Hysteresis peaks

Figure 6.13 illustrates the glass transition temperature of small beads of polystyrene, measured by standard DSC [19]. One can observe, besides the glass transition with a small hysteresis peak, that on first heating, shown by the solid curves. 

Besides T, the breadth of the ttansition region, and ACp, partial crystallization affects also the hysteresis behavior (enthalpy relaxation) of the amorphous fraction. Figure 6.129 in its bottom curve depicts a typical enthalpy relaxation in amorphous PET that can be used to study the thermal history, as outlined for polystyrene in Figs. 4.125-127. The DSC curves for the samples with increasing crystallinity show that the hysteresis disappears faster than the step in ACp at the glass transition. The top sample in Fig. 6.129 of 31% crystallinity shows a smaller change in heat capacity than expected for an amorphous content of 69%, due to some rigid-amorphous fraction, but the hysteresis peak seems to have disappeared completely. 

A drawn film was also included in the analysis [66]. The quasi-isothermal TMDSC of this drawn sample is reproduced in Fig. 6.109. It was produced out of practically amorphous PET by biaxial drawing at 368 K. The sample retains no residual cold crystalhzation and has a higher rigid-amorphous fraction than the semicrystalline reference PET of Fig. 3.92. Long-time aimealing causes an aimealing peak of the crystals, as described in Sect. 6.22, but displays no hysteresis peak, as was also observed for the slowly cooled, undrawn PET samples with similar crystallinity used as an example in Fig. 6.129. 

Figure 8.10 shows a hysteresis peak associated with the glass transition. Such hysteresis peaks appear frequently, but not all the time. They are most often associated with some physical relaxation, such as residual orientation. On repeated measurement, the hysteresis peak usually disappears. 

An improved method of separating a transient phenomenon such as the hysteresis peak from the reproducible result of the change in heat capacity is obtained via the use of modulated DSC (25,26) see Table 8.5. Here, a sine wave is imposed on the temperature ramp. A real-time computer analysis... 

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 2.29. The DSC heating curve of a poly(ethylene 2,6-naphthalenedicarboxylate) sample at a heating rate of 10 °C/min previously cooled at 1 °C/min from the melt.There is no hysteresis peak at the glass transition see that part of the curve in the inset. Endothermis down. [PEN sample from Aldrich (435317-10(Xj) (Menczel, unpublished results).]...

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],...

The glass transition curve usually shows an enthalpy relaxation or hysteresis peak as well. The intensity of the hysteresis peak can be greatly increased if one anneals the glass just below the glass transition temperature or if the material was slowly cooled (see Section 2.7, on phase transitions in thermoplastic polymers). TTiis poses two problems (1) obtaining a comparative glass transition temperature and (2) evaluating the hysteresis behavior. 

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). 

Another, presently unsolved problem in hysteresis of glassy materials is found in semicrystalline polymers. In these samples one finds that the remaining amorphous fraction is showing reduced or no hysteresis when compared to the fully amorphous polymer. The data on poly(ethyIene terephthalate) showed, for example, that a 10% crystallinity is enough to make the hysteresis peak disappear. 

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