Hysteresis endotherm

An endothermic (4.18kJ mol ) structural transition is observed at 118°C from the orthorhombic low-temperature structure to the hexagonal high-temperature structure. The high-temperature structure melts at 287 °C with a latent heat of 7.56 kJ mol . Both transitions are reversible, although a small hysteresis in temperature was observed. 

C c = 0.5 wt%, sharp transition, no hysteresis [357] Shows endotherm during the phase transition [357]... 

As stated above, the hysteresis of ELPs in the course of the ITT transition results from the overlap of two kinetically different processes a fast endothermic process, which corresponds to the destruction of the ordered hydrophobic hydration, and a second exothermic process arising from the P-spiral chain folding. 

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. 

Systems far from equilibrium exhibit unusual kinetic behavior. Feedback may lead to instability and ultimately explosion or to undamped (or weakly damped) oscillations about an inaccessible steady state. Here we treat another example of chemical instability— the abrupt switching between steady states with an attendant chemical hysteresis.The coupled variables are temperature and the progress variable for a chemical reaction, the same coupling that creates thermal explosion (Section 7.2) and thermal oscillation (Section 7.6.4). The difference is that here the chemical reaction is endothermic so explosion is ruled out. Instability is induced by heat flow into the system. Unlike the examples considered previously, the mathematical description of this coupled system is simple. The resulting equations may be solved and detailed theoretical predictions verified. 

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.

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.

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],...

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). 

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