Differential Thermal Methods

Very often the limiting parameter for the useful application of a polymer is dimensional or chemical stability. The temperatures at which physical transitions occur are important to the first criterion, and the temperature of the onset of chemical 

FIGURE 18.2 Typical transmission spectra in the IR region. The ordinate in each diagram is 0%-100% transmission. The incompletely hydrolyzed sample of polyfvinyl acetate) illustrates the additivity of spectra. (Data from Nyquist, R. A., Infrared Spectra of Plastics and Resins, 2nd edn., Dow Chemical Co., Midland, TX, 1961.) 

FIGURE 18.4 Differential scanning calorimeter (power compensation type). The platinum resistance thermometer (PRT) sensor detects a small error signal between the programmed temperature and the sample temperature and calls for more or less power to each heater to keep both holders on program. The difference in power required between the sample and the reference is amplified and recorded. (Courtesy of PerkinElmer Corporation, Norwalk, CT.) 

FIGURE 18.5 The glass transition for plasticized PVC as determined by DSC. The heating rate was 40°C/min. (Reprinted from Brennan, W. P. et ah, Amer. Lab., 20, 36,1988 copyright by International Scientific Communications, Inc.) 

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Boerio-Goates J and Callanan J E 1992 Differential thermal methods Physical Methods of Chemistry... 

Boerio-Goates J and Callanan J E 1992 Differential thermal methods Determination of Thermodynamic Properties, Physical Methods of Chemistry 2nd edn vol VI ed B W Rossiter and R C Baetzold (New York Wiley)... 

Over the past few years several papers have been published describing the application of differential thermal methods to the determination... 

DTA is a widely used tool for studying and characterizing polymers. Figure 31-7 illustrates the types of physical and chemical changes in polymeric materials that can be studied by differential thermal methods. Note that thermal transitions for a polymer often take place over an extended temperature range because even a pure polymer is a mixture of homologs and not a single chemical species. 

J.Boerio-Goates, J.E. Callen Differential Thermal Methods in Determination of Thermodynamic Properties (B.W. Rossiter, R.C. Beatzold, edts.), Wiley, New York 1992... 

Assessing the Thermal Stability of Chemicals by Methods of Differential Thermal Analysis, American Society for Testing and Materials, Philadelphia. 

Thermal Properties. The thermal stabiUty of cellulose esters is deterrnined by heating a known amount of ester in a test tube at a specific temperature a specified length of time, after which the sample is dissolved in a given amount of solvent and its intrinsic viscosity and solution color are deterrnined. Solution color is deterrnined spectroscopically and is compared to platinum—cobalt standards. Differential thermal analysis (dta) has also been reported as a method for determining the relative heat stabiUty of cellulose esters (127). 

Crystallization kinetics have been studied by differential thermal analysis (92,94,95). The heat of fusion of the crystalline phase is approximately 96 kj/kg (23 kcal/mol), and the activation energy for crystallization is 104 kj/mol (25 kcal/mol). The extent of crystallinity may be calculated from the density of amorphous polymer (d = 1.23), and the crystalline density (d = 1.35). Using this method, polymer prepared at —40° C melts at 73°C and is 38% crystalline. Polymer made at +40° C melts at 45°C and is about 12% crystalline. 

Thermoanalytical method is one of the widespread methods of physical and chemical reseai ches. Considerably broadens its possibilities combination with methods differential-thermal (DTA), differential-thermogravimetric (DTG), thermogravimetric (TG) analyses. 

The procedures of measuring changes in some physical or mechanical property as a sample is heated, or alternatively as it is held at constant temperature, constitute the family of thermoanalytical methods of characterisation. A partial list of these procedures is differential thermal analysis, differential scanning calorimetry, dilatometry, thermogravimetry. A detailed overview of these and several related techniques is by Gallagher (1992). 

It is possible to distinguish between SBR and butyl rubber (BR), NR and isoprene rubber (IR) in a vulcan-izate by enthalpy determination. In plastic-elastomer blends, the existence of high Tg and low Tg components eases the problems of experimental differentiation by different types of thermal methods. For a compatible blend, even though the component polymers have different Tg values, sometimes a single Tg is observed, which may be verified with the help of the following equation ... 

In addition to these standardised test methods set by regulation (in particular the transport regulations of dangerous substances), there are laboratory methods that can provide more details regarding substance behaviour. In particular, there is differential thermal analysis (DTA), thermal gravimetric analysis, calorimetry and thermomanometry, which will not be described here. 

Phase diagrams give valuable information about the compounds that can form in a system of components. These compounds can then be prepared and studied. For the experimental determination of phase diagrams the following methods are used. In differential thermal analysis (DTA) a sample of a given composition is heated or cooled slowly... 

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... 

ASTM E 1356-98, ASTM Book of Standards 2002. Standard Test Method for Assignment of the Glass Transition Temperature by Differential Scanning Calorimetry or Differential Thermal Analysis . ASTM International, Conshohocken, PA. 

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