Endothermic hysteresis peaks

Figure 2.22. DSC curves showing glass transitions of amorphous polymers with (a) an endothermic hysteresis peak on high-temperature side of glass transition (endotherm down) and (b) an exothermic hysteresis peak on low-temperature side of glass transition (endotherm down). The heat capacity of the glass is extrapolated to higher temperatures to make the broad exothermic peak more visible.

In the bottom graph of Fig. 4.34, actual data on polystyrene glasses are reproduced. All samples were heated at 5 K/min. The different thermal histories were produced by cooling the samples at the rates indicated in the legend. The endothermic hysteresis peak for the slowly cooled samples is clearly apparent. The exothermic hysteresis for the fast cooled sample is less obvious (recently some doubts were raised about the proper description of the exotherm via the hole theory). 

Figure 2.20. Determiaation of glass transition temperatnre in a heating experiment from an idealized DSC cirrve (endotherm down). In this DSC curve no hysteresis peak is present, which is a very rare case. The height of the double arrow is proportional to the heat capacity increase at the glass transition.

It was mentioned before that endothermic or exothermic hysteresis peaks are common in the DSC curves of the glass transition. How can the presence... 

Figure 2.32. The glass transition of (semicrystalline) PET recorded on cooling (CR = 1 °C/min) and on reheating (HR = 10°C/inin) since the two areas between the two curves are equal, there is no hysteresis peak (Endotherm is down) [from Menczel and Jaffe (2006, 2007) reprinted with permission of Springer-Verlag and the North American Thermal Analysis Society],...

Almost all endothermic peaks have corresponding exothermic peaks at lower temperatures, indicating a temperature hysteresis. The exceptions are a small en-dothoinic peak for CTADeS and a transition below for CTADDS i.e., they do not have corresponding exotherms in the cooling cycle. 

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