Some thermodynamic aspects of nitrides in materials science

The energies of combustion in fluorine of gallium nitride and indium nitride in wurzite crystalline structure have been measured in a two-compartment calorimetric bomb, and new standard molar enthalpies of formation have been calculated: Δ_fH_m⁰(GaN(cr) 298.15 K)= –(163.7±4.2) kJ mol^–1 and Δ_fH_m⁰(InN(cr) 298.15 K)= –(146.5±4.6) kJ mol^–1 . Comparison with the recommended values of the Δ_fH_m⁰ nitrides from the literature is also presented.     

Multiple glass transitions and local composition effects on polymer solvent mixtures

Recent differential scanning calorimetry (DSC) results on polystyrene–solvent mixtures show two distinct glass transitions whose positions and widths vary with composition. Parallel work on the dynamic response in polymer blends has focused on how segmental mobilities are controlled by local composition variations within a “cooperative volume” containing the segment. Such variations arise from both chain connectivity and composition fluctuations. We account for both using a lattice model for polymer–solvent mixtures that yields the composition distribution around polymer and solvent segments. Insights from our lattice model lead us to simplified calculations of the mean and variance of local composition, both in good agreement with lattice results. Applying our model to compute DSC traces leads to an estimate of the cooperative volume, since a larger cooperative volume both reduces the biasing effect of connectivity, and narrows the composition distribution. Comparing our results to data, we are able to account for the composition-dependent broadening with a cooperative length scale of about 2.5 nm. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3528–3545, 2006     

Solute-solvent interactions in water-tert-butyl alcohol mixtures. VI. Enthalpies of solution for halo-para-substituted benzoates

Enthalpies of solution of sodium benzoate, potassium benzoate, and potassium halo-substituted benzoates are reported at 298.15°K in water and in nine water-tert-butyl alchol mixtures. Transfer enthalpies from water to the mixed solvent go through a maximum for about 0.055 mole fraction of alcohol. Additivity of ionic contributions in the enthalpies of transfer is verified. Substituent effects on the transfer enthalpies of benzoates are discussed in terms of size of the solutes and cohesion of the solvent mixtures. For Part V, see ref. 1.     

Temperature-controlled combustion and kinetics of different rank coal samples

Differential scanning calorimetry (DSC) and thermogravimetry (TG/DTG) has been used to obtain information on the temperature-controlled combustion characteristics of seventeen coals of different origin from Thrace basin of Turkey. Experiments were performed in air atmosphere up to 600°C at a heating rate of 10°C min^-1. The DSC/TG curves clearly demonstrate distinct transitional stages in the entire coal samples studied. Reaction intervals, peak and burn-out temperatures of the coal samples are also determined. Two different kinetic methods known as, Arrhenius and Coats-Redfern, were used to analyze the kinetic data and the results are discussed.     

Thermodynamic quantities for the interaction of SO 4 2− with H+ and Na+ in aqueous solution from 150 to 320°C

The aqueous reactions,

Study of poly(bisphenol A carbonate) relaxation kinetics at the glass transition temperature

In this work, the variations of the relaxation times are investigated above and below the glass transition temperature of a model amorphous polymer, the polycarbonate. Three different techniques (calorimetric, dielectric and thermostimulated currents) are used to achieve this goal. The relaxation time at the glass transition temperature was determined at the temperature dependence convergence of the relaxation times calculated with dynamic dielectric spectroscopy (DDS) for the liquid state and thermostimulated depolarisation currents (TSDC) for the vitreous state. We find a value of τ(T_g) = 110 s for PC samples. The knowledge of the temperature dependence, τ(T), and the value τ(T_g) enables to determine the glass-forming liquid fragility index, m. We find m = 178 ± 5.     

Modulated differential scanning calorimetry in the glass transition region

Temperature-modulated calorimetry (TMC) allows the experimental evaluation of the kinetic parameters of the glass transition from quasi-isothermal experiments. In this paper, model calculations based on experimental data are presented for the total and reversing apparent heat capacities on heating and cooling through the glass transition region as a function of heating rate and modulation frequency for the modulated differential scanning calorimeter (MDSC). Amorphous poly(ethylene terephthalate) (PET) is used as the example polymer and a simple first-order kinetics is fitted to the data. The total heat flow carries the hysteresis information (enthalpy relaxation, thermal history) and indications of changes in modulation frequency due to the glass transition. The reversing heat flow permits the assessment of the first and higher harmonics of the apparent heat capacities. The computations are carried out by numerical integrations with up to 5000 steps. Comparisons of the calculations with experiments are possible. As one moves further from equilibrium, i.e. the liquid state, cooperative kinetics must be used to match model and experiment.On leave from Toray Industries, Inc., Otsu, Shiga 520, Japan.This work was supported by the Division of Materials Research, National Science Foundation, Polymers Program, Grant # DMR 90-00520 and the Division of Materials Sciences, Office of Basic Energy Sciences, U. S. Department of Energy at Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. for the U. S. Department of Energy, under contract number DE-AC05-96OR22464. Support for instrumentation came from TA Instruments, Inc. Research support was also given by ICI Paints, and Toray Industries, Inc.     

Evaluation of thermodynamic properties of irreversible protein thermal unfolding measured by DSC

Summary We assessed the applicability of the extrapolation procedure at infinite scanning rate to differential scanning calorimetry (DSC) data related to irreversible protein unfolding. To this aim, an array of DSC curves have been simulated on the basis of the Lumry-Eyring model N↔U→F. The results obtained confirmed that when the apparent equilibrium constant K_{app (T=T1/2)} is lower than 3, the application of the extrapolation procedure provides accurate thermodynamic parameters. Although this procedure applies only to monomeric proteins for which the Lumry-Eyring model is a reasonable approximation, it will hopefully contribute to increase the potential of DSC in obtaining reliable thermodynamic information regarding the folding/unfolding equilibrium.     

Crystallization of poly(N-methyldodecano-12-lactam) in blends with poly(styrene-stat-acrylic acid)

Crystallization behaviour of blends of poly(N-methyldodecano-12-lactam) (PMDL) with statistical copolymer poly(styrene-stat-acrylic acid) (PSAA) has been studied by the DSC and WAXD methods. The blend films prepared from dioxane solutions were crystallized at laboratory temperature for five days. Approximate crystallinities of as-prepared neat lower- PMDL 5 and higher-molecular weight PMDL 45 were 28% and 19%, respectively. With increasing PSAA content in the blends the crystallinities decreased sharply. The melting point of the primary crystalline structure of PMDL showed a decreasing dependence on PSAA content in the blends, confirming miscibility of the PMDL-PSAA pair. Recrystallization was strongly suppressed in the blends. The lower-melting endotherm appearing at about 10-15 °C above the crystallization temperature was attributed to melting to less perfect structures formed during secondary crystallization. In neat PMDL, the extent of secondary crystallization was approximately 5-10%. In the blends containing 20% PSAA approximate relative proportion of secondary crystallites on total crystallinity was 40% and 60% for the blends with PMDL 5 and PMDL 45, respectively. WAXD measurements did not reveal any change in crystal modification on blending. Increased T_g in blends of flexible PMDL cannot play a significant role in suppression of primary in favour of secondary crystallization. This was attributed to low mobility of PMDL chains due to dilution effect and specific interactions with the amorphous copolymer component, and, in case of the higher-molecular-weight PMDL, a greater involvement of entanglements. Higher T_g of blends was involved in retardation of non-isothermal crystallization on cooling and subsequent cold crystallization.     

Pyrolysis Analysis and Kinetics of Crude Oils

This research presents the results of an experimental study on the determination of pyrolysis behaviour and kinetics of six crude oils by differential scanning calorimetry (DSC) and thermogravimetry (TG/DTG). Crude oil pyrolysis indicated two main temperature ranges where loss of mass was observed. The first region between ambient to 400°C was distillation. The second region between 400 and 600°C was visbreaking and thermal cracking. Arrhenius-type kinetic model is used to determine the kinetic parameters of crude oils studied. It was observed that as crude oils gets heavier (°API decreases) cracking activation energy increases. Activation energy of cracking also show a general trend with asphaltene content. This revised version was published online in July 2006 with corrections to the Cover Date.