Thermochimica Acta最新文献

Cholecalciferol, generally known as vitamin D₃ (VD), and calcium (Ca²⁺) are very common dietary co-supplements in the pharmaceutical formulations, as they are symbiotically and pharmacologically dependent. Development of the VD/Ca²⁺ formulation is highly challenging due to stability and solubility issues, mainly for VD instability toward temperature, light, oxygen and pH. In this study, VD was loaded into carrier which consisted of amorphous calcium carbonate (ACC) and hydroxypropyl methylcellulose acetate succinate (HPMCAS), yielding amorphous VD/ACC/HPMCAS formulation with various composition ratios. The structural and thermal stability study of the formulations was conducted to find that VD was a promising molecule for the stabilization of ACC even with the trace amount (0.6 %). On the other hand, ACC stabilized the amorphous state of VD; thus, they had a bidirectional stabilizing effect. The amount of VD played a significant role in thermal stabilization of the VD/ACC/HPMCAS formulations, for which kinetic analysis was performed. Using isoconversional expanded Friedman's model (FRM) activation energies of decomposition of the organic part were determined as 190, 133, and 114 kJ mol⁻¹ for VD/ACC/HPMCAS = 0.6/64.0/35.4, 2.3/56.8/40.9, and 4.9/52.6/42.5 formulations, respectively, revealing that the formulation with the highest amount of VD (4.9 %) was the least stable. The dissolution advantage for VD by amorphization was also demonstrated.

Here we suggest that integral isoconversional method, when applied in a mathematically correct way, can lead to satisfactory results with the least number of adjustable parameters. Differential and incremental methods are used in cases when the apparent activation energy, E, varies with the degree of conversion, α. However, in some cases the observed E(α) dependence can spuriously be induced by small variations in α(T) curves and there is only little to no benefit gained from allowing arbitrary change of E between adjacent conversion levels. As a result, the E(α) dependences are highly “fragile” and subject to minor variations in the experimental data. On the other hand, when the activation energy is optimized globally for all isoconversional levels, a significantly more robust estimate is obtained and the agreement between the experimental and simulated data is still plausible. The approach is demonstrated on two datasets which were evaluated with both variable E(α) dependence and with constant value of E.

In prior work (N. Shamim, Y. P. Koh, S. L. Simon and G. B. McKenna, "The glass transition of trinitrotoluene (TNT) by Flash DSC," Thermochimica Acta, 620, 36–39 (2015)) results for the glass transition temperature T_g of TNT based on measurements using a rapid chip calorimeter were reported. In that work a silver paste was used to contain the liquid TNT on the chip sensor. In the present work we show that there was an interaction between the paste and the TNT that cause the reported T_g to be approximately 10 °C higher than what we find for the material when placed directly on the chip calorimeter sensor. The reasons for the differences in T_g values and similarities in the glassy fragility index m from the different sample preparations are presented and discussed.

Monomers consisting of a methacrylate moiety attached to a short poly(ethylene glycol) (PEG) chain can be polymerized to form hydrogels with several applications such as the removal of dyes and heavy metals from wastewater. In this study, the radical copolymerization kinetics of oligo(ethylene glycol) methyl methacrylate (OEGMMA), and oligo(ethylene glycol) hydroxyethyl methacrylate (OEGHEMA), with acrylic acid (AAc) was investigated. In both cases, hydrogels were formed with cross-linked structure. The rate of polymerization and degree of conversion were measured using differential scanning calorimetry (DSC) operating under non-isothermal conditions, at several constant heating rates, or under isothermal conditions, at different constant reaction temperatures. Isoconversional kinetics were employed to estimate the effective activation energy of the polymerization. It was found that, in the homopolymer (POEGHEMA) bearing hydroxyl groups, polymerization under both isothermal and non-isothermal conditions proceeds faster compared to the polymer with methoxy groups (POEGMMA). This behavior is attributed to the interaction between the hydrogen in the hydroxyl group with the carbonyl oxygen of the methacrylic ester, which results in a reduction of the electron density at the double bond and therefore increases its reactivity. Monomer–monomer association through hydroxyl groups results in initially lower activation energy of POEGHEMA. As polymerization proceeds, the existence of aggregated hydroxyl structures in the POEGHEMA macromolecular chains result in higher activation energies and a more abrupt increase in the conversion time curve. The addition of acrylic acid results in higher copolymerization rates for both methacrylates, with P(OEGMMA-AAc) affected more, since in the case of OEGHEMA specific functional (i.e. hydroxyl) groups already exist in the macromolecules. The association of monomer-AAc through hydroxyl‑carbonyl groups, results in lower activation energies for both copolymers compared to the corresponding homopolymers. The significant contribution of isoconversional methods in the study of polymerization kinetics of hydrogels it was thus verified.

Ternary blends comprising poly(L-lactide) (PLLA), ethylene-ethyl acrylate-glyceryl methacrylate terpolymer (EGA), and poly(D-lactide) (PDLA) were melt blended using low-temperature method to obtain blends with balanced properties. The formation of stereocomplex polylactide (SC-PLA) crystals was verified by torque changes and differential scanning calorimetry (DSC) results. The SC-PLA crystals formed a percolating network in the PLLA matrix at a concentration of 2 wt% PDLA, and blend melt presented solid-like rheological behavior. Attributing to the increased viscosity ratio between PLLA and EGA, the size of dispersed EGA phase increased. The crystallization of PLLA was significantly promoted because of the nucleation effect of SC-PLA crystals. The PLLA/EGA/2 %PDLA ternary blends with elongation at break of 138 % and impact strength of 72.3 kJ/m² exhibited excellent toughness. These ternary blends demonstrated exceptional crystallization ability and tailored rheological and mechanical properties, opening a new path for developing high-performance PLLA-based materials in packaging industry.

From measurements of the terahertz spectra of commercial polyethylene terephthalate (PET) bottles, a linear correlation is observed between the melting point of the plastic and the absorption intensity around 4 THz. This result indicates that the melting point could be estimated using a non-contact method from the 4 THz peak intensity of the PET bottles. The melting points of the PET bottles used as samples in this study varied from 500 to 503 K. In the present-day recycling process, all waste PET is heated to the same temperature above the highest of the melting points. Based on the data obtained in this study, if could be possible to sense the melting point of waste PET in advance without heating and therefore to optimize the thermal budget of the recycling process.

This paper explores an efficient and eco-friendly epoxidation process using the phase transfer catalyst ([(C₁₈H₃₇)₂(CH₃)₂N]₃{PO₄[W(O)(O₂)₂]₄}), which offers more advantages over the use of carboxylic and inorganic acids as catalysts in the Prileschajew epoxidation process. Consequently, a study of the process's thermal hazards is imperative. The paper conducts a comprehensive analysis of the process, employing a combination of calorimetric techniques. The critical runaway temperature, stabilization temperature, and required heat dissipation rate to prevent thermal runaway reactions were calculated using the Semenov model. On-line Fourier transform infrared spectroscopy and reaction calorimetry were used to relate the reaction mechanism and exothermic behavior of the actual production process, and a reliable model was developed for the calculation of reaction enthalpy. The findings indicate that the thermal risk depends on the rate of double bond epoxidation, which provides offering valuable insights for safe industrial-scale ESO production.

Metal-free and green α‑hydroxy acids (AHA) such as l-malic (MA), DL-mandelic acid (MDL), and citric acid (CA) were successfully and effectively utilized as an effective initiator for the solvent-free ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). The performance of AHAs in the polymerization of ε-CL was completely and powerfully investigated via the non-isothermal differential scanning calorimetry (DSC). The proceed of ROP of ε-CL with AHAs could be real-time monitored by the obtained polymerization exotherms at different heating rates. The polymerization exotherms obtained from the ROP of ε-CL with CA occurred at a lower temperature range than MA, and MDA, respectively. From the kinetics study, the average activation energy (E_a) values for the ROP of ε-CL with CA (38.0 ± 1.8 kJ mol⁻¹) were lower than MA (45.2 ± 3.6 kJ mol⁻¹) and MDA (48.8 ± 6.2 kJ mol⁻¹). Using the first-order model fitting, the values of pre-exponential factor (lnA₀) for the ROP of ε-CL with CA, MA, and MDA were 7.0 ± 0.3, 8.5 ± 0.2, and 8.9 ± 0.3, respectively. The effectiveness of AHAs in the synthesis of PCL was clarified by conducting a larger-scale (4.0000 g) polymerization using conventional heating and microwave (MW) irradiation methods. From conventional heating, the green AHAs produced PCL with the number average molecular weight (M_n) and dispersity (Đ) values in the range of 5.18 × 10³ - 1.43 × 10⁴ g mol⁻¹ and 1.14–2.02, respectively. The irradiation by MW could enhance the synthesis of PCL by reducing the synthesis time. By using the MW power of 450 W and irradiation time of 30 min, the PLC was obtained from the ROP of ε-CL with MA and MDA initiators, and the M_n of the obtained PCL from MW heating was 4.28 × 10³ - 8.45 × 10³ g mol⁻¹. The mechanism for the ROP of ε-CL with all AHAs was proposed through the activated monomer mechanism.

This study used various analytical techniques to explore the structural characteristics of anthracite, lean coal, and bituminous coal. Thermogravimetric analysis assessed the combustion performance of individual coals and their blends. Results revealed distinct structural differences among the three coals. Although lean coal's functional groups distribution resembled bituminous coal, its carbon ordering aligned with anthracite. Increasing the proportion of bituminous coal minimally improved combustion in air. Under O₂ + N₂, rising oxygen levels lowered ignition and burnout temperatures, enhancing the comprehensive combustion index. At high heating rates, 30 %O₂ + 70 %CO₂ outperformed 30 %O₂ + 70 %N₂. The fitting performance of the double parallel random pore model (DRPM) was superior to that of the random pore model (RPM). Kinetic analysis suggested a DRPM for lean coal and bituminous coal co-combustion, unfolding in two stages. Activation energy increased with O₂ concentration in O₂ + N₂ but remained lower than in O₂ + CO₂.

Aliphatic polyamides (nylons) show a remarkable variability in terms of crystallographic structures, polymorphic transitions and crystal morphology despite all polymers of this family have a simple constitution that is based on amide groups and polymethylene segments. Nylons derived from diamines and dicarboxylic acids having different parity (e.g., even or odd) have peculiar characteristics due to the difficulty of establishing an optimal hydrogen-bonding geometry when molecular chains adopt a typical all trans conformation. In this work, two isomeric odd-even (nylon 7,10) and even-odd (nylon 10,7) polyamides with the same methylene/amide ratio have been studied. Specifically, crystallization kinetics have been evaluated from calorimetric data, while thermal degradation mechanisms from thermogravimetric analysis. Classical methods (e.g., Avrami) together with isoconversional analyses have been considered for crystallization studies, being found significant differences between both nylons in terms of nucleation and activation energies. The isoconversional analyses of the non-isothermal crystallization allowed to determine the temperature dependence of both the crystal growth and the overall crystallization rate that points out the slower crystallization process of nylon 10,7. Isoconversional methods (integral and differential) were applied to evaluate thermal degradation. The mechanism was similar for both nylons (e.g., A_3/2 and A_1.8 for nylons 7,10 and 10,7, respectively), although a remarkable difference was determined for the corresponding activation energies (175 and 210 kJ/mol for nylons 7,10 and 10,7, respectively).