This study reexamines an early literature report of an unusual phenomenon of a dramatic difference in the activation energy for the thermal decomposition of malonic (propanedioic) acid in the liquid and solid states. The study has been carried out via thermogravimetric analysis (TGA) to probe possible differences between the kinetics in three condensed phases: solid, liquid, and supercooled liquid. An advanced isoconversional method has been applied to determine the activation energy, preexponential factor, and reaction model. The activation energy as well as preexponential factor for the decomposition processes in all three condensed phases have been determined to be practically the same: E = 110 ± 10 kJ mol −1 and log(A/min −1 ) = 13 ± 1. In all phases, the reaction kinetics has been found to follow the reaction order model. The reaction order values have been similar for the liquid and supercooled liquid phases but markedly smaller for the solid-phase decomposition. The results obtained have not confirmed the phenomenon reported in the early literature. A practically important conclusion of the present study is that the liquid-phase kinetic data can be used to obtain a reasonable estimate for the thermal stability of this compound in the solid phase.
This work explores the effect of an inert gas pressure on the kinetics of reversible thermal decomposition of solids. The nature of this effect is diffusional. Theoretical analysis of the effect suggests that the process rate should decelerate with increasing inert gas pressure and that the deceleration should occur at the expense of a decrease in the pre-exponential factor. The effect is illustrated by applying high-pressure differential scanning calorimetry to the process of the thermal dehydration of lithium sulfate monohydrate (Li2SO4·H2O). An increase in nitrogen pressure from 0.1 to 7 MPa shifts the reaction to higher temperature by more than 10 °C. Kinetic analysis by means of the Kissinger and advanced isoconversional methods indicates that the activation energy remains practically unchanged by changing pressure. The pre-exponential factor has been found to decrease in proportion to an increase in pressure.
In this study, differential scanning calorimetry (DSC) has been applied to measure the kinetics of nonisothermal crystallization of potassium nitrate and ammonium perchlorate from unsaturated and saturated aqueous solutions. DSC data have been analyzed by an advanced isoconversional method that demonstrates that the process is represented by negative values of the effective activation energy, which varies with the progress of crystallization. The classical nucleation model can be used to predict and understand the experimentally observed variation in the effective activation energy. The saturated and unsaturated solutions have demonstrated distinctly different crystallization kinetics. It is suggested that the unsaturated solutions undergo a change in crystallization mechanism from homogeneous to heterogeneous nucleation.
Linear polyphosphonates with the generic formula –[P(Ph)(X)OR′O]n– (X = S or Se) have been synthesized by polycondensations of P(Ph)(NEt2)2 and a diol (HOR′OH = 1,4‐cyclohexanedimethanol, 1,4‐benzenedimethanol, tetraethylene glycol, or 1,12‐dodecanediol) followed by reaction with a chalcogen. Random copolymers have been synthesized by polycondensations of P(Ph)(NEt2)2 and mixture of two of the diols in a 2:1:1 mol ratio followed by reaction with a chalcogen. Block copolymers with the generic formula –[P(Ph)(X)OR′O](x + 2) –[P(Ph)(X)OR′O](x + 3)– (X = S or Se) have been synthesized by the polycondensations of Et2N[P(Ph)(X)OR′O](x + 2)P(Ph)NEt2 oligomers with HOR′O[P(Ph)(X)OR′O](x + 3)H oligomers followed by reaction with a chalcogen. The Et2N[P(Ph)(X)OR′O](x + 2)P(Ph)NEt2 oligomers are prepared by the reaction of an excess of P(Ph)(NEt2)2 with a diol while the HOR′O[P(Ph)(X)OR′O](x + 3)H oligomers are prepared by the reaction of P(Ph)(NEt2)2 with an excess of the diol. In each case the excess, x is the same and determines the average block sizes. All of the polymers were characterized using 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, TGA, DSC, and SEC. 31P{1H} NMR spectroscopy demonstrates that the random and block copolymers have the expected arrangements of monomers and, in the case of block copolymers, verifies the block sizes. All polymers are thermally stable up to ~300°C, and the arrangements of monomers in the copolymers (block vs. random) affect their degradation temperatures and Tg profiles. The polymers have weight average MWs of up to 3.8 × 104 Da.
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