Differential scanning calorimetry first-order phase transitions

In this paper, a thermodynamic phase transition is studied using Differential Scanning Calorimetry (DSC). This phase transition, which will be described according to the current thermodynamic theories as a first order or a second order one, is recorded on the DSC trace as an anomalous change in the differential power ZP, different from the normal IP variation only due to the heat capacity of the material. This variation, sharp or smooth, will be called the "transition peak". We define the height h of the peak as the distance between the heat capacity trace, or baseline, and the maximum during the course of the phase transition. In the case of a pure second order phase transition, this height is the diffe-... 

Synchrotron-x>ray powder-diffraction and differential-scanning-calorimetry measurements on solid Cso reveal a first-order phase transition from a low-temperature simple-cubic structure with a four-molecule basis to a face-centered-cubic structure at 249 K. The free-energy change at the transition is approximately 6.7 J/g. Model fits to the diffraction intensities are consistent with complete orientational disorder at room temperature, and with the development of orientational order rather than molecular displacements or distortions at low temperature. 

Differential scanning calorimetry can be used to provide an insight into the thermal behavior of a gel. On heating, it is possible to observe an endotherm associated with the gel-sol transition, and on cooling, an exotherm can be observed as the sol reforms the gel. However, it should be noted that the conversion of a gel to a sol is not a first-order phase transition, as it is a multistep hierarchical process, and for this reason, the observed endotherms/exotherms can be very broad. However, if such peaks are observed, then integration can provide data about the enthalpy of the transition (A//gei-soi on heating and A//soi-gei on cooling). This method can also provide detailed information on the reversibility and repeatability of the gel-sol-gel transitions. 

At the nominal melting point Tm there is a first-order phase transition from the crystal to the mesophase with the usual discontinuities in the extensive properties (e.g. volume and entropy). In Fig. 5.7, we schematically illustrate a hypothetical differential-scanning-calorimetry (DSC) trace and the variation in volume of the sample versus temperature for an ideal nematic. The values for the changes in enthalpy (AH 45 kJ mol" ) and volume (A V 10%) at are typical of those changes in extensive properties that occur on melting ordinary organic molecular crystals. However, if you continue to heat the opalescent-looking mesophase, there is a second transition to a transparent isotropic state above Td. Nematic melts... 

Furthermore, assuming a first-order phase transition, the VPT can be detected by caloric methods. With a differential scanning calorimetry (DSC) measurement, flie heat change, which provides information about the internal free energy or latent heat, can be determined (Otake et al. 1990 Hirotsu 1993, 1994). Figure 1 shows two curves of the DSC method for PNIPAAm with different cross-linking concentrations. The cross-linker used for this case was layered siUcates (Haraguchi et al. 2002 Ferse 2007). 

Temperature-modulated differential scanning calorimetry (T-MDSC) applies a thermal modulation in temperature to a conventional DSC mn and determines a dynamic heat capacity from the relationship between the modulation components of temperature and of heat flow. Primary application of this technique has been the measurement of specific heat capacity and the examination of the anomaly in a relaxation process such as alpha process related to the glass transition. An application to the first-order phase transitions of crystallisation and melting of polymer crystals has recently been suggested. The method and typical results are described. 13 refs. 

Similarly, if a quantity such as the volume exhibits an abrupt change in slope, which occurs at the T, then there is a discontinuity in quantities associated with first derivatives of this parameter, or second derivatives of the free energy (with respect to appropriate thermodynamic variables), such as the specific heat (Figure 10-19). Accordingly, the Tg may be related to a second-order phase transition, but this remains in dispute. The experimentally observed transition is clearly governed by kinetics and the standard method of measuring this transition is by differential scanning calorimetry (DSC), which measures the specific heat. 

Differential scanning calorimetry was used by Murrill and co-workers (43-45) to elucidate solid - solid phase transitions in a large number of organic compounds. First-order transitions were reported for tetrahedral compounds of the type CR1R2R2R4, where R is methyl, methylol, amino, nitro, and carboxy, as well as for octahedral-type compounds. This technique was also used to detect phase transitions in alkali metal stearates (46), some dibenzazepines, carbazoles, and phenothiazines (16), and the half esters of O-phthalic acid (31). The solid-state decomposition kinetics and activation parameters of N-aryl-N -tosyl-oxydt-imide N-oxides were determined using DSC by Dorko et al. (49). 

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