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Abstract  

DSC study of native and denatured biopolymers with different chemical and steric structure was carried out in a wide range of temperatures and water contents. It was shown that all the native and denatured humid biopolymers studied are glassy systems. The residues of native structures surviving after partial dehydration prevent the glass transition at the glass transition temperatures of the denatured biopolymers. In dehydrated native biopolymers the processes of melting and glass transition take place in the same temperature range that leads to a large change of the heat capacity across denaturation.

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Abstract  

A study of the thermal dehydration of α-NiSO4·6H2O has been performed by power compensation differential scanning calorimetry in flowing nitrogen. No significant differences in behaviour were observed using either uncrushed crystalline powders or single crystal slabs cleaved parallel to {001}. In good agreement with previous findings, the kinetic analysis of the thermal curves confirms the validity of an=2 Avrami-Erofeev equation (AE2) in isothermal experiments at low (338–343 K) temperatures or in the initial portions of variable temperature runs. The kinetic obedience is however of an ‘order of reaction’ type for the main portion of the variable temperature runs and, for isothermal experiments, in the upper part of the temperature range investigated. Values of activation energies and frequency factors are reported. Parallel studies by optical microscopy showed relevant changes of surface texture when partially (thermally or vacuum) dehydrated {001} cleaved surface were submitted to rehydration. This phenomenon (named orange peel formation) indicates that a dehydrated layer forms on the crystal surfaces preceding the appearance of product crystals (germination or nucleation). Microscopy also revealed that reaction goes on inside the crystal and that product formation takes place in the bulk phase, following lattice collapse in experiments at high heating rates. Combined with previous results, these new experimental findings allow us to formulate a mechanism for the present transformation, comprising three main rate processes: i)  the reaction (detachment of water molecules from their lattice positions in the reactant); ii)  the migration of the water molecules freed by the reaction through the initially formed, water-depleted layer enveloping the reactant crystal; iii)  the crystallization of such a layer to form the product.

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Abstract  

The dehydration of a series of VPI-5 and H3 samples, synthesized under various conditions, as well as the solid state transformation of VPI-5 to AlPO4-8 have been investigated using combined TG-DTG-DSC and high-resolution solid state31P-NMR. The TG curves show a quasi-continuous release of water, the total loss being characteristic for each sample. Complete dehydration is achieved when the samples are heated from 20°C to about 150°C at various beating rates. Besides the main dehydration effect, several weak endothermic peaks are observed. These generally non-reproducible modulated peaks, recorded at high heating rates, are presumably due to the interactions of the water molecules leaving the channels of VPI-5 with the randomly positioned fragments stemming from the destruction of the water triple helix assemblage. The non-isothermal kinetic parameters of the dehydration have been evaluated from the TG and DTG curves recorded at low heating rates.

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Summary  

Kinetics of dehydration of equilibrium swollen poly(acrylic acid) hydrogel was investigated using methods of non-isothermal thermal analysis. Methods of Kissinger, Coats-Redfern, Van Krevelen and Horowitz-Metzger were applied for determination the kinetics parameters: activation energy (E), pre-exponent (lnA) as well as the kinetics model ƒ(69) for the process of hydrogel dehydration under different heating rates. An existence of good agreement between determined values of kinetic parameters (Eand A), which were obtained applying different methods under the same heating rate. Functional relationship between changes of kinetic parameters of dehydration and changes of heating rate was established. An existence of compensation effect is accepted and explanation of compensation effect appearance during the hydrogel dehydration is suggested.

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Abstract

The literature reveals that the mechanisms of some solid-state dehydrations are more complicated than has been generally accepted. Reactions at a thin advancing reactant-product interface provide the geometric models on which the most widely employed rate equations are based. For some systems, this “thin interface” model is a simplification of observed behaviour. Elimination of water from crystallographic sites may occur to a significant extent within a much thicker zone of reactant towards which the active interface is progressing. Consequently the region of chemical change may not coincide with the region of structural transformation. Limited initial dehydration may occur across all crystal faces prior to the onset of a nucleation and growth process that is usually regarded as the dominant rate process in the dehydrations of many large crystals. Experimental observations for solid-state dehydrations are discussed and reaction mechanisms with different rate controlling processes are distinguished. Studies of dehydrations have contributed substantially to the theory of solid-state reactivity, and advances in understanding may have wider application to other solid-state reactants.

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Abstract  

The thermal behaviour of Ba[Cu(C2O4)2(H2O)]·5H2O in N2 and in O2 has been examined using thermogravimetry (TG) and differential scanning calorimetry (DSC). The dehydration starts at relatively low temperatures (about 80°C), but continues until the onset of the decomposition (about 280°C). The decomposition takes place in two major stages (onsets 280 and 390°C). The mass of the intermediate after the first stage corresponded to the formation of barium oxalate and copper metal and, after the second stage, to the formation of barium carbonate and copper metal. The enthalpy for the dehydration was found to be 311±30 kJ mol−1 (or 52±5 kJ (mol of H2O)−1). The overall enthalpy change for the decomposition of Ba[Cu(C2O4)2] in N2 was estimated from the combined area of the peaks of the DSC curve as −347 kJ mol−1. The kinetics of the thermal dehydration and decomposition were studied using isothermal TG. The dehydration was strongly deceleratory and the α-time curves could be described by the three dimensional diffusion (D3) model. The values of the activation energy and the pre-exponential factor for the dehydration were 125±4 kJ mol−1 and (1.38±0.08)×1015 min−1, respectively. The decomposition was complex, consisting of at least two concurrent processes. The decomposition was analysed in terms of two overlapping deceleratory processes. One process was fast and could be described by the contracting-geometry model withn=5. The other process was slow and could also be described by the contracting-geometry model, but withn=2. The values ofE a andA were 206±23 kJ mol−1 and (2.2±0.5)×1019 min−1, respectively, for the fast process, and 259±37 kJ mol−1 and (6.3±1.8)×1023 min−1, respectively, for the slow process.

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Abstract  

The dehydration process of Co(II), Cu(II) and Zn(II) methanesulfonates was studied by thermogravimetry/derivative thermogravimetry (TG/DTG) and differential scanning calorimetry (DSC) techniques in dynamic N2 atmosphere. The TG/DTG curves show that all of them contain four crystallization water molecules, which are lost in two steps. The peak temperature and dehydration enthalpies ΔH were measured from DSC curves for each compound. The effect of procedural variables on the TG and DSC curves was investigated. In this work, the procedural variables included heating rate, Al pan state (unsealed and sealed) and sample mass.

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Abstract  

The dehydration of LiCl·H2O was studied under inert helium atmosphere by DTA/TG for different heating rates. The dehydration of LiCl·H2O proceeds through a two step reaction between 99–110 and 160–186°C, respectively. It leads to the formation of LiCl·0.5H2O as intermediate compound. The proposed mechanism is:

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Based on the temperature peak of the DTA signals the activation energies of the two reactions were determined to be 240 kJ mol−1 (step 1) and 137 kJ mol−1 (step 2), respectively.

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Abstract  

Transpiration method was used to measure the equilibrium water vapor pressures of the dehydration of the respective hydrates, such as oxalates, sulfates,chlorides and acetate, and the enthalpies of dehydrations (ΔH Tr 0) of these hydrates were obtained. The heats of dehydrations (ΔH DSC 0) were also determined by TG-DSC method. From the comparison with ΔH Tr 0 of ΔH DSC 0, the relation of ΔH DSC 0H Tr 0=R (=dehydration molar number determined by TG-DSC peak/stoichiometric dehydration molar number) was yielded. From these results, the following relations were found: ΔH DSC 0(corrected)=ΔH DSC 0/RH Tr 0

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Abstract  

Miscibility and dehydration of poly(2-hydroxyethyl methacrylate) and poly(methacrylic acid) (PHEMA/PMAA) blends were investigated by temperature modulated DSC (TMDSC), TG and solid-state 13C NMR methods. TMDSC spectra and 1H spin-relaxation times showed that the blends are homogeneous on a scale of 5-10 nm for all compositions. From TG and 13C NMR, we elucidated that the mass loss of the blends at 300C is ascribed to the dehydration between the hydroxyl group of PHEMA and the carboxyl group of PMAA.

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