Isothermal Melt Research Articles

Застосування фторидних вогнетривів для отримання функціо-наль¬них сплавів, що містять хімічно активні компоненти Ti, Zr, Hf, Nb, V

Refractory crucibles, cups and other products made of material based on alkaline earth metals fluorides, which are used for casting, isothermal melting, high-temperature homogenization of chemically aggressive alloys containing a large amount of Ti, Zr, Hf, V, Nb, have been developed and manufactured. However, due to the large difference in coefficients of thermal linear expansion (KTLE) of fluorides and metals, mechanical compression of the crystallizing alloy in the crucible occurs.Such a difference in KTLE leads to cracking and destruction of crucibles. Crucibles that can be disassembled into several parts have been developed. This makes it possible to use such crucibles a large number of times for high-temperature homogenization and melting of chemically active alloys containing Ti, Zr, V, Nb.The developed refractory and special cups made from it also provide new opportunities for scientific research: studying the capillary characteristics (surface tension) of metal melts with a high content of reactive metals, which was previously practically impossible, as well as determining thermodynamic properties (enthalpy of mixing and formation of alloys, activity of components) for these alloys. Keywords: functional metal materials, refractory materials, fluorides of alkaline earth metals, collapsible crucibles, chemically aggressive metals.

Estimating the activation energy of crystallization in blends of poly L‐lactic acid and poly(propylene 2,5‐furan dicarboxylate) during isothermal melt crystallization using temperature dependent infrared spectroscopy

Estimating the activation energy of crystallization in blends of poly L‐lactic acid and poly(propylene 2,5‐furan dicarboxylate) during isothermal melt crystallization using temperature dependent infrared spectroscopy

Crystallization Kinetics and Melting Behavior of Novel Biobased Poly(butylene 2,5‐thiophenedicarboxylate)

AbstractPoly(butylene 2,5‐thiophenedicarboxylate) (PBTh) is a novel biobased semicrystalline polyester with superior thermal, mechanical, and gas barrier properties; however, the crystallization kinetics and melting behavior of PBTh have not been studied in detail so far. The crystallization kinetics of PBTh was first studied in this research. The Ozawa equation failed to describe the nonisothermal melt crystallization kinetics of PBTh due to the secondary crystallization. The crystallization activation energy value was calculated by the Friedman method, which was negative and exhibited the conversion dependence. The isothermal melt crystallization kinetics of PBTh was well described by the Avrami equation in a wide range of crystallization temperature from 84 to 120 °C. PBTh showed the fastest crystallization rate at 104 °C; moreover, the Avrami exponent value slightly varied between 2.2 and 2.7, suggesting the same crystallization mechanism within the investigated temperature range. PBTh displayed one or two melting endotherms depending on crystallization temperature, which was well explained by the melting, recrystallization, and remelting mechanism. In addition, the equilibrium melting point of PBTh was estimated to be 163.1 °C with the Hoffmann−Weeks equation.

The Isothermal Melting Kinetics of Ultra-High Molecular Weight Polyethylene Crystals.

The isothermal melting behaviors of ultra-high molecular weight polyethylene (UHMWPE) with different entangled states (i.e., nascent and melt-crystallized samples) are studied. For two kinds of UHMWPE samples, the result shows that the relative content of survived crystals (Xs) exponentially decreases with time and reaches a constant value. It is suggested that such a melting behavior is related to the observed nonlinear growth of crystals induced by the kinetically rejected entanglements accumulated at the growth front. Additionally, the exponential decay of Xs with time provides a characteristic melting time (τ) for the melting process. Compared to the melt-crystallized UHMWPE, the τ value of nascent UHMWPE is generally longer even in a higher temperature range, which is mainly because the former has a larger entanglement density difference. Furthermore, these observations demonstrate that UHMWPEs with different entangled states have an analogous melting mechanism since they exhibit a similar melting activation energy (≈1300kJ mol-1).

Influence of low contents of cellulose nanocrystals on the crystallization behavior of biobased Poly(propylene 2,5-furandicarboxylate)

To address the slow crystallization problem of poly(propylene 2,5-furandicarboxylate) (PPF), fully biobased PPF/cellulose nanocrystals (CNC) composites were prepared via a solution and casting method in this work at low CNC contents. The influence of CNC on the crystallization of PPF was studied in detail under various conditions, including nonisothermal and isothermal melt crystallization as well as nonisothermal and isothermal cold crystallization. The results revealed that the crystallization of PPF was enhanced under various crystallization conditions by CNC due to the heterogeneous nucleating agent role. From the isothermal crystallization kinetics study, the presence of CNC did not alter the crystallization mechanism of PPF. In addition, the thermogravimetric analysis results showed that both PPF and PPF/CNC composites displayed an excellent thermal stability. On the basis of the wide angle X-ray diffraction study, CNC did not change the crystal structure of PPF.

Synthesis and thermal investigation of poly(propylene terephthalate-co-2,5-furan dicarboxylate) copolyesters

Herein, we report the synthesis of new poly(propylene terephthalate-co-2,5-furan dicarboxylate) (PPTF) copolyesters with high molecular weight. Copolymers having various PT/PF molar fractions were synthesized via a two-stage melt polycondensation reaction from the dimethyl esters of terephthalic acid (TPA) or 2,5-furandicarboxylic acid (FDCA) with 1,3-propanediol. The combination of the crystallization ability of PPT with the biobased nature of PPF could make the PPTF copolyesters very appealing. Based on nuclear magnetic resonance spectroscopy, the structure and the actual molar composition of the copolyesters were identified and all the copolymers were found to be random. The investigation of the cocrystallization behavior with XRD showed that isodimorphism predominates, and DSC confirmed the random nature of the copolymers by the presence of a minimum value of the melting temperature as a function of copolymers’ composition. Moreover, isothermal melt crystallization studies were conducted. The melting point depression was investigated through the implementation of a plethora of thermodynamic models, and it was found that the incorporation of the PF units into the PPT crystal was preferred. In all cases, the effect of the furan ring was noticeable, as the copolyesters with higher PT content demonstrated higher crystallization rates. Finally, through polarized light microscopy the spherulitic morphology of the polymers was examined.

Improved Triplex-Forming Isoorotamide PNA Nucleobases for A-U Recognition of RNA Duplexes.

Four new isoorotamide (Io)-containing PNA nucleobases have been designed for A-U recognition of double helical RNA. New PNA monomers were prepared efficiently and incorporated into PNA nonamers for binding A-U in a PNA:RNA2 triplex. Isothermal titration calorimetry and UV thermal melting experiments revealed slightly improved binding affinity for singly modified PNA compared to known A-binding nucleobases. Molecular dynamics simulations provided further insights into binding of Io bases in the triple helix. Together, the data revealed interesting insights into binding modes including the notion that three Hoogsteen hydrogen bonds are unnecessary for strong selective binding of an extended nucleobase. Cationic monomer Io8 additionally gave the highest affinity observed for an A-binding nucleobase to date. These results will help inform future nucleobase design toward the goal of recognizing any sequence of double helical RNA.

Topochemical polymerization in microparticles of crystalline triazine-based monomers: Study by conventional and ultra-fast chip calorimetry

The work addresses complex thermal behavior of two triazine-based azide-alkyne monomers: 2-azido-4,6-bis(prop‑2-yn-1-yloxy)-1,3,5-triazine (ABPOT) and 2,4-diazido-6-(prop‑2-yn-1-yloxy)-1,3,5-triazine (DAPOT). Despite the similarity of the two AB2-type structures, the isothermal aging kinetics of DAPOT occurs significantly faster than that of ABPOT. During storage at room temperature, the chemical transformation starts in the crystalline state via topochemical reactions. Calorimetry, along with X-ray diffraction, makes it possible to probe transformations in isolated microcrystals including melting, oligomerization, polymerization and decomposition. The thermal behavior of microparticles can strongly differ from that of the same material in bulk due to enhanced contribution of the monomer evaporation at the particle surface at extremely high heating rates. The isothermal melting caused by the topochemical reaction and subsequent polymerization during long-term storage can be modeled using the correlated two-stage Avrami equation. The Avrami parameter corresponding to the amorphization process is close to unity while that of the polymerization is approximately 0.5. Consequently, both processes can likely be viewed as one-dimensional, with diffusion control dominating the polymerization process. The observed impact of long-term storage on the polymerization capability of the monomers may prove beneficial in synthesizing hyperbranched polymers through "green" topochemical methods.

Controllable cell structures of poly(ether-block-amide) foams via isothermal melt crystallization-foaming in supercritical CO2

A simple isothermal melt crystallization-foaming method was proposed to regulate the crystallinity (Xc) of PEBA to prepare poly(ether-block-amide) (PEBA) foams with uniform or bimodal cell structures by supercritical CO2. PEBA with different Xc values was obtained by controlling the isothermal crystallization temperature (Tiso) and time (tiso). The rheological and DSC results showed that the viscoelasticity and Xc of the samples increased with decreasing Tiso and increasing tiso. PEBA foams with Xc above 1.1% exhibited a distinct bimodal cell structure. The increased crystals acted as heterogeneous nucleation sites to increase the cell density from 6.8 × 106 cells/cm3 to 3.58 × 1010 cells/cm3. The size of large cells decreased from 206.4 µm to 48.0 µm, and the number ratio of small to large cells increased from 2.34 to 65 times. Finally, a mechanism to explain the cell structure variation in PEBA foams was proposed, providing a new strategy for regulating the cell structure of polymeric foams.

Methane hydrate exploitation characteristics and thermodynamic non-equilibrium mechanisms by long depressurization method

Methane hydrate as a clean energy source has attracted worldwide attention. Yet, thermodynamic evolution of methane hydrate-bearing deposits during long depressurization are still unclear. In this study, five methane yields (1.17, 0.77, 0.41, 0.21 and 0.13 Ls/min) for exploitation were conducted to simulate long depressurization of thousands of minutes. The rates of both depressurization and hydrate dissociation are linearly enhanced by increasing methane yield. Icing is always observed when the pressure reaches approximately 2.4 MPa unless the hydrate dissociation already finished. The accelerating effect of icing on hydrate dissociation can be strengthened by high methane yields. Thermodynamic responses at different exploitation stages are different because of the varied combinations of some non-equilibrium processes including gas expansion, hydrate dissociation, icing and melting as well as heat transfer. In this study, there are four thermodynamic non-equilibrium phenomena, respectively are gas expansion production, non-isothermal heat transfer, hydrate stationary dissociation and ice isothermal melting. The thermodynamic controlling mechanism and influencing factors of different exploitation stages are analyzed qualitatively or quantitatively, which reveals the temperature response of methane hydrate exploitation during long depressurization process. These results are helpful for understanding complex coupling mechanism of phase change and heat and mass transfer in methane hydrate exploitation.

Interaction of an anticancer benzopyrane derivative with DNA: Biophysical, biochemical, and molecular modeling studies

BackgroundSIMR1281 is a potent anticancer lead candidate with multi- target activity against several proteins; however, its mechanism of action at the molecular level is not fully understood. Revealing the mechanism and the origin of multitarget activity is important for the rational identification and optimization of multitarget drugs. MethodsWe have used a variety of biophysical (circular dichroism, isothermal titration calorimetry, viscosity, and UV DNA melting), biochemical (topoisomerase I & II assays) and computational (molecular docking and MD simulations) methods to study the interaction of SIMR1281 with duplex DNA structures. ResultsThe biophysical results revealed that SIMR1281 binds to dsDNA via an intercalation-binding mode with an average binding constant of 3.1 × 106 M−1. This binding mode was confirmed by the topoisomerases' inhibition assays and molecular modeling simulations, which showed the intercalation of the benzopyrane moiety between DNA base pairs, while the remaining moieties (thiazole and phenyl rings) sit in the minor groove and interact with the flanking base pairs adjacent to the intercalation site. ConclusionsThe DNA binding characteristics of SIMR1281, which can disrupt/inhibit DNA function as confirmed by the topoisomerases' inhibition assays, indicate that the observed multi-target activity might originate from ligand intervention at nucleic acids level rather than due to direct interactions with multiple biological targets at the protein level. General significanceThe findings of this study could be helpful to guide future optimization of benzopyrane-based ligands for therapeutic purposes.

Crystallization kinetics in the chiral compound showing the vitrification of the smectic CA* phase for moderate cooling rates

ABSTRACT The crystallization of the 3F5FPhF6 compound from the liquid crystalline antiferroelectric smectic CA* phase is investigated in various conditions using differential scanning calorimetry and polarizing optical microscopy. The kinetics of isothermal melt and cold crystallization is studied on the basis of the Avrami model. The kinetics of non-isothermal cold crystallization is analysed using the Ozawa model and the activation energy is determined by the Augis-Bennett, Friedman’s differential isoconversional and Matusita methods. The border temperature between the diffusion-controlled crystallization and thermodynamically-controlled crystallization is estimated. The high fragility index of 105 is determined from the α-process in the smectic CA* phase by the broadband dielectric spectroscopy. The metastable, conformationally disordered crystal phase, forming only via cold crystallization, is reported.

Synthesis, Thermal and Mechanical Properties of Fully Biobased Poly (hexamethylene succinate-co-2,5-furandicarboxylate) Copolyesters

Poly (hexamethylene succinate) (PHS) is a biobased and biodegradable polyester. In this research, two fully biobased high-molecular-weight poly (hexamethylene succinate-co-2,5-furandicarboxylate) (PHSF) copolyesters with low hexamethylene furandicarboxylate (HF) unit contents (about 5 and 10 mol%) were successfully synthesized through a two-step transesterification/esterification and polycondensation method. The basic thermal behavior, crystal structure, isothermal crystallization kinetics, melting behavior, thermal stability, and tensile mechanical property of PHSF copolyesters were studied in detail and compared with those of PHS. PHSF showed a decrease in the melt crystallization temperature, melting temperature, and equilibrium melting temperature while showing a slight increase in the glass transition temperature and thermal decomposition temperature. PHSF copolyesters displayed the same crystal structure as PHS. Compared with PHS, PHSF copolyesters showed the improved mechanical property. The presence of about 10 mol% of HF unit increased the tensile strength from 12.9 ± 0.9 MPa for PHS to 39.2 ± 0.8 MPa; meanwhile, the elongation at break also increased from 498.5 ± 4.78% to 1757.6 ± 6.1%.

Synthesis, Thermal Behavior, and Mechanical Properties of Fully Biobased Poly(Hexamethylene 2,5-Furandicarboxylate-Co-Sebacate) Copolyesters.

In this research, three fully biobased poly(hexamethylene 2,5-furandicarboxylate-co-sebacate) (PHFSe) copolyesters with low contents of hexamethylene sebacate (HSe) unit (10 mol%, 20 mol%, and 30 mol%) were successfully synthesized through a two-step transesterification/esterification and polycondensation method. The chemical structure and actual composition of PHFSe copolyesters were confirmed by hydrogen nuclear magnetic resonance. The thermal behavior and mechanical property of PHFSe copolyesters were investigated and compared with those of the poly(hexamethylene 2,5-furandicarboxylate) (PHF) homopolymer. Both PHFSe copolyesters and PHF showed the high thermal stability. The basic thermal parameters, including glass transition temperature, melting temperature, and equilibrium melting temperature, gradually decreased with increasing the HSe unit content. PHFSe copolyesters crystallized more slowly than PHF under both the nonisothermal and isothermal melt crystallization conditions; however, they crystallized through the same crystallization mechanism and crystal structure. In addition, the mechanical property, especially the elongation at break of PHFSe copolyesters, was obviously improved when the HSe unit content was greater than 10 mol%. In brief, the thermal and mechanical properties of PHF may be easily tuned by changing the HSe unit content to meet various practical end-use requirements.

Phase Sequence, Kinetics of Crystallization and Molecular Dynamics of the Chiral Liquid Crystalline Compound Forming a Hexatic Smectic Glass

The vitrification of the antiferroelectric hexatic smectic XA* phase and cold crystallization are reported for (S)-4′-(1-methylheptylcarbonyl)biphenyl-4-yl 4-[5-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]benzoate. The kinetics of isothermal cold crystallization and melt crystallization are investigated, revealing that both are controlled mainly by diffusion, as indicated by decrease in the characteristic crystallization time with increasing temperature of crystallization, with an activation energy of 114 kJ/mol. A weak relaxation process is detected in a crystal phase, with an activation energy of 38 kJ/mol, implying the conformationally disordered crystal phase. The estimated fragility parameter of the investigated glass former is equal to 94.5, which indicates rather high fragility.

Revisiting Non-Conventional Crystallinity-Induced Effects on Molecular Mobility in Sustainable Diblock Copolymers of Poly(propylene adipate) and Polylactide

This work deals with molecular mobility in renewable block copolymers based on polylactide (PLA) and poly(propylene adipate) (PPAd). In particular, we assess non-trivial effects on the mobility arising from the implementation of crystallization. Differential scanning calorimetry, polarized light microscopy and broadband dielectric spectroscopy were employed in combination for this study. The materials were subjected to various thermal treatments aiming at the manipulation of crystallization, namely, fast and slow cooling, isothermal melt- and cold-crystallization. Subsequently, we evaluated the changes recorded in the overall thermal behavior, semicrystalline morphology and molecular mobility (segmental and local). The molecular dynamics map for neat PPAd is presented here for the first time. Unexpectedly, the glass transition temperature, Tg, in the amorphous state drops upon crystallization by 8-50 K. The drop becomes stronger with the increase in the PPAd fraction. Compared to the amorphous state, crystallization leads to significantly faster segmental dynamics with severely suppressed cooperativity. For the PLA/PPAd copolymers, the effects are systematically stronger in the cold- as compared to the melt-crystallization, whereas the opposite happens for neat PLA. The local βPLA relaxation of PLA was, interestingly, recorded to almost vanish upon crystallization. This suggests that the corresponding molecular groups (carbonyl) are strongly involved and immobilized within the semicrystalline regions. The overall results suggest the involvement of either spatial nanoconfinement imposed on the mobile chains within the inter-crystal amorphous areas and/or a crystallization-driven effect of nanophase separation. The latter phase separation seems to be at the origins of the significant discrepancy recorded between the calorimetric and dielectric recordings on Tg in the copolymers. Once again, compared to more conventional techniques such as calorimetry, dielectric spectroscopy was proved a powerful and quite sensitive tool in recording such effects as well as in providing indirect indications for the polymer chains' topology.

Investigating the elastic, mechanical, and thermal properties of polycrystalline Mo2C under high pressure and high temperature

Understanding pressure-induced acoustic velocity-elasticity behavior has extraordinary significance in the fields of materials science and earth science. Herein, we used an advanced high-pressure high-temperature (HPHT) method to synthesize pure-phase β-Mo2C bulk ceramics. The sound velocity-elasticity behavior, thermodynamic properties, and mechanical behavior of the synthesized β-Mo2C ceramics were systematically investigated by in-situ high-pressure ultrasonic interferometry and theoretical calculations. The compressive and shear wave velocities, isothermal bulk modulus, shear modulus, Vickers hardness, fracture toughness, Debye temperature, and melting temperature of polycrystalline β-Mo2C exhibited a monotonic increase with increasing pressure. The experimental and calculation results showed that β-Mo2C had strong ductile behavior and was a ductile ceramic. Additionally, the temperature-hardness relationship of the as-synthesized polycrystalline β-Mo2C was investigated by in-situ high-temperature Vickers indentation measurements. The hardness of β-Mo2C gradually decreased with increasing temperature, and this ceramic still maintained a hardness as high as 12.3 GPa at 500 °C. These results suggest that the intrinsic mechanical and thermodynamic properties of β-Mo2C are dominated by its unique electronic structure and bonding mode.

Thermochromic Insulation Materials for Thermal Sensing and Overheat Prewarning

The real-time monitoring and scientific prewarning of local overheating for electrical equipment and electronic devices are of great significance but remain challenging. Therefore, in this work, a temperature-responsive microcapsule is prepared to endow insulating materials with the ability to thermally sense external temperature fields. Specifically, in the microcapsule/epoxy composites, the epoxy resin served as the polymer matrix was doped with reversible thermochromic microcapsules, acting as a temperature indicator whose color transition temperature is centralized between 45 °C and 55 °C. The test results indicate that the color of the prepared composites can sensitively vary in response to the fluctuations in external ambient temperature, which was illustrated by the behavior of isothermal melting. Further analysis reveals that composites are able to precisely locate multiple tiny overheated areas and present remarkable thermal durability undergoing 480 h of aging at 70 °C. The thermochromic materials designed in this article provide a new technological route for large-scale thermal sensing applications of electrical equipment.

Enhancement of the properties of poly(L-lactide)/poly(butylene adipate-co-terephthalate) blends by introduction of stereocomplex polylactide crystallites

Herein, poly(L-lactide) (PLLA), poly(butylene adipate-co-terephthalate) (PBAT), and poly(D-lactide) (PDLA) were melt compounded by a low-temperature method to prepare ternary blends with balanced properties. Differential scanning calorimetry results and torque changes during melt blending confirmed that the stereocomplex polylactide (SC-PLA) crystallites were formed. Network structure composed of SC-PLA crystallites was formed when the PDLA content was 5 wt%, at which point the melts of the blend exhibited solid-like rheological behavior. The formation of SC-PLA crystallites increased the viscosity ratio between PLLA and PBAT melt, which led to an increase of the size of dispersed PBAT phase. Furthermore, the SC-PLA crystallites could greatly promote both nonisothermal and isothermal melt crystallization behaviors of PLLA/PBAT/PDLA ternary blends because of their effective heterogeneous nucleation effect. The ternary blend containing 2 wt% PDLA showed good toughness and had an elongation at break of 211% compared to the elongation at break of 3.5% for neat PLLA. These biodegradable blends showed excellent melt crystallization abilities as well as tailored rheological and mechanical properties, which could demonstrate their potential as traditional non-biodegradable plastic substitutes.

Solid-liquid phase change subjected to unipolar charge injection from a circular wire electrode

A numerical investigation of isothermal melting of a dielectric phase change material in a square cavity subjected to unipolar charge injection from a circular wire electrode is reported. Governing equations for the solid-liquid phase change under the influence of an electric field include the Navier–Stokes equations for fluid flow, energy equation for thermal transport, Poisson equation for electric potential, and the Nernst-Planck equation for charge conservation. An enthalpy source-term based fixed grid approach is adopted to model the melting process. The governing equations that consider the coupled interactions between flow, thermal and electric fields are solved using the finite volume framework of OpenFOAM®. Time evolution of the melting rate, maximum flow velocity, mean Nusselt number, and mean Coulomb force in the electrohydrodynamic flow assisted melting process is mapped. The mechanism and role of electrohydrodynamic forces on influencing the net flow and melt interface morphology is studied. The competing viscous, buoyancy, and electric forces result in an altered flow pattern that aids the melting process. The enhancement in the rate of melting at different strengths of buoyancy and electrostatic forces is quantified. Furthermore, the effects of the radius of the circular wire electrode in relation to the cavity size are highlighted. A maximum of 63% enhancement in melting rate is observed within the considered parameter space.