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Investigating Thermal History Effects on PET Glass Transition and Melting Behavior Using Differential Scale Calorimetry
Understanding the thermal history of polymers is essential for comprehending the relationship between the internal structure and processing conditions of polymer materials. This brief presents an example of Differential Scanning Calorimetry (DSC) measurement conducted on polyethylene terephthalate (PET).
Hitachi NEXTA® DSC
Measurements
Effects of Cooling Rate on Glass Transition
PET samples were subjected to various cooling rates from temperatures above the glass transition temperature to investigate the effects of cooling rate on the glass transition. The results, as shown in Figure 1, indicate that there was minimal difference in heat capacity before and after the glass transition across all DSC curves. However, with the exception of the quenched PET sample, the curves displayed distinctive endothermic peaks at the end of the glass transition phase. Furthermore, higher cooling rates resulted in smaller endothermic peaks. The thermal history near the glass transition can be deduced from the characteristics of these endothermic peaks.
Figure 1: The Effects of Cooling Rate on Glass Transition
Changes in Melting Peaks Due to Different Thermal Processing
PET samples were thermally processed at various temperatures after being cooled from a molten state to examine the effects on melting peaks. As observed in Figure 2, except for the quenched PET sample, the DSC curves revealed small endothermic peaks just before the melting peak at temperatures slightly higher than the thermal processing temperatures. The nature of these endothermic peaks varied depending on the thermal processing conditions. The thermal history of the samples can be inferred from the shapes of these melting peaks.
Figure 2: Changes In Melting Peaks by Different Thermal Processing
Conclusion
This study highlights the importance of thermal history in determining the relationship between the internal structure and processing conditions of polymers. Through DSC measurements on polyethylene terephthalate (PET), it was observed that cooling rates and thermal processing conditions significantly influence the glass transition and melting peaks. The presence of endothermic peaks and their characteristics provide valuable insight into the thermal history and processing history of the material. This understanding can be leveraged to optimize processing conditions for better material performance and quality control in polymer manufacturing.
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References - Hitachi