Differential scanning calorimetry exothermic transition

Although there are other ways, one of the most convenient and rapid ways to measure AH is by differential scanning calorimetry. When the temperature is reached at which a phase transition occurs, heat is absorbed, so more heat must flow to the sample in order to keep the temperature equal to that of the reference. This produces a peak in the endothermic direction. If the transition is readily reversible, cooling the sample will result in heat being liberated as the sample is transformed into the original phase, and a peak in the exothermic direction will be observed. The area of the peak is proportional to the enthalpy change for transformation of the sample into the new phase. Before the sample is completely transformed into the new phase, the fraction transformed at a specific temperature can be determined by comparing the partial peak area up to that temperature to the total area. That fraction, a, determined as a function of temperature can be used as the variable for kinetic analysis of the transformation. 

Differential Scanning Calorimetry. Some structural information is provided by the thermal behavior of the polymer. The homopolymer of DPP crystallizes when heated above the glass transition temperature. A crystallization exotherm at the appropriate temperature therefore indicates the presence of DPP blocks, either as the homopolymer or in a block copolymer. 

Figure 24.1 Differential scanning calorimetry (DSC) traces for the different N,N -dimethylpyr-rolidinium (Pn) species. The y-axis has been adjusted to allow comparison between species. Thermal transitions are recorded as peaks (endothermic) or troughs (exothermic) and indicate changes of phase. The structurally similar dicyanamide (DCA) and thiocyanate species both show broad phase transitions in comparison to the TFSA species.

Figure 5.1. Schematic representation of typical differential scanning calorimetry curves showing a glass transition and possible exothermal and endothermal relaxations.

Differential scanning calorimetry monitors the energy required to maintain the sample and a reference at the same temperature as they are heated. A plot of heat flow (W/g or J/g) versus temperature is obtained. A thermal transition which absorbs heat (melting, volatilization) is called endothermic. If heat is released during a thermal transition (crystallization, degradation), it is called exothermic. The area under a DSC peak is directly proportional to the heat absorbed or released and integration of the peak results in the heat of transition. 

Frequently when using differential scanning calorimetry as an analysis technique, one can observe an endothermic peak corresponding to a phase transition, followed by a second endothermic peak corresponding to melting. Sometimes there is an exothermic peak between the two endotherms, representing a crystallization step. In these cases it is often... 

In differential scanning calorimetry (DSC), the heat flow to the sample is compared to the heat flowing to an inert reference as both are heated at the same rate. When an endothermic transition occurs in the sample, the recorder shows a peak, the area of which is proportional to the amount of heat absorbed by the sample. When an exothermic transition occurs in the sample, the opposite effect is seen. Because the difference in electrical power is monitored as the sample and reference have their temperature changed, what is measured is dH/tfT for the sample. But dH/dT = Cp, the heat capacity at constant pressure. From thermodynamics, we know that... 

Perhaps the most useful techniques now in use to locate the are differential thermal analysis (DTA) and differential scanning calorimetry (DSC) (see Chapter 10). Both methods provide a means of characterizing thermal transitions as either exothermic or endothermic peaks (melting and crystalhzation), or as changes in heat capacity (glass transition). 

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