol(-1) more stable than the Z form (C(1) point group; N-C-C horizontal lineO dihedral: ca. +/- 42 degrees ). In consonance with both the large energy difference and low energy barrier between the Z and E conformers, upon isolation in low temperature argon and xenon matrices, only the E form could be observed and characterized spectroscopically. In the argon matrix, this conformer was found to exist in at least three matrix sites, of different stability. In a supersonic jet, besides the monomer (E), the most stable dimer of PZA with two equivalent NH...O horizontal line hydrogen bonds could also be identified. Its spectrum reveals rapid energy flow out of the excited NH stretching mode mediated by one of the heteroatoms in the ring. Finally, the IR spectra of the amorphous solid resulting from fast cooling of the vapor of the compound (initially in the alpha crystalline phase) onto the cold substrate of the cryostat (10 K) and of the crystalline phase resulting from warming the amorphous solid were also recorded and interpreted. The obtained crystalline phase was found to be the thermodynamically most stable delta polymorph of PZA.