Calorimetry scanning,

Differential scanning calorimetry (DSC) is a technique for measuring the thermal behavior of a substance. In DSC, the difference between the heat flow (J/s, or W) to or from a sample and a reference material is measured as a function of temperature or time while the sample and the reference material are subjected together to a controlled 

In the context of milk fat, DSC is most widely used to measure temperatures and heats of transitions (phase changes). It is also used to measure specific heat, solid fat content, crystallization kinetics constants and fat purity, and in the study of fat crystal polymorphism. 

In any DSC instrument, the sample and reference material are placed in small individual pans or crucibles, which may be open or hermetically sealed, of 10-20 p,L capacity (Harwalkar and Ma, 1990). Sample size is 

When a thermal event occurs in the sample, a temperature difference tends to develop between the sample and the reference. This results in a heat flux between the two via the heat flux plate. This heat flux ensures that the temperature difference between the sample and the reference is always very small. This is important for ensuring that both sample and reference are subjected to essentially the same temperature programme (Bhadeshia, 2002). 

However, it is this small temperature difference between sample and reference that is measured as a function of temperature during the applied temperature scan. It is converted by the instrument s software, using instrument calibration data, to the difference between the power absorbed or released from the sample and that absorbed or released from the reference. The final instrument output is a thermogram of this differential power plotted against temperature, as for PC instruments. 

Differential scanning calorimetry is often combined with thermogravi-metric analysis of some type, which is thermal desorption or adsorption. The method yields fine details in the analysis. Adsorption experiments are performed by addition of the adsorbate at a rate that is 

slow enough that the system is very close to equilibrium but 

fast enough to obtain a temperature increase enough to measure in the differential mode. 

The first criterion can be checked by doing some kinetic studies, either gravimetric or volumetric. The second criterion would probably be obvious during the calorimetry experiment. The calorimetry system has been described by Rouguerol et al. [10]. It provides details of the thermodynamics of 

The differential scanning calorimeter has the advantage that the heat of adsorption or desorption is compared to a standard using a differential temperature measuring method, usually two thermopiles for which the voltage difference between them is measured. Fig. 30 is a schematic of the system that Rouquerol et al. employed. ( TCP indicates the thermopile, S the sample chamber, M a matching reference chamber and L is a slow He leak. 

The technique of differential scanning calorimetry (DSC) is very similar to DTA. The peaks in a DTA thermogram represent a difference in temperature between the sample and reference, whereas the peaks in a DSC thermogram represent the amount of electrical energy supplied to the system to keep the sample and reference at the same temperature. 

The areas under the DSC peaks will be proportional to the enthalpy change of the reaction. 

DSC is often used for the study of equilibria, heat capacities and kinetics of explosive reactions in the absence of phase changes, whereas DTA combined with TGA is mainly used for thermal analysis. 

A summary of the nitration techniques for some military and commercial explosives is presented in Table 7.1. 

C-Nitration Picric acid TNT HNS Mixture of nitric and sulfuric acids Mixture of nitric and sulfuric acids Mixture of nitric and sulfuric acids 

Copyright 2011 RoyaJ Society of Chemisi Retrieved from www.krwvel.com 

The Chemistry of Explosives, 3rd Edition By Jacqueline Akhavan J. Akhavan 2011 

Calorimetric Methods.—Differential Scanning Calorimetry (d.s.c.). The use of commercially available d.s.c. apparatusto study phase separation in polymer and copolymer solutions is described. In this context perhaps the main advantage of the method is to distinguish between liquid-liquid phase separation, where the enthalpy change is usually small, and crystallization of polymer (see Chapter 12). The method is much used to study the glass transition in polymer mixtures (see p. 320). 

Microcalorimetry. Skerjanc and Pavlin note the use of standard equimpent to obtain heats of mixing in ionic polymer solutions. Gusenkov and Krestov have considered the interpretation of microcalorimetric data to obtain thermodynamic parameters and illustrated their analysis with results from an aqueous biopolymer solution. Filisko et al. have studied enthalpy in amorphous liquid polymers from measurements of their heats of solution. 

Since AH is proportional to the area of the DTA peak, one ought to be able to measure heats of reaction directly, using the equation 3.5.22. Indeed we can and such is the basis of a related method called Differential Scanning Calorimetry (DSC), but only if the apparatus is modified suitably. We find that it is difficult to measure the area of the peak obtained by DTA accurately. Although one could use an integrating recorder to convert the peak to an electrical signal, there is no way to use this signal in a control-loop feed-back to produce the desired result. 

A more practical way to do this is to control the rate of heating, i.e.-dT/dt, and provide a separate signal to obtain a heating differential. One such way that became the basis of DSC is shown in the following diagram  

The apparatus consists of a DSC-head within a furnace, like the DTA apparatus. However, there is also a silver block which encloses the DSC head as well. This ensures complete and even heat dispersion. There are 

We use the same approach for DSC as we did for DTA. We start with the thermal heat flow equation which is similar to Ohm s Law, vis- 

We Ccm define the heat change Involved with the sample as dh/dt so as to get the equation  

Thermal analysis is a term used to cover a group of techniques in which a physical property of a substance and/or its reaction product(s) is measured as a function of temperature. This experiment is confined to the area of differential thermal analysis (DTA) and, more specifically, its quantitative development, differential scanning calorimetry (DSC) [1-15]. 

DTA/DSC curves reflect changes in the energy of the system under investigation—changes that may be either physical or chemical in origin. DSC measures the heat required to maintain the same temperature in the sample versus an appropriate reference material in a furnace [9]. Enthalpy changes due to a change of state of the sample are determined. DTA differs from DSC in that the temperature difference is determined, rather than enthalpy differences between the sample and the reference material [9]. 

A number of important physical changes in a polymer may be measured by DSC. These include the glass transition temperature (Tg), the crystallization temperature (Tc), the melt temperature (Tm), and the degradation or decomposition temperature (TD). Chemical changes due to polymerization reactions, degradation reactions, and other reactions affecting the sample can be determined (Table 16.1). A typical DSC trace showing these transitions is shown in Fig. 16.1. 

In DSC, differences in heat flow into a reference and sample are measured vs. the temperature of the sample. The difference in heat flow is a difference in energy DSC is a calorimetric technique, and results in more accurate measurement of changes in enthalpy and heat capacity than that obtained by DTA. 

The DSC peak area must be calibrated for enthalpy measurements. The same types of high purity metals and salts from NIST discussed for calibration of DTA equipment are also used to calibrate DSC instmments. As an example, NIST SRM 2232 is a 1 g piece of high purity indium metal for calibration of DSC and DTA equipment. The indium SRM is certified to have a temperature of fusion equal to 156.5985°C + 0.00034°C and a certihed enthalpy of fusion equal to 28.51 + 0.19 J/g. NIST offers a range of similar standards. These materials and their certified values can be found on the NIST website at www.nist.gov. Government standards organizations in other countries offer similar reference materials. 

The resultant thermal curve is similar in appearance to a DTA thermal curve, but the peak areas are accurate measures of the enthalpy changes. Differences in heat capacity can also be accurately measured and are observed as shifts in the baseline before and after an endothermic or exothermic event or as isolated baseline shifts due to a glass transition. Because DSC provides accurate quantitative analytical results, it is now the most used of the thermal analysis techniques. A typical DSC thermal curve for polyethylene tereph-thalate, the polymer used in many soft drink bottles, is shown in Fig. 16.22. 

DSC is used to study all of the types of reactions and transitions that can be studied using DTA, with the added advantage of accurate quantitative measurements of A// and ACp. 

Polymer chemists use DSC extensively to study percent crystallinity, crystallization rate, polymerization reaction kinetics, polymer degradation, and the effect of composition on the glass transition temperature, heat capacity determinations, and characterization of polymer blends. Materials scientists, physical chemists, and analytical chemists use DSC to study corrosion, oxidation, reduction, phase changes, catalysts, surface reactions, chemical adsorption and desorption (chemisorption), physical adsorption and desorption (physisorp-tion), fundamental physical properties such as enthalpy, boiling point, and equdibrium vapor pressure. DSC instruments permit the purge gas to be changed automatically, so sample interactions with reactive gas atmospheres can be studied. 

Two main types of commercial DSC instruments are in use, namely heat flux (hf) and power compensation (pc) instruments (c/r. ref. [21]). The power compensating version, originally developed by Perkin-Elmer Co. [56], employs two different ovens. DSC in the heat flux mode with one oven is similar in operation to a conventional 

except that the quantitative compensation for Table 2.8. Main characteristics of DSC 

For DSC measurements the sample is contained in a metal pan and the reference is an empty pan of the same material (usually aluminium). In a typical DSC, a sample may have a mass of 20 mg and shows a heat capacity of about 50 rnl/K. DSC can examine materials between - 170°C and - -750°C. As heat is 

Accurate temperature calibration using NIST-ICTA melting point standards (indium, tin, lead, aluminium, zinc of purity 99.999% accuracy to O.DC or better [59]) is essential. Richardson [60] has critically described standardisation and quality assurance of DSC. Much work has been done to implement peak separation software techniques. 

DSC thermograms have been published [67] the reader is referred to a recent book [68] and reviews on DTA and DSC [69-71] for further details. Quantitative DSC has been reviewed [72]. Mathot [73] has reviewed recent advances in DSC, including (very) high pressure DSC (up to 550 MPa). Modulated DSC has been reviewed by ref. [21] and its practical applicability to polymeric systems has been described [74], Reading et al. [75] have discussed origin and interpretation of MTDSC. Special issues on temperature modulated calorimetry [76] and MTDSC [77,78] have appeared. 

If a sample of paint is seanned with increased temperature per unit time (e.g., 10°C/min or 50°F/min) plot of heat capacitance versus temperature, changes in heat capacitance will occur. Major thermal transitions can be observed, such as glass transition (Tg) temperature. The Tg indicates different types of binders, such as alkyd, linseed oil, or epoxy. Although temperature transitions do not indicate chemical substances, they can differentiate between thermoplastic and thermoset (cured) binders, since the former possess a melting temperature (TJ, and the latter possess Tg. For example, a vinyl chloride binder usually melts, while an alkyd binder does not. 

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The crystal stmcture of glycerides may be unambiguously determined by x-ray diffraction of powdered samples. However, the dynamic crystallization may also be readily studied by differential scanning calorimetry (dsc). Crystallization, remelting, and recrystallization to a more stable form may be observed when Hquid fat is solidified at a carefully controlled rate ia the iastmment. Enthalpy values and melting poiats for the various crystal forms are shown ia Table 3 (52). 

The product must be formulated and frozen in a manner which ensures that there is no fluid phase remaining. To achieve this, it is necessary to cool the product to a temperature below which no significant Hquid—soHd phase transitions exist. This temperature can be deterrnined by differential scanning calorimetry or by measuring changes in resistivity (94,95). 

Other PDMS—sihca-based hybrids have been reported (16,17) and related to the ceramer hybrids (10—12,17). Using differential scanning calorimetry, dynamic mechanical analysis, and saxs, the microstmcture of these PDMS hybrids was determined to be microphase-separated, in that the polysiUcate domains (of ca 3 nm in diameter) behave as network cross-link junctions dispersed within the PDMS oligomer-rich phase. The distance between these... 

The process known as transimidization has been employed to functionalize polyimide oligomers, which were subsequentiy used to produce polyimide—titania hybrids (59). This technique resulted in the successhil synthesis of transparent hybrids composed of 18, 37, and 54% titania. The effect of metal alkoxide quantity, as well as the oligomer molecular weight and cure temperature, were evaluated using differential scanning calorimetry (dsc), thermogravimetric analysis (tga) and saxs. 

Most hydrocarbon resins are composed of a mixture of monomers and are rather difficult to hiUy characterize on a molecular level. The characteristics of resins are typically defined by physical properties such as softening point, color, molecular weight, melt viscosity, and solubiHty parameter. These properties predict performance characteristics and are essential in designing resins for specific appHcations. Actual characterization techniques used to define the broad molecular properties of hydrocarbon resins are Fourier transform infrared spectroscopy (ftir), nuclear magnetic resonance spectroscopy (nmr), and differential scanning calorimetry (dsc). 

Glass-transition temperatures are commonly determined by differential scanning calorimetry or dynamic mechanical analysis. Many reported values have been measured by dilatometric methods however, methods based on the torsional pendulum, strain gauge, and refractivity also give results which are ia good agreement. Vicat temperature and britde poiat yield only approximate transition temperature values but are useful because of the simplicity of measurement. The reported T values for a large number of polymers may be found ia References 5, 6, 12, and 13. 

The compositional distribution of ethylene copolymers represents relative contributions of macromolecules with different comonomer contents to a given resin. Compositional distributions of PE resins, however, are measured either by temperature-rising elution fractionation (tref) or, semiquantitatively, by differential scanning calorimetry (dsc). Table 2 shows some correlations between the commercially used PE characterization parameters and the stmctural properties of ethylene polymers used in polymer chemistry. 

The cure of novolaks with hexa has been studied with differential scanning calorimetry (dsc) and torsional braid analysis (tba) (46) both a high ortho novolak and a conventional acid-cataly2ed system were included. The dsc showed an exothermic peak indicating a novolak—hexa reaction ca 20°C higher than the gelation peak observed in tba. Activation energies were also calculated. 

Cure kinetics of thermosets are usually deterrnined by dsc (63,64). However, for phenohc resins, the information is limited to the early stages of the cure because of the volatiles associated with the process. For pressurized dsc ceUs, the upper limit on temperature is ca 170°C. Differential scanning calorimetry is also used to measure the kinetics and reaction enthalpies of hquid resins in coatings, adhesives, laminations, and foam. Software packages that interpret dsc scans in terms of the cure kinetics are supphed by instmment manufacturers. 

Temperature-risiag elution fractionation (tref) is a technique for obtaining fractions based on short-chain branch content versus molecular weight (96). On account of the more than four days of sample preparation required, stepwise isothermal segregation (97) and solvated thermal analysis fractionation (98) techniques usiag variatioas of differeatial scanning calorimetry (dsc) techniques have been developed. 

Thermal analysis iavolves techniques ia which a physical property of a material is measured agaiast temperature at the same time the material is exposed to a coatroUed temperature program. A wide range of thermal analysis techniques have been developed siace the commercial development of automated thermal equipment as Hsted ia Table 1. Of these the best known and most often used for polymers are thermogravimetry (tg), differential thermal analysis (dta), differential scanning calorimetry (dsc), and dynamic mechanical analysis (dma). 

Transitions such as T and are rapidly and conveniently studied using differential scanning calorimetry (dsc). This technique monitors changes in... 

Fig. 5. Differential scanning calorimetry thermogram. Amorphous PPS is heated from room temperature to 325°C at 20°C/min.

Onset value as measured by differential scanning calorimetry. 

The glass-tiansition tempeiatuiesfoi solution-polymeiized SBR as well as ESBR aie loutinely determined by nuclear magnetic resonance (nmr), differential thermal analysis (dta), or differential scanning calorimetry (dsc). 

The thermal glass-transition temperatures of poly(vinyl acetal)s can be determined by dynamic mechanical analysis, differential scanning calorimetry, and nmr techniques (31). The thermal glass-transition temperature of poly(vinyl acetal) resins prepared from aliphatic aldehydes can be estimated from empirical relationships such as equation 1 where OH and OAc are the weight percent of vinyl alcohol and vinyl acetate units and C is the number of carbons in the chain derived from the aldehyde. The symbols with subscripts are the corresponding values for a standard (s) resin with known parameters (32). The formula accurately predicts that resin T increases as vinyl alcohol content increases, and decreases as vinyl acetate content and aldehyde carbon chain length increases. 

By differential scanning calorimetry from 30 to 100°C on dried resia. 

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