The Basics of Differential Scanning Calorimetry

As previously mentioned in 2.1, ASTM standard E473 defines differential scanning calorimetry (DSC) as a technique in which the heat flow rate difference into a substance and a reference is measured as a function of temperature while the substance and reference are subjected to a controlled temperature program. It should be noted that the same abbreviation, DSC, is used to denote the technique (differential scanning calorimetry) and the instrument performing the measurements (differential scanning calorimeter). 

As Wunderlich (1990) mentioned, no heat flow meter exists that could directly measure the heat flowing into or out of the sample, so other, indirect techniques must be used to measure the heat. Differential scanning calorimetry is one of these techniques it uses the temperature difference developed between the sample, and a reference for calculation of the heat flow. An exotherm indicates heat flowing out of the sample, while an endotherm indicates heat flowing in. 

Since the sample is heated from one specific source (usually from outside), potenfially significant temperature gradients exist within the sample. It is an important task in thermal analysis to create conditions in which the temperature gradients within the sample can be minimized. The temperature gradient is the unequal distribution of temperature within the sample. The temperature gradient in the sample depends on the heating rate, the sample size, and the thermal diffusivity of the sample and the sample holder. Thermal diffusivity (m /s) is determined as the ratio of thermal conductivity A, [W/(m-K)] and the volumetric heat capacity [(J/(kg-K)] 

The temperature gradient is not to be confused with thermal lag, which is another physical property that should also be minimized in DSC experiments. Thermal lag is the difference between the average sample temperature and the sensor temperature and is caused by so-called thermal resistance, which characterizes the ability of the material to hinder the flow of heat. Thermal lag is smaller in DSC than in DTA because of smaller sample size (milligrams in DSCs), but more types of thermal resistance develop in DSC than in DTA. These effects are caused by introduction of the sample and reference pans into the DSC sample and reference holders. Thus, in DTA thermal resistance develops between the sample holder (in some instruments called the sample pod) and the sample (analogously, between the reference holder and the reference material), and within the sample and the reference materials. On the other hand, in DSC thermal resistance will develop between the sample holder and the bottom of the sample pan and the bottom of the sample pan and the sample (these are called external thermal resistances), and within the sample itself (this is called internal thermal resistance). These thermal resistances should be taken into account since they determine the thermal lag. Let us suppose that the cell is symmetric with regard to the sample and reference pods or holders, the instrumental thermal resistances are identical for the sample and reference holders, the contact between the pans and the pods are intimate, no crosstalk exists between the sample and reference sensors (i.e.. 

Finally, the thermal lag should not be confused with the lag time, also called time to steady state, although the thermal lag does influence the time necessary to reach steady state (the lag time is the time necessary to reach steady state after a DSC run has begun). The thermal lag becomes a factor once steady state has been achieved, while the initial startup of the DSC run characterizes the instrument response time or the time to steady state. This can be illustrated if one proceeds from an isothermal hold (where sample temperature, reference temperature, and block temperature are all identical, i.e., T = Ti= T, see 1, below) to a heating experiment that results in an initial exponential rise of the heat flow until the steady-state condition is achieved. This initial rise represents a nonlinear response, and this part of the DSC curve does not contain information that could be used to evaluate transitions. For this reason the starting temperature should be far away from the thermal event of interest. 

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Mettler Toledo (Mettler) now markets the DSC 1 unit, but at the time that this instrumental brief was written, marketed only the DSC823 (its design is further discussed in Section 2.3, The Basics of Differential Scanning Calorimetry). For both units, Mettler now offers a two-sensor option based on a thermopile construction embedded in a chemically inert and corrosion resistant ceramic material. The sensors differ structurally in their design, although the primary difference is the number of thermocouples incorporated into each thermopile. The performance specifications are given in Table 2.8. 

In this chapter, the basics of differential scanning calorimetry (DSC) analysis and its correlation to polymer morphology for semicrystalline polymeric materials are presented. After a brief review of fundamental concepts, the utility of the technique is illustrated by a series of practical applications. 

The use of differential scanning calorimetry and thermogravimetric analysis as analytical tools in part failure determination is applied. Problems are examined in part performance of moulded 30% glass-reinforced, low-viscosity PEI and a second part moulded from PPS with proprietary additives to improve wear resistance. The importance of checking the composition of the basic material is stressed. 

Several collaborating laboratories (usually five participating laboratories) test the proposed substance using a variety of techniques. The relative reactivity or relative absorbance of the impurities present in a substance must be checked when a nonspecific assay method is employed, e.g. by colorimetry or ultraviolet spectrophotometry. It is particularly important to quantify the impurities when a selective assay is employed. In such a case, it is best to examine the proposed substance by as many methods as practicable, including, where possible, absolute methods. For acidic and basic substances, titration with alkali or acid is simple but other reactions which are known to be stoichiometric may be used. Phase solubility analysis and differential scanning calorimetry may also be employed in certain cases. 

A rather different study of the kinetics of decomposition of solid complexes of [VO(dbm)2(L)] (dbm = dibenzoylmethanato, L = py and several methyl-, dimethyl- and amino-pyridines) used differential scanning calorimetry (DSC).531 Using the temperature that corresponds to the loss of the molecule L in equation (37), a linear relationship was found between it and the basicity of L, except for 4-amino- and 4-methyl-pyridine. 

Spectroscopy has become a powerful tool for the determination of polymer structures. The major part of the book is devoted to techniques that are the most frequently used for analysis of rubbery materials, i.e., various methods of nuclear magnetic resonance (NMR) and optical spectroscopy. One chapter is devoted to (multi) hyphenated thermograviometric analysis (TGA) techniques, i.e., TGA combined with Fourier transform infrared spectroscopy (FT-IR), mass spectroscopy, gas chromatography, differential scanning calorimetry and differential thermal analysis. There are already many excellent textbooks on the basic principles of these methods. Therefore, the main objective of the present book is to discuss a wide range of applications of the spectroscopic techniques for the analysis of rubbery materials. The contents of this book are of interest to chemists, physicists, material scientists and technologists who seek a better understanding of rubbery materials. 

It was outlined in chapter 2 in detail that screening tests primarily have the purpose, to provide a first characterization of the safety relevant substance properties as part of the basic assessment. It was further explained that the determination of the thermal stability of a substance is of the greatest importance. The most fi-equently used methods for this puipose are those that investigate thermal stability using very small amounts of sample material only. The most widely used test equipments to perform such investigations are the DTA ( difference thermal analysis ) and DSC ( differential scanning calorimetry). 

This concludes the discussion of thermometry and dilatometry. The tools to measure temperature, length, and volume have now been analyzed. The tools for measurement of heat, the central theme of this book, will take the next three sections and deal with calorimetry, differential scanning calorimetry, and temperature-modulated calorimetry. The mechanical properties which involve dilatometry of systems exposed to different and changing forces, ate summarized in Sect. 4.5. The measurement of the final basic variable of state, mass, is treated in Sect. 4.6 which deals with thermogravimetry. 

The Basics of Modulated Temperature Differential Scanning Calorimetry... 

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