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Thermal Analysis - Fundamentals and Applications to Polymer Science Part 11

Tham khảo tài liệu 'thermal analysis - fundamentals and applications to polymer science part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Page 142 Figure 6.21. Glow curve of polyacrylonitrile. The low- and high-temperature peaks are attributed to relaxations in the crystalline regions and melting of the crystalline regions. respectively in Figure 6.21. By using curve-fitting software the glow curve can be resolved into two distinct peaks. The low-temperature peak corresponds to relaxations occurring in the semi-crystal-line regions of the film and the high-temperature peak is attributed to relaxations of the amorphous regions. The following items should be included when presenting the results of a TL measurement type of TL apparatus sample preparation including dimensions and method of attachment temperature range and heating rate irradiation source and total dose. 6.7 Alternating Current Calorimetry ACC Alternating current calorimetry ACC measures the alternating temperature change produced in a sample by an alternating heating current from which the heat capacity of the material can be estimated. Assuming that heat does not leak from the sample during heating and a constant current amplitude and frequency the amplitude of the alternating temperature of the sample is given by 6.1 where Cp is the heat capacity of the sample Qe is the heat flux and co the angular frequency. A block diagram of an AC calorimeter is presented in Page 143 Figure 6.22. Schematic diagram of an alternating current calorimeter Figure 6.22. The output of a white light source is modulated with a variable-frequency beam chopper so that a square wave is produced which illuminates one surface of the sample. The fluctuating alternating temperature is measured on the other surface using a thermocouple. Figure 6.23 shows the variation in Tac as a function of time for materials of large and small heat capacity. Owing to recent improvements in the design of lock-in amplifiers which can operate at low frequencies ACC can be applied to a broad range of materials including polymers. Figure 6.23. I In ACC the sample is illuminated by a .