Differential scanning calorimetry heat capacity determination

The measurement of the amount of heat released or absorbed in a reaction also included in this category are determinations of heat capacities, latent heats, and caloric values of fuels. See also Differential Scanning Calorimetry... 

Differential scanning calorimetry (DSC). The DSC analyses were carried out using a Perkin-Elmer DSC-7 and a DuPont 910DSC. Tg was defined as the midpoint of the change in heat capacity occurring over the transition. The samples were first scanned to 95°C, thereafter cooled and recorded a second time. The Tg was determined from the second run. The measurements were carried out under an atmosphere of dry nitrogen at a heating rate of 10°C/min. 

Differential scanning calorimetry is primarily used to determine changes in proteins as a function of temperature. The instrument used is a thermal analysis system, for example a Mettler DSC model 821e. The instrument coupled with a computer can quickly provide a thermal analysis of the protein solution and a control solution (no protein). The instrument contains two pans with separate heaters underneath each pan, one for the protein solution and one for the control solution that contains no protein. Each pan is heated at a predetermined equal rate. The pan with the protein will take more heat to keep the temperature of this pan increasing at the same rate of the control pan. The DSC instrument determines the amount of heat (energy) the sample pan heater has to put out to keep the rates equal. The computer graphs the temperature as a function of the difference in heat output from both pans. Through a series of equations, the heat capacity (Cp) can be determined (Freire 1995). 

Heat capacity is best determined with a calorimeter incorporating an electric heater. The net energy input and the resultant temperature rise are both measured. Procedures and precautions for such direct calorimetry are discussed thoroughly by Sturtevant (1959). Differential scanning calorimetry is convenient to use for the determination of heat capacity (Watson et al. 1964). 

ASTM E1269, 2004. Standard test method for determining specific heat capacity by differential scanning calorimetry. 

Most of the physical properties of the polymer (heat capacity, expansion coefficient, storage modulus, gas permeability, refractive index, etc.) undergo a discontinuous variation at the glass transition. The most frequently used methods to determine Tg are differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical thermal analysis (DMTA). But several other techniques may be also employed, such as the measurement of the complex dielectric permittivity as a function of temperature. The shape of variation of corresponding properties is shown in Fig. 4.1. 

This would not be problematic if standardized, reliable, reproducible, and inexpensive laboratory tests were available to estimate each of the required properties. Although several specialized laboratory tests are available to measure some properties (e.g., specific heat capacity can be determined by differential scanning calorimetry [DSC]), many of these tests are still research tools and standard procedures to develop material properties for fire modeling have not yet been developed. Even if standard procedures were available, it would likely be so expensive to conduct 5+ different specialized laboratory tests for each material so that practicing engineers would be unable to apply this approach to real-world projects in an economically viable way. Furthermore, there is no guarantee that properties measured independently from multiple laboratory tests will provide accurate predictions of pyrolysis behavior in a slab pyrolysis/combustion experiment such as the Cone Calorimeter or Fire Propagation Apparatus. 

Thermochemical data were required for the estimation of ground state strain. Heats of formation ( 0.5 kcal mol-1) were obtained by the experimental determination of heats of combustion 25 -27) using either a stirred liquid calorimeter 25) or an aneroid microcalorimeter 26) heats of fusion and heat capacities were measured by differential scanning calorimetry (DSC), heats of vaporization 21, 25, 27) by several transport methods, or they were calculated from increments 28). For the definition of the strain enthalpies Schleyer s single conformation increments 29) were used and complemented by increments for other groups containing phenyl30) and cyano substituents. 

Differential scanning calorimetry (DSC) can be used to determine experimentally the glass transition temperature. The glass transition process is illustrated in Fig. 1.5b for a glassy polymer which does not crystallize and is being slowly heated from a temperature below Tg. Here, the drop which is marked Tg at its midpoint, represents the increase in energy which is supplied to the sample to maintain it at the same temperature as the reference material. This is necessary due to the relatively rapid increase in the heat capacity of the sample as its temperature is increases pass Tg. The addition of heat energy corresponds to the endothermal direction. 

The other common category of calorimetry is differential methods, in which the thermal behavior of the substance being measured is compared to that of a reference sample whose behavior is known. In differential scanning calorimetry (DSC), the instrument measures the difference in power needed to maintain the samples at the same temperature. In differential thermal analysis (DTA), the samples are heated in a furnace whose temperature is continuously changed (usually linearly), and the temperature difference between the sample and the reference sample as a function of time can yield thermodynamic information. DSC and DTA are most commonly used for determining the temperature of a phase transition, particularly for transitions involving solids. In addition, DSC experiments can yield values for the enthalpy of a phase transition or the heat capacity. Commercial DSC and DTA instruments are available. 

Heat capacities from 10 K to 300 K were determined by adiabatic calorimetry, and by differential scanning calorimetry (DSC) from 300 K to 550 K. In the paper, only rounded values are supplied for the adiabatic calorimetry results for Nil2(cr) except for values near the (structural phase [81KU1/SAN]) transition at approximately 59 K. The authors also supply an equation (A.55)... 

Robie et al. have carried out excellent measurements of the heat capacity of Ni2Si04 olivine between 5 and 387 K by cryogenic adiabatic-shield calorimetry and between 360 and 1000 K by differential scanning calorimetry. At 298.15 K the molar heat capacity and entropy of Ni2Si04 olivine were determined. This molar heat capacity and entropy were accepted by this review. The thermal heat capacity function proposed by Robie et al. for the temperature range between 300 and 1300 K was, however, not adopted by this review, because it is based on measurements which were made only at temperatures up to 1000 K. Combining the heat capacity measurements with results of molten salt calorimetry, thermal decomposition of Ni2Si04 olivine into its constituent oxides, and equilibrium studies, both by CO reduction and solid state electrochemical cell measurements for the reaction ... 

Virtually every chemical process involves a change in the heat capacity of the sample. When measured by differential scanning calorimetry, such changes produce a curve similar to Figure 17.7 (except with a y-axis in cal/sec). The area under the DSC curve is determined in the same manner as in DTA. This area is proportional to the amount of heat evolved or absorbed by the reaction, and the heat of reaction is obtained by dividing this by the moles of sample used. If the heat of reaction is known, the moles of sample present can be calculated from essentially the same equation (i.e., the integral of Equation 17.6). All determinations should be preceded by an analysis of a standard sample of known mass and A/7 in order to calibrate the particular instrument used. 

Any polymer property that changes with temperature and has different values above and below Tg can be used, in principle, to determine Tg. For example, the change in specific volume, heat capacity, or elastic modulus may be used to measure Tg. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) are two common methods for such determinations. An example of the results of DSC analysis Is presented In Fig. 3.46. It is common for different methods to yield slightly different values for Tg. 

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