Thermal Phase Transition Temperature Research Articles

Ionic liquid/polybenzimidazole/SiO2 composite membranes for medium temperature operating

Fuel cells (FC) with proton exchange membranes (PEMs) are seen as an alternative energy source due to their efficiency, power density, low emissions, and reliable energy supply. Proton exchange membranes based on polybenzimidazole have shown potential for operating at high and medium temperatures to enhance FCs performance. New composite membranes made from m-PBI and diethylammonium mesylate [DEAH/MsO] ionic liquid were prepared trough a solution casting method. Silica nanopowder (SiO2) was used as an inorganic filler at varying concentrations (0.5–20 wt%). The ionic liquid content in the membranes ranged from 1 to 2.5 mol per mole of PBI monomer units. Our study is focused on the thermal properties, such as thermal stability and phase transition temperatures, morphology, conductivity, and electrochemical stability of the membranes. The influence of the inorganic filler on these properties was also discussed.

Probabilistic thermal analysis and phase transition temperature optimization for low-rise residential buildings with temperature-adaptive radiative cooling roofs

This paper conducts an optimization study on the energy performance of applying temperature-adaptive radiative cooling (TARC) coatings as building roofs. To provide guidance for the application of TARC roofs, a method for determining the optimal phase transition temperature of TARC coatings for application in buildings is proposed. Then, the thermal performance of the TARC roofs is analyzed for buildings located in five different climatic zones in China based on the probability density function and cumulative distribution function. Finally, the energy efficiency of the TARC roof and the effect of the phase transition temperature on the energy savings potential of the TARC roof are discussed in detail. The results showed that the TARC roof can address the issue of increased heating loads caused by passive daytime cooling (PDRC) roofs during the heating season. Furthermore, the annual total energy savings potential of the TARC roof is 6.3–23.5 MJ/m2 for buildings located in five different climatic zones in China. In addition, the annual energy savings of buildings with TARC roofs can be further improved by 3–7% once the optimal phase transition temperature is adopted.

Photo-triggered Phase Transition of Crystals and Photoactuation.

Photo-triggered phase transition is a new type of phase transition in which a photochromic crystal with a thermal phase transition transforms into an identical high-temperature phase in a temperature region lower than the thermal phase transition temperature upon light irradiation. Here, we report a second crystal that exhibits a photo-triggered phase transition, thereby demonstrating that the photo-triggered phase transition is a general phenomenon that occurs in crystals. When the chiral salicylidenephenylethylamine crystal was irradiated with ultraviolet (UV) light, the photo-triggered phase transition occurred in the temperature range -30 to -10 °C. The photo-triggered phase transition is induced by local stress due to trans-keto molecules produced by photoisomerization near the irradiated surface. Crystal cantilevers exhibited stepwise bending by the combination of the photo-triggered phase transition and photoisomerization. Alternate irradiation with UV and visible light achieved locomotion of single crystals driven by repeated stepwise bending. Finally, a detailed comparison of photo-triggered and non-photo-triggered phase transition crystals revealed that a sufficient molecular conformation change in affordable crystal voids, smooth photoisomerization, and most likely a chiral molecular arrangement are required for inducing the photo-triggered phase transition.

Reactive organophosphorus retardant-enhanced sorbitol epoxy resins with highly thermal and flame-retardant performance

As a low-cost sugar-based chemical, sorbitol has been less reported in recent years in the field of epoxy resins. In view of the low viscosity characteristics of its epoxy monomer, it is of great significance to prepare the advanced resin system via a solvent-free green curing process. As known, most of the existing flame-retardant networks contain organophosphorus flame retardants. To achieve excellent flame- retardant effect, a large amount of phosphorus was added to the resin matrices. As a result, apart from exacerbating the migration of organophosphorus, their thermal stability and phase transition temperature are further reduced, thus weakening the thermal properties of the material to a certain extent. In order to overcome those problems, we introduced ultra-low amounts of phosphorus to achieve efficient flame retardant into the sorbitol -based epoxy resin. It was found that the introduction of EP-DOPO in 0.55 P wt% amount could achieve a UL94-V0 grade flame retardant effect. Different from other phosphorus-containing flame retardants, reactive EP-DOPO favored an improvement in both thermal stability and phase transition temperature, meanwhile, it effectively avoids the phosphorus migration as well.

Significantly enhanced thermal stability of HMX by phase-transition lysozyme coating

A new robust bio-inspired route by using lysozyme aqueous solution for surface modification on 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) was described in this paper. HMX crystals were coated by in situ phase transition of lysozyme (PTL) molecules. The HMX decorated by PTL was characterized by SEM, XRD, FTIR and XPS, demonstrating a dense core-shell coating layer. The coverage of lysozyme on HMX crystal was calculated by the ratio of sulfur content. The surface coverage increased from 60.5% to 93.5% when the content of PTL was changed from 0.5 wt% to 2.0 wt%, indicating efficient coating. The thermal stability of HMX was investigated by in situ XRD and DSC. The thermal phase transition temperature of HMX (β to δ phase) was delayed by 42 °C with 2.0 wt% PTL coating, which prevented HMX from thermal damage and sensitivity by the effect of PTL coating. After heating at 215 °C, large cracks appeared in the naked HMX crystal, while the PTL coated HMX still maintained intact, with the impact energy of HMX dropped dramatically from 5 J to 2 J. However, the impact energy of HMX with 1.0 wt% and 2.0 wt% coating content (HMX@PTL-1.0 and HMX@PTL-2.0) was unchanged (5 J). Present results potentially enable large-scale fabrication of polymorphic energetic materials with outstanding thermal stability by novel lysozyme coating.

Development of empirical correlations for assessing the CuAlMn based shape memory alloy thermal switch phase transition temperatures

Development of empirical correlations for assessing the CuAlMn based shape memory alloy thermal switch phase transition temperatures

Tailoring of thermal expansion and phase transition temperature of ZrW2O8 with phosphorus and enhancement of negative thermal expansion of ZrW1.5P0.5O7.75

ZrW2O8 is a typical isotropic negative thermal expansion material with cubic structure. However, quenching preparation, pressure phase transition and metastable structure influence its practical applications. Adopting P to part-substitute W for ZrW2−x P x O8–0.5x has decreased the sintering temperature and avoided the quenching process. When x = 0.1, ZrW1.9P0.1O7.95 with a stable cubic structure can be obtained at 1150 °C. The thermal expansion coefficient is tailored with the P content, and phase transition temperature is lowered. When x = 0.5, thermal expansion coefficient attains −13.6 × 10−6 °C−1, ZrW1.5P0.5O7.75 exhibits enhance negative thermal expansion property. The difference of electronegativity leads to the decrease of phase transition temperature with the increase of P content. The different radii of ions lead to new structure of materials when P substitutes more. The results suggest that the P atom plays the stabilization role in the crystal structure of ZrW2−x P x O8–0.5x .

Revisiting the phase transitions of the Dicke model

The Dicke model exhibits a variety of phase transitions. The quantum phase transition from the normal phase to the super-radiant phase is marked by a dramatic change in the scaling of the participation ratio. We find that the ground state in the super-radiant phase exhibits multifractality manifest in the participation ratio scaling as the square root of the full Hilbert space dimension. The thermal phase transition temperature, for which we obtain an exact analytical expression, is strikingly captured by the mutual information between two spins. In the excited state quantum phase transition within the super-radiant phase, we discover a new upper cut-off energy; the central energy band between the lower and upper cut-off energies shows distinctly different behaviour. This finding is corroborated with the aid of several eigenvector properties: von Neumann entanglement entropy between spins and bosons, the mean photon number, concurrence between two spins, and participation ratio. Thus we obtain a unified picture for the three different kinds of phase transitions.

Uticaj nanopunila i masterbača na svojstva silokasnih materijala

Nanocomposites based on siloxane and silicon(IV)oxide nanoparticles (with a hydrophilic and hydrophobic surface) were synthesized to design the desired final properties of the composite material. Masterbatch, a mixture of siloxane containing vinyl functional groups and reinforcing fillers, was added to improve the mechanical properties and topology of siloxane networks. Silicon(IV)oxide was added in amounts of 1, 5, 10, and 20 wt% and masterbatch in amounts of 5 and 10 wt% to examine the effect of the amounts of fillers and masterbatch in the synthesized samples. Fourier transform infrared spectroscopy was used to analyze the chemical structure of the obtained materials. Transmission electron microscopy (TEM) was used to examine the dispersion of filler particles in siloxane nanocomposites. To examine the thermal stability and phase transition temperature of siloxane materials, thermogravimetric analyzes (TGA) and differential scanning calorimetry (DSC) were performed. The addition of masterbatch did not lead to a significant difference in melting temperature, but stoichiometry was disturbed, which decreased the thermal stability compared to samples without masterbatch. The addition of masterbatch to nanocomposites with hydrophilic fillers increases both elongations at break and tensile strength. According to the results, the combination of masterbatch and nanofillers affects the properties of siloxane materials, which could enable obtaining materials with the desired properties.

Triethylamine-Based Salts: Protic Ionic Liquids or Molecular Complexes?

Amines can interact with protic acids with different degrees of proton transfer, which can lead to the formation of both hydrogen-bonded complexes and protic ionic liquids (PILs) in which the hydrogen bond between the cation and anion contributes to the formation of ion pairs. This work is devoted to studying the degree of proton transfer from different acids (hydrochloric, nitric, phosphoric, acetic, propionic, benzoic, and salicylic) to triethylamine (TEA). The results of quantum-chemical calculations based on the density functional theory (DFT) and thermal (phase transition and destruction temperatures) and physicochemical (conductivity, viscosity) characteristics of the compounds show that TEA interaction with acetic and propionic acids leads to the formation of hydrogen-bonded complexes. The B3LYP-GD3 method also shows that the interaction between TEA and benzoic acid is more energetically favorable for the formation of a molecular complex, whereas the obtained experimental data are more characteristic of a protic ionic liquid. For the other acids studied, the calculation and experimental data confirm salt formation. The geometric and energy parameters of the H-bond have been calculated both in the molecular complexes and in the ion pairs. The QTAIM theory was used to localize critical points of the hydrogen bonds and to calculate their properties.

Diethylamine-based ionic liquids: quantum chemical calculations and experiment

The structural and energetic characteristics of the compounds formed by the reaction of diethylamine (DEA) with protic acids (sulfuric (H2SO4), methanesulfonic (MsOH), trifluoromethanesulfonic (TfOH), and para-toluenesulfonic (TsOH)) were examined using quantum chemical computations (B3LYP-GD3/6-31++G(d,p)). The strength of the hydrogen bonds in the ion pairs formed was quantitatively estimated by the QTAIM theory and NBO analysis. The results of the quantum chemical computations and the obtained thermal (phase transition and decomposition temperatures) and physicochemical (viscosity and conductivity) characteristics indicate that the reactions of DEA with the acids afford salts. The salts with the melting points higher than 100 °C are formed in the case of DEA/OTf(OTs), while protic ionic liquids are produced in the case of DEA/OMs(HSO4).

Microstructure and performance of hybrid laser-arc welded high-strength low alloy steel and austenitic stainless steel dissimilar joint

Effects of hybrid laser-arc welding process parameters on the weld formation of a dissimilar steel joint were studied. A full penetrated weld with minimum defects was obtained under the optimized parameters. Different thermal conductivities and phase transition temperature resulted in a wider heat affected zone (HAZ) of high-strength low alloy steel (EH36) than that of austenitic stainless steel (316L). Owing to the limited penetration of arc, the laser zone could be only heated by laser while the hybrid zone was heated by not only laser but also arc, resulting in higher heat input and larger grain size in hybrid zone than in laser zone. The addition of filler wire led to higher contents of chromium (Cr) and nickel (Ni) in hybrid zone than laser zone, which inhibited the formation of austenite and led to the formation of martensite in laser zone. Small grain size and existence of martensite resulted in higher hardness of laser zone than hybrid zone. Tensile samples of both laser zone and hybrid zone failed at the EH36 side with massive dimples distributing on the fracture surfaces, indicating good tensile property of the welds and typical characteristics of ductile fracture. Corrosion resistance of laser zone was weaker than that of hybrid zone, which was closely related to the formation of martensite with poor corrosion resistance instead of austenite with good corrosion resistance.

Polymer electrolytes based on PVdF-HFP doped with protic ionic liquids containing different cations

Several protic ionic liquids based on trifluoroacetate anion and different alkylammonium (diethyl-, dimethylethyl-, tributyl-, diisopropylethyl-) cations have been synthesized and investigated by different methods. Based on the results obtained earlier for triethylammonium and triethanolammonium trifluoroacetates, the influence of the cations structure on the thermal (phase transition and decomposition temperatures) and electrochemical (conductivity, electrochemical window, ionicity) properties of the PILs has been studied. For the investigated salts, the specific conductivity within the cation series DMEA > DEA > TEA > TBA decreases. These PILs have been used as dopants to prepare membranes based on (poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP)) whose electrochemical and thermal properties have also been investigated. The electric conductivity of these membranes was lower than that of the PILs themselves because of the confined mobility of the ions. However, the conductivity decrease was not too significant and did not exceed one order of magnitude. Doping PILs into the PVdF-HFP copolymer lowered the polymer crystallinity degree. The values of the electrochemical window of the studied PILs and PVdF-HFP–PILs membranes ranged from 0.9 to 1.3 V and from 6 to 7 V, respectively.

PH/Thermo-Dual Responsive Tunable In Situ Cross-Linkable Depot Injectable Hydrogels Based on Poly(N-Isopropylacrylamide)/Carboxymethyl Chitosan with Potential of Controlled Localized and Systemic Drug Delivery.

In the current study, cytocompatible in situ cross-linkable pH/thermo-dual responsive injectable hydrogels were prepared based on poly(N-isopropylacrylamide) and carboxymethyl chitosan, i.e., poly(CMCS-g-NIPAAm). The prepared formulations were aimed to be used as drug depot of 5-fluorouracil (5-FU) after subcutaneous administration in vivo. The phase transition from sol-gel state under physiologic temperature range was analyzed and confirmed by tube titling and optical transmittance measurements. The viscoelastic properties of gel formulations were confirmed by rheology determination via time sweep, temperature, and continuous ramp test. Oscillatory swelling cycles confirmed temperature effect and structural changes. pH and temperature sensitivity of dual responsive gels were analyzed at different pH and temperature programs. In vitro drug release profile displayed that developed formulations have the highest release in acidic pH at 25°C. The safety of blank gel formulations was evaluated against L929 cell lines via MTT assay and confirmed cytocompatibility with no detectable toxicity. In vitro cytotoxic potential of drug-loaded gels against HeLa and MCF-7 cancer cell lines confirmed that 5-FU has controlled cytotoxic potential in depot form in comparison to free 5-FU solution. The IC50 values for free 5-FU (21 ± 05μg/ml and 18 ± 66μg/ml) were found higher in comparison to the loaded form. The copolymer structure formation was confirmed by NMR and FTIR spectroscopic analysis. TG and DSC analysis proved the thermal stability and phase transition temperatures of pure and copolymer samples, while SEM analysis showed the porous nature of in situ formed hydrogels. It was concluded from the results that the developed formulations have pH/temperature sensitivity with potential of systemic and intratumoral controlled drug delivery properties.

Synthesis and characterization of antimicrobial polylactide via ring‐opening polymerization and click chemistry methods

AbstractNovel biodegradable polylactide (PLA) copolymers bearing pendant antimicrobial agent groups were successfully fabricated with a combination of ring‐opening copolymerization and copper(I)‐catalysed azide–alkyne cycloaddition click reaction in a two‐step reaction procedure. First, biodegradable PLA copolymers bearing azido groups were synthesized by the ring‐opening copolymerization of l‐lactide and 2,2‐ bis(azidomethyl)trimethylene carbonate in the presence of 1‐dodecanol as protic co‐initiator and tin(II) 2‐ethylhexanoate (Sn(Oct)2) as the catalyst. Then, alkyne functionalized quaternary ammonium salts were attached onto the azido groups of the copolymers via a Huisgen 1,3‐dipolar cycloaddition reaction to give PLA imparting antimicrobial activity. The chemical structure and composition of the copolymers were clearly confirmed using 1H NMR and Fourier transform infrared spectroscopies and gel permeation chromatography. Thermal phase transition temperatures (Tm and Tg) and the thermal stability of the polymers were investigated by DSC and TGA experiments, respectively. The antimicrobial activity tests were carried out against Gram‐negative (Escherichia coli) and Gram‐positive (Staphylococcus aureus) bacteria by the drop plate method. It was observed that antimicrobial agents are more active in the polymeric form than in the monomeric form. Also, the activity depends on the compositional ratio and the length of the alkyl group on the ammonium salts. © 2018 Society of Chemical Industry

Mathematical Characterization of Thermo-reversible Phase Transitions of Agarose Gels

ABSTRACTThe thermal phase transition temperatures of high (HMP) and low melting point (LMP) agarose gels were investigated by using UV–vis spectroscopy techniques. Transmitted light intensities from the gel samples with different agarose concentrations were monitored during the heating (gel-sol) and cooling (sol–gel) processes. It was observed that the transition temperatures, Tm, defined as the location of the maximum of the first derivative of the sigmoidal transition paths obtained from the UV–vis technique, slightly increased by increasing the agarose concentration in both the HMP and LMP samples. Here, we express the phase transitions of the agar-water system, as a representative of reversible physical gels, in terms of a modified Susceptible-Infected-Susceptible epidemic model whose solutions are the well-known 5-point sigmoidal curves. The gel point is hard to determine experimentally and various computational techniques are used for its characterization. Based on previous work, we locate the gel point, T0, of sol-gel and gel-sol transitions in terms of the horizontal shift in the sigmoidal transition curve. For the gel-sol transition (heating), T0 is greater than Tm, i.e. later in time, and the difference between T0 and Tm is reduced as the agarose content increases. For the sol-gel transition (cooling), T0 is again greater than Tm, but it is earlier in time for all agarose contents and moves forward in time and gets closer to Tm as the agarose content increases.

Fully bio-based poly(propylene succinate) synthesis and investigation of thermal degradation kinetics with released gases analysis

One of the most important information about polyesters is their thermal stability and phase transition temperatures. These characteristics give information about the promising behavior of the polyester during processing. In this work, linear bio-based polyester polyols were prepared with the use of succinic acid and 1.3-propanediol (both with natural origin). As a polycondensation catalyst was used tetraisopropyl orthotitanate (TPT), which different amount was employed. The thermogravimetric analysis allowed to observe high thermal stability and one step of the thermal decomposition. This analysis affirmed also that the catalyst content did not influence the thermal degradation characteristics of the prepared polyols. Nevertheless, it has huge importance in the context of thermal degradation kinetics. It was determined with the use of Ozawa, Flynn, and Wall and Kissinger's methods to verifying catalyst impact on the thermal degradation kinetics. Moreover, probable mechanism of the prepared bio-based polyols thermal degradation was proposed based on the QMS results.

Effect of phase transition temperature and thermal conductivity on the performance of Latent Heat Storage System

The heat transfer properties of phase change materials (PCMs) are of importance for the efficiency assessment on the heat storage and release in solar thermal systems. Previous research results demonstrate that the increase of thermal conductivity of PCMs can enhance the thermal performance in solar thermal systems; however, the corresponding mechanism is not clear. To this end, this work investigates the influence of PCMs properties on storage performance of solar thermal systems. First, experimental testing was conducted to verify the effectiveness of a thermal simulation model in the heat storage and release process. Then, the proposed simulation model was used to investigate the performance of several commonly used PCMs in the process of melting and solidification. The influence of thermal conductivity and phase transition temperature on the thermal storage properties was analyzed. The analysis results demonstrated that the influence of phase transition temperature on the thermal system performance was greater than that of the thermal conductivity in short time, while the thermal conductivity contributed greater influence on the system performance in long time. The phase transition temperature hardly affected the total system efficiency if given enough heat transfer time. The findings in this work may provide a theoretical reference for the selection of heat storage materials.

Benzimidazole based mesogenic Schiff-bases: Synthesis and characterization

Two homologous series of mesogenic Schiff-bases, N-4-((alkoxy)-(phenyl-3-hydroxy-4-(4-(5-methylbenzimidazol))-2-alkoxysalicylaldimine)benzoate (7a–d) and N-4′-(5-methyl-benzimidazole)-phenyl-4-alkoxysalicylaldimine (8a–d) incorporating benzimidazole moiety have been prepared and the molecular structures studied by FT-IR, NMR and ESI-MS spectrometry. Mesogenic behaviour was investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and variable temperature powder X-ray diffraction (PXRD) techniques. Changing the spacer (ester-linked to non-ester linked) of the Schiff-base results in enhancement of thermal stability and phase transition temperature. The members of series-I show monotropic SmA while those of series-II reflect enantiotropic SmA mesomorphism. An electrochemical study of a representative Schiff base in each series (7d and 8c) showed an electrical band gap 1.26eV and 1.22eV respectively.

LCST-type thermoresponsive behaviour of interpolymer complexes of well-defined poly(poly(ethylene glycol) methacrylate)s and poly(acrylic acid) synthesized by ATRP

Unexpected tremendous effect of poly(acrylic acid) (PAA) as polydonor on the thermoresponsive behaviour of the comb-like poly(poly(ethylene glycol) methacrylate) (PPEGMA500) with short poly(ethylene glycol) (PEG) side chains as H-bond acceptor, both synthesized by atom transfer radical polymerization (ATRP), has been found indicating the formation of a unique class of interpolymer complexes between these well-defined polymers. Strikingly, the aqueous solutions of the PPEGMA500-PAA interpolymer complexes possess LCST-type thermoresponsive behaviour with hysteresis at significantly lower temperatures (∼40–50 °C) than that of PPEGMA500, which is attributed to the decreased hydrophilicity of the complexes. It is revealed that thermal phase transition temperatures (TPT) of the PPEGMA500-PAA interpolymer complexes have a minimum in the near of 1.3 ethylene glycol/acrylic acid (EG/AA) monomer unit molar ratio, in accordance with the dynamic light scattering (DLS) results, and the TPT can be fine tuned by varying the EG/AA ratio, the molecular weight of the PPEGMA500 and pH of the solution.