Phase Boundary-controlled Reaction Research Articles

Isothermal and non-isothermal decomposition mechanisms of bastnaesite during the hydrogen-based mineral phase transformation

The hydrogen-based mineral phase transformation-flotation process can realize the efficient development of iron-bearing rare earth ores. The isothermal and non-isothermal decomposition mechanisms of bastnasite in the hydrogen atmosphere were studied to optimize the hydrogen-based mineral phase transformation process. The thermal decomposition of bastnasite first produced REOF, which would react with oxygen to produce rare earth oxides and fluorides during the analysis process. The thermal decomposition was an endothermic reaction, and the generated gas were mainly CO2 and trace CO, with a mass loss of about 20%. Increasing the temperature can greatly promote the thermal decomposition. After thermal decomposition, the particles showed a layered structure, and many parallel slits run through the particles. The porosity and specific surface area of the particles were significantly improved. The apparent activation energy of isothermal decomposition was 71.80 ± 5.40 kJ/mol, and the kinetic mechanism was random nucleation and growth model (n = 2). The apparent activation energy of non-isothermal decomposition was 201.96 kJ/mol, and the kinetic mechanism was phase-boundary controlled reaction mechanism (n ≈ 2).

Water vapor effect on the physico-geometrical reaction pathway and kinetics of the multistep thermal dehydration of calcium chloride dihydrate.

This study investigated how water vapor influences the reaction pathway and kinetics of the multistep thermal dehydration of inorganic hydrates, focusing on CaCl2·2H2O (CC-DH) transforming into its anhydride (CC-AH) via an intermediate of its monohydrate (CC-MH). In the presence of atmospheric water vapor, the thermal dehydration of CC-DH stoichiometrically proceeded through two distinct steps, resulting in the formation of CC-AH via CC-MH under isothermal conditions and linear nonisothermal conditions at a lower heating rate (β). Irrespective of atmospheric water vapor pressure (p(H2O)), these reaction steps were kinetically characterized by a physico-geometrical consecutive process involving the surface reaction and phase boundary-controlled reaction, which was accompanied by three-dimensional shrinkage of the reaction interface. In addition, a significant induction period was observed for the second reaction step, that is, the thermal dehydration of CC-MH intermediate to form CC-AH. With increasing p(H2O), a systematic increase in the apparent Arrhenius parameters was observed for the first reaction step, that is, the thermal dehydration of CC-DH to form CC-MH, whereas the second reaction step exhibited unsystematic variations of the Arrhenius parameters. At a larger β in the presence of atmospheric water vapor, the first and second reaction steps partially overlapped; moreover, an alternative reaction step of the thermal dehydration of CC-MH to form CaCl2·0.3H2O was observed between these reaction steps. The physico-geometrical phenomena influencing the reaction pathway and kinetics of the multistep thermal dehydration were elucidated by considering the effects of atmospheric and self-generated water vapor in a geometrically constrained reaction scheme.

Pyrolysis mechanism of bastnaesite during roasting in N2 atmosphere: An in-situ study of gas products, phase transition, and kinetics

The application of suspension magnetization roasting technology in ferruginous rare earth bearing ore has gained significant attention. Bastnaesite, due to its pyrolysis characteristics, has been proposed as a reductant for iron minerals. In this study, the pyrolysis of bastnaesite was investigated through various in-situ methods. The results showed that during pyrolysis, CeOF and CO2 were first generated, followed by the reaction of CO2 with Ce2O3 and Ce7O12 to produce CO. Pyrolysis occurred initially on the particle surface and then progressed inward. Increasing the roasting temperature promoted the pyrolysis of bastnaesite and CO generation. The addition of CO2 during the roasting process enhanced the formation of CO. The pyrolysis kinetic mechanisms under isothermal and non-isothermal conditions were phase-boundary controlled reaction mechanism (n = 4) and phase-boundary controlled reaction mechanism (n = 2–4), respectively. This detailed analysis of the pyrolysis behavior of bastnaesite facilitates the efficient and low-carbon development of ferruginous rare earth ores through suspension magnetization roasting.

Preparation of building ceramic bricks using waste residue obtained by mutual treatment of electrolytic manganese residue and red mud

The waste residue (ER) obtained by the mutual treatment of electrolytic manganese residue and red mud. ER is a kind of silicon and aluminum waste residue with low pollution. In order to alleviate the shortage of silicon-aluminum raw materials in ceramic bricks industry. In this paper, ER is used as a part of silicon-aluminum raw material to prepare building ceramic bricks. The results show that the impurity ions (Fe3+, Na+, K+, etc.) in ER can promote the formation of liquid phase during the calcination process. This makes the ceramic bricks can be successfully fired at 1150 °C. The mineral composition of ceramic bricks prepared by ER is anorthite, quartz, corundum, mullite and glass phase, and the corresponding contents are 40.31 wt%, 2.48 wt%, 6.81 wt%, 2.58 wt% and 35.07 wt%, respectively. The layer stacking of plate anorthite is closely combined with the corundum to form a microscopic block. The rod-like mullite is interspersed in the microscopic block to form a skeleton structure. This microstructure makes the mechanical properties of the ceramic bricks superior, and the compressive strength is 138.2 MPa. The sintering process of ceramic bricks prepared by ER can be divided into four stages. Stage 1 (25 °C–200 °C) mainly involved the removal of free water and crystal water and the decomposition of part of AlO(OH) into γ-Al2O3, the corresponding kinetic mechanism is phase boundary chemical second-order reaction mechanism. Stage 2 (200 °C–400 °C) mainly involved the residual AlO(OH) decomposed into γ-Al2O3, the corresponding kinetic mechanism is phase boundary–controlled reaction and one-dimensional movement mechanism. Stage 3 (400 °C–900 °C) mainly involved the γ-Al2O3 transformed into α-Al2O3, CaCO3 and CaSO4 decompose into CaO, and CaO reacts with SiO2 and α-Al2O3 to form (Ca,Na)(Al,Si)4O8 and CaAl2Si2O8, the corresponding kinetic mechanism is random nucleation and growth mechanism. Stage 4 (900 °C–1200 °C) mainly involved the melting of (Ca,Na)(Al,Si)4O8, and the continuous formation of CaAl2Si2O8, Ca(Al, Fe)12O19 and mullite, the corresponding kinetic mechanism is Mampel power law exponential form nucleation reaction mechanism.

Physico-geometrical Kinetic Aspects of the Thermal Dehydration of Trehalose Dihydrate

When linearly heating trehalose dihydrate (TH-DH) at a relatively high heating rate (β ≥ 1.5 K min–1), the thermal dehydration begins in the solid state and transitions to the liquid state at the melting point of TH-DH (∼370 K). The complex mass loss and phase change characteristics observed during the thermal dehydration is due to the contributory effects of various physical phenomena, including melting of TH-DH and anhydride products. The reactions entirely occurred in the solid state under isothermal and nonisothermal conditions at a low heating rate (β ≤ 1.0 K min–1), and its kinetics were described by the physico-geometrical consecutive process comprising the surface reaction and subsequent phase boundary-controlled reaction. The reactions initiated at approximately the melting point of TH-DH under a significantly high atmospheric water vapor pressure and proceeded mostly in the liquid state exhibiting a single-step mass loss behavior. The thermal dehydration accompanied by liquefaction demonstrated an autocatalytic kinetic behavior, which was regulated by the evaporation of the water vapor from the surface of viscous liquid particles. The kinetic findings for the reactions in the solid and liquid states allow the physico-geometrical interpretation for the complex mass loss and phase change behaviors during heating TH-DH at relatively high β values.

Enhancing the thermal reactivity of AP crystals by coating of Al-based bi-metal nanocomposites

A series of AP reactive composites modified by Al/TMs bi-metal compounds including Al/Ti, Al/Ni and Al/Co have been successfully prepared by acoustic resonance mixing. Various analysis techniques have been utilized and results indicate that the Al/Co exhibits the strongest thermal interaction effect on the thermal decomposition of ammonium perchlorate (AP) by merging reaction steps with enhanced thermal reactivity. In comparison to the pure AP, the peak temperatures of high temperature decomposition (HTD) of AP in the presence of Al/Ti, Al/Ni and Al/Co are significantly decreased by 60.3 °C, 76.9 °C and 100.2 °C, respectively. Moreover, the apparent activation energies obtained for AP@Al/Ti, AP@Al/Ni and AP@Al/Co composites are 96.5 kJ·mol−1, 103.5 kJ·mol−1 and 53.6 kJ·mol−1, respectively. The thermolysis physical models of AP in composites of AP@Al/Ti, AP@Al/Ni and AP@Al/Co appear to follow three-dimensional random nucleation and nucleus growth (A3), whereas the A3, chain scission and phase boundary-controlled reaction are found to be the most appropriate models to represent the three-step thermal decomposition of pure AP. Owing to the high thermal interaction activity, as great bi-metal fuels, Al/TMs can also be considered as outstanding additives for AP thermal decomposition.

Thermal properties and pyrolysis kinetics of phosphate-rock acid-insoluble residues

In the phosphorous–sulphur two-step process for the clean production of phosphoric acid, a phosphate-rock acid-insoluble residue (PAIR) is a solid filter residue obtained via the phosphoric acid acidolysis of phosphate rock (PR). PAIR combined with other raw materials can be used to prepare cement, ceramics and glasses, opening a potential avenue for large-scale PAIR utilisation. However, the preparation of such materials requires high-temperatures calcination. Understanding the high-temperature thermal properties of PAIR can enable its more targeted comprehensive utilisation or disposal. In this study, the thermal properties and pyrolysis kinetics of PAIR were systematically studied using a multiple heating rate method based on thermogravimetric analysis and a kinetic model. Results showed that from room temperature to 1200 °C, the main changes in the PAIR were the complete removal of fluorine and sulphur, partial removal of phosphorus and conversion of quartz to cristobalite. Moreover, during these processes, H2O(g), NH3, N2, CO2, SO2, P2O5(g), CO, CF3+ and organic gases were volatilised. Herein, the pyrolysis kinetics of PAIR is divided into five stages. Stage 1 (conversion rate ɑ: 0.05–0.2) conforms to the random nucleation and growth as well as the Avrami–Erofeev (n = 2/3) mechanism; the corresponding mechanism function is F(ɑ) = [−Ln(1 − ɑ)]2/3. Stage 2 (ɑ: 0.2–0.4) conforms to the first-order chemical reaction mechanism; the corresponding mechanism function is F(ɑ) = −Ln(1 − ɑ). Stage 3 (ɑ: 0.4–0.6) conforms to the phase boundary–controlled reaction and one-dimensional movement mechanism; the corresponding mechanism function is F(ɑ) = ɑ. Stage 4 (ɑ: 0.6–0.8) conforms to the three-dimensional diffusion process (Jander model); the corresponding mechanism function is F(ɑ) = [1 − (1 − ɑ)1/3]2. Stage 5 (ɑ: 0.6–0.95) conforms to the one-dimensional diffusion process; the corresponding mechanism function is F(ɑ) = ɑ2.

Thermal reactivity of metastable metal-based fuel Al/Co/AP: Mutual interaction mechanisms of the components

In this paper, a group of novel intermetallic metastable composites Al/Co@AP has been designed, and three methods including ultrasonic dispersion, mechanical grinding and spray-drying have been attempted for their preparation. The latter one has demonstrated to be the most appropriate means, by which the core–shell structured Al/Co@AP with desired properties could be successfully obtained. The thermal reactivity of Al/Co/AP composites prepared differently has been investigated and compared by TG/DSC technique. It has been shown that the heat release rate of AP in DSC curve was largely increased in the presence of Al/Co when spray-drying technique was used, which may be attributed to the increased nuclear site by the intimate contact. The initial reaction temperature of AP in Al/Co@AP was decreased by 7.8 °C and the heat releases by the thermal decomposition of AP and the intermetallic reaction between Al and Co were enhanced by 52.6 % and 67.7% in comparison with that of pure AP and Al/Co. The types of major gaseous products of Al/Co@AP are almost identical to that of pure AP, which include HCl, H2O, N2O and NO2. However, the concentrations of NO2 and N2O in gaseous products for Al/Co@AP are lower than that observed for pure AP, which may be due to the partial consumption of N element by the reaction of Al with acidic substance (HNO3) decomposed from AP. In addition, the AP in Al/Co@AP composite decomposes in one-step with the apparent activation energy (Ea) of 98.8 kJ·cm−3. The decomposition process of the AP in Al/Co@AP composite follows two-dimensional nucleation and growth model(A2), whereas the pure AP follows different physical models, which are close to three-dimensional nucleation and growth, chain scission and phase boundary-controlled reaction (contracting area) models. The intermetallic reaction between Al and Co in Al/Co@AP is merged into one-step following the A2 physical model.

Graphene Oxide–(Ferrocenylmethyl) Dimethylammonium Nitrate Composites as Catalysts for Ammonium Perchlorate Thermolysis

Graphene oxide has attracted attention due to its excellent catalytic properties, and it is expected to be used in the field of burning rate catalysis. To investigate the synergistic effect of graphene oxide and ferrocene derivatives, graphene oxide–(ferrocenylmethyl) dimethylammonium nitrate composites (GO–FcMANO3) were prepared and characterized by X-ray photoelectron spectroscopy (XPS), Raman analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM), and their thermal stability and catalytic effect on ammonium perchlorate (AP) were studied by thermogravimetry–differential scanning calorimetry (TG–DSC) techniques. Based on interaction region indicator (IRI) analysis, electrostatic potential (ESP) and energy decomposition analysis on the basis of forcefield (EDA-FF), the dispersion effect is the dominant component of interaction energy, and the contribution of electrostatic attraction is small. The frontier molecular orbitals illustrate that a higher highest occupied molecular orbital (HOMO) energy makes GO–FcMANO3 more susceptible to oxidization and is more conducive to catalyzing AP decomposition. The TG data illustrated that the presence of GO in the composites increased the thermal stability of FcMANO3 during AP decomposition. The catalytic effect of GO–FcMANO3 for AP decomposition manifested in two ways: it advanced the peak temperature in the second stage of AP decomposition and reduced the exothermic temperature width between two stages; it also increased the heat released during AP decomposition. Combined kinetic analysis and the Friedman method provided more detailed information about the catalytic behavior of composites for AP thermolysis, and GO–FcMANO3 composites decreased the Ea of the second stage of AP decomposition. The introduction of GO influenced the decomposition physical model of FcMANO3 for AP catalysis. The first stage of AP decomposition transformed from random nucleation and two-dimensional growth of nuclei model (A2) to random nucleation and three-dimensional growth of nuclei model (A3), and the second stage changed from the phase boundary-controlled reaction (R2) to random nucleation and two-dimensional growth of nuclei model (A2).

Thermal Dehydration of Lithium Sulfate Monohydrate Revisited with Universal Kinetic Description over Different Temperatures and Atmospheric Water Vapor Pressures

This study aims to universally describe the kinetic features of the thermal dehydration of lithium sulfate monohydrate across different temperatures (T) and atmospheric water vapor pressures (p(H2O)) as a model reaction of the thermal dehydration of crystalline hydrates. The features of the physicogeometrical consecutive process, comprising the induction period (IP)–surface reaction (SR)–phase boundary-controlled reaction (PBR), and the effect of p(H2O) on kinetic behavior were revealed experimentally under various heating conditions. Then, the accommodation function (AF), accounting for the effect of p(H2O) on the kinetic behavior, was derived by considering the consecutive/concurrent elementary steps of SR and PBR at the atomic and molecular levels. The universal kinetic descriptions for the IP and subsequent mass-loss process were realized by introducing the AF into formal kinetic equations and using the isoconversional kinetic relationship. Furthermore, by combining the physicogeometrical consecutive IP–SR–PBR(n) model and the formulated AF, the universal kinetic descriptions for each physicogeometrical reaction step across different T and p(H2O) conditions were obtained, which reveal novel kinetic features of each reaction step and these variations as the reaction step advances. The significance of the revealed kinetic features is discussed through demonstrating the development of the novel kinetic approach.

Isothermal Coal-Based Reduction Kinetics of Fayalite in Copper Slag.

The coal-based reduction of fayalite was characterized using thermogravimetric (TG) and differential TG methods with reduction temperatures from 1123 to 1273 K. The results of fayalite isothermal reduction indicate that the reduction process is divided two stages. The corresponding apparent activation energy E was gained using the isoconversional and model-fitting methods. At the first stage, the effect of temperature on the reduction degree was not clear, and the phase boundary chemical reaction was the controlling step, with an apparent activation energy E value of 175.32–202.37 kJ·mol–1. At the second stage, when the temperature was more than 1123 K, the conversion degree and the reaction rate increased nonlinearly with increasing temperature, and two-dimensional diffusion, three-dimensional diffusion, one-dimensional diffusion, and phase boundary-controlled reaction were the controlling stages, with an apparent activation energy E ranging from 194.81 to 248.96 kJ·mol–1. For the whole reduction process, the average activation energy E and pre-exponential factor A were 185.07–225.67 kJ·mol–1 and 0.796–0.797 min–1, respectively.

Physico-Geometrical Kinetic Modeling of the Thermal Decomposition of Magnesium Hydroxide

The thermal decomposition of Mg(OH)2 was selected to realize an integrated kinetic understanding of the thermal decomposition of inorganic solids by correlating the physico-geometrical mechanisms and the effect of the product gas presented in the reaction atmosphere. Herein, the mechanistic features of the reaction, as revealed by a systematic kinetic study on a reaction in flowing dry N2 gas, were reported as the first part of the study. In spite of the smooth mass loss under various heating conditions, the formal kinetic analysis based on an assumption of single-step reaction indicated a possible multistep reaction comprising the surface reaction (SR) and subsequent internal phase boundary-controlled reaction (PBR). Two physico-geometrical models were applied to find the mechanistic features of the overall reaction. One is a single reactant-body model with an assumption of independent SR and PBR. The other is based on the physico-geometrical consecutive SR–PBR model in the assemblage of reactant particl...

Novel kinetics model for adsorption of pollutant from wastewaters onto zeolites. Kinetics of phenol adsorption on zeolite-type silicalite

The kinetics of isothermal adsorption of phenol from an aqueous solution onto the zeolite-type silicalite was investigated. Zeolite-type silicalite was synthesized and its basic physico-chemical properties were determined. Isothermal adsorption kinetics curves of phenol on zeolite-type silicalite were measured at temperature range from 283 to 313 K. By applying Friedman’s differential isoconversional method it was found that the adsorption of phenol on silicalite has one rate determining step. By using the ‘model-fitting’ method it was established that the kinetic of adsorption can be described with theoretical kinetic model of the two-dimensional phase-boundary controlled reaction (model R2). The kinetic parameters, activation energy ([Formula: see text]) and preexponetial factor ( lnA = 14.1 min−1) of phenol adsorption were calculated. The thermodynamic parameters, standard enthalpy (Δ H*), standard entropy (Δ S*) and standard free Gibbs energy of adsorption (Δ G*) were calculated and discussed. A novel model for the kinetics of pollutant adsorption from wastewaters onto zeolites based on the following: zeolite pores have cylindrical shape with average radius r0, pores in zeolite are filled simultaneously by the model ‘layer by layer’, the rate of phenol adsorption is higher than the rate of the growth of the thickness of the adsorption layer was suggested. It has been found that the adsorption kinetics can be completely described by this kinetic model.

Kinetics and Mechanisms of the Thermal Decomposition of Copper(II) Hydroxide: A Consecutive Process Comprising Induction Period, Surface Reaction, and Phase Boundary-Controlled Reaction

This study focused on the kinetics and mechanisms of the thermal decomposition of Cu(OH)2 as a potential processing route to produce CuO nanoparticles. The physico-geometric reaction behaviors were studied using available physicochemical techniques and microscopic observation. The kinetic modeling of the reaction process to produce CuO nanoparticles was examined via the kinetic analysis of the mass-loss data recorded under different heating conditions. The reaction exhibited specific physico-geometric kinetic characteristics, including an evident induction period, a subsequent sigmoidal mass-loss behavior under isothermal conditions, and a long-lasting reaction tail under linearly increasing temperature conditions. During the first mass-loss step characterized by the sigmoidal mass-loss behavior, the crystallite size of the produced CuO was constant, and the specific surface area increased systematically. The second mass-loss step during the reaction tail was accompanied by the crystal growth of CuO. Ther...

Thermal degradation kinetics and density functional study of a macromonomer containing dual-lactone

A macromonomer containing dual-lactone, MMDL, was characterized by thermal degradation kinetics and density functional theory (DFT) calculations. The frontier molecular orbitals, molecular structural geometry, molecular electrostatic potential (MEP) and electrostatic potential (ESP) maps were determined with the help of structure optimizations based on the DFT method with standard 3–21G* as a basis set that has polarization functions on the second row atoms only. The 3–21G* basis set comprises the same number of primitive Gaussian functions. The electronic properties, such as electron affinity, HOMO–LUMO energies, ionization energy, electronegativity, chemical potential, global hardness and softness, global electrophilicity were computed with the help of the DFT method. The MEP and ESP maps were determined to predict the reactive sites of the macromonomer. Finally, the activation energy and thermal degradation mechanism for the initial part of the decomposition process under non-isothermal conditions were determined from the thermogravimetric analysis by integral approximation methods. The decomposition activation energies of the macromonomer were computed with the help of Flynn–Wall–Ozawa, Coats–Redfern and Tang methods. The kinetic equations showed that the reaction mechanism was an R1 mechanism, which is a phase boundary-controlled reaction (one-dimensional movement) solid-state mechanism.

Characterization of the thermal oxidative degradation kinetics of thermoplastics

ABSTRACTThermal oxidative degradation kinetics of polyethylene, poly(ethylene terephthalate), and polystyrene were investigated by thermogravimetric analysis. The samples included low-density polyethylene, medium-density polyethylene, high-density polyethylene, a starch-based biodegradable thermoplastic polyester, polyethylene natural gas pipe, a poly(ethylene terephthalate) water bottle, and a polystyrene drinking cup. The kinetics were conducted under dynamic conditions at heating rates of 10, 20, 30, and 40°C min−1 between 25 and 650°C in air. The Kissinger, Flynn–Wall–Ozawa, and Coats–Redfern methods were used for investigation of degradation of these polymers. Thermal oxidative degradation of the polymers was compared. Low-density polyethylene, medium-density polyethylene, high-density polyethylene, and polyethylene pipe obeyed a diffusion mechanism for oxidative thermal degradation. The starch-based biodegradable thermoplastic polyester, polystyrene cup, and poly(ethylene terephthalate) bottle followed random nucleation, diffusion, and phase boundary-controlled reaction mechanisms, respectively.

Molecular structure, kinetics and mechanism of thermal decomposition, molecular electrostatic potential, thermodynamic parameters and HOMO–LUMO analysis of coumarin-containing graft copolymer

This paper contains a combined theoretical and experimental study of thermodynamic, thermogravimetric and electronic properties of coumarin-containing graft copolymer. All the theoretical calculations were performed using the DFT (B3LYP) method with 6-311 + G (d, p) basis set in the ground state. From the optimized geometry of the coumarin-containing graft copolymer, atomic charge distributions, molecular electrostatic potential (MEP) surfaces, frontier molecular orbitals (FMOs) and thermodynamic properties such as entropy, enthalpy and specific heat capacity have been calculated theoretically. Finally, the activation energy and thermal degradation mechanism for graft copolymer were obtained by integral approximation methods under non-isothermal conditions. The activation energies (Ea) for thermal degradation of graft copolymer were determined by Flynn–Wall–Ozawa (FWO), Tang and Coats–Redfern (CR) methods. The study of kinetic equations showed that the reaction mechanism for graft copolymer was R3 mechanism, phase boundary-controlled reaction (contracting volume) of solid-state mechanism.

Thermal degradation kinetics of poly(arylene ether nitrile) and its cross-linked polymer

The kinetics of the thermal degradation of poly(arylene ether nitrile) (PEN; cross-linked and uncross-linked) was investigated by thermogravimetric analysis. The corresponding kinetic parameters of PEN were determined using the Flynn–Wall–Ozawa and the Friedman method. The Satava method was also used to explore the probable degradation mechanisms of PEN. The results showed that the activation energy ( E) obtained from the Flynn–Wall–Ozawa method was in good agreement with the value obtained from the Friedman method. The solid-state decomposition mechanisms of uncross-linked and cross-linked PEN were A2 (nucleation and growth) and R2 type (phase boundary–controlled reaction), respectively. The E and initial decomposition temperature of cross-linked PEN were higher than those of uncross-linked PEN, which indicates that cross-linking treatment is effective in enhancing the thermal stability of PEN.

Experimental Determination of the Crystallization Phase-Boundary Velocity in the Halozeotype CZX-1

Isothermal crystallization experiments were performed on the halozeotype CZX-1 with 2D temperature- and time-resolved synchrotron X-ray diffraction (TtXRD) and differential scanning calorimetry (DSC). These crystallization experiments demonstrate that the fundamental materials property, the velocity of the phase boundary of the crystallization front, vpb, can be recovered from the Kolmogorov Johnson and Mehl and Avrami (KJMA) model of phase-boundary controlled reactions by introducing the sample volume into the KJMA rate expression. An additional corrective term is required if the sample volume of the crystallization measurement is anisotropic. The concurrent disappearance of the melt and appearance of the crystalline phase demonstrate that no intermediates exist in the crystallization pathway. The velocity of the phase boundary approaches 0 as the glass transition (Tg ≈ 30 °C) is approached and at about 10° below melting point (Tm = 173 °C). The velocity of the phase boundary reaches a maximum of 30 μm s...

Kinetics of thermo-oxidative degradation of PS-POSS hybrid nanocomposite

The thermo-oxidative degradation kinetics of a hybrid nanocomposite comprised of polystyrene and polyhedral oligomeric silsesquioxane (PS-POSS) was studied by dynamic thermogravimetry. The dependence of the activation energy on the conversion (Eα(T)) was determined by means of a model-free isoconversional method and the kinetic mechanisms involved throughout the degradation process were determined by comparison of convolution functions with master curves of kinetic models. The Eα(T) values remained practically constant in the range of 80 to 120 kJ mol−1 throughout the process, indicating that the degradation is essentially limited by a single step process. The degradation proceeded via Rn mechanisms (phase boundary-controlled reactions) in the range of α = 0 to α ≈ 0.8, whereas for α > 0.80 there was a gradual change to Dn (diffusion-controlled reactions) and Fn (chemically-controlled reactions) mechanisms. This demonstrates that volatilization occurs from the surface toward the center of the sample up to α ≈ 0.8 and then becomes governed by the concentration, reactivity and diffusion of the gases.