Differential scanning calorimetry heat capacity determination using

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). 

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. 

A study of the relaxational transitions and related heat capacity anomalies for galactose and fructose has been described which employs calorimetric methods. The kinetics of solution oxidation of L-ascorbic acid have been studied using an isothermal microcalorimeter. Differential scanning calorimetry (DSC) has been used to measure solid state co-crystallization of sugar alcohols (xylitol, o-sorbitol and D-mannitol), and the thermal behaviour of anticoagulant heparins. Thermal measurements indicate a role for the structural transition from hydrated P-CD to dehydrated P-CD. Calorimetry was used to establish thermodynamic parameters for (1 1) complexation equilibrium of citric acid and P-CD in water. Several thermal techniques were used to study the decomposition of p-CD inclusion complexes of ferrocene and derivatives. DSC and derivative thermogravimetric measurements have been reported for crystalline cytidine and deoxycytidine. Heats of formation have been determined for a-D-glucose esters and compared with semiempirical quantum mechanical calculations. ... 

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). 

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. 

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. 

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. 

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. 

It is seen that the calibration constant disappears, which assumes that it is constant over the experimental conditions. The calculation is carried out using dedicated software. In some circumstances the crucible used for the sample may have to be different from that used for the calibrant. This means that a correction will be required to take into account the difference between the heat capacity of the two crucibles - readily calculated with sufficient accuracy. Measurements can be made at a series of temperatures but are meaningful only within the quasi-steady-state region of the experiment. The specific heat capacity of sapphire has been listed by ASTM in connection with the standard test method E 1269 (1999) for determining specific heat capacity by differential scanning calorimetry. 

E 1269 (1999) Test method for determining specific heat capacity by differential scanning calorimetry E 1354 (1994) Test method for heat and visible smoke release rates for materials and products using an oxygen consumption calorimeter... 

Although the glass transition resembles characteristics of a second-order thermodynamic transition such as changes in the coefficient of expansion and heat capacity, the temperature of the transition is a function of the heating or cooling rate and of the rate of deformation. The methods used to determine Tg are based either on static or dynamic mechanical processes. The former uses volume effects (dilatometry) and heat capacity effects in differential scanning calorimetry (DSC), entailing conditions of very low deformation. The latter utilizes the response to imposed deformation of the system. 

The glass transition temperature is measured using differential scanning calorimetry (DSC) (179,284), by which a polymer sample is heated, and its enthalpic changes are measured in response. The temperature at which the heat capacity of the polymer drops is the glass transition temperature. Dynamic mechanical spectroscopy (DMS) is also used to determine the glass transition temperature. Certain mathematical equations, such as the Fox equation, relate the copolymer composition to the glass transition temperature. 

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are similar techniques. They measure change in the heat capacity of a sample. These techniques can be used to determine various transition temperatures (T , Tg, T , Tp, etc.), specific heat, heat of fusion, percent crystallinity, onset of degradation temperature, induction time, reaction rate, crystallization rate, etc. A DSC instrument operates by compensating electrically for a change in sample heat. The power for heating is controlled in such a way that the temperature of the sample and the reference is the same. The vertical axis of a DSC temperature scan shows the heat flow in cal/s. 

Differential scanning calorimetry is probably the most frequently used method for determining 7. In this method, a change in the expansion coefficient and the heat capacity occurs as a sample material is heated or cooled through this transition region. 

The glass transition temperatures and heat capacity increments for the transitions have been determined for a number of mono- and oligo-saccharides using differential scanning calorimetry. Aqueous gels and micellar fibres prepared from TV-alkylaldonamide mixtures have been studied by electron microscopy. ... 

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