2024 Lecture 9 - Thermal Analysis - Exercises_Solutions.pdf

Lecture 9 – Thermal Analysis General knowledge questions 1. Describe what quantity is measured and how the measurement is performed for each of the following techniques: a) TGA b) DSC c) DTA TGA: Weight changes. A microbalance register weight changes upon...

Lecture 9 – Thermal Analysis General knowledge questions 1. Describe what quantity is measured and how the measurement is performed for each of the following techniques: a) TGA b) DSC c) DTA TGA: Weight changes. A microbalance register weight changes upon heating the sample in a controlled atmosphere. DSC: Heat flow is measured between a reference and the sample to measure. DTA: Temperature changes are measured between the sample and a reference crucible. 2. What information can be obtained by DTA/DSC that cannot be obtained by TGA? Physcal or Chemical transformation that take place without mass changes. 3. What are the advantages of using a combination of DSC and TGA to characterize a sample? It allows gathering information on weight changes (associated with liquid to gas transitions) and characterizing energy exchanges related to that (e.g. exothermal or endothermal events). 4. Why are Pt and quartz commonly used for sample holders in thermal analysis? Because these materials do not undergo oxidation or chemical degradation at high temperatures and are (relatively) inert. 5. The peak area of differential scanning calorimetry curve is proportional to one or more of the following: a) Mass loss b) Mass of the sample c) Heat of decomposition/ phase change c) the other two refeers to TGA transformation 1 Tutorial exercises 6. The following figure shows the TGA thermal curves of a glass-fiber-filled polymer (solid line) and the nonfilled polymer (dotted line). (a) If the original sample mass for the glass-fiber-filled polymer was 23.6 mg, what is the mass % glass fiber in the sample? (b) What would you recommend as the upper temperature limit for the use of this material, assuming you want to build in a safety factor of 20 % (i.e., you want to be no higher than 80 % of the decomposition temperature)? a) The % is approximately 30% (7 mg) b) The decomposition temperature is approximately at 400 C; therefore 320 C should insure a safety factor of 20% 2 7. A commercial sample of “light” margarine is composed of water, vegetable oils (sunflower, linseed, palm, rapeseed) and other components. The DSC thermal curve of a sample of “light” margarine is shown. (Note: This plot has the endothermic peaks pointing up!) (a) What process and what substance do you think give rise to the peak at 4.60 °C? (Think about what might be added to margarine to reduce the calories.) (b) And the peak at −24.6 °C? Explain your answer. a) The transitions are endotherminc, indicating a shift from a more ordered to a less ordered state The peak has an onset at around 0 C, suggesting the presence of water in the margarine and its transition to liquid form b) The melting point of lupine oils is similar to rapeseed oil (−28.44 °C), sunflower oil (−24.03 °C), soybean oil (−27.93 °C) or corn oil (−38.37 °C) are in the area of what is indicated by the DSC. 3 8. The DSC transition of a polymer is reported below. Is the polymer semicrystalline or amorphous? Explain The material is semycrystalline. In a (semi) crystalline polymer the polymer will undergo the glass transition and then melt; when subsequently cooling down the polymer, it crystallizes. If you analyze a 100% amorphous polymer, like atactic polystyrene, you wouldn't a crystallization dip, because such materials don't crystallize. 4