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Thermal Analysis - Fundamentals and Applications to Polymer Science Part 4

Tham khảo tài liệu 'thermal analysis - fundamentals and applications to polymer science part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Page 37 3.5.1 Mass of Sample It is easier to obtain data free from the effects of temperature gradients with low-mass samples where the true sample temperature can be more precisely defined. Generally when measuring a first-order phase transition at a scanning rate of 5-10 K min 1-5 mg of sample is used. The melting temperatures of two samples of a polyethylene film weighing 0.25 and 2.5 mg as a function of heating rate are presented in Figure 3.8. The peak melting temperature of the low-mass sample is approximately constant with heating rate whereas an increase in melting temperature is observed for the heavier sample. The DSC heating curves of 77-hexacosane an 77-alkane measured at 10 K min exhibit two endotherms Figure 3.9 . The low-temperature peak is associated with a crystalto-crystal transition and the high-temperature peak with melting. Plotting the observed transition temperatures as a function of sample mass over the range 0.5-100 pg reveals that the transition temperatures are constant in this low sample mass range. Commercially available DSC instruments are sufficiently sensitive to record phase changes in samples in the region of 0.001 mg. The sample must be representative of the material as a whole and the lower limit of the sample mass may be detennined by the homogeneity of the sample. This is especially true of composites and polymer blends where frequently 10 mg of sample are required before a representative sample can be prepared. When a large amount of sample is to be analysed the heating rate should be decreased accordingly to compensate for the increased temperature gradients in the sample. A precise knowledge of the sample mass is not necessary to estimate the transition temperature. In order to measure the transition enthalpy the sample mass should be known to within 1 or 0. Ipg if possible. During the weighing Heating Rate K tnin Figure 3.8. Melting temperature of polyethylene film as a function of heating rate. Sample mass 0.25 and 2.5 mg