Nonreversing component

Hence, in the simplest terms, tmDSC is a description of the heat flow into the sample resulting from the sinusoidal modulation of the temperature program. Two properties of the sample can be investigated by tmDSC, the heat capacity which is directly related to the reversing component and a kinetically hindered thermal event which is related to the nonreversing component. Conventional DSC provides only a measure of the total heat flux into a sample as a function of temperature whereas tmDSC allows the heat capacity and kinetic components to be separated. However,... 

A variation of DSC is the MDSC (modulated DSC), wherein heat is applied sinusoidally, such that any thermal events are resolved into reversing and nonreversing components to allow complex and even overlapping processes to be deconvoluted. The heat flow signal in conventional DSC is a combination of... 

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 4.126 illustrates that the reversing glass transition measurement is almost independent of the annealing (thermal) history. The still little-explored, small differences are an indication of the changes in relaxation kinetics on annealing of glasses. The corresponding nonreversing components are shown in Fig. 4.127. They permit the quantitative characterization of the thermal history of the glass as mentioned above, the differences, however, between the curves of Fig. 4.126 are not considered in this separation.

Fig. 16.4 Illustration of the use of DSC data for measuring Tg and AH (the activation energy for enthalpy relaxation). Tgon, Tgenj, and ATg indicate the onset, end, and width of the glass transition. Modulated DSC (mDSC) allows the separation of the total heat flow into reversing and nonreversing components. AH can be evaluated from (a) the dependence of Tgon on scanning rate q, (b) ATg, (c) the dependence of the relaxation enthalpy AH (area of the overshoot on annealing time, and (d) the dependence of the complex heat capacity Cp (obtainable by mDSC) on modulation frequency v. (Reproduced with permission from Yu 2001)...

An MTDSC experiment can not only generate the total heat flow similar to the heat flow obtained in conventional DSC but also separate the total heat flow into its reversing and nonreversing components. The total heat flow is the sum of the thermal events and is generally equivalent to the heat flow seen in conventional DSC. The reversing heat flow is the heat capacity component (plus other terms in some cases see text below) of the total heat flow Cp dT/dt as noted in Eq. (2.91)]. 

Modulated temperature DSC offers a number of advantages when analyzing a glass transition. These advantages stem from its ability to separate multiple thermal events. As noted earlier, the glass transition can be separated into reversing and nonreversing components. 

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...

The reversing component of the total heat flow signal (Cpfi) allows the nonreversing component to be calculated using the relationship... 

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