Differential scanning calorimeters types

Adiabatic calorimeters are complex home-made instruments, and the measurements are time-consuming. Less accurate but easy to use commercial differential scanning calorimeters (DSCs) [18, 19] are a frequently used alternative. The method involves measurement of the temperature of both a sample and a reference sample and the differential emphasizes the difference between the sample and the reference. The two main types of DSC are heat flux and power-compensated instruments. In a heat flux DSC, as in the older differential thermal analyzers (DTA), the... 

Figure 12.1 Scheme of a disk-type heat flux differential scanning calorimeter. A cell B furnace C temperature sensors S sample R reference. 

The heat capacity is the amount of energy required to increase the temperature of a unit mass of material. It is commonly measured using a differential scanning calorimeter (DSC). The heat capacity depends on the resin type, additives such as fillers and blowing agents, degree of crystallinity, and temperature. A temperature scan for the resin will reveal the Tg for amorphous resins and the peak melting temperature and heat of fusion for semicrystalline resins. The heat capacities for LDPE and PS resins are shown in Fig. 4.15. 

There are two types of differential scanning calorimeters (a) heat flux (AT) and (b) power compensation (AT). Subsequent sections of this experiment will not distinguish between the two types. In either type of calorimeter, the measurement is compared to that for a reference material having a known specific heat [16,17], As AT and AT have opposite signs there is some potential for confusion [3], e.g., at the melting point, Tm, Ts < Tr, and AT < 0, whereas Ts > Tr and AT > 0 because latent heat must be supplied (subscripts s and r refer to the sample and the reference material, respectively) [3]. 

A further positive reaction to this dramatic incident took place in the central research department of the company. A physico-chemist had the idea of using his differential scanning calorimeter (DSC) to look at the energy involved in this reaction. He performed an experiment with the initial concentration and a second with a higher concentration. The thermograms he obtained were different and he realized that he could have predicted the incident (see Exercise 11.1). As a consequence, it was decided to create a laboratory dedicated to this type of experiment. This was the beginning of the scientific approach of safety assessments using thermo-analytic and calorimetric methods. From this time on, many different methods were developed in different chemical companies and became commercially distributed, often by scientific instrument companies. 

Figure 18.11. Schematic of (A) a differential thermal analyzer and (B) differential scanning calorimeter for a TA Instruments, Inc.-type configuration. Modified from Richardson (1989). Reproduced by permission of Elsevier, Ltd.

According to analysis by differential scanning calorimeter (DSC), water molecules in the membranes can be classified into three types, non-freezing, freezing bound and free water.40 Also, the weight ratios of freezable water and... 

Thermal conductivity was measured by the hot wire method. The principle involved in the measurement has been well explained by Carislaw and Jaeger (1959). Specific heat was measured by DSC (Differential Scanning Calorimeter) System TA 2910 (DuPont, U.S.A.) with heat flux type. The temperature differences between the reference material and the target specimen were measured during heating. Measurements were conducted twice at a specified temperature and temperature is varied from room temperature to 100 °C. Relationship between specific heat and temperature was linear and the result is summarized in table 1 and figure 2. 

Both types of differential scanning calorimeters make use of a crucible to contain the sample. The reference is either an inert material in a crucible of the same type as that used for the sample or simply the empty crucible. Crucibles commonly measure 5-6 mm in diameter, which gives some idea of the overall dimensions of the DSC cell. 

Figure 9 Calvet-type differential scanning calorimeter...

The differential scanning calorimeter evolved from an older instrument known as a differential thermal analyzer, or DTA. The DTA, which is based on the work of Le Chatelier in 1887, was developed in 1899 for identification of specific types of clays, which are difficult to differentiate by more traditional methods. The concept of the DTA is quite simple. A differential thermocouple, which consists of two otherwise identical thermocouples connected in opposing polarities, is placed in a furnace in a position which allows the bead of one thermocouple to be inserted into an inert reference material, while the bead of the other thermocouple is inserted into the sample. The difference in temperature between the reference and sample materials is obtained directly as a function of temperature as the entire assembly is heated at a controlled, usually linear, rate. In the absence of any thermal difference between the sample and reference material, the output of the differential thermocouple will be zero. When a thermal event occurs, c.g., heat released during crystallization, the change in specific heat at the glass... 

The melting point, as determined by differential scanning calorimeter, of these aromatic polyanhydrides is mnch higher than aliphatic polyanhydrides. The melting point of aliphatic-aromatic copolyanhydrides is proportional to aromatic content. For this type of copolymers, there is characteristically a minimnm between 5 and 20mol% of lower-melting component. The introdnction of fatty acids in the copolymer chain lowers the melting point as compared to that of bulk polymer [14]. 

The cell is based on a Du Pont differential scanning calorimeter cell. A view of the interior of the top of this DSC cell, showing the sample and reference platforms, is given in Fig. 4. The Du Pont DSC is a heat flow type DSC cell in which a constantan sheet ( thermo-electric disk ) that supports the sample and reference serves as the major heat flow path for transferring heat to the sample and reference pans and also as the common element of the differential thermocouple. This thermoelectric disk is mounted inside a silver heating block which has a silver lid. The sample and reference are in sealed metal pans so that the thermal environment is reproducible from run to run. 

Power compensation-type differential scanning calorimeter Instrument for measuring the differential electric power supplied between a sample and reference to maintain a minimal temperature difference between the sample and reference, in response to a temperature programme. 

Different types of liquid-crystalline side-chain polymers based on sUoxane backbones were synthesized by hydrosilylation reactions as described in Refs. [3] and [4]. The resulting nematic LC silicones have a broad chain length distribution. The length of the backbones are controlled by GC, H NMR and Si NMR. An example of an LC silicone used for the TCR films is shown in Fig. 1. Its degree of polymerization is about 14 and the phase transition temperatures measured by a differential scanning calorimeter are a glass transition temperature Tg of 18 °C and an isotropic transition temperature Tc of 68 °C. 

The thermal conductivity was measured by a laser flash method. Disk-type samples (12.7 mm in diameter and 1mm in thickness) were set in an electric furnace. Specific heat capacities were measured with a differential scanning calorimeter. Thermal diffusivity (X, Wm K ) was calculated from thermal diffusivity (a, m s ), density (p, g/cm ) and specific heat capacity (C, J g K ) at each temperature using the following ... 

The heats of decomposition studies were carried out on the Perkin-Elmer DSC-4 and DSC-7 differential scanning calorimeters. The thermal scan shown in Figure 9.8 illustrates the type of result DSC technique yields. Figure 9.8 shows the thermal decomposition scan for a sample that contains 30% azo dispersed in low-density polyethylene (LDPE). The endothermic peak at 115 °C is due to the melting of LDPE and the exothermic peak at about 222 °C is due to the decomposition of azo. 

Table 2.1 shows TA apparatus which is commercially available. There are a variety of standard types, such as the thermogravimeter (TG), differential thermal analyzer (DTA), differential scanning calorimeter (DSC), thermomechanical analyzer (TMA) and viscoelastic measurement analyzer (the term thermomechanometery is sometimes used for measurements including TMA, DMA and other viscoelastic measurements) including a dynamic mechanical analyzer... 

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