Nonreversing heat flow curve

Figure 7. TMDSC reversing and nonreversing heat flow curves for the heating cycle of as-received Nitinol SE [37]. Reproduced with permission from Elsevier Science.

Broadj low-temperature exothermic peaks appear on the nonreversing heat flow curves in Figures 7 and 8, indicating transformation within martensite to another structure (M ). Low-temperature peaks had previously been reported from DSC [10] and electrical resistivity measurements [38] of engineering and NiTi orthodontic alloys, respectively. However, our group was unable to confirm these peaks on DSC plots for five representative orthodontic wires[25]. 

Similar results were obtained for TMDSC analyses of the in vivo shape-memory wire, Neo Sentalloy, shown in Figures 9 and 10 [37]. While our previous DSC study [25] suggested that direct transformation of martensite to austenite occurs during heating, the nonreversing heat flow curve in Figure 9 shows that a two-step transformation involving R-phase actually takes place. 

This polyether impression material undergoes a gleiss transition near -SO C and has melting peaks near -20 °C and 50 °C. While there are exothermic peaks indicative of crystallization on the nonreversing heat-flow curve for modulation condition (b) in Figure 18, only endothermic peaks that would not correspond to crystallization are found on the nonreversing heat-flow curve for modulation condition (a) in Figure 17. 

The TMDSC results are shown for the Coe-Flex polysulfide impression material and modulation condition (a) in Figure 19. There is a glass transition near -55 °C and an apparent weak crystalline polymer melting peak near 70 °C. No evidence of an exothermic crystallization peak can be seen on the bottom nonreversing heat-flow curve. The peak near 190 °C requires further study, but is not clinically relevant for the properties of the impression material. 

Figure 2.101. Nonreversing heat flow curves following isothermal aging of PMMA. Annealings for 6 h at a = 140 °C, b = 130 °C, c = 120 °C, d = 115 °C, e = 110 f = 105 °C, g = 103 °C, h = 100 °C, i = 95 °C, j = 90 °C, k = 85 °C. The two figures differ in that panel (a) shows the observed curves while (b) shows the curves after subtraction of the nonaged runs (top curve). [From Kubota et al. (2005) reprinted with permission from Elsevier Ltd.l...

Figure 2.97. The crystallization exotherm during an MDSC heating experiment seen in the nonreversing heat flow (top curve) is confirmed by superimposing it on the modulated heat flow signal for polyamide 12 that had been annealed at 160 °C for one hour. The exothermic direction of this figure is upward [from Judovits (1997) reprinted with permission from the North American Thermal Analysis Society].

The nonreversing effects can be noted in the IsoK baseline (Ye 2006). The IsoK baseline is the heat flow curve at zero ramping or for the isothermal hold after the ramp. In Fig. 2.119 the signals for the step scan DSC are shown, the IsoK baseline can be noted in the raw data following the return to baseline after the ramp segment. Slight overshoots in the heat flow may occur as the calorimeter obtains steady state. 

Figure 15. TMDSC curves showing reversing, nonreversing and total heat flow for the unpigmented MDX 4-4515 silicone after 1 hour processing [41]. Reproduced with permission from Quintessence Publishing Company, Inc.

In Fig. 4.102 a Unearly increasing and an exponentially increasing heat-flow rate of many times the size of the effect to be measured, is added at 250 and 0 s in the top and bottom curves, respectively. The loss of stationarity is in these cases negligible, and a separation of reversing and nonreversing components is possible with high precision. Effects of this type are expected in samples that undergo irreversible... 

Figure 2.119. Separation of a scan of PET into the nonreversing component or IsoK baseline and the reversing or Thermodynamic heat capacity from an SSDSC experiment from the raw data labeled above. Upper curve shows the reversing heat capacity component bottom curve, is the IsoK baseline. An insert of the cold crystallization process demonstrates the temperature steps with the resultant modulated effect on the heat flow with its associated IsoK baseline. [From Ye (2006) courtesy of Perkin-Elmer.l...

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