Thermal stability of gelatin gels: Effect of preparation conditions on the activation energy barrier to melting

10, 20, and 40 wt.% aqueous gelatin gels were prepared under isothermal (annealing at 15, 20, and 25 °C for 15 to 120 min) and nonisothermal (cooling at 1 °C min⁻¹) conditions. Isoconversional kinetic analysis of DSC data on gel melting (gel-sol transition) of all types of gels revealed significant variations in the activation energy throughout the process. Activation energy barrier to melting of isothermally prepared gels was in the range 160-190 kJ mol⁻¹ and found to increase with increasing the annealing temperature that was the major effect discovered. Activation energy barrier to melting of nonisothermally prepared gels was determined to be around 120-140 kJ mol⁻¹ and increase with increasing the concentration. Local reversibility of the gel melting was demonstrated by using temperature modulated DSC.     

Winterisation of waste cooking oil methyl ester to improve cold temperature fuel properties

Waste cooking oil methyl ester (WCOME) was winterised at 1, 0, −1 and −2°C following a 4×2 factorial design with one replication per cell. The process was carried out by filtration and both the filtrate (solid phase) and the liquid phase were analysed by gas chromatography (GC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Cold filter plugging point (CFPP) and calorific values were measured.Temperatures of 0 and −1°C in conjunction with the quickest cooling rate (0.1°C min⁻¹) and 15-24 h of cooling gave the most successful results in terms of fuel properties.Improvements in the low temperature properties of the winterised fuel were reflected by a reduction of saturated fatty acid methyl esters (SFAME) in the composition by 1.5-6%, by a decrease in the CFPP values by 2-4°C and by a shift of the DSC high temperature melting peak (approx. 5°C) towards lower temperatures in comparison to the original fuel. Calorific values of the winterised WCOME did not significantly change and boiling temperatures increased (approx. 26%) in comparison to the non-winterised WCOME.     

High temperature thermosets derived from benzocyclobutene-containing main-chain oligomeric carbosilanes

A series of benzocyclobutene-carbosilane thermosets derived from benzocyclobutene-containing oligomeric silylenephenylene (PBSEPs) were synthesized. DSC analysis results demonstrated that the reaction of PBSEPs presumably took place within the temperature range of 200–350 °C. FT-IR, ¹H NMR and ¹³C NMR results demonstrated that this exothermic reaction was attributed to a [4 + 2] cycloaddition. DSC results further revealed that benzocylcobutene linked with silylene exhibited lower exothermic temperature compared with that linked with Si(CH₃)₂. Chemical simulation results attributed the lower temperature to lower steric hindrance and greater activity. DSC and TGA analysis of the thermosets showed high T_g and thermostability, and good pyrolysis yield.     

Poly(trimethylene terephthalate) copolyester containing ethylene glycol moieties. Part 1: Crystallization kinetics and melting behavior

A copolyester was characterized to have 91 mol% trimethylene terephthalate unit and 9 mol% ethylene terephthalate unit in a random sequence by using ¹³C NMR. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics in the temperature range (T_c) from 180 to 207 °C. The melting behavior after isothermal crystallization was studied by using DSC and temperature modulated DSC (TMDSC). The exothermic behavior in the DSC and TMDSC curves gives a direct evidence of recrystallization. No exothermic flow and fused double melting peaks at T_c = 204 °C support the mechanism of different morphologies. The Hoffman-Weeks linear plot gave an equilibrium melting temperature of 236.3 °C. The kinetic analysis of the growth rates of spherulites and the morphology change from regular to banded spherulites indicated that there existed a regime II → III transition at 196 °C.     

Characterization, crystallization kinetics and melting behavior of poly(ethylene succinate) copolyester containing 5 mol% trimethylene succinate

Copolyester was synthesized and characterized as having 94.4 mol% ethylene succinate units and 5.6 mol% trimethylene succinate units in a random sequence as revealed by NMR. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics of this copolyester in the temperature range (T_c) from 30 to 80 °C. The melting behavior after isothermal crystallization was studied by using DSC and temperature modulated DSC (TMDSC) by varying the T_c, the heating rate and the crystallization time. DSC and TMDSC curves showed triple melting peaks. The melting behavior indicates that the upper melting peaks are primarily due to the melting of lamellar crystals with different stabilities. A small exothermic curve between the main melting peaks gives a direct evidence of recrystallization. As the T_c increases, the contribution of recrystallization gradually decreases and finally disappears. The Hoffman-Weeks linear plot gave an equilibrium melting temperature of 108.3 °C. The kinetic analysis of the spherulitic growth rates indicated that a regime II → III transition occurred at ∼65 °C.     

X-ray studies of multiple melting behavior of poly(butylene-2,6-naphthalate)

Multiple melting behavior of poly(butylene-2,6-naphthalate) (PBN) was studied with X-ray analysis and differential scanning calorimetry (DSC). Double endothermic peaks L and H attributed to the α-form crystal modification, a small peak attributed to the β-form crystal modification, and a new shoulder peak S at a lower temperature of peak H appeared in the DSC melting curves. Wide-angle X-ray diffraction patterns of the samples isothermally crystallized at 200 and 220 °C were obtained at a heating rate of 1 K min⁻¹, successively. In this heating process, change of crystal structure and increase of quantity of the β-form crystallites could not be observed up to the final melting. With increasing temperature, the diffraction intensity decreased gradually and then increased distinctly before a steep decrease due to the final melting. The X-ray analysis clearly proved the melt-recrystallization during heating. The β-form crystal modification was formed during slow heating process in the high temperature region.     

Crystal structure and morphology influenced by shear effect of poly(l-lactide) and its melting behavior revealed by WAXD, DSC and in-situ POM

Shear effect on crystal structure, morphology and melting behavior of poly(l-lactide) (PLLA) were investigated by wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and in-situ polarized optical microscope (POM). Influences of steady shear on PLLA melt was investigated during annealing. The impacts of crystallization temperature (T_c) and shear rate (SR) on crystal structure, crystal growth rate, nucleation, and melting behavior were studied. It could be clarified into the low (<20 s⁻¹) and high shear rate ranges in terms of the crystal structure, growth rate, nucleation and melting behavior. It was found that phase transition and recrystallization during heating completely changed the stack structure. A new stable structure was established after a temperature jump in the melting range. Cylindric morphology arranged with spherulite was also observed in the polymer films at high T_c influenced by shear effect.     

Dehydration reactions and kinetic parameters of gibbsite

Dehydration reactions and the corresponding kinetics of gibbsite [γ-Al(OH)₃] were analyzed using non-isothermal thermoanalysis and the Kissinger equation. It is concluded that the starting temperature of the dehydration reaction of γ-Al(OH)₃ rises with increasing heating rate of the system. At a heating rate of 10 °C min⁻¹, γ-Al(OH)₃ has lost part of its crystalline water, and was completely transformed into boehmite (γ-AlOOH)) at about 317 °C. However, γ-AlOOH did not lose the residual crystalline water entirely, and did not change into amorphous Al₂O₃ until the system was above 700 °C. The kinetic energy needed to convert γ-Al(OH)₃ to γ-AlOOH, and γ-AlOOH to amorphous Al₂O₃, was calculated by differential scanning calorimetry (DSC) with activation energies of 108.50 and 217.24 kJ mol⁻¹, pre-exponential factors of 2.93 × 10⁹ and 8.30 × 10¹³, and reaction orders of 0.96 and 1.06, respectively. The kinetic parameters of dehydration reactions for γ-Al(OH)₃ obtained using the derivative thermogravimetric method (DTG) are very similar to that of obtained by DSC.     

DSC and synchrotron-radiation X-ray diffraction studies on crystallization and polymorphic behavior of palm stearin in bulk and oil-in-water emulsion states

The crystallization and polymorphic behavior of palm stearin (PS) in a bulk state and in oil-in-water (O/W) emulsion droplets (average diameter, 1.7±0.3 μm) was observed by using DSC, optical microscopy, and in situ X-ray diffraction with synchrotron radiation (SR-XRD). For the bulk sample the DSC measurements revealed three main exothermic peaks at approximately 31 (large), 21 (small) and 3°C (medium) on cooling, and broad endothermic peaks at approximate −3 (small), 8, 15 to 25 (medium), and 37 and 53°C upon heating. The SR-XRD patterns taken during cooling from 60 to −5°C clarified that the DSC exothermic peaks around 31 and 3°C corresponded to crystallization of the α form of high-melting and low-melting fractions, respectively, and that the occurrence of β′ corresponded to the small exothermic peak around 21°C. The XR-XRD patterns taken during heating from −5 to 60°C demonstrated that the DSC endothermic peaks corresponded to the following transformation processes: melting of α of the low-melting fraction (−3°C), melt-mediated transformation from α to ∇′ (15–25°C), melting of β′ (36°C), and melting of β (53°C) of the high-melting fraction. As for the O/W emulsion sample, the DSC and SR-XRD measurements during the cooling and heating processes exhibited basically the same behavior as that of PS in the bulk state, except that β′ did not crystallize during the cooling process, and the temperatures of crystallization of α, melt-mediated α→β′→β transformation, and melting of β were lower in the emulsion droplets than in the bulk state.     

Thermal transition behaviors in a liquid crystalline polyesterimide

The crystallization and melting behaviors of the polyesterimide, derived from N,N′-hexane-1,6-diylbis(trimellitimides), 4,4′-dihydroxybenzophenone and p-hydroxybenzoic acid, were investigated by using polarized light microscopy (PLM), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The nematic texture of the polyesterimide was observed on raising temperature to 265 °C, and the nematic phase was found to convert to isotropic melt beginning from about 300 °C, the ordered nematic micro-domains still surviving after 320 °C. Isothermal crystallization of the samples was performed at 180 °C after heating samples at various temperatures in the range of 265-360 °C, and a completed crystallization peak can appear on DSC curves up to the heating temperature of 360 °C in the presence of the nematic phase and the ordered nematic micro-domains. Non-isothermal crystallization of the samples at different cooling rate was carried out, and the melting of the resulting crystals exhibits double endotherms. It is indicated that a fast crystallization in the nematic phase forms relatively more ordered crystals, which melt at higher temperature, and a slow crystallization in the isotropic phase or in the biphasic melt produces poor crystals, which melt at lower temperature. The crystallized polyesterimide was annealed, which has a minor effect on the high-melting peak but leads to a continual shifting of the low-melting peak to higher temperature with increasing annealing temperature or annealing time. WAXD patterns indicated that the structural transform was not found during annealing process.     

Evaluation of polyesters from renewable resources as alternatives to the current fossil-based polymers. Phase transitions of poly(butylene 2,5-furan-dicarboxylate)

Poly(butylene 2,5-furan dicarboxylate) (PBF) is an alipharomatic polyester that can be prepared using monomers derived from renewable resources such as 2,5-furan dicarboxylic acid and 1,4-butanediol. In the present work the thermal behavior of PBF was studied. Multiple melting was observed during heating traces of samples isothermally crystallized from the melt using differential scanning calorimetry (DSC). The wide angle X-ray diffraction (WAXD) patterns did not reveal the presence of a second crystal population, or a crystal transition upon heating. DSC study showed that the phenomena are closely related to recrystallization. Temperature modulated DSC (TMDSC) tests indeed evidenced enhanced recrystallization. The equilibrium melting point was estimated to be 184.5 °C using the linear Hoffman–Weeks extrapolation. The heat of fusion of the pure crystalline polymer was found equal to 129 J/g or (27.35 kJ/mol), a little lower than that of PBT. The Lauritzen–Hoffman secondary nucleation theory was used and the surface energy values and the work of chain folding were found to be comparable to those of PBT, but quite lower than those of poly(ethylene terephthalate) (PET). The non-isothermal crystallization on cooling and the cold-crystallization of quenched samples were also studied. Condensed spherulites were observed on isothermal crystallization under large supercoolings by using polarized optical microscopy (POM), while the spherulites turned to ring-banded morphology at higher temperatures. In every case the nucleation density was high.     

Thermogravimetric analysis and kinetics of coal/plastic blends during co-pyrolysis in nitrogen atmosphere

Investigations into the co-pyrolytic behaviours of different plastics (high density polyethylene, low density polyethylene and polypropylene), low volatile coal and their blends with the addition of the plastic of 5 wt.% have been conducted using a thermogravimetric analyzer. The results indicated that plastic was decomposed in the temperature range 438–521 °C, while the thermal degradation temperature of coal was 174–710 °C. The overlapping degradation temperature interval between coal and plastic was favorable for hydrogen transfer from plastic to coal. The difference of weight loss (?W) between experimental and theoretical ones, calculated as an algebraic sum of those from each separated component, was 2.0–2.7% at 550–650 °C. These experimental results indicated a synergistic effect during plastic and coal co-pyrolysis at the high temperature region. In addition, a kinetic analysis was performed to fit thermogavimetric data, the estimated kinetic parameters (activation energies and pre-exponential factors) for coal, plastic and their blends, were found to be in the range of 35.7–572.8 kJ/mol and 27–1.7 × 10³⁸ min^− 1, respectively.     

Solid-state reorganization,melting and melt-recrystallization of conformationally disordered crystals (α′-phase) of poly (l-lactic acid)

Conformationally disordered α′-crystals of poly (l-lactic acid) were formed by crystallization of the melt at high supercooling at 95 °C. Analysis of their melting temperature as a function of the crystallinity revealed absence of crystal thickening during isothermal crystallization. Annealing of α′-crystals between the crystallization temperature of 95 °C and their zero-entropy production melting temperature of 150 °C leads to their stabilization, mainly by solid-state reorganization. Heating faster than 30 K s⁻¹ suppresses reorganization and permits superheating of the α′-phase. Consequently, isothermal melting followed by melt-recrystallization becomes accessible. Melting is completed within few hundreds of milliseconds, and melt-recrystallization is about two orders of magnitude faster than crystallization of the isotropic melt at identical temperature. The time required for melting decreases with superheating and increases with the lateral dimension of the lamellar crystals. Laterally extended lamellae require long time for melting of the outer crystal layers, which allows stabilization of the central crystal part. These crystal remnants then serve as seed for immediate recrystallization. In case of complete melting of smaller lamellae, melt-recrystallization is retarded but still distinctly faster than cold- and melt-crystallization, due to incomplete isotropization of the melt.     

Morphology, reorganization and stability of mesomorphic nanocrystals in isotactic polypropylene

The morphology and thermodynamic stability of crystals of isotactic polypropylene (iPP) were analyzed as a function of the path of crystallization by atomic force microscopy (AFM) and differential scanning calorimetry (DSC). Samples were melt-crystallized at different rates of cooling using a “controlled rapid cooling technique”, and subsequently annealed at elevated temperature. Mesomorphic equi-axed domains with a size less than 20 nm were obtained by fast cooling from the melt at a rate larger about 100 K s⁻¹. These domains stabilize on heating by growing in chain direction and cross-chain direction, to reach a maximum size of about 40-50 nm at a temperature of 433 K, with the quasi-globular shape preserved. Annealing at 433 K additionally triggers formation of different types of lamellae. It is suggested that these lamellae either develop by coalescence of nodules, or by recrystallization from the melt. The transition from the disordered mesomorphic structure, evident at ambient temperature after fast crystallization, to monoclinic structure on heating at about 340 K occurs at local scale within existing crystals, and cannot be linked to complete melting of mesomorphic domains and recrystallization of the melt. The temperature of melting of initial mesomorphic domains, after reorganization at elevated temperature, is identical to the temperature of melting of rather perfect lamellae, obtained by initial slow melt-crystallization, followed by annealing. The close-to-identical temperatures of melting of these crystals of largely different shapes are confirmed by model calculations, using the Gibbs-Thomson equation. Modeling of the melting temperature reveals that nodular crystals, stabilized by annealing at high temperature, exhibit a similar fold-surface as lamellar crystals.     

Recrystallization studies on isotropic cold-crystallized PET: Influence of heating rate

The nanostructural changes associated to the multiple melting behaviour of isotropic cold-crystallized poly(ethylene terephthalate) (PET) have been investigated by means of simultaneous wide- and small-angle X-ray scattering, using a synchrotron radiation source. Variations in the degree of crystallinity, coherent lateral crystal size and long period values, as a function of temperature, for two different heating rates are reported for cold-crystallized samples in the 100-190 °C range. The Interface Distribution Function analysis is also employed to provide the crystalline and amorphous layer thickness values at various temperatures of interest. Results suggest that samples crystallized at both low (T_a = 100-120 °C) and high (T_a = 160-190 °C) temperatures are subjected to a nearly continuous nanostructural reorganization process upon heating, starting immediately above T_g (≈80 °C) and giving rise to complete melting at ≈260 °C. For all the T_a investigated, a melting-recrystallization mechanism seems to take place once T_a is exceeded, concurrently to the low-temperature endotherm observed in the DSC scans. For low-T_a and slow heating rates (2 °C/min), a conspicuous recrystallization process is predominant within T_a + 30 °C ≤ T ≤ 200 °C. In contrast, for high-T_a, an increasingly strong melting process is observed. For both, high- and low-T_a, an extensive structural reorganization takes place above 200 °C, involving the appearance of new lamellar stacks simultaneously to the final melting process. The two mechanisms should contribute to the high-temperature endotherm in the DSC scan. Finally, the use of a high heating rate is found to hinder the material's overall recrystallization process during the heating run and suggests that the high-temperature endotherm is ascribed to the melting of lamellae generated or thickened during the heating run.     

Rheological properties of selected bituminous coals

The rheological behaviour of several bituminous coals has been evaluated by means of a constant torque Gieseler plastometer, utilizing both steadily increasing (3 °C min⁻¹) and isothermal (410 °C) temperature conditions. The results indicated that under either thermal treatment, the coals investigated showed predominantly pseudoplastic behaviour. The greatest degree of non-Newtonian behaviour occurred at or near the temperature of maximum fluidity for coals heated at 3 °C min⁻¹, or within the early stages of melting when isothermal heating was used. Empirical relationships relating the Gieseler fluidities to apparent viscosity were derived from the data.     

Multiple melting behaviour of poly(ethylene terephthalate)

Differential scanning calorimetry (DSC) and temperature modulated DSC (MTDSC) have been used to investigate the melting behaviour of poly(ethylene terephthalate) (PET). Multiple melting endotherms were observed even at high heating rates, e.g. 160 K min⁻¹ and these have been attributed to the presence of two different distributions of lamella thickness and re-crystallisation (reorganisation) on heating. This has been confirmed by MTDSC—the presence of endotherms and an exotherm in the reversing component of the heat flow during heating. Examination of the endotherms of samples heating stepwise indicated that further crystallisation took place above the isothermal crystallisation temperature (T_c). Some part of this was associated with lamella thickening and crystal perfecting. The multiple melting endotherms observed are a consequence of the balance between the melting and re-crystallisation and the lamella thickness distribution existing within the sample, prior to heating. The triple melting endotherms observed are attributed to the melting of secondary and primary lamellae produced on crystallisation and to thickened lamellae produced during heating to the melting point.     

Hydrophilic hollow fiber membranes for water desalination by the pervaporation method

Hydrophilic ion-exchange membranes based on sulfonated polyethylene hollow fibers were manufactured, and their suitability for a water pervaporation process was studied for possible application in water desalination systems. The effects of the following parameters on the average water flux were determined: membrane properties (diameter (0.4–1.8 mm) and wall thickness (0.05–0.18 mm)); charge density (0.6–1.2 meq g⁻¹); and operating conditions (brine inlet temperature (30–68°C), air sweep velocity (0–6 m s⁻¹), and salt concentration in the feed brine (0–3 M)). A water flux of 0.8–3.3 kg m⁻² h⁻¹ was obtained using this type of hollow fiber with an inlet brine temperature of 25–65°C. It was found that, for our application, the optimal specifications for the ion-exchange hollow fibers were an outside diameter of 1.2 mm, a wall thickness of 0.1 mm, and an ion-charge density of about 1.0 meq g⁻¹. This information is required as basic data for the design of a prototype water desalination system based on a pervaporation system that uses this type of ion-exchange hollow fiber membrane.     

Evolution of the structure and mechanical behaviour of a carbon foam at very high temperatures

Mechanical behaviour of a low density carbon/carbon composite at very high temperature is studied in relation with its microstructure. This composite is a syntactic foam made of carbon microbeads with a binder and voids. The resulting geometrical density is 0.3 g cm⁻³. Compressive tests from room temperature up to 3100 °C with a very high heating rate (180 °C s⁻¹) have been conducted. Intermediate temperature tests have also been performed and show an obvious modification of mechanical behaviour from around 2000 °C. This result is related to a sudden modification of structure and texture of the carbonaceous matter during the high temperature mechanical test. A strong plastic deformation occurs when the mechanical experiment is performed at 3100 °C whereas the material elastically deforms at room temperature.     

The physical–chemical properties and electrocatalytic performance of iridium oxide in oxygen evolution

An IrO₂ anode catalyst was prepared by using the Adams method for the application of a solid polymer electrolyte (SPE) water electrolyzer. The effect of calcination temperature on the physical–chemical properties and the electrochemical performance of IrO₂ were examined to obtain a low loading and a high catalytic activity of oxygen evolution at the electrode. The physical–chemical properties were studied via thermogravimetry–differential scanning calorimetry (TG–DSC), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical activity was investigated by using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry in 0.1 mol L⁻¹ H₂SO₄ at room temperature. The optimum condition was found to be at the calcination temperature of 500 °C, where the total polarization reached a minimum at high current densities (>200 mA cm⁻²). The optimized catalyst was also applied to a membrane electrode assembly (MEA) and stationary current–potential relationships were investigated. With an optimized catalytic IrO₂ loading of 1.5 mg cm⁻² and a 40% Pt/C loading of 0.5 mg cm⁻², the terminal applied potential difference was 1.72 V at 2 A cm⁻² and 80 °C in a SPE water electrolysis cell.