Physical melting enthalpy

A suitable phase change temperature and a large melting enthalpy are the requirements that have always to be met by a PCM. However, there are more requirements that have to be met for most, but not all applications. These are Physical requirements ... 

TTie presence of crystallites is very important for product performance because crystallites serve as physical crosslinking points which reinforce material and improve its viscoelastic properties. It is therefore very important to notice that the mechanism peculiar to PVC allows for melting of crystallites and their formation on cooling. Melting of crystallites improves PVC processability (lowers melt viscosity and improves its flow), and recovery of crystallites on cooling improves performance characteristics of PVC. The gelation depends on the melting enthalpies of primary and secondary crystallites. 

Poly(L-lactic acid) (PLEA) is hydrolytically unstable and does not withstand humid heat. The more and more extended radiation sterilization on medical wear imposes investigation of induced effects. It undergoes random chain scission, when subjected to ionizing radiation consequences of this phenomenon on the crystalline state can reflect the induced modifications. The linear decrease of crystallinity (Table 40) [02K3] describes the constant deterioration of molecular structure. Physical properties like melting enthalpy and crystallization heat which are sensitive to the modification in molecular size and interactions are adequately mitigated (Fig. 58). 

The properties of plastics, and especially the physical aging mechanisms in semicrystalline plastics, depend mainly on their degree of crystallization. The degree of crystallization can be detected using the melt enthalpy measured in DSC ... 

After a week, specific melt enthalpy and the annealing peak remain constant, indicating that the physical aging processes are completed. These processes are more pronounced in oxidative atmosphere because of chemo-crystallization (see Section 1.4.2.2) than in inert atmosphere. Figure 5.168 shows the effects of thermal and thermal-oxidative aging on the mechanical properties of polypropylene. 

Figure 8.12 The melting enthalpies of carbamazepine form III compared with theoretical values for mixtures of carbamazepine form III and carbamazepine form I. In this experiment the individual polymorphs were not physically mixed so that no interaction could take place. The fact that measured and theoretical values are the same (when separated) indicates that, as found in previous experiments, interaction does take place where the materials are in intimate contact.

Some physical properties of the elements are compared in Table 10,2. Germanium forms brittle, grey-white lustrous crystals with the diamond structure it is a metalloid with a similar electrical resistivity to Si at room temperature but with a substantially smaller band gap. Its mp, bp and associated enthalpy changes are also lower than for Si and this trend continues for Sn and Pb which are both very soft, low-melting metals. 

Exothermic events, such as crystallization processes (or recrystallization processes) are characterized by their enthalpies of crystallization (AHc). This is depicted as the integrated area bounded by the interpolated baseline and the intersections with the curve. The onset is calculated as the intersection between the baseline and a tangent line drawn on the front slope of the curve. Endothermic events, such as the melting transition in Fig. 4.9, are characterized by their enthalpies of fusion (AHj), and are integrated in a similar manner as an exothermic event. The result is expressed as an enthalpy value (AH) with units of J/g and is the physical expression of the crystal lattice energy needed to break down the unit cell forming the crystal. 

Table XI gives the room-temperature, atmospheric pressure crystal structures, densities, and atomic volumes, along with the melting points and standard enthalpies of vaporization (cohesive energies), for the actinide metals. These particular physical properties have been chosen as those of concern to the preparative chemist who wishes to prepare an actinide metal and then characterize it via X-ray powder diffraction. The numerical values have been selected from the literature by the authors.

We assume, in this case, that the conduction band has become normal (that is, it has no longer any 5 f character). Thus, physical properties may be usefully compared with those of the lanthanides. In Table 5 we report known basic properties (metallic radii, crystal structures, melting temperatures and enthalpies of sublimation) of the transplutonium metals. 

To avoid the use of the ambiguous term "heat" in connection with "heat content," it is customary to use the term enthalpy. At a given temperature and pressure, every substance possesses a characteristic amount of enthalpy (H), and the heat changes associated with chemical and physical changes at constant pressure are called changes in enthalpy (AH) AHT is the enthalpy of transition. Two common enthalpies of transition are AHf = 1435 cal/mole for the enthalpy of fusion (melting) of ice at 0°C, and AH, = 9713 cal/mole for the enthalpy of vaporization of water at 100°C. 

---